Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2021 • 298 Effect of cooling rate on the microstructure and mechanical properties of a low-carbon low-alloyed steel
    Wang, H. and Cao, L. and Li, Y. and Schneider, M. and Detemple, E. and Eggeler, G.
    Journal of Materials Science 56 11098-11113 (2021)
    Heavy plate steels with bainitic microstructures are widely used in industry due to their good combination of strength and toughness. However, obtaining optimal mechanical properties is often challenging due to the complex bainitic microstructures and multiple phase constitutions caused by different cooling rates through the plate thickness. Here, both conventional and advanced microstructural characterization techniques which bridge the meso- and atomic-scales were applied to investigate how microstructure/mechanical property-relationships of a low-carbon low-alloyed steel are affected by phase transformations during continuous cooling. Mechanical tests show that the yield strength increases monotonically when cooling rates increase up to 90 K/s. The present study shows that this is associated with a decrease in the volume fraction of polygonal ferrite (PF) and a refinement of the substructure of degenerated upper bainite (DUB). The fine DUB substructures feature C-rich retained austenite/martensite-austenite (RA/M-A) constitutes which decorate the elongated micrograin boundaries in ferrite. A further increase in strength is observed when needle-shaped cementite precipitates form during water quenching within elongated micrograins. Pure martensite islands on the elongated micrograin boundaries lead to a decreased ductility. The implications for thick section plate processing are discussed based on the findings of the present work. © 2021, The Author(s).
    view abstractdoi: 10.1007/s10853-021-05974-3
  • 2021 • 297 Testing procedure for fatigue characterization of steel‐cfrp hybrid laminate considering material dependent self‐heating
    Mrzljak, S. and Schmidt, S. and Kohl, A. and Hülsbusch, D. and Hausmann, J. and Walther, F.
    Materials 14 (2021)
    Combining carbon fiber reinforced polymers (CFRP) with steel offers the potential of utilizing the desired characteristics of both materials, such as specific strength/stiffness and fatigue strength of fiber reinforced polymers (FRP) and impact resistance of metals. Since in such hybrid laminates multiple material layers are combined, a gradual failure is likely that can lead to changes in mechanical properties. A failure of the metal partner leads to an increase in stress on the FRP, which under fatigue load results in increased self‐heating of the FRP. Therefore, a suitable testing procedure is required and developed in this study, to enable a reproducible characterization of the mechanical properties under fatigue load. The resulting testing procedure, containing multiple frequency tests as well as load increase and constant amplitude tests, enabled characterization of the fatigue performance while never exceeding a testing induced change in temperature of 4 K. In addition to the development of the testing procedure, an insight into the manufacturing induced residual stresses occurring in such hybrid laminates, which impacts the load‐bearing capacity, was established using finite element simulation. The gathered data and knowledge represents a basis for future in‐depth investigations in the area of residual stress influence on the performance of hybrid laminates and highlights its importance, since not only the used testing procedure determines the measured fatigue performance. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma14123394
  • 2020 • 296 Crystal structure and composition dependence of mechanical properties of single-crystalline NbCo2 Laves phase
    Luo, W. and Kirchlechner, C. and Zavašnik, J. and Lu, W. and Dehm, G. and Stein, F.
    Acta Materialia 184 151-163 (2020)
    Extended diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases with concentration gradients covering their entire homogeneity ranges were produced by the diffusion couple technique. Single-phase and single-crystalline micropillars of the cubic and hexagonal NbCo2 Laves phases were prepared in the diffusion layers by focused ion beam (FIB) milling. The influence of chemical composition, structure type, orientation and pillar size on the deformation behavior and the critical resolved shear stress (CRSS) was studied by micropillar compression tests. The pillar orientation influences the activated slip systems, but the deformation behavior and the CRSS are independent of orientation. The deformation of the smallest NbCo2 micropillars (0.8 µm in top diameter) appears to be dislocation nucleation controlled and the CRSS approaches the theoretical shear stress for dislocation nucleation. The CRSS of the 0.8 µm-sized NbCo2 micropillars is nearly constant from 26 to 34 at.% Nb where the C15 structure is stable. It decreases as the composition approaches the Co-rich and Nb-rich boundaries of the homogeneity range where the C15 structure transforms to the C36 and the C14 structure, respectively. The decrease in the CRSS at these compositions is related to the reduction of shear modulus and stacking fault energy. As the pillar size increases, stochastic deformation behavior and large scatter in the CRSS values occur and obscure the composition effect on the CRSS. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.036
  • 2020 • 295 Effect of the bias voltage on the structural and tribo-mechanical properties of Ag-containing amorphous carbon films
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Nienhaus, A. and Thomann, C.A. and Wittrock, A. and Moldenhauer, H. and Debus, J.
    Diamond and Related Materials 105 (2020)
    The modification of amorphous carbon films by a mixture of Ag atoms is a promising approach to reduce the residual stresses in the coating and to improve its adhesion to the substrate. Besides the Ag concentration, the bias voltage has a crucial impact on the properties of carbon-based films. Therefore, the effect of the bias voltage on the structural and tribo-mechanical properties of hydrogen free a-C:Ag is investigated. The a-C:Ag films are sputtered from graphite targets with varying number of Ag pellets by setting the bias voltage to −100, −150, and −200 V. A non-modified a-C and two a-C:Ag film systems with different Ag content are synthetized to obtain a comprehensive understanding about the influence of the bias voltage on the properties of the a-C:Ag films. A high bias voltage leads to a reduction in the amount of Ag within the a-C:Ag films, since impinging ions remove Ag atoms during the film growth. Additionally, XRD analyses show the formation of large Ag nanocrystallites with rising bias voltage. In Raman scattering studies, an Ag-induced graphitization of the a-C films is identified. The graphitization is less pronounced at low Ag concentrations and high bias voltages. The residual stresses increase with rising bias voltage and decreasing Ag content, which also favor greater values of hardness and elastic modulus. While a high bias voltage results in a poor adhesion strength for the a-C films, a good adhesion behavior is observed for the a-C:Ag films. It is ascribed to lower stresses in the a-C:Ag films as compared to that in a-C. The friction behavior of the a-C:Ag films is not influenced by the bias voltage, since the coefficients of friction vary from 0.26 to 0.32 against a steel counterpart in tribometer tests. An agglomeration of Ag particles in the tribological contact is observed for all a-C:Ag films which contributes to the slightly higher friction when compared to non-modified a-C films. On the whole, it is demonstrated that the tribo-mechanical properties of a-C:Ag are not only affected by the Ag content, but also by the applied bias voltage. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.diamond.2020.107803
  • 2020 • 294 Effects of acetylene flow rate and bias voltage on the structural and tribo-mechanical properties of sputtered a-C:H films
    Tillmann, W. and Ulitzka, H. and Lopes Dias, N.F. and Stangier, D. and Thomann, C.A. and Moldenhauer, H. and Debus, J.
    Thin Solid Films 693 (2020)
    The properties of sputtered a-C:H films are significantly influenced by the C2H2 flow rate and bias voltage. A suitable Design of Experiments allows to consider their effects on the mechanical and tribological properties. The a-C:H films are deposited by varying the C2H2 flow rate from 5.9 to 34.1 sccm and the bias voltage from −83 to −197 V, following the Central Composite Design. In Raman scattering studies, the presence of C[sbnd]H bands with increasing C2H2 flow rate is identified. Additionally, a decrease of the I(D)/I(G) ratio is observed with increasing C2H2 flow rate. Both observations indicate the formation of sp³-hybridized C[sbnd]H bonds. In contrast, a low C2H2 flow rate and a high bias voltage result in a higher I(D)/I(G) ratio and a lower intensity of the C[sbnd]H stretching bands, indicating a lower amount of C[sbnd]H bonds. The mechanical properties are also considerably influenced by these parameters. A higher C2H2 proportion results in a lower hardness and elastic modulus, which are related to a higher H content. However, a higher bias voltage increases the hardness and elastic modulus due to densification mechanisms, which increase the degree of distortion of the a-C:H films. Consequently, a low C2H2 flow rate and a high bias voltage ensure a high hardness of up to ~24 GPa due to a lower amount of C[sbnd]H bonds and a higher degree of distortion. In tribometer tests, most a-C:H films exhibit a low coefficient of friction against steel, ranging from 0.23 to 0.25. All a-C:H films are marked by a deformative wear, indicating a high resistance against abrasive wear when sliding against steel. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2019.137691
  • 2020 • 293 Gas atomization and laser additive manufacturing of nitrogen-alloyed martensitic stainless steel
    Boes, J. and Röttger, A. and Theisen, W. and Cui, C. and Uhlenwinkel, V. and Schulz, A. and Zoch, H.-W. and Stern, F. and Tenkamp, J. and Walther, F.
    Additive Manufacturing 34 (2020)
    Nitrogen as an alloying element can improve the corrosion resistance and the mechanical properties of stainless steels. Therefore, nitrogen-alloyed martensitic stainless steels, such as X30CrMoN151, have been developed in recent decades and conventional processing of this steel by casting or powder metallurgy is well understood. However, only very few attempts to process nitrogen-alloyed martensitically hardenable stainless steels containing more than 0.2 mass-% of carbon by laser powder bed fusion (L-PBF) have been reported so far. In this study, X30CrMoN15-1 steel powder has been produced from quasi nitrogen-free X30CrMo15-1 steel by gas atomization using N2 as the process gas to introduce nitrogen into the steel. The gas-atomized powder was characterized in terms of nitrogen content, particle size distribution, particle morphology, and flow properties. The powder was then processed by L-PBF under an N2 gas atmosphere, and microstructural investigations were performed on the L-PBF-built samples using scanning electron microscopy and X-ray computed tomography. Additionally, a first impression of the mechanical properties of the L-PBF-built steel in the as-built and quenched and tempered condition was obtained by means of fatigue tests. It was shown that a nitrogen content of 0.16 mass-% could be introduced into the steel during gas atomization. The resulting powder was successfully processed by means of L-PBF, and specimens with a high density were produced. During fatigue testing, a large amount of retained austenite in the as-built condition resulted in a greater damage tolerance of the specimens compared to the heat-treated condition. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2020.101379
  • 2020 • 292 Influence of Process Parameters on the Aerosol Deposition (AD) of Yttria-Stabilized Zirconia Particles
    Mishra, T.P. and Singh, R. and Mücke, R. and Malzbender, J. and Bram, M. and Guillon, O. and Vaßen, R.
    Journal of Thermal Spray Technology (2020)
    Aerosol deposition (AD) is a novel deposition process for the fabrication of dense and rather thick oxide films at room temperature. The bonding of the deposited ceramic particles is based on a shock-loading consolidation, resulting from the impact of the ceramic particles on the substrate. However, the deposition mechanism is not fully understood. In addition, many technical challenges have been observed for achieving a successful deposition of the oxides with higher efficiency. In this work, the influence of different processing parameters on the properties of the deposited layer is studied. Proof of concept was done using 8 mol.% yttria-stabilized zirconia (8YSZ) powder as starting material. The window of deposition with respect to carrier gas flows for successful deposition was identified. The influence of this carrier gas flow, the substrate materials and the carrier gas species on the coating thickness, interface quality and coating microstructure was systematically investigated. The derived mechanical characteristics revealed an unexpected behavior related to a gradient microstructure. This study supports understanding of the mechanism of room-temperature impact consolidation and its effect on the mechanical properties of the deposited layer. © 2020, ASM International.
    view abstractdoi: 10.1007/s11666-020-01101-x
  • 2020 • 291 Joint investigation of strain partitioning and chemical partitioning in ferrite-containing TRIP-assisted steels
    Tan, X. and Ponge, D. and Lu, W. and Xu, Y. and He, H. and Yan, J. and Wu, D. and Raabe, D.
    Acta Materialia 186 374-388 (2020)
    We applied two types of hot-rolling direct quenching and partitioning (HDQ&P) schemes to a low-C low-Si Al-added steel and obtained two ferrite-containing TRIP-assisted steels with different hard matrix structures, viz, martensite or bainite. Using quasi in-situ tensile tests combined with high-resolution electron back-scattered diffraction (EBSD) and microscopic digital image correlation (µ-DIC) analysis, we quantitatively investigated the TRIP effect and strain partitioning in the two steels and explored the influence of the strain partitioning between the soft and hard matrix structures on the TRIP effect. We also performed an atomic-scale analysis of the carbon partitioning among the different phases using atom probe tomography (APT). The results show that the strain mainly localizes in the ferrite in both types of materials. For the steel with a martensitic hard-matrix, a strong strain contrast exists between ferrite and martensite, with the local strain difference reaching up to about 75% at a global strain of 12.5%. Strain localization bands initiated in the ferrite rarely cross the ferrite/martensite interfaces. The low local strain (2%–10%) in the martensite regions leads to a slight TRIP effect with a transformation ratio of the retained austenite of about 7.5%. However, for the steel with bainitic matrix, the ferrite and bainite undergo more homogeneous strain partitioning, with an average local strain in ferrite and bainite of 15% and 8%, respectively, at a global strain of 12.5%. The strain localization bands originating in the ferrite can cross the ferrite/bainite (F/B) interfaces and increase the local strain in the bainite regions, resulting in an efficient TRIP effect. In that case the transformation ratio of the retained austenite is about 41%. The lower hardness difference between the ferrite and bainite of about 178 HV, compared with that between the ferrite and martensite of about 256 HV, leads to a lower strain contrast at the ferrite/bainite interfaces, thus retarding interfacial fracture. Further microstructure design for TRIP effect optimization should particularly focus on adjusting the strength contrast among the matrix structures and tuning strain partitioning to enhance the local strain partitioning into the retained austenite. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.12.050
  • 2020 • 290 Microstructure and mechanical properties of 316L austenitic stainless steel processed by different SLM devices
    Röttger, A. and Boes, J. and Theisen, W. and Thiele, M. and Esen, C. and Edelmann, A. and Hellmann, R.
    International Journal of Advanced Manufacturing Technology 108 769-783 (2020)
    In this work, we examined the influence of different types of selective laser melting (SLM) devices on the microstructure and the associated material properties of austenitic 316L stainless steel. Specimens were built using powder from the same powder batch on four different SLM machines. For the specimen build-up, optimized parameter sets were used, as provided by the manufacturers for each individual SLM machine. The resulting microstructure was investigated by means of scanning electron microscopy, which revealed that the different samples possess similar microstructures. Differences between the microstructures were found in terms of porosity, which significantly influences the material properties. Additionally, the build-up direction of the specimens was found to have a strong influence on the mechanical properties. Thus, the defect density defines the material’s properties so that the ascertained characteristic values were used to determine a Weibull modulus for the corresponding values in dependence on the build-up direction. Based on these findings, characteristic averages of the mechanical properties were determined for the SLM-manufactured samples, which can subsequently be used as reference parameters for designing industrially manufactured components. © 2020, The Author(s).
    view abstractdoi: 10.1007/s00170-020-05371-1
  • 2020 • 289 Microstructure and Mechanical Properties of Reactive-Air-Brazed 3YSZ/Crofer 22 APU Joints at Ambient Temperature
    Tillmann, W. and Anar, N.B. and Wojarski, L. and Lassner, J.J.
    Metallography, Microstructure, and Analysis 9 529-540 (2020)
    The growth of inventive high-temperature electrochemical devices such as solid oxide fuel cells constitutes a major task in brazing technology of ceramic–metal joints. In this work, reactive air brazing was used and the joining characteristics of 3YSZ with Crofer 22 APU have been systematically analyzed for three different brazing temperatures (1000, 1050 and 1100 °C) and two dwell times (5 and 30 min). The joints have been brazed successfully using the Ag–4CuO filler alloy. This braze filler metal was manufactured by an arc PVD (physical vapor deposition) process. Further, sufficient wetting of the zirconium oxide was achieved. The morphology of the oxide reaction layer at the steel side had a major influence on the shear strength of the brazed joints. A maximum average shear strength of 101 ± 4 MPa was obtained for a temperature of 1050 °C and a dwell time of 5 min. © 2020, ASM International.
    view abstractdoi: 10.1007/s13632-020-00663-0
  • 2020 • 288 Nitrogen doping of MoSx thin films sputtered by reactive High Power Impulse Magnetron Sputtering
    Tillmann, W. and Wittig, A. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
    Thin Solid Films 713 (2020)
    Incorporating nitrogen into non-stoichiometric molybdenum disulfide (MoSx) thin films is a promising approach in order to improve the mechanical properties. Nevertheless, the adhesion between the film and the substrate is still challenging and the interaction between the mechanical and the tribological properties is not fully understood yet. Subsequently, reactive High Power Impulse Magnetron Sputtering (HiPIMS) is used to deposit nitrogen doped MoSx thin films with different nitrogen amounts on 16MnCr5 steel. The interaction between the structural changes, the mechanical properties and the tribological behavior depending on the nitrogen amount is investigated. The results prove that an increasing amount of nitrogen significantly affects the structure and the tribo-mechanical properties of the thin films. X-ray diffraction analysis reveals a transformation from crystalline to amorphous with an increasing amount of nitrogen from (7.1 ± 0.3) at.-% to (19.5 ± 0.5) at.-%. This transformation is related to a suppression of the columnar microstructure as well as an increasing hardness and Young‘s modulus from (0.14 ± 0.02) GPa, and (5.28 ± 0.32) GPa for the undoped film, to (5.12 ± 0.32) GPa and (92.5 ± 6.2) GPa, for the film with the highest nitrogen amount. The results of the Rockwell indentation tests show that the films with a small amount of nitrogen exhibit an improved adhesion behavior. The wear coefficient can be reduced to a quarter of the value of the undoped MoSx film, whereas coefficients of friction are at similar level of 0.2 in ambient air. Reactive HiPIMS has proven to be promising to deposit nitrogen doped MoSx thin films on steel substrates, which reveal improved mechanical properties and an excellent transfer film built-up during the tribo-tests without failures. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2020.138267
  • 2020 • 287 Sintering and biocompatibility of blended elemental Ti-xNb alloys
    Chen, Y. and Han, P. and Dehghan-Manshadi, A. and Kent, D. and Ehtemam-Haghighi, S. and Jowers, C. and Bermingham, M. and Li, T. and Cooper-White, J. and Dargusch, M.S.
    Journal of the Mechanical Behavior of Biomedical Materials 104 (2020)
    Titanium-niobium (Ti–Nb) alloys have great potential for biomedical applications due to their superior biocompatibility and mechanical properties that match closely to human bone. Powder metallurgy is an ideal technology for efficient manufacture of titanium alloys to generate net-shape, intricately featured and porous components. This work reports on the effects of Nb concentrations on sintered Ti-xNb alloys with the aim to establish an optimal composition in respect to mechanical and biological performances. Ti-xNb alloys with 33, 40, 56 and 66 wt% Nb were fabricated from elemental powders and the sintering response, mechanical properties, microstructures and biocompatibility assessed and compared to conventional commercial purity titanium (CPTi). The sintered densities for all Ti-xNb compositions were around 95%, reducing slightly with increasing Nb due to increasing open porosity. Higher Nb levels retarded sintering leading to more inhomogeneous phase and pore distributions. The compressive strength decreased with increasing Nb, while all Ti-xNb alloys displayed higher strengths than CPTi except the Ti–66Nb alloy. The Young's moduli of the Ti-xNb alloys with ≥40 wt% Nb were substantially lower (30–50%) than CPTi. In-vitro cell culture testing revealed excellent biocompatibility for all Ti-xNb alloys comparable or better than tissue culture plate and CPTi controls, with the Ti–40Nb alloy exhibiting superior cell-material interactions. In view of its mechanical and biological performance, the Ti–40Nb composition is most promising for hard tissue engineering applications. © 2020
    view abstractdoi: 10.1016/j.jmbbm.2020.103691
  • 2020 • 286 Strain rate-dependent characterization of carbon fibre-reinforced composite laminates using four-point bending tests
    Rojas-Sanchez, J.F. and Schmack, T. and Boesl, B. and Bjekovic, R. and Walther, F.
    Journal of Reinforced Plastics and Composites 39 165-174 (2020)
    This research addresses the problem of accurately quantifying the strain rate effect of carbon fibre-reinforced plastics by proposing a method with a simple specimen manufacturing and experiment execution based on four-point bending tests. By easing the strain rate-dependent characterization of carbon fibre-reinforced plastics, less conservative designs can be achieved. The method proposed uses Euler–Bernoulli and Timoshenko’s beam theories to obtain the longitudinal compressive and tensile modulus, compressive strength, shear modulus, and shear yielding point. Transverse properties could not be obtained due to limitations of the fixture employed. A strain-dependent material characterization was done using the proposed method and compared to the characterization of the same material using traditional uniaxial tests. Most of the material properties obtained with different methods correlated within approximately 10%. More work needs to be done to determine how this discrepancy affects simulation results. © The Author(s) 2019.
    view abstractdoi: 10.1177/0731684419879235
  • 2020 • 285 The influence of oxygen on the chemical composition and mechanical properties of Ti-6Al-4V during laser powder bed fusion (L-PBF)
    Dietrich, K. and Diller, J. and Dubiez-Le Goff, S. and Bauer, D. and Forêt, P. and Witt, G.
    Additive Manufacturing 32 (2020)
    In Laser powder bed fusion (L-PBF), metal powders, sensitive to humidity and oxygen, like AlSi10Mg or Ti-6Al-4 V are used as starting material. Titanium-based materials are influenced by oxygen and nitrogen due to the formation of oxides and nitrides, respectively. During this research, the oxygen concentration in the build chamber was controlled from 2 ppm to 1000 ppm using an external measurement device. Built Ti-6Al-4 V specimens were evaluated regarding their microstructure, hardness, tensile strength, notch toughness, chemical composition and porosity, demonstrating the importance of a stable atmospheric control. It could be shown that an increased oxygen concentration in the shielding gas atmosphere leads to an increase of the ultimate tensile strength by 30 MPa and an increased (188.3 ppm) oxygen concentration in the bulk material. These results were compared to hot isostatic pressed (HIPed) samples to prevent the influence of porosity. In addition, the fatigue behavior was investigated, revealing increasingly resistant samples when oxygen levels in the atmosphere are lower. © 2019
    view abstractdoi: 10.1016/j.addma.2019.100980
  • 2019 • 284 Achieving ultra-high strength and ductility in equiatomic CrCoNi with partially recrystallized microstructures
    Slone, C.E. and Miao, J. and George, E.P. and Mills, M.J.
    Acta Materialia 165 496-507 (2019)
    Despite having otherwise outstanding mechanical properties, many single-phase medium and high entropy alloys are limited by modest yield strengths. Although grain refinement offers one opportunity for additional strengthening, it requires significant and undesirable compromises to ductility. This work therefore explores an alternative, simple processing route to achieve strength by cold-rolling and annealing an equiatomic CrCoNi alloy to produce heterogeneous, partially recrystallized microstructures. Tensile tests reveal that our approach dramatically increases the yield strength (to ∼1100 MPa) while retaining good ductility (total elongation ∼23%) in the single-phase CrCoNi alloy. Scanning and transmission electron microscopy indicate that the strengthening is due to the non-recrystallized grains retaining their deformation-induced twins and very high dislocation densities. Load-unload-reload tests and grain-scale digital image correlation are also used to study the accumulation of plastic deformation in our highly heterogeneous microstructures. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.015
  • 2019 • 283 Bitumen rheology and the impact of rejuvenators
    Ganter, D. and Mielke, T. and Maier, M. and Lupascu, D.C.
    Construction and Building Materials 222 414-423 (2019)
    Bitumen is a material used in many industrial applications. It is the primary binding material for road pavements in asphalt. To improve sustainability, it is important to be aware of the finite nature of conventional oil reserves. As a cost and energy saving material, reclaimed asphalt pavement can help preserve our resources. Virgin binder ages significantly during its service life time yielding declining mechanical properties. As material properties of bitumen are responsible for the endurance of asphalt pavements, adding rejuvenators is supposed to restore the mechanical properties like the original bitumen properties. In this paper, three different rejuvenators were studied using one common polymer modified bitumen PmB 25/55-55 A. The virgin binder was aged in two different aging steps (short and long-term) and rheological properties were determined by traditional bitumen tests and dynamic shear rheometer (DSR) tests. For a better understanding of rejuvenation on the microscale, virgin, aged, and modified bitumen samples were measured using Optical Microscopy and Atomic Force Microscopy (AFM). Aging has significant influence on the macro- and micro-Rheology of bitumen. It causes changes in viscosity and at the same time clear changes in its surface structure. All Rejuvenators positively influence the rheological properties to different extents. Atomic Force Microscopy revealed considerable changes of the morphology between virgin, aged, and rejuvenated binders. The combination of rheological properties and micro structure imaging is an important tool in advancing and optimizing reclaimed asphalt pavement. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.conbuildmat.2019.06.177
  • 2019 • 282 Columnar to equiaxed transition and grain refinement of cast CrCoNi medium-entropy alloy by microalloying with titanium and carbon
    Liu, X.W. and Laplanche, G. and Kostka, A. and Fries, S.G. and Pfetzing-Micklich, J. and Liu, G. and George, E.P.
    Journal of Alloys and Compounds 775 1068-1076 (2019)
    Thermomechanical processing has been used to control the grain size/shape of the equiatomic CrCoNi medium-entropy alloy (MEA) and obtain excellent strength and ductility. However, in the cast state, the alloy has coarse columnar grains with average widths and lengths of approximately 120 and 1000 μm, respectively, resulting in inferior mechanical properties. To overcome this deficiency, here we microalloyed with Ti and C and successfully changed the grain shape (from columnar to equiaxed) and refined the grain size. The degree to which the microstructure changes depends on the amount of Ti and C added, with the best results obtained at 0.4 at.% each. In the optimal alloy [(CrCoNi)99.2Ti0.4C0.4], the as-cast grains were nearly equiaxed with a uniform size of ∼75 μm. Associated with this change in grain shape/size was a significant improvement of yield strength, ultimate tensile strength and elongation to fracture at both 293 and 77 K. The columnar to equiaxed transition is attributed to the strong mutual affinity of C and Ti, which leads to their build-up ahead of the solid-liquid interface and, in turn, to enhanced constitutional undercooling. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2018.10.187
  • 2019 • 281 Crystal plasticity finite element simulation and experiment investigation of nanoscratching of single crystalline copper
    Wang, Z. and Zhang, H. and Li, Z. and Li, G. and Zhang, J. and Zhang, J. and Hassan, H.U. and Yan, Y. and Hartmaier, A. and Sun, T.
    Wear 430-431 100-107 (2019)
    Mechanical properties of crystalline materials strongly correlate with deformation behaviour at the grain level. In the present work, we establish a 3D crystal plasticity finite element model of nanoscratching of single crystalline copper using a Berkovich probe, which is capable of addressing the crystallography influence. In particular, nanoindentation experiments and high resolution electron back-scatter diffraction characterization are jointly carried out to precisely calibrate parameters used in the crystal plasticity finite element model. Subsequent finite element simulations of nanoscratching are performed to reveal fundamental deformation behaviour of single crystalline copper in terms of mechanical response and surface pile-up topography, as well as their dependence on crystallographic orientation. Furthermore, nanoscratching experiments with the same parameters used in the finite element simulations are carried out, the results of which are further compared with predication results by the finite element simulations. Simulation data and experimental results jointly demonstrate the strong anisotropic characteristics of single crystalline copper under nanoscratching, due to the crystallographic orientation dependent coupled effects of intrinsic dislocation slip and extrinsic discrete stress distribution by probe geometry. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2019.04.024
  • 2019 • 280 Development of a high flow rate aerodynamic lens system for inclusion of nanoparticles into growing PVD films to form nanocomposite thin films
    Kiesler, D. and Bastuck, T. and Kennedy, M.K. and Kruis, F.E.
    Aerosol Science and Technology 53 630-646 (2019)
    Hard coatings for wear protection of tools, bearings, and sliding parts play an important role in industrial manufacturing. Nanocomposite coatings are being used in this context to improve the mechanical properties. The technology applied therefore is often based on physical vapor deposition (PVD), in which the different materials are co-deposited. In these processes it is not possible to control the properties of the disperse phase and continuous phase independently. Here, we present a technology which combines aerosol technology with thin film technology to produce nanocomposite coatings directly, which gives us full control over both phases. It is based on an upscaled three-stage aerodynamic lens, which allows to bring nanoparticles from an atmospheric-pressure aerosol reactor into a PVD vacuum chamber operating at low pressure (2 Pa). This requires the use of a higher mass flow rate than conventionally used in aerodynamic lenses, so that a rational upscaling strategy for designing an aerodynamic lens for larger mass flow rates is proposed. Here, an array consisting of eight parallel three-stage aerodynamic lenses having each a mass flow rate of 0.6 slm using argon and 0.71 slm using nitrogen is built and optimized, assisted by CFD and numerical trajectory analysis. The transfer efficiency has been investigated numerically and experimentally. It is possible to transfer 80% of the particles with only 1.3% of the gas into the deposition chamber. A number of coatings consisting of titanium carbonitride nanoparticles embedded in a PVD chromium oxynitride film with varying nanoparticle content were produced. Electron microscopy shows the successful incorporation of the nanoparticles in the thin film. A reduction in film crystallite size with increasing nanoparticle content was found. A reverse Hall–Petch behavior was observed. Copyright © 2019 American Association for Aerosol Research. © 2019, © 2019 American Association for Aerosol Research.
    view abstractdoi: 10.1080/02786826.2019.1587149
  • 2019 • 279 Emergence and impact of Al2TiO5 in Al2O3-TiO2 APS coatings
    Richter, A. and Berger, L.-M. and Conze, S. and Sohn, Y.J. and Vaßen, R.
    IOP Conference Series: Materials Science and Engineering 480 (2019)
    Despite numerous studies and decades of industrial application, there is still a lack of understanding about the formation and the impact of aluminum titanate (Al2TiO5) in Al2O3-TiO2 thermal spray coatings. Especially the influence of the feedstock powder characteristics on the phase composition has only crudely been investigated so far. Therefore, in this work we have characterized commercial fused and crushed Al2O3-TiO2 feedstock powders: Three of them containing 13 wt.% TiO2 and three containing 40 wt.% TiO2. The effect of the varying phase compositions of the powders and their relevance on the deposition efficiency, the phase compositions, the porosity, and the hardness of the respective APS coatings is described in detail. While detrimental to the mechanical properties of 40 wt.% TiO2 coatings, we have found an enhancement of the hardness for 13 wt.% TiO2 coatings with a high Al2TiO5/Al6Ti2O13 content in the feedstock powder. Furthermore, it was found that Al2TiO5 may reform during APS when sprayed from an Al2TiO5-free powder. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/480/1/012007
  • 2019 • 278 Gradients of microstructure, stresses and mechanical properties in a multi-layered diamond thin film revealed by correlative cross-sectional nano-analytics
    Gruber, D.P. and Todt, J. and Wöhrl, N. and Zalesak, J. and Tkadletz, M. and Kubec, A. and Niese, S. and Burghammer, M. and Rosenthal, M. and Sternschulte, H. and Pfeifenberger, M.J. and Sartory, B. and Keckes, J.
    Carbon 144 666-674 (2019)
    Thin diamond films deposited by chemical vapour deposition (CVD) usually feature cross-sectional gradients of microstructure, residual stress and mechanical properties, which decisively influence their functional properties. This work introduces a novel correlative cross-sectional nano-analytics approach, which is applied to a multi-layered CVD diamond film grown using microwave plasma-enhanced CVD and consisting of a ∼8 μm thick nanocrystalline (NCD) base and a ∼14.5 μm thick polycrystalline (PCD) top diamond sublayers. Complementary cross-sectional 30 nm beam synchrotron X-ray diffraction, depth-resolved micro-cantilever and hardness testing and electron microscopy analyses reveal correlations between microstructure, residual stress and mechanical properties. The NCD sublayer exhibits a 1.5 μm thick isotropic nucleation region with the highest stresses of ∼1.3 GPa and defect-rich nanocrystallites. With increasing sublayer thickness, a 110 fibre texture evolves gradually, accompanied by an increase in crystallite size and a decrease in stress. At the NCD/PCD sublayer interface, texture, stresses and crystallite size change abruptly and the PCD sublayer exhibits the presence of Zone T competitive grain growth microstructure. NCD and PCD sublayers differ in fracture stresses of ∼14 and ∼31 GPa, respectively, as well as in elastic moduli and hardness, which are correlated with their particular microstructures. In summary, the introduced nano-analytics approach provides complex correlations between microstructure, stresses, functional properties and deposition conditions. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.carbon.2018.12.093
  • 2019 • 277 Hierarchical microstructure design to tune the mechanical behavior of an interstitial TRIP-TWIP high-entropy alloy
    Su, J. and Raabe, D. and Li, Z.
    Acta Materialia 163 40-54 (2019)
    We demonstrate a novel approach of utilizing a hierarchical microstructure design to improve the mechanical properties of an interstitial carbon doped high-entropy alloy (HEA) by cold rolling and subsequent tempering and annealing. Bimodal microstructures were produced in the tempered specimens consisting of nano-grains (∼50 nm) in the vicinity of shear bands and recovered parent grains (10–35 μm) with pre-existing nano-twins. Upon annealing, partial recrystallization led to trimodal microstructures characterized by small recrystallized grains (<1 μm) associated with shear bands, medium-sized grains (1–6 μm) recrystallized through subgrain rotation or coalescence of parent grains and retained large un-recrystallized grains. To reveal the influence of these hierarchical microstructures on the strength-ductility synergy, the underlying deformation mechanisms and the resultant strain hardening were investigated. A superior yield strength of 1.3 GPa was achieved in the bimodal microstructure, more than two times higher than that of the fully recrystallized microstructure, owing to the presence of nano-sized grains and nano-twins. The ductility was dramatically improved from 14% to 60% in the trimodal structure compared to the bimodal structure due to the appearance of a multi-stage work hardening behavior. This important strain hardening sequence was attributed to the sequential activation of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects as a result of the wide variation in phase stability promoted by the grain size hierarchy. These findings open a broader window for achieving a wide spectrum of mechanical properties for HEAs, making better use of not only compositional variations but also microstructure and phase stability tuning. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.10.017
  • 2019 • 276 Impact of Al2O3-40 wt.% TiO2 feedstock powder characteristics on the sprayability, microstructure and mechanical properties of plasma sprayed coatings
    Richter, A. and Berger, L.-M. and Sohn, Y.J. and Conze, S. and Sempf, K. and Vaßen, R.
    Journal of the European Ceramic Society 39 5391-5402 (2019)
    Atmospheric plasma sprayed (APS) Al2O3-TiO2 coatings have found a wide range of industrial application due to their favorable properties, combined with low costs and a high availability. However, the detailed effect of the phase composition and the element distribution of the feedstock powders on the coating properties and the spraying process have only crudely been investigated so far. Here the impact of aluminum titanate (Al2TiO5) on the microstructural features and mechanical properties of Al2O3-40 wt.% TiO2 APS coatings is demonstrated by investigating the detailed phase composition and the distribution of aluminum and titanium in three fused and crushed feedstock powders and the respective coatings. Thereby, a direct influence of Al2TiO5 content on the deposition efficiency, the porosity, the elastic modulus, and the hardness of the coatings is revealed. The results emphasize the need for a more detailed specification of commercial Al2O3-TiO2 feedstock powders to ensure a high reliability of the coating properties. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jeurceramsoc.2019.08.026
  • 2019 • 275 Influence of chemical postprocessing on mechanical properties of laser-sintered polyamide 12 parts
    Wörz, A. and Wiedau, L.C. and Wudy, K. and Wegner, A. and Witt, G. and Drummer, D.
    Journal of Polymer Engineering 39 830-837 (2019)
    A limiting factor for industrial usage of laser-sintered parts is the high surface roughness due to the semi-molten or attaching powder particles resulting from tool and pressureless manufacturing. An approach to improve the surface quality is the postprocessing with acids to smoothen the surface as it enables improvement without geometrical restrictions of the parts. The present work deals with the usage of nitric, hydrochloric, and trifluoroacetic acids, and exhibits the influence on the resulting surface morphology, dimensional accuracy, and the mechanical properties. The results exhibit different interaction mechanics and show great differences in the resulting part properties. © 2019 Walter de Gruyter GmbH, Berlin/Boston.
    view abstractdoi: 10.1515/polyeng-2019-0110
  • 2019 • 274 Influence of Microstructural Features on the Strain Hardening Behavior of Additively Manufactured Metallic Components
    Biswas, A. and Prasad, M.R.G. and Vajragupta, N. and ul Hassan, H. and Brenne, F. and Niendorf, T. and Hartmaier, A.
    Advanced Engineering Materials 21 (2019)
    Additive manufacturing (AM) has recently become one of the key manufacturing processes in the era of Industry 4.0 because of its highly flexible production scheme. Due to complex thermal cycles during the manufacturing process itself and special solidification conditions, the microstructure of AM components often exhibits elongated grains together with a pronounced texture. These microstructural features significantly contribute to an anisotropic mechanical behavior. In this work, the microstructure and mechanical properties of additively manufactured samples of 316L stainless steel are characterized experimentally and a micromechanical modeling approach is employed to predict the macroscopic properties. The objective of this work is to study the effects of texture and microstructural morphology on yield strength and strain hardening behavior of face-centered cubic additively manufactured metallic components. To incorporate the texture in synthetic representative volume elements (RVE), the proposed approach considers both the crystallographic and grain boundary textures. The mechanical behavior of these RVEs is modeled using crystal plasticity finite element method, which incorporates size effects through the implementation of strain gradients. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201900275
  • 2019 • 273 Influence of plasma nitriding pretreatments on the tribo-mechanical properties of DLC coatings sputtered on AISI H11
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D.
    Surface and Coatings Technology 357 1027-1036 (2019)
    The duplex treatment, consisting of plasma nitriding and the deposition of a DLC coating, was carried out on the hot-work tool steel AISI H11. The coating structure, composed of Cr-based interlayers and a hydrogenated carbon layer, was sputtered on non-nitrided, nitrided, as well as nitrided-repolished AISI H11 steel with an either annealed or quenched and tempered base condition to examine the influence of the pretreatment condition on the tribo-mechanical properties of the DLC coating. Besides the graded hardness profile, plasma nitriding leads to a roughness increase, which affects the microstructure as well as the mechanical properties of the DLC coating. The rougher surface favors a film growth of a carbon layer with larger cluster-like structures. As a result, these DLC coatings exhibit hardness values below 22 GPa, while the coating systems sputtered on substrates with smoother surfaces reach values of approximately 26 GPa and showed a good adherence. The heat treatment condition influences the load-bearing capacity of the nitrided substrate as the higher core hardness enhances the mechanical support of the coating and reaches the highest adhesion class HF1 in the Rockwell C tests. Due to the lower film adhesion and the low hardness of the DLC coatings sputtered on nitrided non-repolished AISI H11, high coefficients of frictions and wear coefficients of up to 0.59 and 3.19 ∗ 10−5 mm3/N∗m were determined in tribometer tests against WC/Co counterparts. In contrast, the nitrided repolished steel exhibits a low coefficient of friction of 0.12 as well as a low wear coefficient of 0.06 ∗ 10−5 mm3/N∗m. Therefore, a repolishing of the nitrided AISI H11 with quenched and tempered base condition ensures the highest load-bearing capability of the substrate as well as an improved friction and wear behavior of the DLC coating. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2018.11.002
  • 2019 • 272 Interaction effects of cathode power, bias voltage, and mid-frequency on the structural and mechanical properties of sputtered amorphous carbon films
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Bayer, M. and Moldenhauer, H. and Debus, J. and Schmitz, M. and Berges, U. and Westphal, C.
    Applied Surface Science 487 857-867 (2019)
    The Design of Experiments is a promising method to investigate the cause-effect relation between the mid-frequency magnetron sputtering parameters on the structural and mechanical properties of amorphous carbon (a-C) films. Based on the Central Composite Design, the cathode power of two graphite targets, bias voltage, and mid-frequency were simultaneously varied from 1500 to 4000 W, −100 to −200 V, and 20 to 50 kHz, respectively. The chemical bonding state was characterized using UV and visible Raman spectroscopy with excitation wavelengths of 266 and 532 nm. Corresponding measurements were performed by X-ray photoelectron spectroscopy (XPS) using synchrotron radiation. Additionally, hardness and elastic modulus of the sputtered a-C films were determined in nanoindentation tests. Multi-wavelength Raman spectroscopy identified an sp3 content below 20%, with most a-C films having an sp3 value in the range of 12 to 18%. The formation of sp3 bonded atoms is negatively influenced by a high cathode power and bias voltage, whereas the highest sp3 content is obtained with a-C films sputtered with a cathode power and bias voltage of 2750 W and −150 V. However, higher values of the cathode power and bias voltage result in a film delamination and decrease of the sp3 concentration. The bonding state affects the mechanical properties, as high hardness and elastic modulus result from a high sp3 content. Therefore, a targeted adjustment of cathode power and bias voltage is necessary to obtain a-C films with a high hardness. In contrast, the mid-frequency does not have a significant impact on the mechanical properties. In conclusion, the Central Composite Design has proven to be a suitable method to investigate the cause-effects of the sputtering parameters on the properties of the a-C film. © 2019
    view abstractdoi: 10.1016/j.apsusc.2019.05.131
  • 2019 • 271 Investigation of the tribo-mechanical properties of sputtered a-C:Si films using design of experiments
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D.
    Diamond and Related Materials 91 127-137 (2019)
    Adding silicon to sputtered amorphous carbon films is a promising approach to enhance tribo-mechanical properties. Although most works analyse the influence of one specific deposition parameter, the Design of Experiments is a more suitable method to investigate the cause-effect relation between the parameters on the properties of silicon-containing amorphous carbon (a-C:Si) films. In a sputtering process, the cathode power of the silicon target and the bias voltage were simultaneously varied from 217 to 783 W and −83 to −197 V, based on the Central Composite Design. The modification of the chemical composition was evaluated by means of glow-discharge optical emission spectroscopy. To analyse the influence of the silicon content and bias voltage on the tribo-mechanical properties, the hardness and elastic modulus were determined by nanoindentation, while the friction and wear behaviour were investigated in tribometer tests utilizing 100Cr6 counterparts. The silicon content increases linearly from 8.6 to 31.7 at.-% with an increasing cathode power without any impact of the bias voltage. However, an interaction between the two parameters is observed for the mechanical properties. At a high Si content and high bias voltage, the a-C:Si films show a high hardness of 22.0 ± 1.0 GPa and a high elastic modulus of 231.7 ± 10.6 GPa. In contrast, the lowest hardness of 18.1 ± 0.7 GPa and lowest elastic modulus of 203.2 ± 9.6 GPa are obtained at a low Si content and low bias voltage. The lowest coefficient of friction of 0.083 ± 0.029 as well as the lowest wear coefficients of 0.094 ± 0.025 × 10−5 mm3/N × m were obtained by a-C:Si films with a small amount of silicon. It was determined that the tribo-mechanical properties of a-C:Si films are interactively influenced by the silicon content and bias voltage. In this context, the Central Composite Design is an efficient method to obtain fundamental knowledge concerning the interrelation between these parameters. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.diamond.2018.11.014
  • 2019 • 270 Iron Aluminides
    Palm, M. and Stein, F. and Dehm, G.
    Annual Review of Materials Research 49 297-326 (2019)
    The iron aluminides discussed here are Fe-Al-based alloys, in which the matrix consists of the disordered bcc (Fe,Al) solid solution (A2) or the ordered intermetallic phases FeAl (B2) and Fe3Al (D03). These alloys possess outstanding corrosion resistance and high wear resistance and are lightweight materials relative to steels and nickel-based superalloys. These materials are evoking new interest for industrial applications because they are an economic alternative to other materials, and substantial progress in strengthening these alloys at high temperatures has recently been achieved by applying new alloy concepts. Research on iron aluminides started more than a century ago and has led to many fundamental findings. This article summarizes the current knowledge of this field in continuation of previous reviews. © 2019 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-matsci-070218-125911
  • 2019 • 269 Local quasi-static and cyclic deformation behaviour of brazed AISI 304L/BAu-4 joints characterised by digital image correlation
    Schmiedt, A. and Manka, M. and Tillmann, W. and Walther, F.
    Welding in the World 63 501-509 (2019)
    For a reliable design of brazed components, the degradation of mechanical properties due to the corrosive attack by aggressive operating environments has to be considered. In this study, the effect of a condensate corrosion, which is performed according to VDA test sheet 230-214 up to 6 weeks, on the mechanical behaviour of brazed AISI 304L/BAu-4 stainless steel joints is investigated. A time-dependent reduction of the tensile and fatigue strength values down to 42% of the as-received condition is determined. As standard strain measurements are not appropriate to characterise the local strain distributions of heterogeneous material systems, the optical digital image correlation technique is used to evaluate the local quasi-static and cyclic deformation behaviour of the 50 μm wide brazing seam. A novel triggered image acquisition enables measurements in fatigue tests at a frequency of 10 Hz. The reduction of the virtual gauge length from 12.5 down to 0.5 mm leads to an increase of the total strain and ratcheting strain values, which is more pronounced for higher stresses and enhanced for pre-corroded brazed joints. For a microstructure-related analysis of the damage processes, scanning electron microscopy was used. © 2019, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-018-00693-x
  • 2019 • 268 Mechanical properties of honeycomb structured zr-based bulk metallic glass specimens fabricated by laser powder bed fusion
    Wegner, J. and Frey, M. and Stiglmair, P. and Kleszczynski, S. and Witt, G. and Busch, R.
    South African Journal of Industrial Engineering 30 32-40 (2019)
    Laser powder bed fusion of bulk metallic glasses offers great potential to overcome the existing restrictions of the geometrical size and complexity of bulk metallic glasses in conventional manufacturing routes due to high cooling rates during laser powder bed fusion. Bulk metallic glasses exhibit extraordinary strength, paired with high elasticity. Yet insights into additive manufactured bulk metallic glasses, especially of complex structures, are limited. The present article investigates the mechanical behaviour of Zr-based bulk metallic glasses, fabricated into honeycomb structures through laser powder bed fusion, by performing three-point bending tests. The results reveal a significant increase in specific strength, quasi-plasticity, and high elastic elongation. These structures thus offer great potential for light-weight applications and compliant mechanisms. © 2019, South African Institute of Industrial Engineering. All rights reserved.
    view abstractdoi: 10.7166/30-3-2265
  • 2019 • 267 Microstructural and tribological properties of sputtered AlCrSiWN films deposited with segmented powder metallurgic target materials
    Tillmann, W. and Fehr, A. and Stangier, D.
    Thin Solid Films 687 (2019)
    When synthesizing magnetron sputtered films with a complex stoichiometry, integrating the desired coating constituents into one target material is favorable in order to avoid nanolaminar film depositions and to enable a homogenous film growth. In contrast to alloyed targets, segmented plug targets allow to merge elements with different physical properties in one target material. Two targets, amalgamating 20 and 48 hot-pressed 85.6Cr9.9Si4.5W (at. %) plugs, respectively, into a monolithic aluminum target were fabricated and employed in a direct current magnetron sputtering process to deposit AlCrSiWN films on high-speed steel (AISI M3:2, 1.3344). The cathode powers for the Al(CrSiW)20 and Al(CrSiW)48 targets were varied between 7.5 and 17.5 W/cm2 to analyze how differently composed targets and various cathode powers affect the microstructure and tribological properties of the sputtered films. The results revealed that the chemical composition as well as the thickness of the films strongly depend on the target setup. All AlCrSiWN films exhibited a Cr/Si/W ratio of approximately 84/11/6. The Cr and Al contents were dominant (19–29 at. %), while the Si and W contents varied between 2 and 3 at. %. Especially the Al/Cr ratio of the films is affected by the varying Al/CrSiW surface area ratio of the manufactured plug targets. Furthermore, the mechanical properties are significantly influenced by the Al/Cr ratio, which is responsible for a dense coating growth and the crystalline structure of the films. All AlCrSiWN films were (111) textured indicating a B1 (Al, Cr, W)N structure, which exhibited a finer crystalline growth with an increasing cathode power on the Al(CrSiW)20 target. Tribological analyses of the films against Al2O3 balls further revealed that thinner films resulted in a decreased wear coefficient. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2019.137465
  • 2019 • 266 Microstructure and mechanical properties of as-built and heat-treated electron beam melted Ti–6Al–4V
    Syed, A.K. and Awd, M. and Walther, F. and Zhang, X.
    Materials Science and Technology (United Kingdom) 35 653-660 (2019)
    This paper investigates the effect of post-deposition heat treatment on porosity, microstructure, and mechanical properties of Ti–6Al–4V produced via an Electron Beam Melting process. Samples were studied in the conditions of as-built and heat treated at 920°C and 1030°C. The as-built samples were characterised by columnar β grains consists of α+β microstructure with Widmanstätten and colony morphologies were found. Heat treatment resulted in increased α lath width. The yield strength and ultimate tensile strength was greater in the as-built condition than wrought material. Porosity re-growth occurred after heat treatment but it did not affect the tensile properties. Greater ductility after heat treatment was attributed to the larger α lath width which increases effective slip length. © 2019, © 2019 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1080/02670836.2019.1580434
  • 2019 • 265 On the influence of the heat treatment on microstructure formation and mechanical properties of near-α Ti-Fe alloys
    Sandlöbes, S. and Korte-Kerzel, S. and Raabe, D.
    Materials Science and Engineering A 748 301-312 (2019)
    We study the microstructure formation and mechanical properties of Ti-1Fe (wt%) and Ti-3Fe (wt%) alloys for different heat treatments in the β-phase and α + β-phase regions. By applying different heat treatment routes, we observe different microstructure formation mechanisms causing a wide range of mechanical properties from high strength (1.3 GPa) and low ductility (2%) to intermediate strength (700 MPa) and high ductility (30%) in these simple binary alloys. We performed microstructure characterizsation using scanning electron microscopy, transmission electron microscopy and atom probe tomography to show that the alloying content and heat treatment significantly affect the local martensitic and / or diffusional phase transformations causing the substantial changes in the mechanical behavior. © 2018
    view abstractdoi: 10.1016/j.msea.2018.12.071
  • 2019 • 264 On the mechanism of extraordinary strain hardening in an interstitial high-entropy alloy under cryogenic conditions
    Wang, Z. and Lu, W. and Raabe, D. and Li, Z.
    Journal of Alloys and Compounds 734-743 (2019)
    We investigate the cryogenic deformation response and underlying mechanisms of a carbon-doped interstitial high-entropy alloy (iHEA) with a nominal composition of Fe49.5Mn30Co10Cr10C0.5 (at. %). Extraordinary strain hardening of the iHEA at 77 K leads to a substantial increase in ultimate tensile strength (∼1300 MPa) with excellent ductility (∼50%) compared to that at room temperature. Prior to loading, iHEAs with coarse (∼100 μm) and fine (∼6 μm) grain sizes show nearly single face-centered cubic (FCC) structure, while the fraction of hexagonal close-packed (HCP) phase reaches up to ∼70% in the cryogenically tensile-fractured iHEAs. Such an unusually high fraction of deformation-induced phase transformation and the associated plasticity (TRIP effect) is caused by the strong driving force supported by the reduced stacking fault energy and increased flow stress at 77 K. The transformation mechanism from the FCC matrix to the HCP phase is revealed by transmission electron microscopy (TEM) observations. In addition to the deformation-induced phase transformation, stacking faults and dislocation slip contribute to the deformation of the FCC matrix phase at low strains and of the HCP phase at medium and large strains, suggesting dynamic strain partitioning among these two phases. The combination of TRIP and dynamic strain partitioning explain the striking strain hardening capability and resulting excellent combination of strength and ductility of iHEAs under cryogenic conditions. The current investigation thus offers guidance for the design of high-performance HEAs for cryogenic applications. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2018.12.061
  • 2019 • 263 Rods glued in engineered hardwood products part I: Experimental results under quasi-static loading
    Grunwald, C. and Vallée, T. and Fecht, S. and Bletz-Mühldorfer, O. and Diehl, F. and Bathon, L. and Myslicki, S. and Scholz, R. and Walther, F.
    International Journal of Adhesion and Adhesives 90 163-181 (2019)
    Glued-in Rods (GiR) represent an adhesively bonded structural connection widely used in timber engineering. Up to now, common practice largely focused on softwood. Most structural adhesives have been, accordingly, specifically formulated to perform on softwood, in particular spruce. The increased use of hardwood, and corresponding engineered wood products (EWP), calls for deeper insights regarding GiR for the connection thereof. This paper, the first of a two part series, presents an overview over extensive research carried with 9 adhesives, 3 EWP, and 4 types of rods. Investigations started at component level, by fully characterising all adhesives, EWP, and rods. They were then extended to characterise the behaviour of interfaces, providing by this a methodology for selecting adhesives. Investigations at full scale followed, involving 5 different adhesives, 3 EWP, and 4 rod types. A total of 180 individual samples were tested. The results allowed to draw conclusions about the relationship between performance of GiR connections, and mechanical properties of their components. This relationship, however, has been found to be relatively weak. The companion paper will present a design methodology based on the material properties determined herein, and explain the ambiguous relationship between performance of the GiR and the mechanical properties of the adhesive, wood, and rods © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijadhadh.2018.05.003
  • 2019 • 262 Sintering behavior of columnar thermal barrier coatings deposited by axial suspension plasma spraying (SPS)
    Zhou, D. and Malzbender, J. and Sohn, Y.J. and Guillon, O. and Vaßen, R.
    Journal of the European Ceramic Society 39 482-490 (2019)
    During the last decade, Suspension Plasma Spraying (SPS) attracted a lot of interest as an alternative process to produce columnar Thermal Barrier Coatings (TBCs). In this study, columnar TBCs were deposited with SPS. After spraying, samples were isothermally annealed at 1373 K for 1 h, 3 h, 10 h and 50 h, respectively. Microstructures and mechanical properties of the ceramic coatings were investigated as a function of annealing time. Annealing resulted in healing of micro-cracks, coarsening of pores, growth of domain size, companied with a decrease of porosity within columns. The change of coating microstructure led to change of mechanical properties. In addition, residual stress in SPS coatings was also investigated. Furthermore, as-sprayed coatings and pre-annealed coatings were subjected to burner rig tests. Short time pre-annealing allowed to enhance thermal cycling lifetime of such SPS coatings. The thermal cycling results were related to microstructure modifications of coatings. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.jeurceramsoc.2018.09.020
  • 2019 • 261 Site-specific quasi in situ investigation of primary static recrystallization in a low carbon steel
    Diehl, M. and Kertsch, L. and Traka, K. and Helm, D. and Raabe, D.
    Materials Science and Engineering A 755 295-306 (2019)
    Low-alloyed steels with body-centered cubic crystal structure are a material class that is widely used for sheet metal forming applications. When having an adequate crystallographic texture and microstructure, their mechanical behavior is characterized by an isotropic in-plane flow behavior in combination with a low yield strength. The decisive processing steps for obtaining these beneficial mechanical properties are cold rolling and subsequent annealing. While for the former the number of passes, the deformation rates, and the total thickness reduction are the main processing parameters, the latter is described mainly by the heating rate and the holding temperature and time. Primary static recrystallization during annealing subsequent to the cold rolling process alters mainly two aspects of the material state: It firstly replaces the elongated and heavily deformed grains of the cold rolled microstructure by small, globular grains with low dislocation density and secondly it changes the crystallographic texture insofar as it typically diminishes the α- and strengthens the γ-fiber texture components. In the present work, the recrystallization behavior of a commercial non-alloyed low carbon steel is studied. A quasi in situ setup that enables site-specific characterization is employed to gain a local picture of the nucleation and recrystallization process. From the Kernel Average Misorientation (KAM) values of the deformation structure, the tendency to be consumed by new grains can be predicted. Crystallographic analysis shows that the most deformed regions have either a γ-fiber orientation or belong to heavily fragmented regions. New grains nucleate especially in such highly deformed regions and inherit often the orientation from the deformation microstructure. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.02.032
  • 2019 • 260 Structural and mechanical properties of carbon incorporation in DC/HiPIMS CrAlN coatings
    Tillmann, W. and Stangier, D. and Roese, P. and Shamout, K. and Berges, U. and Westphal, C. and Debus, J.
    Surface and Coatings Technology 374 774-783 (2019)
    Incorporating carbon into ternary nitride coatings to tune the mechanical and tribological properties of thin films is of great interest in order to improve the performance of tools and components. Especially, the approach to tailor CrAlN coatings by doping transition metals has been extensively studied in recent years. Nevertheless, the microstructural changes, induced by carbon incorporation, especially into Al-rich CrAlN coatings, are not yet fully understood. Thus, detailed investigations of the microstructure, performed by means of synchrotron radiation, using x-ray diffraction and x-ray photoelectron spectroscopy and Raman scattering with different laser excitation (355 nm and 532 nm), were conducted to understand the evolution of the mechanical properties of CrAlCN coatings depending on the carbon content. The results prove that an increasing carbon content significantly influences the microstructure, residual stresses, as well as the mechanical properties of the coatings. The presence of C[dbnd]C and C[dbnd]N bonds was proven by investigating the C 1s orbital. Furthermore, the increasing amount of carbon forms amorphous Cr[sbnd]C structures, which were detected by analyzing the Cr 3p orbital. These results were confirmed for the amorphous phases by Raman scattering additionally indicating the formation of nanocomposite structures due to the formation of carbon nano-onoin like structures. The investigations of the crystalline structure using XRD reveal the existence of a fcc structure for the CrAlN phase as well as small amounts of hexagonal AlN in the CrAlCN coating with the highest carbon content. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2019.06.053
  • 2019 • 259 Structure and mechanical properties of hafnium nitride films deposited by direct current, mid-frequency, and high-power impulse magnetron sputtering
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Tolan, M. and Paulus, M.
    Thin Solid Films 669 65-71 (2019)
    The structural properties of hafnium nitride films are mainly influenced by the deposition conditions, which are affected by the sputtering technique. A suitable use of the different sputtering modes allows to control the structural development of the films and thus to adjust the profile of the properties. NaCl-type hafnium nitride films were deposited using direct current magnetron sputtering (dcMS), mid-frequency magnetron sputtering (mfMS), and high-power impulse magnetron sputtering (HiPIMS). dcMS produces films with a columnar microstructure, whereas a fully-dense morphology is achieved by mfMS and HiPIMS. X-ray diffraction patterns show that films sputtered in dcMS mode have a (200) orientation, whereas mfMS and HiPIMS favor an orientation with the (111) plane parallel to the samples’ surface. mfMS leads to films with the largest crystal sizes and lowest stresses, which is ascribed to recrystallization mechanisms during the film growth. Hafnium films with an overstoichiometric composition show the highest hardness values. In this context, the dcMS-Hf49.8N50.2, mfMS-Hf50.4N49.6, and HiPIMS-Hf49.0N51.0 have a hardness of 28.2 ± 2.1, 32.4 ± 3.4, and 30.4 ± 3.1 GPa, respectively. In summary, the sputtering technique has a crucial role on the properties of the film and can be suitable used to adjust the structure and hardness of HfN films. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2018.10.035
  • 2019 • 258 Synthesis and mechanical testing of grain boundaries at the micro and sub-micro scale
    Malyar, N.V. and Springer, H. and Wichert, J. and Dehm, G. and Kirchlechner, C.
    Materialpruefung/Materials Testing 61 5-18 (2019)
    The important role of grain boundaries for the mechanical properties of polycrystalline materials has been recognized for many decades. Up to now, the underlying deformation mechanisms at the nano- and micro scale are not understood quantitatively. An overview of the synthesis and subsequent mechanical testing of specific grain boundaries at the micro and sub-micro scale is discussed in the present contribution, including various methods for producing one or multiple specific, crystallographically well-defined grain boundaries. Furthermore, established micromachining methods for isolating and measuring local dislocation-grain boundary interactions are portrayed. Examples of the techniques described are shown with to the aid of copper grain boundaries. © Carl Hanser Verlag, München
    view abstractdoi: 10.3139/120.111286
  • 2019 • 257 Temperature and load-ratio dependent fatigue-crack growth in the CrMnFeCoNi high-entropy alloy
    Thurston, K.V.S. and Gludovatz, B. and Yu, Q. and Laplanche, G. and George, E.P. and Ritchie, R.O.
    Journal of Alloys and Compounds 794 525-533 (2019)
    Multiple-principal element alloys known as high-entropy alloys have rapidly been gaining attention for the vast variety of compositions and potential combinations of properties that remain to be explored. Of these alloys, one of the earliest, the ‘Cantor alloy’ CrMnFeCoNi, displays excellent damage-tolerance with tensile strengths of ∼1 GPa and fracture toughness values in excess of 200 MPa√m; moreover, these mechanical properties tend to further improve at cryogenic temperatures. However, few studies have explored its corresponding fatigue properties. Here we expand on our previous study to examine the mechanics and mechanisms of fatigue-crack propagation in the CrMnFeCoNi alloy (∼7 μm grain size), with emphasis on long-life, near-threshold fatigue behavior, specifically as a function of load ratio at temperatures between ambient and liquid-nitrogen temperatures (293 K–77 K). We find that ΔKth fatigue thresholds are decreased with increasing positive load ratios, R between 0.1 and 0.7, but are increased at decreasing temperature. These effects can be attributed to the role of roughness-induced crack closure, which was estimated using compliance measurements. Evidence of deformation twinning at the crack tip during fatigue-crack advance was not apparent at ambient temperatures but seen at higher stress intensities (ΔK ∼ 20 MPa√m) at 77 K by post mortem microstructural analysis for tests at R = 0.1 and particularly at 0.7. Overall, the fatigue behavior of this alloy was found to be superior, or at least comparable, to conventional cryogenic and TWIP steels such as 304 L or 316 L steels and Fe-Mn steels; these results coupled with the remarkable strength and fracture toughness of the Cantor alloy at low temperatures indicate significant promise for the utility of this material for applications at cryogenic environments. © 2019
    view abstractdoi: 10.1016/j.jallcom.2019.04.234
  • 2019 • 256 Tungsten carbide as a deoxidation agent for plasma-facing tungsten-based materials
    Šestan, A. and Zavašnik, J. and Kržmanc, M.M. and Kocen, M. and Jenuš, P. and Novak, S. and Čeh, M. and Dehm, G.
    Journal of Nuclear Materials 524 135-140 (2019)
    Tungsten (W) and various composites are being considered as the primary plasma-facing materials for fusion reactors. Like all engineering materials, they contain certain levels of impurities, which can have an important impact on mechanical properties. In the present work, oxygen was identified as a major impurity in our starting tungsten powder. At elevated temperatures, the presence of interstitial elements such as oxygen leads to the formation of an oxide-rich tungsten phase at the tungsten grain boundaries. In this study, we determined the capacity of tungsten carbide (WC) nanoparticles to remove the oxide impurities from a tungsten body. Tungsten composites with 0.05, 0.25 and 0.51 wt. % carbon (C) in the form of WC were sintered using a field-assisted sintering technique (FAST) at 1900 °C for 5 min. The sintered samples were characterized using field-emission scanning and transmission electron microscopy. Thermodynamic and kinetic considerations allowed us to determine the optimum theoretical amount of WC to prevent the in-situ formation of WO2. © 2019 Andreja Šestan, Janez Zavašnik, Marjeta Maček Kržmanc, Matej Kocen, Petra Jenuš, Saša Novak, Miran Čeh, Gerhard Dehm
    view abstractdoi: 10.1016/j.jnucmat.2019.06.030
  • 2019 • 255 Understanding the role of cu and clustering on strain hardening and strain rate sensitivity of al-mg-si-cu alloys
    Langille, M. and Diak, B.J. and De Geuser, F. and Guiglionda, G. and Meddeb, S. and Zhao, H. and Gault, B. and Raabe, D. and Deschamps, A.
    Minerals, Metals and Materials Series 143-151 (2019)
    Increased demand for light-weighting in passenger vehicles has created a need for strong, light, ductile materials to be used in body-in-white applications. The AA6xxx-series of aluminum alloys are suitable candidates meeting most requirements but can fall short of the formability demands of designers, necessitating an understanding of what controls the formability in this alloy series. This work examines the effect of copper alloying in AA6xxx on the pre-ageing and natural ageing responses of the microstructure and mechanical properties. The changes in microstructure observed by differential scanning calorimetry and hardness testing are related to the work-hardening and strain-rate sensitivity parameters for these alloys measured by tensile testing. An observed asymmetry in the measured strain-rate sensitivity associated with increasing versus decreasing strain rate changes suggests that a different mechanism operates for the two conditions. It is postulated how this asymmetry in strain-rate sensitivity will impact the necking and ductility behaviour of these alloys. © 2019, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-030-05864-7_20
  • 2018 • 254 Ab initio simulation of hydrogen-induced decohesion in cementite-containing microstructures
    McEniry, E.J. and Hickel, T. and Neugebauer, J.
    Acta Materialia 150 53-58 (2018)
    In high-strength carbon steels suitable for use in the automotive industry, hydrogen embrittlement (HE) is a potential barrier to the widespread application of these materials. The behaviour of hydrogen within the most prevalent carbide, namely cementite, has been investigated via ab initio simulation. In order to examine possible decohesion effects of hydrogen on carbon steels, the binding and diffusion of hydrogen at the interface between ferrite and cementite has been examined. In order to understand the effect of hydrogen on the mechanical properties of carbon steels, simulated ab initio tensile tests have been performed on the ferrite-cementite bicrystal. The results of the tensile tests can be combined with thermodynamic considerations in order to obtain the expected hydrogen concentrations at such ferrite-cementite phase boundaries. We find that the effect of hydrogen on the cohesion of the phase boundary may be significant, even when the bulk hydrogen concentration is low. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.03.005
  • 2018 • 253 Advanced Materials through Assembly of Nanocelluloses
    Kontturi, E. and Laaksonen, P. and Linder, M.B. and Nonappa and Gröschel, A.H. and Rojas, O.J. and Ikkala, O.
    Advanced Materials 30 (2018)
    There is an emerging quest for lightweight materials with excellent mechanical properties and economic production, while still being sustainable and functionalizable. They could form the basis of the future bioeconomy for energy and material efficiency. Cellulose has long been recognized as an abundant polymer. Modified celluloses were, in fact, among the first polymers used in technical applications; however, they were later replaced by petroleum-based synthetic polymers. Currently, there is a resurgence of interest to utilize renewable resources, where cellulose is foreseen to make again a major impact, this time in the development of advanced materials. This is because of its availability and properties, as well as economic and sustainable production. Among cellulose-based structures, cellulose nanofibrils and nanocrystals display nanoscale lateral dimensions and lengths ranging from nanometers to micrometers. Their excellent mechanical properties are, in part, due to their crystalline assembly via hydrogen bonds. Owing to their abundant surface hydroxyl groups, they can be easily modified with nanoparticles, (bio)polymers, inorganics, or nanocarbons to form functional fibers, films, bulk matter, and porous aerogels and foams. Here, some of the recent progress in the development of advanced materials within this rapidly growing field is reviewed. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adma.201703779
  • 2018 • 252 Boron doped ultrastrong and ductile high-entropy alloys
    Seol, J.B. and Bae, J.W. and Li, Z. and Chan Han, J. and Kim, J.G. and Raabe, D. and Kim, H.S.
    Acta Materialia 151 366-376 (2018)
    A new class of materials called high-entropy alloys (HEAs) constitutes multiple principal elements in similar compositional fractions. The equiatomic Fe20Mn20Cr20Co20Ni20 (at%) HEA shows attractive mechanical properties, particularly under cryogenic conditions. Yet, it lacks sufficient yield and ultimate tensile strengths at room temperature. To strengthen these materials, various strategies have been proposed mainly by tuning the composition of the bulk material while no efforts have been made to decorate and strengthen the grain boundaries. Here, we introduce a new HEA design approach that is based on compositionally conditioning the grain boundaries instead of the bulk. We found that as little as 30 ppm of boron doping in single-phase HEAs, more specific in an equiatomic FeMnCrCoNi and in a non-equiatomic Fe40Mn40Cr10Co10 (at%), improves dramatically their mechanical properties, increasing their yield strength by more than 100% and ultimate tensile strength by ∼40% at comparable or even better ductility. Boron decorates the grain boundaries and acts twofold, through interface strengthening and grain size reduction. These effects enhance grain boundary cohesion and retard capillary driven grain coarsening, thereby qualifying boron-induced grain boundary engineering as an ideal strategy for the development of advanced HEAs. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.04.004
  • 2018 • 251 Comparison between the optical properties of injection molded and additive manufactured components
    Kuehn, C. and Mehl, O. and Laumer, T. and Witt, G.
    Procedia CIRP 74 259-263 (2018)
    Fused Layer Manufacturing (FLM) is an additive technology based on polymer material extrusion. Due to variations in temperature during the manufacturing process and the resulting stress between the stacked layers, the final parts show anisotropic mechanical properties. One possible approach for their reduction is the immediate local preheating of the surface via laser radiation. At first, our research examines the influence of laser parameters as wavelength, power, velocity and area of impact for the preheating of the surface. In addition, an overview of possible parameter combinations is given based on the selection of raw materials, its colors, thicknesses and the manufacturing process. Initially, the absorption level of the materials regarding the emitted wavelength is detected using a spectrophotometer. Subsequently, preheating tests are conducted with different laser types while the temperature is determined by a thermal camera. The selected laser type is planned to get mounted on a prototype-machine for further in-situ preheating experiments on FLM parts during the manufacturing process. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.
    view abstractdoi: 10.1016/j.procir.2018.08.106
  • 2018 • 250 Construction of statistically similar representative volume elements for discontinuous fiber composites
    Sasagawa, T. and Tanaka, M. and Omote, R. and Balzani, D.
    Composite Structures 203 193-203 (2018)
    A computational method is proposed for the construction of statistically similar representative volume elements (SSRVEs) for discontinuous fiber composites (DFCs) in order to enable an efficient calculation of material properties based on computational homogenization. The SSRVEs are obtained by solving an optimization problem which minimizes the difference between the power spectral density of a target microstructure and its simplified one. The SSRVEs are constructed for target microstructures serving as examples for DFCs, which are validated by means of comparing the mechanical properties of the target microstructures with the ones of the SSRVEs. The results show that the mechanical properties of the SSRVEs agree with the target microstructures and that the SSRVEs can extremely reduce the computational costs of finite element analyses to derive macroscopic material properties of DFCs. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.compstruct.2018.06.014
  • 2018 • 249 Coupled effect of crystallographic orientation and indenter geometry on nanoindentation of single crystalline copper
    Wang, Z. and Zhang, J. and Hassan, H.U. and Zhang, J. and Yan, Y. and Hartmaier, A. and Sun, T.
    International Journal of Mechanical Sciences 148 531-539 (2018)
    Surface pile-up topography is very significant for property extraction in nanoindentation tests. In the present work, we perform crystal plasticity finite element simulations of Berkovich nanoindentation of single crystalline copper with different crystallographic orientations, which derive quantitatively comparable mechanical properties and surface pile-up topographies with experimental data. Simulation results demonstrate that there is a coupled effect of crystallographic orientation of indented material and indenter geometry on surface pile-up behavior, due to the interaction between intrinsic dislocation slip events and extrinsic discrete stress distribution patterns. Based on the relative spatial orientation between crystallographic orientation of indented material and indenter geometry, a surface pile-up density factor mp is proposed to qualitatively characterize the propensity of surface pile-up behavior in nanoindentation tests of single crystalline copper. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijmecsci.2018.09.007
  • 2018 • 248 Damage mechanisms and mechanical properties of high-strength multiphase steels
    Heibel, S. and Dettinger, T. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Materials 11 (2018)
    The usage of high-strength steels for structural components and reinforcement parts is inevitable for modern car-body manufacture in reaching lightweight design as well as increasing passive safety. Depending on their microstructure these steels show differing damage mechanisms and various mechanical properties which cannot be classified comprehensively via classical uniaxial tensile testing. In this research, damage initiation, evolution and final material failure are characterized for commercially produced complex-phase (CP) and dual-phase (DP) steels in a strength range between 600 and 1000 MPa. Based on these investigations CP steels with their homogeneous microstructure are characterized as damage tolerant and hence less edge-crack sensitive than DP steels. As final fracture occurs after a combination of ductile damage evolution and local shear band localization in ferrite grains at a characteristic thickness strain, this strain measure is introduced as a new parameter for local formability. In terms of global formability DP steels display advantages because of their microstructural composition of soft ferrite matrix including hard martensite particles. Combining true uniform elongation as a measure for global formability with the true thickness strain at fracture for local formability the mechanical material response can be assessed on basis of uniaxial tensile testing incorporating all microstructural characteristics on a macroscopic scale. Based on these findings a new classification scheme for the recently developed high-strength multiphase steels with significantly better formability resulting of complex underlying microstructures is introduced. The scheme overcomes the steel designations using microstructural concepts, which provide no information about design and production properties. © 2018 by the authors.
    view abstractdoi: 10.3390/ma11050761
  • 2018 • 247 Densification of nanocrystalline NdFeB magnets processed by electro-discharge sintering – Microstructure, magnetic, and mechanical properties
    Leich, L. and Röttger, A. and Theisen, W. and Krengel, M.
    Journal of Magnetism and Magnetic Materials 460 454-460 (2018)
    This work investigates the densification process of nanocrystalline NdFeB powder by electro-discharge sintering (EDS) and the associated magnetic properties. The EDS technique is used as a fast and energy-saving compaction process for metal powders. A large current is discharged from capacitors into a pre-compacted loose powder, thus resulting in complete compaction. In this study, the microstructure, magnetic, and mechanical properties of the compacted, hard magnetic NdFeB specimens were investigated under variation of the energy EEDS and compression load pEDS. For all specimens, the intrinsic coercivity HcJ decreases on increasing the discharge energy. However, the compaction load has apparently no influence on the coercivity HcJ, whereas the residual induction Br decreases only with increasing discharge energy. An increase in the compression load pEDS causes an increase in the specimens’ density and thus promotes residual induction Br. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross-section of the EDS-densified specimens. Volume fractions of the three different microstructures that form during the EDS process determine the resulting mechanical and magnetic properties of the specimens. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2018.04.035
  • 2018 • 246 Development of high modulus steels based on the Fe – Cr – B system
    Baron, C. and Springer, H. and Raabe, D.
    Materials Science and Engineering A 724 142-147 (2018)
    We present a novel alloy design strategy for cost-efficient high modulus steels with an increased stiffness / mass density ratio. The concept is based on the liquid metallurgy synthesis of Fe – Cr – B based alloys, straightforward processability, and well tuneable mechanical properties via plain heat treatments. The base alloy Fe – 18 Cr – 1.6 B (wt%) contained 14–17 vol% of (Cr,Fe)2B particles of ellipsoidal morphology in a ferritic matrix. Hot rolled materials revealed a specific modulus of 32.8 GPa g−1 cm3, exceeding that of conventional Fe-Cr steels by almost 30%. Mechanical properties obtained are comparable to TiB2 based high modulus steels. Addition of 1 wt% Cu to the base alloy did not interact with the formation, fraction, size and morphology of (Cr,Fe)2B particles, and allowed to mildly increase the strength values by ageing treatments, however at the price of a reduction of the specific modulus. C additions of 0.2 wt% did not affect the (Cr,Fe)2B particle microstructure greatly, but free C dissolved in the matrix enables for the first time to utilize the wide range of microstructures and mechanical properties of established C-containing high strength steels also in high modulus steels. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2018.03.082
  • 2018 • 245 Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel
    Benzing, J.T. and Poling, W.A. and Pierce, D.T. and Bentley, J. and Findley, K.O. and Raabe, D. and Wittig, J.E.
    Materials Science and Engineering A 711 78-92 (2018)
    The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3 mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.11.017
  • 2018 • 244 Experimental and Numerical Investigations on Interdiffusion Profiles in Compounds Produced by Sinter-Cladding
    Blüm, M. and Theisen, W. and Weber, S.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 4991-5000 (2018)
    Tools used for mineral processing applications are affected by strong abrasive wear and high dynamic loads. This results in opposing demands on the mechanical properties of these tools. Therefore, modern concepts for the manufacturing of mineral processing tools include a composite tool concept consisting of a low-alloyed substrate and a high-alloyed, wear-resistant cladding material. These coatings can be applied using different production processes such as composite casting, deposit welding, and HIP cladding. During the deposition of the cladding, interdiffusion between the substrate and cladding material occurs. This interdiffusion may have a negative impact on the compound, since characteristics such as wear resistance, mechanical properties, and the local microstructure are influenced. This article is focused on the investigation and simulation of interdiffusion processes in supersolidus-sintered compounds using computational thermodynamics, diffusion calculations, optical emission spectrometry, hardness profiles, and microstructural investigations. It is shown that the interdiffusion processes between the solid substrate and the semisolid cladding can be simulated using a dispersed phase model to give results with a close concordance to optical emission spectrometry measurements. © 2018, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-018-4750-9
  • 2018 • 243 Hardness and modulus of Fe2B, Fe3(C,B), and Fe23(C,B)6 borides and carboborides in the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Materials Characterization 135 192-202 (2018)
    This work provides a comparative and comprehensive study of the indentation hardness and indentation modulus of iron-rich borides and carboborides of types Fe2B, Fe3(C,B), and Fe23(C,B)6. In addition, the hardness and elastic modulus of Cr-rich M7C are investigated for comparative purposes. We investigated the impact of increasing B content and indentation size effect (ISE). The phases of interest were stabilized in cast Fe-C-B alloys that varied with respect to the B / (B + C) ratio and heat treatment. The resulting microstructures were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength X-ray spectroscopy (WDS). Dynamic in-situ nanoindentation experiments based on the method of continuous stiffness measurement (CSM) were coupled to SEM and EBSD investigations to determine the mechanical properties of the individual borides and carboborides as a function of the indentation depth. The results were compared to values obtained for the Cr-rich M7C3 carbide. It was found that the hardness of the B-rich Fe3(C,B) phase is considerably higher than pure Fe3C and increases with increasing B content. The ISE was present in all investigated phases, and the hardness decreased as a function of indentation depth. The hardness at infinite indentation depth H0 was estimated according to the model of Nix and Gao. The Fe2B phase was found to be the hardest phase (H0 = 19.04 GPa), followed by M7C3 (H0 = 16.43 GPa), Fe3(C,B) (H0 = 11.18 to 12.24 GPa), and Fe23(C,B)6 (H0 = 10.39 GPa). © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.matchar.2017.11.012
  • 2018 • 242 Hydrogen embrittlement of tungsten induced by deuterium plasma: Insights from nanoindentation tests
    Fang, X. and Kreter, A. and Rasinski, M. and Kirchlechner, C. and Brinckmann, S. and Linsmeier, C. and Dehm, G.
    Journal of Materials Research 33 3530-3536 (2018)
    Hydrogen exposure has been found to result in metal embrittlement. In this work, we use nanoindentation to study the mechanical properties of polycrystalline tungsten subjected to deuterium plasma exposure. For the purpose of comparison, nanoindentation tests on exposed and unexposed reference tungsten were carried out. The results exhibit a decrease in the pop-in load and an increase in hardness on the exposed tungsten sample after deuterium exposure. No significant influence of grain orientation on the pop-in load was observed. After a desorption time of td ≥ 168 h, both the pop-in load and hardness exhibit a recovering trend toward the reference state without deuterium exposure. The decrease of pop-in load is explained using the defactant theory, which suggests that the presence of deuterium facilitates the dislocation nucleation. The increase of hardness is discussed based on two possible mechanisms of the defactant theory and hydrogen pinning of dislocations. © 2018 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2018.305
  • 2018 • 241 Imaging Inelastic Fracture Processes in Biomimetic Nanocomposites and Nacre by Laser Speckle for Better Toughness
    Verho, T. and Karppinen, P. and Gröschel, A.H. and Ikkala, O.
    Advanced Science 5 (2018)
    Mollusk nacre is a prototypical biological inorganic–organic composite that combines high toughness, stiffness, and strength by its brick-and-mortar microstructure, which has inspired several synthetic mimics. Its remarkable fracture toughness relies on inelastic deformations at the process zone at the crack tip that dissolve stress concentrations and stop cracks. The micrometer-scale structure allows resolving the size and shape of the process zone to understand the fracture processes. However, for better scalability, nacre-mimetic nanocomposites with aligned inorganic or graphene nanosheets are extensively pursued, to avoid the packing problems of mesoscale sheets like in nacre or slow in situ biomineralization. This calls for novel methods to explore the process zone of biomimetic nanocomposites. Here the fracture of nacre and nacre-inspired clay/polymer nanocomposite is explored using laser speckle imaging that reveals the process zone even in absence of changes in optical scattering. To demonstrate the diagnostic value, compared to nacre, the nacre-inspired nanocomposite develops a process zone more abruptly with macroscopic crack deflection shown by a flattened process zone. In situ scanning electron microscopy suggests similar toughening mechanisms in nanocomposite and nacre. These new insights guide the design of nacre-inspired nanocomposites toward better mechanical properties to reach the level of synergy of their biological model. © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/advs.201700635
  • 2018 • 240 Inconel 625 lattice structures manufactured by selective laser melting (SLM): Mechanical properties, deformation and failure modes
    Leary, M. and Mazur, M. and Williams, H. and Yang, E. and Alghamdi, A. and Lozanovski, B. and Zhang, X. and Shidid, D. and Farahbod-Sternahl, L. and Witt, G. and Kelbassa, I. and Choong, P. and Qian, M. and Brandt, M.
    Materials and Design 157 179-199 (2018)
    Additive Manufacture (AM) enables the fabrication of highly complex lattice structures with exceptional engineering properties. Inconel is a technically useful material in that it provides high resistance to oxidisation, creep and loss of mechanical properties at elevated temperatures. The combination of Inconel material properties and the geometric freedom of AM provides a unique opportunity for the fabrication of engineered structures with exceptional strength and stiffness at elevated temperatures, as for example is required for high temperature turbomachinery. Despite the associated technical opportunities, there exists no design data on the mechanical response, deformation characteristics and failure modes of AM Inconel 625 lattice structures. This research provides a comprehensive reference for the mechanical response of Inconel 625 lattice structures fabricated by Selective Laser Melting (SLM). Furthermore, the high ductility of Inconel 625 lattice enables novel insight into the structural mechanics of AM lattice, and the associated deformation photography provides a reference for the validation and verification of numerical models of AM lattice behaviour. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2018.06.010
  • 2018 • 239 Investigation of multiple laser shock peening on the mechanical property and corrosion resistance of shipbuilding 5083Al alloy under a simulated seawater environment
    Wang, H. and Huang, Y. and Zhang, W. and Ostendorf, A.
    Applied Optics 57 6300-6308 (2018)
    To investigate the effect of laser shock peening (LSP) with different LSP impacts on the mechanical properties in artificial seawater and corrosion resistance of shipbuilding 5083Al alloy in 3.5% NaCl solution, wear property and electrochemical corrosion resistance tests were performed by a ball-on-disk sliding wear tester and electrochemical workstation. The wear mass losses of the samples treated by 1 and 3 LSP impacts are much lower, by 55.22% and 65.94%, respectively, than those of untreated specimens in artificial seawater. Compared with the untreated sample, the electrochemical corrosion rate of the treated samples decreased by 74.91% and 95.03% after being treated by 1 and 3 LSP impacts, respectively. The reasons for the enhancement of the wear properties and electrochemical corrosion behavior were caused by the increased residual stress and microhardness after the LSP treatment. © 2018 Optical Society of America.
    view abstractdoi: 10.1364/AO.57.006300
  • 2018 • 238 Laser additive manufacturing of oxide dispersion strengthened steels using laser-generated nanoparticle-metal composite powders
    Wilms, M.B. and Streubel, R. and Frömel, F. and Weisheit, A. and Tenkamp, J. and Walther, F. and Barcikowski, S. and Schleifenbaum, J.H. and Gökce, B.
    Procedia CIRP 74 196-200 (2018)
    A new route for the synthesis of powder composites suitable for processing with laser additive manufacturing is demonstrated. The powder composites, consisting of micrometer-sized stainless steel powder, homogenously decorated with nano-scaled Y2O3 powder particles, are manufactured by laser processing of colloids and electrostatic deposition. Consolidated by laser metal deposition and selective laser melting, the resulting specimens show superior mechanical properties at elevated temperatures, caused by the nano-sized, homogenously distributed dispersoids. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.
    view abstractdoi: 10.1016/j.procir.2018.08.093
  • 2018 • 237 Mechanical characterization of friction drilled internal threads in AZ91 profiles
    Wittke, P. and Teschke, M. and Walther, F.
    International Journal of Advanced Manufacturing Technology 99 3111-3122 (2018)
    In this study, the influence of friction drilling tool pre-heating on mechanical properties of chipless manufactured internal threads in thin-walled AZ91 magnesium casting alloy profiles is investigated. In this context, the influence of manufacturing processes on microstructure and the resulting fracture behavior during mechanical loading are in focus for the determination of failure mechanisms. Two batches were investigated, whereas specimens were manufactured without and with pre-heating the friction drilling tool before manufacturing. The mechanical properties were determined in tensile and fatigue tests in tensile loading range. The mechanical results were correlated with the profile qualities in form of computed tomography analyses and hardness mappings. Light and electron microscopic investigations of fractured surfaces were performed to analyze the fracture behavior in cyclic tests. Process-related and stress-related work hardening effects were determined at the edge area of the threads. Differences in fracture behavior under quasi-static and cyclic loads were determined. Turns of internal threads connected to the threaded rod were sheared off in tensile tests without visible cracks on the exterior surface of the flat profile specimens, whereas cyclically tested specimens provided fractured surfaces for fractographic failure analyses. Crack initiation at thread root and two stages of crack propagation until complete failure due to overload fracture were investigated. Pre-heating of the friction drilling tool during manufacturing of the threads had no influence on quasi-static and fatigue properties, respectively. © 2018, Springer-Verlag London Ltd., part of Springer Nature.
    view abstractdoi: 10.1007/s00170-018-2698-y
  • 2018 • 236 Microstructure and mechanical properties in the thin film system Cu-Zr
    Oellers, T. and Raghavan, R. and Chakraborty, J. and Kirchlechner, C. and Kostka, A. and Liebscher, C.H. and Dehm, G. and Ludwig, Al.
    Thin Solid Films 645 193-202 (2018)
    A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library. © 2017
    view abstractdoi: 10.1016/j.tsf.2017.10.030
  • 2018 • 235 Microstructure and mechanical properties of Al0.7CoCrFeNi high-entropy-alloy prepared by directional solidification
    Liu, G. and Liu, L. and Liu, X. and Wang, Z. and Han, Z. and Zhang, G. and Kostka, A.
    Intermetallics 93 93-100 (2018)
    The high-entropy-alloy Al0.7CoCrFeNi (molar ratio) was prepared by vacuum arc melting followed by directional solidification (DS) with &lt;001&gt; oriented seed. The unique lamellar-dendrite microstructure was obtained over a wide cooling rate range. During solidification, Fe and Co are prone to segregate to the dendrite, while Cr and Al segregate to interdendrite. The solute pile-up of Cr and Al at the solid/liquid interface leads to the dendritic solidification. During the following cooling process, the BCC phase precipitates from the FCC dendrite to form the lamellar structure, while the ordered B2 phase precipitates from the interdendrite. Moreover, the lamellar spacing is significantly refined with increasing cooling rate, resulting in the higher hardness and compressive yield strength. Directional solidification is proved to be an efficient way to improve the mechanical properties of multi-phases high-entropy alloys. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.intermet.2017.11.019
  • 2018 • 234 Microstructure and Mechanical Properties of CMSX-4 Single Crystals Prepared by Additive Manufacturing
    Körner, C. and Ramsperger, M. and Meid, C. and Bürger, D. and Wollgramm, P. and Bartsch, M. and Eggeler, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 1-12 (2018)
    Currently, additive manufacturing (AM) experiences significant attention in nearly all industrial sectors. AM is already well established in fields such as medicine or spare part production. Nevertheless, processing of high-performance nickel-based superalloys and especially single crystalline alloys such as CMSX-4® is challenging due to the difficulty of intense crack formation. Selective electron beam melting (SEBM) takes place at high process temperatures (~ 1000 °C) and under vacuum conditions. Current work has demonstrated processing of CMSX-4® without crack formation. In addition, by using appropriate AM scan strategies, even single crystals (SX SEBM CMSX-4®) develop directly from the powder bed. In this contribution, we investigate the mechanical properties of SX SEBM CMSX-4® prepared by SEBM in the as-built condition and after heat treatment. The focus is on hardness, strength, low cycle fatigue, and creep properties. These properties are compared with conventional cast and heat-treated material. © 2018 The Author(s)
    view abstractdoi: 10.1007/s11661-018-4762-5
  • 2018 • 233 Mn-Alloyed High-Strength Steels with a Reduced Austenitization Temperature: Thermodynamic Calculations and Experimental Investigations
    Windmann, M. and Opitz, T. and Klein, S. and Röttger, A. and Theisen, W.
    Steel Research International 89 (2018)
    High-strength steels (e.g., 1.5528–22MnB5), processed by direct press-hardening, are widely used for security-relevant structures in automotive bodyworks. In this study, the austenitization temperature AC3 of the steel 22MnB5 (approx. 840 °C) is decreased to enable a reduction in the heat-treatment temperature. Thermodynamic calculations using the CALPHAD method are used to assess the effect of alloying elements on the α–γ transformation temperatures. On this account, 22MnB5 steel is alloyed with 6 to 9.5 mass% manganese, which decreases the α–γ transformation temperature to 744 °C. Simultaneously, the martensite finish temperature decreases below room temperature, which is accompanied by the presence of retained austenite after hardening. Furthermore, ϵ-martensite is formed. High Mn-alloyed steel 22MnB5 (9.5 mass% Mn, AC3 = 744 °C) possesses a high strength of Rm = 1618 MPa, similar to the initial material 22MnB5. Elongation-to-fracture decreases to A5 = 3.5% due to the formation of ϵ-martensite. The material strength of the steel alloyed with 6 mass% manganese (AC3 = 808 °C) strongly increases to Rm = 1975 MPa as a result of α-martensite and solid-solution strengthening by the element manganese. This steel possesses a higher elongation-to-fracture of A5 = 7%. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201800166
  • 2018 • 232 Modifying the nanostructure and the mechanical properties of Mo2BC hard coatings: Influence of substrate temperature during magnetron sputtering
    Gleich, S. and Soler, R. and Fager, H. and Bolvardi, H. and Achenbach, J.-O. and Hans, M. and Primetzhofer, D. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Materials and Design 142 203-211 (2018)
    A reduction in synthesis temperature is favorable for hard coatings, which are designed for industrial applications, as manufacturing costs can be saved and technologically relevant substrate materials are often temperature-sensitive. In this study, we analyzed Mo2BC hard coatings deposited by direct current magnetron sputtering at different substrate temperatures, ranging from 380 °C to 630 °C. Transmission electron microscopy investigations revealed that a dense structure of columnar grains, which formed at a substrate temperature of 630 °C, continuously diminishes with decreasing substrate temperature. It almost vanishes in the coating deposited at 380 °C, which shows nanocrystals of ~1 nm in diameter embedded in an amorphous matrix. Moreover, Argon from the deposition process is incorporated in the film and its amount increases with decreasing substrate temperature. Nanoindentation experiments provided evidence that hardness and Young's modulus are modified by the nanostructure of the analyzed Mo2BC coatings. A substrate temperature rise from 380 °C to 630 °C resulted in an increase in hardness (21 GPa to 28 GPa) and Young's modulus (259 GPa to 462 GPa). We conclude that the substrate temperature determines the nanostructure and the associated changes in bond strength and stiffness and thus, influences hardness and Young's modulus of the coatings. © 2018 The Authors
    view abstractdoi: 10.1016/j.matdes.2018.01.029
  • 2018 • 231 On the accumulation of irreversible plastic strain during compression loading of open-pore metallic foams
    Matz, A.M. and Matz, B.S. and Jost, N. and Eggeler, G.
    Materials Science and Engineering A 728 40-44 (2018)
    The accumulation of plastic strain as an essential element of the compression behavior of metal foams is investigated by analyzing effective stress-strain curves which were recorded during testing. By applying loading/unloading cycles within the low-strain region until reaching the stress plateau, it is studied how reversible elastic deformation is gradually transformed into irreversible plastic deformation and it is shown that both, elastic and plastic strains, contribute to the total strain ε. This behavior is found to be independent on the investigated mesostructural foam morphologies. Furthermore, a method is derived which can be used to determine a proof stress σϕPl=0.5 at which yielding dominates the deformation of a metal foam. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2018.05.012
  • 2018 • 230 On the grain boundary strengthening effect of boron in γ/γ′ Cobalt-base superalloys
    Kolb, M. and Freund, L.P. and Fischer, F. and Povstugar, I. and Makineni, S.K. and Gault, B. and Raabe, D. and Müller, J. and Spiecker, E. and Neumeier, S. and Göken, M.
    Acta Materialia 145 247-254 (2018)
    Boron is an essential solute element for improving the grain boundary strength in several high temperature metallic alloys especially in Ni- and Co-base superalloys although the detailed strengthening mechanisms are still not well understood. In superalloys, boron leads to the formation of borides and precipitate depleted zones around the grain boundaries and alters the bond strength among the grains directly. In this paper, we explore in detail the role of the boron content in ternary γ/γ′ Co-9Al-9W alloys. Local as well as bulk mechanical properties were evaluated using nanoindentation and compression testing and correlated to near-atomic scale microstructure and compositions obtained from electron microscopy and atom probe tomography. The alloy variant with low B content (0.005 at.% B) reveals an increase in yield strength at room temperature and 600 °C and atom probe tomography investigations show that solute B segregates to the grain boundaries. However, in the bulk B exclusively partitions to the γ′ phase. Additionally, the γ′/γ′ grain boundaries are depleted in W and Al with the concentration locally shifted towards the γ composition forming a very thin γ layer at the γ′/γ′ grain boundaries, which supports dislocation mobility in the γ′/γ′ grain boundary region during deformation. Higher content of B (0.04 at.% B) promotes formation of W-rich borides at the grain boundaries that leads to undesirable precipitate depleted zones adjacent to these borides that decrease the strength of the alloy drastically. However, it was also found that a subsequent annealing heat treatment eliminates these detrimental zones by re-precipitating γ′ and thus elevating the strength of the alloy. This result shows that, if a precipitate depleted zone can be avoided, B significantly improves the mechanical properties of polycrystalline Co-base superalloys by strengthening the γ′ phase and by improving grain boundary cohesion. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.12.020
  • 2018 • 229 Overview on micro- and nanomechanical testing: New insights in interface plasticity and fracture at small length scales
    Dehm, G. and Jaya, B.N. and Raghavan, R. and Kirchlechner, C.
    Acta Materialia 142 248-282 (2018)
    Micro- and nanomechanical testing has seen a rapid development over the last decade with miniaturized test rigs and MEMS-based devices providing access to the mechanical properties and performance of materials from the micrometer down to the tenths of nanometer length scale. In this overview, we summarize firstly the different testing concepts with excursions into recent imaging and diffraction developments, which turn micro- and nanomechanical testing into “quantitative mechanical microscopy” by resolving the underlying material physics and simultaneously providing mechanical properties. A special focus is laid on the pitfalls of micro-compression testing with its stringent boundary conditions often hampering reliable experiments. Additionally, the challenges of instrumented micro- and nanomechanical testing at elevated temperature are summarized. From the wide variety of research topics employing micro- and nanomechanical testing of materials we focus here on miniaturized samples and test rigs and provide three examples to elucidate the state-of-the-art of the field: (i) probing the “strength” of individual grain boundaries in metals, (ii) temperature dependent deformation mechanisms in metallic nanolayered and -alloyed structures, and (iii) the prospects and challenges of fracture studies employing micro- and nanomechanical testing of brittle and ductile monolithic materials, and materials containing interfaces. Proven concepts and new endeavors are reported for the topics discussed in this overview. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.019
  • 2018 • 228 Thermal stability of nanocomposite Mo2BC hard coatings deposited by magnetron sputtering
    Gleich, S. and Breitbach, B. and Peter, N.J. and Soler, R. and Bolvardi, H. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Surface and Coatings Technology 349 378-383 (2018)
    The investigation of hard coatings under thermal load is crucial in order to obtain information on the thermal stability and possible changes of microstructure and mechanical properties. In addition, advanced heating studies may also provide feedback for the grain growth mechanism occurring during annealing and thus, help to predict optimum post-growth annealing conditions for producing high-performance hard coatings. Here, we investigate the thermal response of Mo2BC, deposited by bipolar pulsed direct current magnetron sputtering in an industrial chamber on a silicon substrate at a substrate temperature of 380 °C. Ex-situ and in-situ X-ray diffraction and transmission electron microscopy studies are performed at elevated temperatures to track changes in the structure. Whereas the as-deposited nanocomposite coating exhibits small spherical nanocrystals (1.2 nm in diameter) embedded in an amorphous matrix, a fully crystalline structure, mainly consisting of elongated and interconnected crystals with lengths of up to 1 μm, is obtained at elevated annealing temperatures. Hardness and Young's modulus increase by ~8% and ~47%, respectively, compared to the as-deposited coating. Delamination from the silicon substrate only occurs at temperatures above 840 °C. Thus, our detailed study of the micro- and nanostructure evolution upon thermal annealing suggests that heat treatments below 840 °C are a suitable method to improve the crystallinity and mechanical properties of nanocomposite Mo2BC coatings. © 2018
    view abstractdoi: 10.1016/j.surfcoat.2018.06.006
  • 2018 • 227 Thermophysical and Mechanical Properties of Advanced Single Crystalline Co-base Superalloys
    Volz, N. and Zenk, C.H. and Cherukuri, R. and Kalfhaus, T. and Weiser, M. and Makineni, S.K. and Betzing, C. and Lenz, M. and Gault, B. and Fries, S.G. and Schreuer, J. and Vaßen, R. and Virtanen, S. and Raabe, D. and Spiecker, E...
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 4099-4109 (2018)
    A set of advanced single crystalline γ′ strengthened Co-base superalloys with at least nine alloying elements (Co, Ni, Al, W, Ti, Ta, Cr, Si, Hf, Re) has been developed and investigated. The objective was to generate multinary Co-base superalloys with significantly improved properties compared to the original Co-Al-W-based alloys. All alloys show the typical γ/γ′ two-phase microstructure. A γ′ solvus temperature up to 1174 °C and γ′ volume fractions between 40 and 60 pct at 1050 °C could be achieved, which is significantly higher compared to most other Co-Al-W-based superalloys. However, higher contents of Ti, Ta, and the addition of Re decrease the long-term stability. Atom probe tomography revealed that Re does not partition to the γ phase as strongly as in Ni-base superalloys. Compression creep properties were investigated at 1050 °C and 125 MPa in 〈001〉 direction. The creep resistance is close to that of first generation Ni-base superalloys. The creep mechanisms of the Re-containing alloy was further investigated and it was found that the deformation is located preferentially in the γ channels although some precipitates are sheared during early stages of creep. The addition of Re did not improve the mechanical properties and is therefore not considered as a crucial element in the design of future Co-base superalloys for high temperature applications. Thermodynamic calculations describe well how the alloying elements influence the transformation temperatures although there is still an offset in the actual values. Furthermore, a full set of elastic constants of one of the multinary alloys is presented, showing increased elastic stiffness leading to a higher Young’s modulus for the investigated alloy, compared to conventional Ni-base superalloys. The oxidation resistance is significantly improved compared to the ternary Co-Al-W compound. A complete thermal barrier coating system was applied successfully. © 2018, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-018-4705-1
  • 2018 • 226 Tribo-mechanical properties of CrC/a-C thin films sequentially deposited by HiPIMS and mfMS
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D.
    Surface and Coatings Technology 335 173-180 (2018)
    A sequence of high power impulse magnetron sputtering (HiPIMS) and mid-frequency magnetron sputtering (mfMS) was carried out to deposit a chromium carbide (CrC) interlayer and an amorphous carbon (a-C) top layer. The deposition of the interlayer by HiPIMS results in a higher adhesion strength and hence affects the properties of the a-C layer. The mechanical and tribological properties of a group of CrC/a-C films consisting of CrC with growing C content are investigated. In this context, single CrC and CrC/a-C films were systematically analyzed in order to evaluate the influence of the CrC interlayer on the properties of CrC/a-C. A higher amount of C changes the morphology of CrC films from a columnar to fully dense microstructure. The hardness decreases from 13.8 to 12.3 GPa with a growing C content, but the H/E- and H3/E2-ratios increase to 0.073 and 0.068 GPa, respectively. In contrast to the CrC interlayers, the CrC/a-C film systems are marked by a higher hardness of up to 19.8 GPa. The H/E- and H3/E2-ratios are also significantly higher with values of 0.090 and 0.155 GPa when compared to the CrC single films. The CrC layers exhibit the best adhesion class HF1 in Rockwell tests and a maximum critical load Lc3 of 41 N in scratch tests. The adhesion strength of CrC/a-C is strongly affected by the CrC interlayers; as generally similar failure mechanisms are observed for both film systems. The friction behavior of the CrC/a-C films is only influenced by the a-C top layer. The high adhesion strength of CrC prevents delamination failures when tribologically loading CrC/a-C films. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2017.12.035
  • 2018 • 225 Viscoelasticity of short fiber composites in the time domain: from three-phases micromechanics to finite element analyses
    Breuer, K. and Schöneich, M. and Stommel, M.
    Continuum Mechanics and Thermodynamics 1-10 (2018)
    Micromechanical models can be used to calculate the mechanical properties of short glass fiber reinforced thermoplastics. In the present work, a three-step framework is used to validate a three-phases micromechanical model (RDI model) in the time domain, since the analysis of technical components by the finite element method is usually carried out in the time domain. The framework includes mechanical characterization, the implementation of the RDI model and a finite element analysis. The characterization delivers necessary information about the material phases of the composite. A dynamic mechanical analysis is performed to characterize the matrix material in order to obtain the linear viscoelastic properties. The mechanical properties of the matrix–fiber interphase are determined with an inverse calculation. In the second step, the RDI model is used to calculate the frequency depended effective stiffness of the composite. A new developed approach transforms the effective stiffness from the frequency domain into the time domain thus avoiding an explicit inverse Laplace–Carson transformation. In the third step, the RDI model is experimentally validated. © 2018 Springer-Verlag GmbH Germany, part of Springer Nature
    view abstractdoi: 10.1007/s00161-018-0686-y
  • 2018 • 224 Water infiltration impact on tensile strength and breaking strain of architectural fabrics
    Asadi, H. and Uhlemann, J. and Stranghöner, N.
    Advances in Structural Engineering 21 2605-2616 (2018)
    Architectural fabrics consist of woven base cloth protected by a coating on both sides. Corrosive liquids or vapours may diffuse through the matrix material and corrosion effects on fibres may lead to substantial reduction in mechanical properties. Tensile strength is of most importance for the safety of a structure and strain properties govern its serviceability. Wetting is one of the main environmental impacts. Due to rain, condensation or snow melting cycles, the membrane gets wet. If the fabric under the coating gets wet and to which amount depends on the condition of the covering coating over the lifetime of the architectural structure and on the wicking properties of the fabric material. Water penetration influences the fabric’s properties. How and to what extent is the field of investigations of this article. The influence of water on the tensile strength and the breaking strain of two common architectural fabrics, polyester (PES)/ polyvinylchloride (PVC) and glass/polytetrafluoroethylene (PTFE), are investigated. Virgin and aged materials are examined. © The Author(s) 2018.
    view abstractdoi: 10.1177/1369433218756005
  • 2017 • 223 A TRIP-assisted dual-phase high-entropy alloy: Grain size and phase fraction effects on deformation behavior
    Li, Z. and Tasan, C.C. and Pradeep, K.G. and Raabe, D.
    Acta Materialia 131 323-335 (2017)
    We present a systematic microstructure oriented mechanical property investigation for a newly developed class of transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) with varying grain sizes and phase fractions. The DP-HEAs in both, as-homogenized and recrystallized states consist of a face-centered cubic (FCC) matrix containing a high-density of stacking faults and a laminate hexagonal close-packed (HCP) phase. No elemental segregation was observed in grain interiors or at interfaces even down to near-atomic resolution, as confirmed by energy-dispersive X-ray spectroscopy and atom probe tomography. The strength-ductility combinations of the recrystallized DP-HEAs (Fe50Mn30Co10Cr10) with varying FCC grain sizes and HCP phase fractions prior to deformation are superior to those of the recrystallized equiatomic single-phase Cantor reference HEA (Fe20Mn20Ni20Co20Cr20). The multiple deformation micro-mechanisms (including strain-induced transformation from FCC to HCP phase) and dynamic strain partitioning behavior among the two phases are revealed in detail. Both, strength and ductility of the DP-HEAs increase with decreasing the average FCC matrix grain size and increasing the HCP phase fraction prior to loading (in the range of 10–35%) due to the resulting enhanced stability of the FCC matrix. These insights are used to project some future directions for designing advanced TRIP-HEAs through the adjustment of the matrix phase's stability by alloy tuning and grain size effects. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.03.069
  • 2017 • 222 Ab initio assisted design of quinary dual-phase high-entropy alloys with transformation-induced plasticity
    Li, Z. and Körmann, F. and Grabowski, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 136 262-270 (2017)
    We introduce a new class of high-entropy alloys (HEAs), i.e., quinary (five-component) dual-phase (DP) HEAs revealing transformation-induced plasticity (TRIP), designed by using a quantum mechanically based and experimentally validated approach. Ab initio simulations of thermodynamic phase stabilities of Co20Cr20Fe40-xMn20Nix (x = 0–20 at. %) HEAs were performed to screen for promising compositions showing the TRIP-DP effect. The theoretical predictions reveal several promising alloys, which have been cast and systematically characterized with respect to their room temperature phase constituents, microstructures, element distributions and compositional homogeneity, tensile properties and deformation mechanisms. The study demonstrates the strength of ab initio calculations to predict the behavior of multi-component HEAs on the macroscopic scale from the atomistic level. As a prototype example a non-equiatomic Co20Cr20Fe34Mn20Ni6 HEA, selected based on our ab initio simulations, reveals the TRIP-DP effect and hence exhibits higher tensile strength and strain-hardening ability compared to the corresponding equiatomic CoCrFeMnNi alloy. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.07.023
  • 2017 • 221 Characterization of hybrid joining techniques for FRP/Steel-structures under combined mechanical and thermal loading
    Hoepfner, M. and Becker, T. and Hülsbusch, D. and Walther, F.
    Key Engineering Materials 742 KEM 358-365 (2017)
    In order to optimize the design of vibrating screening machines and realize significant weight reductions, the use of hybrid structures is gaining importance. In this context, the joining of FRP and steel and their interactions due to different material properties were investigated. Therefore, quasi-static tests with combined mechanical and thermal loads were carried out. To realize the simultaneous application of physical measurement techniques, e.g. optical and acoustic measurements, and thermal loads, short-wave infrared emitter technique was used instead of thermal chambers. Thus, the mechanical characteristics and acoustic emissions could be determined and assessed. The results show different structural mechanisms of hybrid joining at room and elevated temperatures. The characteristics of failure modes, shear stresses, strains and acoustic emissions could be correlated to determine the damage developments and mechanisms. © 2017 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2017 • 220 Comparative evaluation of the sand blasting, acid etching and electron beam surface treatments of titanium for medical application
    Grubova, I. and Chudinova, E. and Surmeneva, M. and Surmenev, R. and Ivanova, A. and Kravchuk, K. and Shugurov, V. and Teresov, A. and Koval, N. and Prymak, O. and Epple, M.
    Proceedings of the 11th International Forum on Strategic Technology, IFOST 2016 69-72 (2017)
    Modification of the surface topography and chemistry are commonly used to achieve the desired biological response to the implants. The influence of the different treatment methods on the physicochemical and mechanical properties of titanium is reported. All samples were divided into 2 groups. First group was sandblasted with 250-320 μm Al2O3 at two pressures 0.45 MPa and 0.61 MPa followed by the chemical etching in a fluorine-containing solution. The second group was acid-etched in the same solution followed by electron beam modification with the energy density 8 J/cm2. The samples were investigated by SEM, EDX, XRD, nanoindentation and sessile drop method. The studies revealed that all groups have nano/micro-patterned surfaces. The EDX analysis detected only titanium in all groups. The XRD results revealed the presence of diffraction peaks corresponding to titanium. The nanoindentation studies revealed significant differences in the mechanical properties between group 1 and 2. The elastic strain to failure and plastic deformation resistance of the group 2 were determined to be 0.035 and 5∗10-3, respectively, which were significantly higher than those of group 1. The obtained results of water contact angle for group 1 revealed moderately hydrophilic properties of treated surfaces. The water contact angle was increased up to 80.85 ± 8.3 ° for group 2. © 2016 IEEE.
    view abstractdoi: 10.1109/IFOST.2016.7884191
  • 2017 • 219 Comparison of microstructure and mechanical properties of Scalmalloy® produced by selective laser melting and laser metal deposition
    Awd, M. and Tenkamp, J. and Hirtler, M. and Siddique, S. and Bambach, M. and Walther, F.
    Materials 11 (2017)
    The second-generation aluminum-magnesium-scandium (Al-Mg-Sc) alloy, which is often referred to as Scalmalloy®, has been developed as a high-strength aluminum alloy for selective laser melting (SLM). The high-cooling rates of melt pools during SLM establishes the thermodynamic conditions for a fine-grained crack-free aluminum structure saturated with fine precipitates of the ceramic phase Al3-Sc. The precipitation allows tensile and fatigue strength of Scalmalloy® to exceed those of AlSi10Mg by ~70%. Knowledge about properties of other additive manufacturing processes with slower cooling rates is currently not available. In this study, two batches of Scalmalloy® processed by SLM and laser metal deposition (LMD) are compared regarding microstructure-induced properties. Microstructural strengthening mechanisms behind enhanced strength and ductility are investigated by scanning electron microscopy (SEM). Fatigue damage mechanisms in low-cycle (LCF) to high-cycle fatigue (HCF) are a subject of study in a combined strategy of experimental and statistical modeling for calculation of Woehler curves in the respective regimes. Modeling efforts are supported by non-destructive defect characterization in an X-ray computed tomography (μ-CT) platform. The investigations show that Scalmalloy® specimens produced by LMD are prone to extensive porosity, contrary to SLM specimens, which is translated to ~30% lower fatigue strength. © 2017 by the author.
    view abstractdoi: 10.3390/ma11010017
  • 2017 • 218 Duplex stainless steel fabricated by selective laser melting - Microstructural and mechanical properties
    Hengsbach, F. and Koppa, P. and Duschik, K. and Holzweissig, M.J. and Burns, M. and Nellesen, J. and Tillmann, W. and Tröster, T. and Hoyer, K.-P. and Schaper, M.
    Materials and Design 133 136-142 (2017)
    In the scope of the present study, microstructural and mechanical characterizations of duplex stainless steel UNS S31803 processed by selective laser melting (SLM) are conducted. The findings shed light on the phase arrangement evolving in the as-built condition and in several heat-treated conditions. In the as-built condition, austenite formation is almost suppressed due to process-related high cooling rates. Therefore, several heat treatments ranging from 900 °C to 1200 °C for 5 min each were performed in order to adjust to the desired austenitic-ferritic microstructure. Results generated by transmission electron microscopy (TEM) reveal a high dislocation density induced during SLM fabrication, such that a recrystallized microstructure prevails after the heat treatment. Tensile tests display the severe impact of the heat treatment on the resulting mechanical response. The nearly complete ferritic as-built specimens obtain a higher ultimate tensile strength and a reduced elongation at fracture compared to the heat-treated specimens. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2017.07.046
  • 2017 • 217 Embedment of eutectic tungsten carbides in arc sprayed steel coatings
    Tillmann, W. and Hagen, L. and Kokalj, D.
    Surface and Coatings Technology 331 153-162 (2017)
    Tungsten carbide reinforced deposits have already evolved into a predominant coating system in order to protect stressed surfaces against wear. Among thermal spraying processes, due to a high deposition rate, arc spraying is a promising process to manufacture cost-saving, wear resistant coatings. However, inherent process characteristics prevailing in arc spraying as well as the utilization of tungsten carbides, as a filling for cored wires, could lead to undesirable phase evolutions, which in turn provoke the degradation of the mechanical properties. The embedment of tungsten carbides into the surrounding metallic matrix is affected by metallurgical interactions with molten spray particles. Within the scope of this study, an external injection of tungsten carbides was applied in order to analyze the embedment of tungsten carbides in arc sprayed low alloyed steel. Accordingly, metallographic investigations were carried out, which address the reactive layer at the interface of embedded tungsten carbides to the surrounded iron-based matrix. Microstructural characteristics such as mechanical properties and phase composition were scrutinized by means of nanoindentation, energy dispersive X-ray spectroscopy, and X-ray diffraction, respectively. It was found that the embedment of tungsten carbides, which have been externally injected into the arc burning zone, differs from that obtained from deposits produced with the use of cored wire with tungsten carbide as filling. Thus, externally injected tungsten carbides are less inclined to form eta carbides due to dissolution, which again results in differences in the mechanical properties across the reactive layer. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2017.10.044
  • 2017 • 216 Influence of compositional inhomogeneity on mechanical behavior of an interstitial dual-phase high-entropy alloy
    Li, Z. and Raabe, D.
    Materials Chemistry and Physics (2017)
    In this study we present and discuss the influence of compositional inhomogeneity on the mechanical behavior of an interstitially alloyed dual-phase non-equiatomic high-entropy alloy (Fe49.5Mn30Co10Cr10C0.5). Various processing routes including hot-rolling, homogenization, cold-rolling and recrystallization annealing were performed on the cast alloys to obtain samples in different compositional homogeneity states. Grain sizes of the alloys were also considered. Tensile testing and microstructural investigations reveal that the deformation behavior of the interstitial dual-phase high-entropy alloy samples varied significantly depending on the compositional homogeneity of the specimens probed. In the case of coarse-grains (∼300 μm) obtained for cast alloys without homogenization treatment, ductility and strain-hardening of the material was significantly reduced due to its compositional inhomogeneity. This detrimental effect was attributed to preferred deformation-driven phase transformation occurring in the Fe enriched regions with lower stacking fault energy, promoting early stress-strain localization. The grain-refined alloy (∼4 μm) with compositional heterogeneity which was obtained for recrystallization annealed alloys without homogenization treatment was characterized by almost total loss in work-hardening. This effect was attributed to large local shear strains due to the inhomogeneous planar slip. These insights demonstrate the essential role of compositional homogeneity through applying corresponding processing steps for the development of advanced high-entropy alloys. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2017.04.050
  • 2017 • 215 Influence of the processing parameters on the fiber-matrix-interphase in short glass fiber-reinforced thermoplastics
    Sambale, A.K. and Schöneich, M. and Stommel, M.
    Polymers 9 (2017)
    The interphase in short fiber thermoplastic composites is defined as a three-dimensional, several hundred nanometers-wide boundary region at the interface of fibers and the polymer matrix, exhibiting altered mechanical properties. This region is of key importance in the context of fiber-matrix adhesion and the associated mechanical strength of the composite material. An interphase formation is caused by morphological, as well as thermomechanical processes during cooling of the plastic melt close to the glass fibers. In this study, significant injection molding processing parameters are varied in order to investigate the influence on the formation of an interphase and the resulting mechanical properties of the composite. The geometry of the interphase is determined using nano-tribological techniques. In addition, the influence of the glass fiber sizing on the geometry of the interphase is examined. Tensile tests are used in order to determine the resulting mechanical properties of the produced short fiber composites. It is shown that the interphase width depends on the processing conditions and can be linked to the mechanical properties of the short fiber composite. © 2017 by the authors.
    view abstractdoi: 10.3390/polym9060221
  • 2017 • 214 Investigation of the influence of the vanadium content on the high temperature tribo-mechanical properties of DC magnetron sputtered AlCrVN thin films
    Tillmann, W. and Kokalj, D. and Stangier, D. and Paulus, M. and Sternemann, C. and Tolan, M.
    Surface and Coatings Technology 328 172-181 (2017)
    The forming of high-strength steels or new aluminum alloys leads to a steady increase of the load of tools and coatings. One approach is to positively influence the manufacturing process by using thin solid films with self-lubricating features, provided by oxides at high temperatures with low decohesion energies. For the purpose of this study, AlCrN provides the matrix, while vanadium oxides are used to enhance the frictional and wear properties. However, it is not yet clear which minimum amount of vanadium has to be incorporated in DC magnetron sputtered AlCrN coatings to improve the tribological behavior. Therefore, in this study, AlCrVN coatings are synthesized with an increasing vanadium content by means of reactive DC magnetron sputtering. Additionally, a vanadium-free AlCrN coating is used as reference for the tribo-mechanical investigations. The coatings were synthesized up to a vanadium content of 13.5 at.-% and no phase change could be detected by means of x-ray diffraction. Moreover, no hexagonal AlN phase, which reduces the mechanical properties and the oxidation resistance, was formed. In contrast to the vanadium-free coating, the hardness of the coatings containing vanadium is slightly reduced. The coating with the smallest vanadium content shows the highest hardness of all analyzed coatings. A heat treatment at 400 °C does not lead to any significant changes with respect to mechanical properties, but at 700 °C hardness, modulus of elasticity and critical load decreased for all coatings, indicating a significant change in mechanical properties. The ball-on-disc test at room-temperature, 400 °C, and 700 °C shows the highest wear coefficient for the coating with the lowest vanadium content, due to the poor adhesion of the coating, although this coating shows the highest H/E-ratio. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2017.08.046
  • 2017 • 213 Material Testing and Chip Formation Simulation for Different Heat Treated Workpieces of 51CrV4 Steel
    Zabel, A. and Rödder, T. and Tiffe, M.
    Procedia CIRP 58 181-186 (2017)
    The heat treatment has a major impact on the mechanical properties of steel alloys and therefore on the condition of a machining processes. In this paper, the low alloy steel 51CrV4 with different heat treatments is investigated in terms of its mechanical properties under high dynamic conditions using a Split Hopkinson Pressure Bar (SHPB) and by means of orthogonal cutting tests. The latter provide a detailed insight in the ongoing processes during chip formation by analyzing the present microstructure of the generated chips. Furthermore, the obtained data from the SHPB tests is used as an input for material models applied for the simulation of chip formation with the Finite-Element-Method. The results reveal fundamental differences in the chip formation mechanisms between the differently heat treated workpiece materials. © 2017 The Authors.
    view abstractdoi: 10.1016/j.procir.2017.03.218
  • 2017 • 212 Mechanical properties of AlXFeY intermetallics in Al-base coatings on steel 22MnB5 and resulting wear mechanisms at press-hardening tool steel surfaces
    Windmann, M. and Röttger, A. and Hahn, I. and Theisen, W.
    Surface and Coatings Technology 321 321-327 (2017)
    Hard and brittle intermetallic AlXFeY phases formed in the Al-base coating applied on high-strength steel 22MnB5 promote strong wear of press-hardening tools during forming and quenching (approx. 800–100 °C). In this study, bulk materials of the intermetallic phases Al13Fe4, Al5Fe2, Al2Fe, and AlFe were produced by remelting stoichiometric powder mixtures. These were then used for mechanical and wear investigations. We found that the dominating wear mechanisms on the tool steel surfaces are strongly influenced by the temperature and depend on the mechanical properties of the respective intermetallic phases. Phases of type Al13Fe4, Al5Fe2, and Al2Fe possess a high hardness of 850–1090 HV0.5 and a low fracture toughness of 0.9–1.6 MPa √ m at room temperature, whereas the AlFe phase has a much lower hardness (520 HV0.5) and a higher fracture toughness (26 MPa √ m ). The hardness of all phases decreases with increasing temperature. At high temperatures (500–800 °C), the intermetallic phases lead mainly to adhesive wear of the tool steel surfaces. At lower temperatures, also abrasive wear occurs due to delamination of hard and brittle intermetallic particles. We found that abrasive wear of the tool steel surface could be decreased by adapting the phase composition in the Al-base coating. © 2017
    view abstractdoi: 10.1016/j.surfcoat.2017.04.075
  • 2017 • 211 Micro-CT defect analysis and hardness distribution of flat-face extruded en AW6060 aluminum chips
    Goerlich, P. and Scholz, R. and Walther, F.
    Materialpruefung/Materials Testing 59 613-617 (2017)
    Besides the energy-intensive secondary metallurgical recycling route, aluminum chips can alternatively be extruded to final profiles by extrusion. The mechanical properties of the extruded profiles have a dependency on the quality of the weldments of the chips, which differs locally due to the batch process. To characterize the influence of this dependency on the mechanical properties, round pre-compacted chip blocks consisting of EN AW-6060 were pre-heated for six hours at 550 °C, extruded with flatface dies at a recipient temperature of 450 °C and divided into three zones: Profile, transition and contact zone. A micro-computed tomographic defect analysis was performed on the samples. It has been shown that the profile samples of both geometries have a very low defect quantity and volume, while towards the contact zone the number and volume increases significantly and small delaminations occur on the surface. For the determination of the hardness distribution, a macro hardness mapping was performed. The coarse grain edge in the outer region of the specimens, which has resulted from increased temperature as a result of the recipient friction and shear stress, shows a slightly increased hardness. Round profiles show a concentric hardness decrease and square profiles a linear drop towards the center of the cross section of the specimens. © 2017 Carl Hanser Verlag, München.
    view abstractdoi: 10.3139/120.111050
  • 2017 • 210 Progress in friction stir welding of Ni alloys
    Lemos, G.V.B. and Hanke, S. and Dos Santos, J.F. and Bergmann, L. and Reguly, A. and Strohaecker, T.R.
    Science and Technology of Welding and Joining 22 643-657 (2017)
    In recent years, interest has been increasing in application of Nickel alloys in the oil industry. For subsea engineering, the possibility to weld high-strength materials in an effective manner is essential. Friction Stir Welding (FSW) is alternative to join several materials retaining their properties or even improving them. This fact is relevant for Corrosion-Resistant Alloys (CRA) used in deep-water exploitation of hydrocarbons. Publications up to now have focused on FSW of Inconel® series as alloy 600, 625, and 718. To provide a solid basis for development, this review discusses the crucial points for FSW. The tool materials are described, as well as the joint microstructure and properties achieved. Furthermore, the basics of the corrosion resistance and the early corrosion studies of FSW joints are presented. It is concluded that FSW is a promising process for Ni alloys, but depends on upcoming research regarding tool technology and corrosion investigations. © 2017 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute.
    view abstractdoi: 10.1080/13621718.2017.1288953
  • 2017 • 209 Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi
    Laplanche, G. and Kostka, A. and Reinhart, C. and Hunfeld, J. and Eggeler, G. and George, E.P.
    Acta Materialia 128 292-303 (2017)
    The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77 K) and room temperature (293 K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2&lt;110&gt; dislocations dissociated into 1/6&lt;112&gt; Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22 ± 4 mJ m−2, which is ∼25% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16 μm grain size is 260 ± 30 MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.02.036
  • 2017 • 208 Spray Characteristics and Tribo-Mechanical Properties of High-Velocity Arc-Sprayed WC-W2C Iron-Based Coatings
    Tillmann, W. and Hagen, L. and Kokalj, D.
    Journal of Thermal Spray Technology 1-16 (2017)
    In terms of arc-sprayed coatings, the lamellar coating microstructure is mainly affected by the atomization behavior of the molten electrode tips. When using compressed air, oxide formations occur during atomization, across the particle-laden spray plume and when the molten droplets splash onto the substrate. Within the scope of this study, the potential of a high-velocity arc-spraying process due to elevated atomization gas pressures and its effect on the spray and coating characteristics was analyzed using a cast tungsten carbide (CTC)-reinforced FeCMnSi cored wire. Since the atomization behavior corresponds with the electrode phenomena, the power spectrum and the droplet formation were observed during spraying. The tribo-mechanical properties of CTC-FeCMnSi coatings were examined in dry sliding experiments and indentation tests. In addition, adhesion tests and metallographic investigations were carried out to analyze the bonding strength, cohesive behavior, and lamellar microstructure. The occurrence of oxide phases was evaluated by x-ray diffraction and electron microscopy. Moreover, the oxygen content was determined by using glow discharge optical emission spectroscopy as well as energy-dispersive x-ray spectroscopy. With respect to elevated atomization gas pressures, a dense microstructure with improved adhesion to the substrate and reduced surface roughness was observed. Dry sliding experiments revealed an advanced wear behavior of specimens, when using above average increased atomization gas pressures. Analytic methods verified the existence of oxide phases, which were generated during spraying. A significant change of the extent and type of oxides, when applying an increased flow rate of the atomization gas, cannot be observed. Besides an enhanced coating quality, the use of increased atomization gas pressure exhibited good process stability. © 2017 ASM International
    view abstractdoi: 10.1007/s11666-017-0605-y
  • 2017 • 207 Strong and Ductile Non-equiatomic High-Entropy Alloys: Design, Processing, Microstructure, and Mechanical Properties
    Li, Z. and Raabe, D.
    JOM 69 2099-2106 (2017)
    We present a brief overview on recent developments in the field of strong and ductile non-equiatomic high-entropy alloys (HEAs). The materials reviewed are mainly based on massive transition-metal solute solutions and exhibit a broad spectrum of microstructures and mechanical properties. Three relevant aspects of such non-equiatomic HEAs with excellent strength–ductility combination are addressed in detail, namely phase stability-guided design, controlled and inexpensive bulk metallurgical processing routes for appropriate microstructure and compositional homogeneity, and the resultant microstructure–property relations. In addition to the multiple principal substitutional elements used in these alloys, minor interstitial alloying elements are also considered. We show that various groups of strong and ductile HEAs can be obtained by shifting the alloy design strategy from single-phase equiatomic to dual- or multiphase non-equiatomic compositional configurations with carefully designed phase instability. This design direction provides ample possibilities for joint activation of a number of strengthening and toughening mechanisms. Some potential research efforts which can be conducted in the future are also proposed. © 2017, The Author(s).
    view abstractdoi: 10.1007/s11837-017-2540-2
  • 2016 • 206 A microstructurally based continuum model of cartilage viscoelasticity and permeability incorporating measured statistical fiber orientations
    Pierce, D.M. and Unterberger, M.J. and Trobin, W. and Ricken, T. and Holzapfel, G.A.
    Biomechanics and Modeling in Mechanobiology 15 229-244 (2016)
    The remarkable mechanical properties of cartilage derive from an interplay of isotropically distributed, densely packed and negatively charged proteoglycans; a highly anisotropic and inhomogeneously oriented fiber network of collagens; and an interstitial electrolytic fluid. We propose a new 3D finite strain constitutive model capable of simultaneously addressing both solid (reinforcement) and fluid (permeability) dependence of the tissue’s mechanical response on the patient-specific collagen fiber network. To represent fiber reinforcement, we integrate the strain energies of single collagen fibers—weighted by an orientation distribution function (ODF) defined over a unit sphere—over the distributed fiber orientations in 3D. We define the anisotropic intrinsic permeability of the tissue with a structure tensor based again on the integration of the local ODF over all spatial fiber orientations. By design, our modeling formulation accepts structural data on patient-specific collagen fiber networks as determined via diffusion tensor MRI. We implement our new model in 3D large strain finite elements and study the distributions of interstitial fluid pressure, fluid pressure load support and shear stress within a cartilage sample under indentation. Results show that the fiber network dramatically increases interstitial fluid pressure and focuses it near the surface. Inhomogeneity in the tissue’s composition also increases fluid pressure and reduces shear stress in the solid. Finally, a biphasic neo-Hookean material model, as is available in commercial finite element codes, does not capture important features of the intra-tissue response, e.g., distributions of interstitial fluid pressure and principal shear stress. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s10237-015-0685-x
  • 2016 • 205 A new view on boundary conditions in the Grioli-Koiter-Mindlin-Toupin indeterminate couple stress model
    Madeo, A. and Ghiba, I.-D. and Neff, P. and Münch, I.
    European Journal of Mechanics, A/Solids 59 294-322 (2016)
    In this paper we consider the Grioli-Koiter-Mindlin-Toupin linear isotropic indeterminate couple stress model. Our main aim is to show that, up to now, the boundary conditions have not been completely understood for this model. As it turns out, and to our own surprise, restricting the well known boundary conditions stemming from the strain gradient or second gradient models to the particular case of the indeterminate couple stress model, does not always reduce to the Grioli-Koiter-Mindlin-Toupin set of accepted boundary conditions. We present, therefore, a proof of the fact that when specific "mixed" kinematical and traction boundary conditions are assigned on the boundary, no "a priori" equivalence can be established between Mindlin's and our approach. © 2016 Elsevier Masson SAS. All rights reserved.
    view abstractdoi: 10.1016/j.euromechsol.2016.02.009
  • 2016 • 204 An investigation of the tribological behaviour of high-speed tool steels at elevated temperatures
    Walter, M. and Egels, G. and Boes, J. and Röttger, A. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 71 148-153 (2016)
    The tribological behaviour of work roll materials is a key issue during hot rolling process of metals. The characteristics of the material (hardness and wear resistance) at elevated temperatures are of great interest for many industrial applications. The study investigates the mechanical properties and the sliding wear behaviour of HS 6-5-3-5 (HSS) high-speed tool steel, which is a common work roll material of the intermediate and finishing stands of hot rolling manufacturing lines. Experimental analysis focuses on the mechanical properties of steel HS 6-5-3-5 at elevated temperatures and on the microstructural surface changes of this material during metallic sliding wear. The results give an overview about the absolute hardness value of HS 6-5-3-5 at elevated temperatures and its evolution at constant operating temperatures. To conclude interdependencies between mechanical properties, microstructure and wear behaviour at elevated temperatures, results are discussed and connected with wear investigations. Findings reveal that high temperature wear behaviour is mainly dependent on the formation of a tribochemical wear layer on the wear bodies' surfaces. Layers suppress direct metallic contact and change the characteristics of the tribological system. Discussed issues of high temperature sliding wear are the formation and stability of tribochemical wear layers, their connection to and support by the bulk material, as well as the fracturing and damage of the layer-bulk-material compound. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/105.110290
  • 2016 • 203 Comparison of microstructure and mechanical properties of 316 L austenitic steel processed by selective laser melting with hot-isostatic pressed and cast material
    Röttger, A. and Geenen, K. and Windmann, M. and Binner, F. and Theisen, W.
    Materials Science and Engineering A 678 365-376 (2016)
    Besides the chemical composition, the manufacturing route primarily determines a material's properties. In this work, the influence of the manufacturing process of the 316 L grade austenitic steel on the microstructure and the resulting material properties were investigated. Thus, the microstructure and mechanical properties of cast and solution annealed, as well as steel powder densified by hot-isostatic pressing (HIP), selective laser melting (SLM) and SLM+HIP, were compared. A SLM parameter study illustrates that the porosity of SLM-densified specimens can be reduced with direction of a higher exposure time and a smaller point distance. With an additional treatment by HIP, the porosity scarcely changes, while cracks are reduced. The mechanical properties were investigated depending on the manufacturing process, and the influence of the sample build up by SLM was examined. High mechanical values have been obtained; in particular, the yield strength in the SLM-densified condition is much higher than in cast or HIP condition, as a result of the smaller grain size. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.10.012
  • 2016 • 202 Computational modeling of dual-phase steels based on representative three-dimensional microstructures obtained from EBSD data
    Brands, D. and Balzani, D. and Scheunemann, L. and Schröder, J. and Richter, H. and Raabe, D.
    Archive of Applied Mechanics 86 575-598 (2016)
    The microstructure of dual-phase steels consisting of a ferrite matrix with embedded martensite inclusions is the main contributor to the mechanical properties such as high ultimate tensile strength, high work hardening rate, and good ductility. Due to the composite structure and the wide field of applications of this steel type, a wide interest exists in corresponding virtual computational experiments. For a reliable modeling, the microstructure should be included. For that reason, in this paper we follow a computational strategy based on the definition of a representative volume element (RVE). These RVEs will be constructed by a set of tomographic measurements and mechanical tests. In order to arrive at more efficient numerical schemes, we also construct statistically similar RVEs, which are characterized by a lower complexity compared with the real microstructure but which represent the overall material behavior accurately. In addition to the morphology of the microstructure, the austenite–martensite transformation during the steel production has a relevant influence on the mechanical properties and is considered in this contribution. This transformation induces a volume expansion of the martensite phase. A further effect is determined in nanoindentation test, where it turns out that the hardness in the ferrite phase increases exponentially when approaching the martensitic inclusion. To capture these gradient properties in the computational model, the volumetric expansion is applied to the martensite phase, and the arising equivalent plastic strain distribution in the ferrite phase serves as basis for a locally graded modification of the ferritic yield curve. Good accordance of the model considering the gradient yield behavior in the ferrite phase is observed in the numerical simulations with experimental data. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00419-015-1044-1
  • 2016 • 201 Corrosion fatigue assessment of creep-resistant magnesium alloy Mg-4Al-2Ba-2Ca in aqueous sodium chloride solution
    Wittke, P. and Klein, M. and Dieringa, H. and Walther, F.
    International Journal of Fatigue 83 59-65 (2016)
    Low corrosion resistance of magnesium alloys strongly limits their application range. This study aims at the investigation of corrosion influence on microstructure and depending mechanical properties of newly developed magnesium alloy Mg-4Al-2Ba-2Ca. The fatigue properties of this creep-resistant magnesium alloy were investigated under three corrosive environments: double distilled water, 0.01 and 0.1 mol L-1 NaCl solutions. Potentiodynamic polarization measurements and immersion tests were performed to estimate the corrosion behaviour. Specimen surfaces were observed using light and scanning electron microscopy for microstructure-related assessment of corrosion mechanisms. The corrosion fatigue behaviour was characterized in continuous load increase tests using plastic strain and electrochemical measurements. Continuous load increase tests allow estimating the fatigue limit and determining the failure stress amplitude with one single specimen. Fatigue results showed a significant decrease in the estimated fatigue limit and determined failure stress amplitude with increasing corrosion impact of the environments. This corrosion-structure-property relation was quantitatively described by means of model-based correlation approaches and failure hypotheses. Plastic strain amplitude and deformation-induced changes in electrochemical measurands can be equivalently applied for precise corrosion fatigue assessment. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijfatigue.2015.04.001
  • 2016 • 200 Direction-dependent mechanical characterization of cellulose-based composite vulcanized fiber
    Scholz, R. and Mittendorf, R. M. and Engels, J. K. and Hartmaier, A. and Kunne, B. and Walther, F.
    Materials Testing 58 813--817 (2016)
    Vulcanized fiber is a macromolecular cellulose-based composite material manufactured using the parchmentizing process. The cellulose is produced from the chemical digestion of plant-based raw materials (wood, cotton) or textile waste. Chemical additives used during manufacturing are completely removed. After the process, vulcanized fiber possesses improved properties concerning mechanical strength and abrasion as well as corrosion resistance in comparison to its raw materials. Concerning its economic life cycle assessment, low density, electrical insulating capability and balanced properties, vulcanized fiber has a potential, up to now unused, as a light and renewable structural material for applications in automotive or civil engineering industries. Research activities concerning the mechanical properties are insufficient and existing standards are out-of-date. In this work, for the first time a direction-dependent characterization of the process-related anisotropic mechanical properties of the material is realized with the aim to formulate an adequate material model for numerical simulations in the next step.
    view abstractdoi: 10.3139/120.110929
  • 2016 • 199 Effect of intercritical deformation on microstructure and mechanical properties of a low-silicon aluminum-added hot-rolled directly quenched and partitioned steel
    Tan, X.-D. and Xu, Y.-B. and Ponge, D. and Yang, X.-L. and Hu, Z.-P. and Peng, F. and Ju, X.-W. and Wu, D. and Raabe, D.
    Materials Science and Engineering A 656 200-215 (2016)
    Here, we applied hot-rolling in conjunction with direct quenching and partitioning (HDQ&P) processes with different rolling schedules to a low-C low-Si Al-added steel. Ferrite was introduced into the steel by intercritical rolling and air cooling after hot-rolling. The effect of intercritcal deformation on the microstructure evolution and mechanical properties was investigated. The promotion of austenite stabilization and the optimization of the TRIP effect due to a moderate degree of intercritical deformation were systematically explored. The results show that the addition of 1.46 wt% of Al can effectively promote ferrite formation. An intercritical deformation above 800 °C can result in a pronounced bimodal grain size distribution of ferrite and some elongated ferrite grains containing sub-grains. The residual strain states of both austenite and ferrite and the occurrence of bainite transformation jointly increase the retained austenite fraction due to its mechanical stabilization and the enhanced carbon partitioning into austenite from its surrounding phases. An intercritical deformation below 800 °C can profoundly increase the ferrite fraction and promote the recrystallization of deformed ferrite. The formation of this large fraction of ferrite enhances the carbon enrichment in the untransformed austenite and retards the bainite transformation during the partitioning process and finally enhances martensite transformation and decreases the retained austenite fraction. The efficient TRIP effect of retained austenite and the possible strain partitioning of bainite jointly improve the work hardening and formability of the steel and lead to the excellent mechanical properties with relatively high tensile strength (905 MPa), low yield ratio (0.60) and high total elongation (25.2%). © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.01.040
  • 2016 • 198 Effects of Mn additions on microstructure and properties of Fe–TiB2 based high modulus steels
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 111 185-191 (2016)
    We studied the effects of Mn additions from 0 to 30 wt.% on microstructure, mechanical and physical properties of liquid metallurgy synthesised high modulus steels in hypo- and hyper-eutectic TiB2 concentrations. While Mn has little effect on density, both Young's modulus and mechanical properties were strongly affected by the achieved wide spectrum of matrix microstructures, ranging from ferrite to martensite, reverted austenite, ε-martensite and austenite. Mn additions of 20 and 30 wt.% did not translate into enhanced mechanical performance despite the higher inherent ductility of the predominantly austenitic matrix, and instead eliminate the intended weight saving potential by significantly reducing the Young's modulus. Martensitic matrices of Mn concentrations of 10 wt.%, on the other hand, are favourable for improved matrix/particle co-deformation without sacrificing too much of the composites' stiffness. In hypo-eutectic Fe – TiB2 based steels, mechanical properties on the level of high strength dual phase steels could be achieved (~ 900 MPa UTS and 20% tensile elongation) but with an enhanced Young's modulus of 217 GPa and reduced density of 7460 kg m− 3. These significantly improved physical and mechanical properties render Mn alloyed high modulus steels promising candidate materials for next generation lightweight structural applications. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.09.003
  • 2016 • 197 Fabrication and physico-mechanical properties of thin magnetron sputter deposited silver-containing hydroxyapatite films
    Ivanova, A.A. and Surmeneva, M.A. and Tyurin, A.I. and Pirozhkova, T.S. and Shuvarin, I.A. and Prymak, O. and Epple, M. and Chaikina, M.V. and Surmenev, R.A.
    Applied Surface Science 360 929-935 (2016)
    As a measure of the prevention of implant associated infections, a number of strategies have been recently applied. Silver-containing materials possessing antibacterial activity as expected might have wide applications in orthopedics and dentistry. The present work focuses on the physico-chemical characterization of silver-containing hydroxyapatite (Ag-HA) coating obtained by radio frequency (RF) magnetron sputtering. Mechanochemically synthesized Ag-HA powder (Ca10-xAgx(PO4)6(OH)2-x, x = 1.5) was used as a precursor for sputtering target preparation. Morphology, composition, crystallinity, physico-mechanical features (Young's modulus and nanohardness) of the deposited Ag-HA coatings were investigated. The sputtering of the nanostructured multicomponent target at the applied process conditions allowed to deposit crystalline Ag-HA coating which was confirmed by XRD and FTIR data. The SEM results revealed the formation of the coating with the grain morphology and columnar cross-section structure. The EDX analysis confirmed that Ag-HA coating contained Ca, P, O and Ag with the Ca/P ratio of 1.6 ± 0.1. The evolution of the mechanical properties allowed to conclude that addition of silver to HA film caused increase of the coating nanohardness and elastic modulus compared with those of pure HA thin films deposited under the same deposition conditions. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.11.087
  • 2016 • 196 Improving the mechanical properties of Fe – TiB2 high modulus steels through controlled solidification processes
    Zhang, H. and Springer, H. and Aparicio-Fernández, R. and Raabe, D.
    Acta Materialia 118 187-195 (2016)
    We investigated novel pathways to improve the mechanical properties of liquid metallurgy produced Fe – TiB2 based high modulus steels (HMS) by controlled solidification kinetics and subsequent thermo-mechanical treatments. The solidification rate was varied by casting of hyper-eutectic alloys (20 vol% TiB2) into moulds with differing internal thickness. Ingots between 5 and 40 mm thickness exhibited irregular particle microstructure consisting of sharp-edged coarse primary particles (increasingly clustered with slower solidification) and closely spaced irregular lamellae. Casting defects can be alleviated by hot rolling, but the mechanical properties remain unsatisfactory. Increasing the solidification rate results only at mould thicknesses of 4 mm and below in a significant refinement of the particle microstructure, necessitating liquid metal deposition techniques to utilise it for obtained improved mechanical performance of HMS. Decreasing the solidification rate causes density-induced floatation of the primary particles, which can be used in block-casting for the production of alloys consisting of small and spheroidised eutectic particles, exhibiting high ductility and superior toughness. Annealing just above the solidus-temperature allows the eutectic zones to liquefy and sink, leaving only primary TiB2 particles behind in the top zone of the alloy. Despite the increased particle fraction up to 24 vol%, both strength, specific modulus and ductility are improved over standard processed HMS alloys with 20 vol% TiB2. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.07.056
  • 2016 • 195 In-situ metal matrix composite steels: Effect of alloying and annealing on morphology, structure and mechanical properties of TiB2 particle containing high modulus steels
    Aparicio-Fernández, R. and Springer, H. and Szczepaniak, A. and Zhang, H. and Raabe, D.
    Acta Materialia 107 38-48 (2016)
    We systematically study the morphology, size and dispersion of TiB2 particles formed in-situ from Fe-Ti-B based melts, as well as their chemical composition, crystal structure and mechanical properties. The effects of 5 wt.% additions of Cr, Ni, Co, Mo, W, Mn, Al, Si, V, Ta, Nb and Zr, respectively, as well as additional annealing treatments, were investigated in order to derive guidelines for the knowledge based alloy design of steels with an increased stiffness/density ratio and sufficiently high ductility. All alloying elements were found to increase the size of the coarse primary TiB2 particles, while Co led to the most homogeneous size distribution. The size of the eutectic TiB2 constituents was decreased by all alloying additions except Ni, while their aspect ratio was little affected. No clear relation between chemical composition, crystal structure and mechanical properties of the particles could be observed. Annealing of the as-cast alloys slightly increased the size of the primary particles, but at the same time strongly spheroidised the eutectics. Additions of Co and Cr appear thus as the best starting point for designing novel in-situ high modulus metal matrix composite steels, while using Mn in concert with thermo-mechanical processing is most suited to adapt the matrix' microstructure and optimise the particle/matrix co-deformation processes. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.048
  • 2016 • 194 Influence of conventional and powder-metallurgical manufacturing on the cavitation erosion and corrosion of high interstitial CrMnCN austenitic stainless steels
    Niederhofer, P. and Richrath, L. and Huth, S. and Theisen, W.
    Wear 360-361 67-76 (2016)
    High interstitial CrMnCN austenitic stainless steels combine superior mechanical properties with high resistance to corrosion. The first is caused by the strengthening effect of C and N and the low stacking fault energy leading to intense cold work hardening and e.g. increased resistance to fatigue, which implies a high resistance to cavitation erosion. Corrosion resistance is provided by the elements chromium and molybdenum. Usual manufacturing consists of casting, often followed by hot working. An alternative approach uses pre-alloyed, gas-atomized powders, which can be compacted either by hot isostatic pressing or supersolidus liquid phase sintering. The latter provides the possibility of adapting the nitrogen content via sintering atmosphere. This results in fully dense materials exhibiting comparable mechanical properties like the cast and hot worked alloys. In this study, high interstitial CrMnCN steels with different nitrogen contents were tested in an ultrasonic vibratory cavitation rig and analyzed by electrochemical polarization measurements using different electrolytes. The results indicate positive influences of increasing nitrogen content on both cavitation erosion and corrosion resistance. A comparison with cast and hot worked alloys is included. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2016.04.017
  • 2016 • 193 Investigation and optimization of the tribo-mechanical properties of CrAlCN coatings using Design of Experiments
    Tillmann, W. and Stangier, D. and Schröder, P.
    Surface and Coatings Technology 308 147-157 (2016)
    The control of friction as well as its adaption is essential for forming operations. Thin hard coatings have a significant influence on the performance of production processes and the service life of tools, especially for Sheet-Bulk Metal Forming (SBMF) processes with high contact normal stresses and issues concerned with the filling of filigree functional elements. To handle these challenges, the CrAlCN coating system is generated by means of bipolar-pulsed reactive magnetron sputtering, using Design of Experiments. A Central Composite Design is selected to investigate the cathode power, bias voltage, as well as the reactive gas flow composition (nitrogen and acetylene). The aim is to evaluate the correlations and the interaction of the investigated process parameters on the mechanical as well as the tribological behavior of the CrAlCN coating, and to develop models to obtain the desired coating properties. The generated coatings show a clear dependency on the selected process parameters. An increased acetylene flow leads to a reduction of the mechanical properties (hardness and Young's modulus) as well as a decreased adhesion of the CrAlCN coating. In contrast to the influence of the acetylene flow, a higher negative bias voltage leads to improved mechanical properties in the context of wear resistant thin films. The tribological properties revealed that the coefficient of friction is related to the chemical composition of the coating which can, on the one hand, be adjusted by the acetylene flow and, on the other hand, by the cathode power. The optimized CrAlCN coating was deposited onto forming punches and the friction was evaluated using DC04 and DP600 specimens in an adapted ring-compression test. In comparison to polished and heat-treated ASP®2023 steel (62 HRC) and a CrAlN reference coating, the developed coating shows a significant reduction of friction due to the carbon incorporation. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.07.110
  • 2016 • 192 Investigation of the brazing characteristics of a new iron-based brazing filler metal
    Tillmann, W. and Wojarski, L. and Manka, M. and Trelenberg, A.
    Welding in the World 60 869-875 (2016)
    High temperature applications of new class of iron-based filler metals provide brazements with high corrosion resistance and mechanical properties. These brazements are cost-effective alternative to those made of the conventional brazing alloys. However, a wiser usage demands a deeper understanding of the wetting as well as gap filling behavior in conjunction with the resulting microstructure, which is mainly influenced by the applied brazing cycles. Therefore, this paper presents results of the investigation of specific brazing fundamentals for the new iron-based brazing alloy Fe-24Cr-20Ni-10Cu-7P-5Mn-5Si. Followed by DTA/DSC measurements, the spreading and gap filling behavior were examined by using stainless steel AISI 304 as base material. In wetting tests and wedge-gap experiments, the influence of the applied brazing temperature and the dwell time were investigated for vacuum brazing processes. The resulting microstructure was evaluated using a scanning electron microscope (SEM), equipped with an energy dispersive X-ray spectroscopy (EDS). Finally, strength tests were conducted in order to determine the influence of the brazing parameters on the mechanical properties of the joint. © 2016, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-016-0346-4
  • 2016 • 191 Modeling of low-alloyed trip-steels based on direct micro-macro simulations
    Prüger, S. and Gandhi, A. and Balzani, D.
    ECCOMAS Congress 2016 - Proceedings of the 7th European Congress on Computational Methods in Applied Sciences and Engineering 2 2280-2291 (2016)
    Low-alloyed TRIP steels are often used in the automotive industry due to their favorable mechanical properties such as high ductility and strength and their moderate production costs. These steels possess a heterogeneous multiphase microstructure, initially consisting of ferrite, bainite and retained austenite which is responsible for the mechanical properties. Upon deformation, a diffusionless, stress-induced, martensitic phase transformation from austenite to martensite is observed, enhancing ductility and strength. We focus on multi-scale methods in the sense of FE2 to describe the macroscopic behavior of low-alloyed TRIP-steels, because this approach allows for a straightforward inclusion of various influencing factors such as residual stress distribution, graded material properties which can hardly included in phenomenological descriptions of these heterogeneous multiphase materials. In order to allow for efficient computations, a simplified microstructure is used in an illustrative direct micro-macro simulation. The inelastic processes in the austenitic inclusions involve the phase transformation from austenite to martensite and the inelastic deformation of these two phases. The isotropic, rate-independent, hyperelastic-plastic material model of Hallberg et al. (IJP, 23, pp. 1213-1239, 2007), originally proposed for high-alloyed TRIP steel, is adopted here for the inclusion phase. Minor modifications of the model are proposed to improve its implementation and performance. The influence of various material parameters associated with the phase transformation on the evolution of retained austenite is studied for different homogeneous deformation states. The non-monotonic stress-state dependence observed in experiments is clearly captured by the model. A numerical two-scale calculation is carried out to enlighten the ductility enhancement in low-alloyed TRIP-steels due to the martensitic phase transformation.
    view abstractdoi: 10.7712/100016.1959.7726
  • 2016 • 190 Plasticity of the ω-Al7Cu2Fe phase
    Laplanche, G. and Bonneville, J. and Joulain, A. and Gauthier-Brunet, V. and Dubois, S.
    Journal of Alloys and Compounds 665 144-151 (2016)
    Polycrystalline samples with the Al0.693Cu0.201Fe0.106 composition, corresponding to the tetragonal P4/mnc ω-Al7Cu2Fe crystallographic structure, were synthesised by spark plasma sintering and deformed in compression under constant strain-rate conditions, ε = 2 × 10-4 s-1, over the temperature range 650 K-1000 K. A brittle-to-ductile transition is evidenced between 700 K and 750 K. The stress-strain curves exhibit a yield point followed by softening or steady state conditions only. The upper yield stress, σUYS, shows a strong temperature dependence suggesting that the rate controlling deformation mechanisms are highly thermally activated. The strain-rate sensitivity of stress characterised either by stress exponents, nexp, or by activation volumes, Vexp, was measured by the load relaxation technique. High nexp values, i.e., larger than 7, associated with low Vexp, typically smaller than 1 nm3, are measured. The Gibbs free activation energy, ΔG, deduced by integrating Vexp with respect to stress varies from nearly 2 eV at 790 K to 4 eV at 1000 K. Because plasticity of the ω-Al7Cu2Fe phase takes place at temperatures at which diffusion processes are considered as dominant, the results are interpreted in the frame of dislocation climb models proposed to account for high temperature plasticity of crystalline phases. © 2016 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2015.12.161
  • 2016 • 189 Quality Control of Laser-Beam-Melted Parts by a Correlation Between Their Mechanical Properties and a Three-Dimensional Surface Analysis
    Grimm, T. and Wiora, G. and Witt, G.
    JOM 69 544-550 (2016)
    Good correlations between three-dimensional surface analyses of laser-beam-melted parts of nickel alloy HX and their mechanical properties were found. The surface analyses were performed with a confocal microscope, which offers a more profound surface data basis than a conventional, two-dimensional tactile profilometry. This new approach results in a wide range of three-dimensional surface parameters, which were each evaluated with respect to their feasibility for quality control in additive manufacturing. As a result of an automated surface analysis process by the confocal microscope and an industrial six-axis robot, the results are an innovative approach for quality control in additive manufacturing. © 2016 The Minerals, Metals & Materials Society
    view abstractdoi: 10.1007/s11837-016-2190-9
  • 2016 • 188 Revealing the relationships between chemistry, topology and stiffness of ultrastrong Co-based metallic glass thin films: A combinatorial approach
    Schnabel, V. and Köhler, M. and Evertz, S. and Gamcova, J. and Bednarcik, J. and Music, D. and Raabe, D. and Schneider, J.M.
    Acta Materialia 107 213-219 (2016)
    An efficient way to study the relationship between chemical composition and mechanical properties of thin films is to utilize the combinatorial approach, where spatially resolved mechanical property measurements are conducted along a concentration gradient. However, for thin film glasses many properties including the mechanical response are affected by chemical topology. Here a novel method is introduced which enables spatially resolved short range order analysis along concentration gradients of combinatorially synthesized metallic glass thin films. For this purpose a CoZrTaB metallic glass film of 3 μm thickness is deposited on a polyimide foil, which is investigated by high energy X-ray diffraction in transmission mode. Through the correlative chemistry-topology-stiffness investigation, we observe that an increase in metalloid concentration from 26.4 to 32.7 at% and the associated formation of localized (hybridized) metal - metalloid bonds induce a 10% increase in stiffness. Concomitantly, along the same composition gradient, a metalloid-concentration-induced increase in first order metal - metal bond distances of 1% is observed, which infers itinerant (metallic) bond weakening. Hence, the metalloid concentration induced increase in hybridized bonding dominates the corresponding weakening of metallic bonds. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.060
  • 2016 • 187 Setting mechanical properties of high strength steels for rapid hot forming processes
    Löbbe, C. and Hering, O. and Hiegemann, L. and Tekkaya, A.E.
    Materials 9 (2016)
    Hot stamping of sheet metal is an established method for the manufacturing of light weight products with tailored properties. However, the generally-applied continuous roller furnace manifests two crucial disadvantages: The overall process time is long and a local setting of mechanical properties is only feasible through special cooling techniques. Hot forming with rapid heating directly before shaping is a new approach, which not only reduces the thermal intervention in the zones of critical formability and requested properties, but also allows the processing of an advantageous microstructure characterized by less grain growth, additional fractions (e.g., retained austenite), and undissolved carbides. Since the austenitization and homogenization process is strongly dependent on the microstructure constitution, the general applicability for the process relevant parameters is unknown. Thus, different austenitization parameters are analyzed for the conventional high strength steels 22MnB5, Docol 1400M, and DP1000 in respect of the mechanical properties. In order to characterize the resulting microstructure, the light optical and scanning electron microscopy, micro and macro hardness measurements, and the X-ray diffraction are conducted subsequent to tensile tests. The investigation proves not only the feasibility to adjust the strength and ductility flexibly, unique microstructures are also observed and the governing mechanisms are clarified. © 2016 by the authors.
    view abstractdoi: 10.3390/ma9040229
  • 2016 • 186 Tempering behavior of a low nitrogen boron-added 9%Cr steel
    Fedorova, I. and Kostka, A. and Tkachev, E. and Belyakov, A. and Kaibyshev, R.
    Materials Science and Engineering A 662 443-455 (2016)
    The effect of tempering temperature on microstructure and mechanical properties was studied in a low-nitrogen, high-boron, 9%Cr steel. After normalizing and low-temperature tempering, cementite platelets precipitated within the martensitic matrix. This phase transformation has no distinct effect on mechanical properties. After tempering at 500 °C, M23C6 carbides appeared in the form of layers and particles with irregular shapes along the high-angle boundaries. Approximately, 6% of the retained austenite was observed after normalizing, which reduced to 2% after tempering at 550 °C. This is accompanied by reduction in toughness from 40 J/cm2 to 8.5 J/cm2. Further increase of the tempering temperature led to spheroidization and coagulation of M23C6 particles that is followed by a significant increase in toughness to 250 J/cm2 at 750 °C. Three-phase separations of M(C,N) carbonitrides to particles enriched with V, Nb and Ti were detected after high-temperature tempering. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.03.092
  • 2016 • 185 The role of process temperature and rotational speed in the microstructure evolution of Ti-6Al-4V friction surfacing coatings
    Fitseva, V. and Hanke, S. and Santos, J.F.D. and Stemmer, P. and Gleising, B.
    Materials and Design 110 112-123 (2016)
    Friction surfacing is a solid state technique for depositing metallic coatings. Coating materials are thermo-mechanically processed at high temperatures during deposition. The high degree of deformation involved leads to a dynamically recrystallised fine grained microstructure. For Ti-6Al-4V, the microstructure and mechanical properties of coatings generated by friction surfacing have not been studied yet. The current work focuses on investigating effects of rotational speed on microstructure, grain size evolution and mechanical properties of the coating material. Various rotational speeds in a wide range, exceeding the range of deformation used in many other severe plastic deformation processes, were used to generate Ti-6Al-4V coatings by friction surfacing. Their influence on the thermal cycle and consequently on microstructure formation was revealed. The β grain size is related to the rotational speed and thermal cycle. Grain refinement at low rotational speed was observed, while higher rotational speeds and corresponding increase in maximum temperature led to grain coarsening. Although the peak temperature dominates the grain size evolution, dynamic recrystallisation during friction surfacing counteracts this effect, reducing the grain size by one order of magnitude. The coatings exhibit a hardness ascent about 15% due to martensite formation, high dislocation density and oxide precipitations. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.07.132
  • 2016 • 184 Three-dimensional surface measurement for the quantification of mechanical properties of laser-sintered parts
    Grimm, T. and Witt, G. and Wiora, G.
    Materialpruefung/Materials Testing 58 293-301 (2016)
    A good correlation (σ = -0.912) is found between three dimensional surface features and mechanical properties of laser-sintered parts. This study identifies correlating surface parameters and features out of the wide range of existing evaluation methods resulting from the new possibilities of a three dimensional surface measurement in contrast to a conventional tactile profilometry. Therefore, surface analyses were performed using a confocal microscope and tensile tests according to ISO 527-1 were conducted. Especially the motifs dale and volume of islands analyses show a good correlation concerning the tensile strength, the Young's modulus and the elongation at break. The differences with regard to each feature and their influence on the mechanical properties are discussed. © 2016 Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/120.110851
  • 2015 • 183 3D structural and atomic-scale analysis of lath martensite: Effect of the transformation sequence
    Morsdorf, L. and Tasan, C.C. and Ponge, D. and Raabe, D.
    Acta Materialia 95 366-377 (2015)
    To improve the fundamental understanding of the multi-scale characteristics of martensitic microstructures and their micro-mechanical properties, a multi-probe methodology is developed and applied to low-carbon lath martensitic model alloys. The approach is based on the joint employment of electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), atom probe tomography (APT) and nanoindentation, in conjunction with high precision and large field-of-view 3D serial sectioning. This methodology enabled us to resolve (i) size variations of martensite sub-units, (ii) associated dislocation sub-structures, (iii) chemical heterogeneities, and (iv) the resulting local mechanical properties. The identified interrelated microstructure heterogeneity is discussed and related to the martensitic transformation sequence, which is proposed to intrinsically lead to formation of a nano-composite structure in low-carbon martensitic steels. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.05.023
  • 2015 • 182 Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Somsen, C. and Kostka, A. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Dlouhy, A. and Müller, J. and Spiecker, E. and Demtroder, K. and Schreuer, J. and Neuking, K. and Eggeler, G.
    Advanced Engineering Materials 17 216-230 (2015)
    In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known γ/γ′ microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can showthat elements such as Re, Co, andCr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while g channels show higher concentrations of Co, Cr, Re, andW.We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140mm x 100mm x 20mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201400136
  • 2015 • 181 Alloying effects on microstructure formation of dual phase steels
    Schemmann, L. and Zaefferer, S. and Raabe, D. and Friedel, F. and Mattissen, D.
    Acta Materialia 95 386-398 (2015)
    In dual-phase (DP) steels, inherited microstructures and elemental distributions affect the kinetics and morphology of phase transformation phenomena and the mechanical properties of the final material. In order to study the inheritance process, we selected two model materials with the same average DP steel composition but with different initial microstructures, created by coiling at different temperatures after hot rolling. These samples were submitted to a DP-steel heat treatment consisting of a short isothermal annealing in the pure austenite region and a quenching process. The evolution of microstructure, chemical composition and mechanical properties (hardness) during this treatment was investigated. The initial samples had a bainitic-martensitic (B + M) microstructure for the material coiled at lower temperature and a ferritic-pearlitic (P + F) microstructure for that coiled at higher temperature. The P + F microstructure had a much more inhomogeneous distribution of substitutional elements (in particular of Mn) and of carbon. After complete heat treatment, both materials showed a typical DP microstructure (martensite islands embedded in ferrite) but the P + F material showed lower hardness compared to the B + M material. It was found that the inhomogeneous elemental distribution prevailed in the P + F material. The inheritance process was studied by combining measurements of the elemental distribution by Wavelength-Dispersive X-ray spectroscopy (WDX), simulations of the evolution of the elemental composition via the DICTRA (diffusion-controlled reactions) computer programme, dilatometry to observe the kinetics of phase transformation, and observation and quantification of the microstructures by Electron Backscatter Diffraction (EBSD) measurements. For the P + F material it was found that the α-γ transformation during annealing is slowed down in regions of lower Mn content and is therefore not completed. During the subsequent cooling the incompletely autenitized material does not require ferrite nucleation and the γ-α transformation starts at relative high temperatures. For B + M, in contrast, nucleation of ferrite is needed and the transformation starts at lower temperatures. As a result the B + M material develops a higher martensite content as well as a higher density of geometrically necessary dislocations (GNDs). It is speculated that for the B + M material the γ-α transformation occurs through a bainitic (i.e. partly displacive) process while the transformation at higher temperatures in the P + F material proceeds exclusively in a diffusive way. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.05.005
  • 2015 • 180 Atom probe tomography investigation of heterogeneous short-range ordering in the 'komplex' phase state (K-state) of Fe-18Al (at.%)
    Marceau, R.K.W. and Ceguerra, A.V. and Breen, A.J. and Palm, M. and Stein, F. and Ringer, S.P. and Raabe, D.
    Intermetallics 64 23-31 (2015)
    We study an Fe-18Al (at.%) alloy after various thermal treatments at different times (24-336 h) and temperatures (250-1100 °C) to determine the nature of the so-called 'komplex' phase state (or "K-state"), which is common to other alloy systems having compositions at the boundaries of known order-disorder transitions and is characterised by heterogeneous short-range-ordering (SRO). This has been done by direct observation using atom probe tomography (APT), which reveals that nano-sized, ordered regions/particles do not exist. Also, by employing shell-based analysis of the three-dimensional atomic positions, we have determined chemically sensitive, generalised multicomponent short-range order (GM-SRO) parameters, which are compared with published pairwise SRO parameters derived from bulk, volume-averaged measurement techniques (e.g. X-ray and neutron scattering, Mössbauer spectroscopy) and combined ab-initio and Monte Carlo simulations. This analysis procedure has general relevance for other alloy systems where quantitative chemical-structure evaluation of local atomic environments is required to understand ordering and partial ordering phenomena that affect physical and mechanical properties. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2015.04.005
  • 2015 • 179 Biopolymer foaming with supercritical CO2 - Thermodynamics, foaming behaviour and mechanical characteristics
    Frerich, S.C.
    Journal of Supercritical Fluids 96 349-358 (2015)
    Polymer foams, especially those based on biodegradable polymers, are in high demand for energy saving applications, for example as thermal insulations or packaging materials. To understand and predict the quality and material properties of polymer foams, concise knowledge of the factors influencing the foaming behaviour, especially pressure and temperature, is required. Therefore, three biodegradable polyesters, namely poly (lactide) (PLA), poly (butylene succinate) (PBS) and a blend of poly (lactide) and poly (hydroxy butyrate) (PLA-PHB), have been subjected to a direct foaming procedure using compressed carbon dioxide as blowing agent, studying the influence of saturation temperature (ranging from 95 °C to 175°C) and applied pressure (ranging from atmospheric pressure to 30 MPa) on the resulting foam material. As these results are strongly depending on the melting behaviour of the polymers, all three polymers were subjected to calorimetric analysis in a scanning transitiometer that allows for applying pressure levels of up to 45 MPa. The created porous materials were characterized by determining their density, porosity and morphology, using SEM analysis. Their mechanical behaviour was investigated by using compressive strength tests. It is shown that the quality of the produced foam structures and its properties is strongly depending on the foaming conditions. In order to obtain foams with a high quality, the saturation temperature and pressure have to be adapted to the phase transition liquid-solid of the polymer-gas system. The results obtained via scanning transitiometer represent the SLG-line of the binary systems polymer and CO2, and their influence on the foaming behaviour enabled the identification of ideal foaming conditions for the three polymers regarded in this study. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.supflu.2014.09.043
  • 2015 • 178 Cold extrusion of hot extruded aluminum chips
    Haase, M. and Tekkaya, A.E.
    Journal of Materials Processing Technology 217 356-367 (2015)
    The direct conversion of aluminum alloy machining chips into finished parts by hot extrusion with subsequent cold extrusion was investigated. While the process of hot extrusion was utilized to break the oxides covering the individual chips and to lead to bonding of the pure metal, the processes of cold forward rod extrusion as well as cold backward can extrusion were used for the production of chip-based finished parts. For the hot extrusion process, a flat face die and a die with integrated equal channel angular pressing (iECAP die) were used in order to investigate the influence of the deformation route on the quality of the chip-based finished parts. The flat-face die is a conventional tool for the fabrication of solid sections, while the iECAP die is an experimental tool that integrates the severe plastic deformation process of equal channel angular pressing into a conventional hot extrusion die. Tensile tests revealed superior mechanical properties of chips extruded through the iECAP die compared to those of chips extruded through the flat-face die. The hot extruded chips were further processed at room temperature by either backward can extrusion to cans with different wall thicknesses or by forward rod extrusion to shafts with different values of extrusion ratio and cone angle. For all fabricated chip-based finished parts, the mechanical properties and the microstructure were analyzed. Backward can extrusion of chip-based extrudates fabricated with the iECAP die resulted in defect-free cans for all investigated wall thicknesses, while the cans obtained from flat-face die processed chips showed cracks within the walls. Shafts without visible internal defects could be produced by forward rod extrusion of previously hot extruded chips, independent of the hot extrusion die design. However, subsequent compression tests revealed a dependency of the mechanical properties of chip-based shafts on the hot extrusion die design. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2014.11.028
  • 2015 • 177 Correlations between microstructure and room temperature tensile behavior of a duplex TNB alloy for systematically heat treated samples
    Kabir, M.R. and Bartsch, M. and Chernova, L. and Kelm, K. and Wischek, J.
    Materials Science and Engineering A 635 13-22 (2015)
    The mechanical properties of TiAl alloys are very sensitive to the inherent microstructure. For an in-depth understanding of microstructural influences on mechanical properties a duplex type TNB (Nb-containing TiAl) alloy has been investigated. For varying the microstructure of this alloy controlled heat treatments (HT) have been performed with eight distinct maximum temperatures, ranging from 1230. °C to 1300. °C with a 10. °C temperature increment. The series of annealing processes resulted in duplex microstructures with a gradual change of the ratio of globular grains and lamellar colonies, keeping the global chemical composition unchanged. Microstructure of each sample was characterized using SEM and TEM before and after mechanical testing to correlate the morphology and microstructure features to the tensile properties. Quantitative data analysis from these results revealed how the evolution of duplex microstructures influences the room temperature tensile properties: i.e. the elastic stiffness, room temperature ductility, work hardening, fracture stress, and fracture strain. The results are discussed with respect to deformation mechanisms as understood from the tensile test results and fracture surface investigations. From the observed correlations between microstructure and properties an optimized constellation of globular and lamellar microstructure for relevant properties can be predicted. Furthermore, the required heat-treatment window for properties targeted can be defined. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.03.041
  • 2015 • 176 Crack propagation behavior of solution annealed austenitic high interstitial steels
    Schymura, M. and Stegemann, R. and Fischer, A.
    International Journal of Fatigue 79 25-35 (2015)
    Austenitic stainless steels provide a beneficial combination of chemical and mechanical properties and have been used in a wide field of applications for over 100 years. Further improvement of the chemical and mechanical properties was achieved by alloying nitrogen. But the solubility of N within the melt is limited and can be increased in substituting Ni by Mn and melting under increased pressure. In order to avoid melting under pressure and decrease production costs, a part of N can also be substituted by C. This leads to austenitic high interstitial steels (AHIS). Within the solution annealed state strength and ductility of AHIS is comparable or even higher of those of AHNS and can be further improved by cold working. Unfortunately the endurance limit does not follow this trend as it is known from cold-worked austenitic CrNi steels. This is due to the differences of the slip behavior which is governed by the stacking fault energy as well as other near field effects. Construction components operating under cyclic loads over long periods of time cannot be considered being free of voids or even cracks. Thus the crack propagation behavior is of strong interest as well. This contribution presents the tensile, fatigue, crack propagation and fracture toughness properties of AHNS and AHIS in comparison to those of CrNi-steels. The differences are discussed in relation to microstructural characteristic as well as their alterations under cyclic loading. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2015.04.014
  • 2015 • 175 Effect of substitution on elastic stability, electronic structure and magnetic property of Ni-Mn based Heusler alloys: An ab initio comparison
    Roy, T. and Gruner, M.E. and Entel, P. and Chakrabarti, A.
    Journal of Alloys and Compounds 632 822-829 (2015)
    First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson's ratio and Young's modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt's and Reuss's method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.255
  • 2015 • 174 Evolution of microstructure and mechanical properties of coated Co-base superalloys during heat treatment and thermal exposure
    Webler, R. and Ziener, M. and Neumeier, S. and Terberger, P.J. and Vaßen, R. and Göken, M.
    Materials Science and Engineering A 628 374-381 (2015)
    New γ'-strengthened Co-base superalloys show an interesting potential for high temperature applications. However, protective coatings are needed as for Ni-base superalloys to ensure sufficient oxidation and corrosion resistance. Therefore the properties of a commercial coating on a multinary new γ'-strengthened Co-base superalloy have been studied. Especially the influence of the coating process on the substrate also after long term annealing is discussed. It was found that the highly deformed areas at the coating-substrate interface indicated by a high local misorientation and caused by the sandblasting process led to a recrystallization of the interdiffusion zone during the age hardening heat treatment. A chemical gradient of γ and γ' promoting elements was found in the interdiffusion zone causing a change in hardness as measured by nanoindentation. Depending on the composition two separate recrystallized regions formed in the interdiffusion zone, one with single phase γ-(Co,Ni) and the other with a cellular two phase microstructure of discontinuously grown γ and γ'. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.060
  • 2015 • 173 Formation of dislocation networks in a coherent Cu Σ3(1 1 1) twin boundary
    Jeon, J.B. and Dehm, G.
    Scripta Materialia 102 71-74 (2015)
    Molecular dynamics simulations were performed to investigate dislocation network formations in a coherent twin boundary in Cu. Depending on the activated glide system, the initial flawless twin boundary can be heavily or sparsely decorated by a dislocation network. The dislocation mechanism leading to a heavy dislocation network at the twin boundary and its consequence on mechanical properties will be discussed. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2015.02.016
  • 2015 • 172 From hard to rubber-like: mechanical properties of resorcinol–formaldehyde aerogels
    Schwan, M. and Naikade, M. and Raabe, D. and Ratke, L.
    Journal of Materials Science 50 5482-5493 (2015)
    Four types of resorcinol–formaldehyde (RF) aerogels, stiff, brittle, low-flexible, and super-flexible are studied in this work. Despite several studies on mechanical properties on RF aerogels their response when exposed to compressive loading and their fracture behavior are not well investigated. Here, we cover aerogels with a very broad density range of 0.08–0.3 g cm−3 and compressive moduli from 0.12 to 28 MPa. We relate the microstructure of the synthesized aerogels and their behavior under uniaxial compression. Additionally, this work is the first, to our knowledge, to implement the usage of digital image correlation for characterizing the deformation of RF aerogels. The comparison of surface strain distribution of four types of aerogels provides an insight to their reaction on compressive loading. © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-015-9094-x
  • 2015 • 171 Hydrogen diffusion and segregation in α iron ∑ 3 (111) grain boundaries
    Hamza, M. and Hatem, T.M. and Raabe, D. and El-Awady, J.A.
    ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 9-2015 (2015)
    Polycrystalline material generally exhibits degradation in its mechanical properties and shows more tendency for intergranular fracture due to segregation and diffusion of hydrogen on the grain boundaries (GBs). Understanding the parameters affecting the diffusion and binding of hydrogen within GBs will allow enhancing the mechanical properties of the commercial engineering materials and developing interface dominant materials. In practice during forming processes, the coincidence site lattice (CSL) GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑ 3 111 [11 0] GB structure and its angular deviations in BCC iron within the range of Brandon criterion will be studied comprehensively using molecular statics (MS) simulations. The clean GB energy will be quantified, followed by the GB and free surface segregation energies calculations for hydrogen atoms. Rice-Wang model will be used to assess the embrittlement impact variation over the deviation angles. The results showed that the ideal GB structure is having the greatest resistance to embrittlement prior GB hydrogen saturation, while the 3° deviated GB is showing the highest susceptibility to embrittlement. Upon saturation, the 5° deviated GB appears to have the highest resistance instead due to the lowest stability of hydrogen atoms observed in the free surfaces of its simulation cell. Molecular dynamics (MD) simulations are then applied to calculate hydrogen diffusivity within the ideal and deviated GB structure. It is shown that hydrogen diffusivity decreases significantly in the deviated GB models. In addition, the 5° deviated GB is representing the local minimum for diffusivity results suggesting the existence of the highest atomic disorder and excessive secondary dislocation accommodation within this interface. Copyright © 2015 by ASME.
    view abstractdoi: 10.1115/IMECE2015-53118
  • 2015 • 170 Importance of dislocation pile-ups on the mechanical properties and the Bauschinger effect in microcantilevers
    Kapp, M.W. and Kirchlechner, C. and Pippan, R. and Dehm, G.
    Journal of Materials Research 30 791-797 (2015)
    Copper microcantilevers were produced by focused ion beam milling and tested in situ using a scanning electron microscope. To provide different interfaces for piling up dislocations, cantilevers were fabricated to be single crystalline, bicrystalline, or single crystalline with a slit in the region of the neutral axis. The aim of the experiment was to study the influence of dislocation pile-ups on (i) strength and (ii) Bauschinger effects in micrometer-sized, focused ion beam milled bending cantilevers. The samples were loaded monotonically for several times under displacement control. Even though the cantilevers exhibited the same nominal strain gradient the strength varied by 34% within the three cantilever geometries. The Bauschinger effect can be promoted and prohibited by the insertion of different interfaces. © 2015 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2015.49
  • 2015 • 169 Influence of Cold Work on Pitting Corrosion and Cavitation Erosion of High Interstitial FeCrMnCN Austenites
    Niederhofer, P. and Siebert, S. and Huth, S. and Theisen, W. and Berns, H.
    Steel Research International 86 1439-1446 (2015)
    High interstitial steels (HIS) are based on the joint addition of carbon and nitrogen, which resulted in an austenitic FeCrMn steel grade. In contrast to high nitrogen steels (HNS), they can be produced by conventional metallurgy and offer a unique combination of mechanical properties and corrosion resistance. This makes them promising candidates for the use in environments featuring corrosive or wear attack or even both. Corrosion resistance can be improved by the addition of molybdenum, particularly in the case of media containing chloride ions. In this study, different FeCrMnCN alloys were investigated by means of pitting corrosion testing in sodium chloride solution, as well as cavitation erosion resistance. These properties were examined depending on prestraining, since the latter is used to strengthen this kind of alloys. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201400381
  • 2015 • 168 Influence of process-induced microstructure and imperfections on mechanical properties of AlSi12 processed by selective laser melting
    Siddique, S. and Imran, M. and Wycisk, E. and Emmelmann, C. and Walther, F.
    Journal of Materials Processing Technology 221 205-213 (2015)
    Selective laser melting (SLM) offers high potential for manufacturing complex geometries and custom-made parts due to its unique layer-wise production process. A series of samples of AlSi12 have been manufactured by SLM process to study the effect of process parameters and post-build heat treatment on the microstructure and the corresponding mechanical properties. Optical microscope, scanning electron microscope, quasistatic tests, continuous load increase fatigue tests and constant amplitude fatigue tests have been employed for characterization. A remarkable eutectic microstructure, with dendritic width changing with SLM process parameters, has been observed. Relationship between SLM process parameters, resulting microstructure and the consequent changes in mechanical properties has been discussed. Base plate heating has been found critical in controlling the in-process microstructure. Mechanical properties of SLM parts outperform those of conventionally manufactured alloy, and can be varied as per requirement, by altering the build rate, keeping the process costs in control. Fatigue scatter can also be controlled by heating the base plate during the process. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2015.02.023
  • 2015 • 167 Manganese tetraboride, MnB4: High-temperature crystal structure, p-n transition, 55Mn NMR spectroscopy, solid solutions, and mechanical properties
    Knappschneider, A. and Litterscheid, C. and Brgoch, J. and George, N.C. and Henke, S.c and Cheetham, A.K. and Hu, J.G. and Seshadri, R. and Albert, B.
    Chemistry - A European Journal 21 8177-8181 (2015)
    The structural and electronic properties of MnB<inf>4</inf> were studied by high-temperature powder X-ray diffraction and measurements of the conductivity and Seebeck coefficient on spark-plasma-sintered samples. A transition from the room-temperature monoclinic structure (space group P2<inf>1</inf>/c) to a high-temperature orthorhombic structure (space group Pnnm) was observed at about 650K. The material remained semiconducting after the transition, but its behavior changed from p-type to n-type. 55Mn NMR measurements revealed an isotropic chemical shift of -1315ppm, confirming an oxidation state of Mn close to I. Solid solutions of Cr<inf>1-x</inf>Mn<inf>x</inf>B<inf>4</inf> (two phases in space groups Pnnm and P2<inf>1</inf>/c) were synthesized for the first time. In addition, nanoindentation studies yielded values of (496±26) and (25.3±1.7)GPa for the Young's modulus and hardness, respectively, compared to values of 530 and 37GPa obtained by DFT calculations. Phase transition: Monoclinic manganese tetraboride can be transformed into an orthorhombic phase thermally or by forming solid solutions with chromium tetraboride. The structural phase transition of semiconducting MnB<inf>4</inf> is accompanied by a p-n transition. 55Mn NMR spectroscopy confirmed the oxidation state I of the metal atom, and nanoindentation experiments resulted in hardness values that are in accordance with DFT calculations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201406631
  • 2015 • 166 Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation
    Schuh, B. and Mendez-Martin, F. and Völker, B. and George, E.P. and Clemens, H. and Pippan, R. and Hohenwarter, A.
    Acta Materialia 96 258-268 (2015)
    An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to ∼520 HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of ∼630 HV at about 450 °C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 HV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5 min at 450 °C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites. © 2015 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2015.06.025
  • 2015 • 165 Micro-tension study of miniaturized cu lines at variable temperatures
    Wimmer, A. and Heinz, W. and Leitner, A. and Detzel, T. and Robl, W. and Kirchlechner, C. and Dehm, G.
    Acta Materialia 92 243-254 (2015)
    In this study, tension experiments on Cu micro-samples at temperatures between 143 and 873 K were performed in order to analyze the influence of grain size, temperature and strain rate on the mechanical properties and fracture mode. The activation energy and evolution of the dislocation density have been analyzed to identify the deformation mechanisms. A transition from bulk-like to stochastic, small-scale behavior has been found with increasing grain size. Furthermore, dependent on the grain size and temperature a change from dislocation based plasticity to diffusion controlled deformation was observed. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.03.056
  • 2015 • 164 Microstructural evolution of a CoCrFeMnNi high-entropy alloy after swaging and annealing
    Laplanche, G. and Horst, O. and Otto, F. and Eggeler, G. and George, E.P.
    Journal of Alloys and Compounds 647 548-557 (2015)
    Abstract The processing parameters which govern the evolution of microstructure and texture during rotary swaging and subsequent heat treatments were studied in an equiatomic single-phase CoCrFeMnNi high-entropy alloy. After vacuum induction melting and casting, the diameter of the 40 mm cast ingot was reduced at room temperature to a final diameter of 16.5 mm by rotary swaging (diameter reduction of 60%/area reduction of 80%) and the alloy was then annealed at different temperatures for 1 h. The resulting microstructures were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction and correlated with results of microhardness measurements. It was found that the microhardness first increases slightly upon annealing below the recrystallization temperature but then drops steeply at higher annealing temperatures due to the onset of recrystallization. Special emphasis was placed on how the microstructure evolves with respect to the radial and longitudinal position in the rod. Finally, a combination of swaging and heat treatment parameters were identified that can produce CoCrFeMnNi high-entropy alloys with a homogeneous composition and grain size and almost no texture. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2015.05.129
  • 2015 • 163 Microstructure design and mechanical properties in a near-α Ti-4Mo alloy
    Tarzimoghadam, Z. and Sandlöbes, S. and Pradeep, K.G. and Raabe, D.
    Acta Materialia 97 291-304 (2015)
    Abstract We study the effects of different heat treatment routes on microstructure engineering and the resulting mechanical response in a plain binary Ti-4Mo (wt%) model alloy. We observe a broad variety of microstructure formation mechanisms including diffusion driven allotropic phase transformations as well as shear and/or diffusion dominated modes of martensitic transformations, enabling a wealth of effective microstructure design options even in such a simple binary Ti alloy. This wide variety of microstructures allows tailoring the mechanical properties ranging from low yield strength (350 MPa) and high ductility (30-35% tensile elongation) to very high yield strength (1100 MPa) and medium ductility (10-15% tensile elongation) as well as a variety of intermediate states. Mechanical testing and microstructure characterization using optical microscopy, scanning electron microscopy based techniques, transmission electron microscopy and atom probe tomography were performed revealing that minor variations in the heat treatment cause significant changes in the resulting microstructures (e.g. structural refinement, transition between diffusive and martensitic transformations). The experimental results on microstructure evolution during the applied different heat treatment routes are discussed with respect to the mechanical properties. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.06.043
  • 2015 • 162 Multiphase microstructures via confined precipitation and dissolution of vessel phases: Example of austenite in martensitic steel
    Belde, M. and Springer, H. and Inden, G. and Raabe, D.
    Acta Materialia 86 1-14 (2015)
    We present a novel method to locally control the constitution, morphology, dispersion and transformation behavior of multiphase materials. The approach is based on the targeted, site-specific formation and confined dissolution of precipitated carbides or intermetallic phases. These dispersoids act as "vessels" or "containers" for specific alloying elements forming controlled chemical gradients within the microstructure upon precipitation and subsequent (partial) dissolution at elevated temperatures. The basic processing sequence consists of three subsequent steps, namely: (i) matrix homogenization (conditioning step); (ii) nucleation and growth of the vessel phases (accumulation step); and (iii) (partial) vessel dissolution (dissolution step). The vessel phase method offers multiple pathways to create dispersed microstructures by the variation of plain thermomechanical parameters such as time, temperature and deformation. This local microstructure design enables us to optimize the mechanical property profiles of advanced structural materials such as high strength steels at comparatively lean alloy compositions. The approach is demonstrated on a 11.6Cr-0.32C (wt.%) steel, where by using M23C6 carbides as a vessel phase, Cr and C can be locally enriched so that the thus-lowered martensite start temperature allows the formation of a significant quantity of retained austenite (up to 14 vol.%) of fine dispersion and controlled morphology. The effects of processing parameters on the obtained microstructures are investigated, with a focus on the dissolution kinetics of the vessel carbides. The approach is referred to as vessel microstructure design. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.11.025
  • 2015 • 161 Nanoindentation studies of the mechanical properties of the μ phase in a creep deformed Re containing nickel-based superalloy
    Rehman, H.U. and Durst, K. and Neumeier, S. and Parsa, A.B. and Kostka, A. and Eggeler, G. and Göken, M.
    Materials Science and Engineering A 634 202-208 (2015)
    Addition of Re in nickel-based superalloys results in an increase of the creep life. However, Re is also known to segregate to the dendrite core and to promote the formation of topologically closed packed (TCP) phases. In the present work, the local segregation of Re was studied in the heat treated and creep deformed state of a nickel-based superalloy. Tensile creep deformation at 1050°C resulted in the formation of TCP phases in the dendritic regions. Characterization using TEM confirmed the presence of μ phase that grows on {111} planes. Measurements with a nanoindenting AFM show that the μ phase is harder and shows less work hardening than both the γ and the γ' phases. Furthermore, in the creep deformed state the hardness of the matrix phase is very similar in the dendrite core and in interdendritic areas, although Re is still enriched in the dendrite core. It is shown that Re is consumed in the dendrite core by the TCP phases. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.03.045
  • 2015 • 160 Nitrogen uptake of nickel free austenitic stainless steel powder during heat treatment-an XPS study
    Weddeling, A. and Lefor, K. and Hryha, E. and Huth, S. and Nyborg, L. and Weber, S. and Theisen, W.
    Surface and Interface Analysis 47 413-422 (2015)
    In austenitic stainless steel nitrogen stabilizes the austenitic phase improves the mechanical properties and increases the corrosion resistance. Nitrogen alloying enables to produce austenitic steels without the element nickel which is high priced and classified as allergy inducing. A novel production route is nitrogen alloying of CrMn-prealloyed steel powder via the gas phase. This is beneficial as the nitrogen content can be adjusted above the amount that is reached during conventional casting. A problem which has to be overcome is the oxide layer present on the powder surface which impedes both the sintering process and the uptake of nitrogen. This study focuses on whether heat treatment under pure nitrogen is an appropriate procedure to enable sintering and nitrogen uptake by reduction of surface oxides. X-ray photoelectron spectroscopy (XPS) in combination with scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) are used to investigate the surface of powdered FeMn19Cr17C0.4N heat treated under nitrogen atmosphere. The analyses showed reduction of iron oxides already at 500 °C leading to oxide-free metallic surface zones. Mn and Cr oxides are reduced at higher temperatures. Distinct nitrogen uptake was registered, and successful subsequent sintering was reached. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.5730
  • 2015 • 159 Post-polymerization of urease-induced calcified, polymer hydrogels
    Rauner, N. and Buenger, L. and Schuller, S. and Tiller, J.C.
    Macromolecular Rapid Communications 36 224-230 (2015)
    Urease-induced calcification is an innovative method to artificially produce highly filled CaCO3-based composite materials by intrinsic mineralization of hydrogels. The mechanical properties of these hybrid materials based on poly(2-hydroxyethylacrylate) cross-linked by triethylene glycol dimethacrylate are poor. Increasing the degree of calcification to up to 94 wt% improves the Young's moduli (YM) of the materials from some 40 MPa to more than 300 MPa. The introduction of calcium carbonate affine groups to the hydrogel matrix by copolymerizing acrylic acid and [2-(methacryloyloxy) ethyl]trimethylammonium chloride, respectively, does not increase the stiffness of the composites. A Young's modulus of more than 1 GPa is achieved by post-polymerization (PP) of the calcified hydrogels, which proves that the size of the contact area between the matrix and calcium carbonate crystals is the most crucial parameter for controlling the stiffness of hybrid materials. Switching from low Tg to high Tg hydrogel matrices (based on poly(N,N-dimethyl acrylamide)) results in a YM of up to 3.5 GPa after PP. (Chemical Equation Presented). © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/marc.201400426
  • 2015 • 158 Processing, Microstructure and Mechanical Properties of the CrMnFeCoNi High-Entropy Alloy
    Gludovatz, B. and George, E.P. and Ritchie, R.O.
    JOM 67 2262-2270 (2015)
    Equiatomic multi-component alloys, referred to variously as high-entropy alloys, multi-component alloys, or compositionally complex alloys in the literature, have recently received significant attention in the materials science community. Some of these alloys can display a good combination of mechanical properties. Here, we review recent work on the processing, microstructure and mechanical properties of one of the first and most studied high-entropy alloys, namely the single-phase, face-centered cubic alloy CrMnFeCoNi, with emphasis on its excellent damage tolerance (strength with toughness) in the temperature range from room temperature down to liquid nitrogen temperature. © 2015, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-015-1589-z
  • 2015 • 157 Soft Nanocomposites-From Interface Control to Interphase Formation
    Pihan, S.A. and Emmerling, S.G.J. and Butt, H.-J. and Berger, R. and Gutmann, J.S.
    ACS Applied Materials and Interfaces 7 12380-12386 (2015)
    We report measurements of structure, mechanical properties, glass transition temperature, and contact angle of a novel nanocomposite material consisting of swellable silsesquioxane nanoparticles with grafted poly(ethyl methacrylate) (PEMA) brushes and PEMA matrices with varying molecular weight. We measured the interparticle distance at the surface of the composites using scanning probe microscopy (SPM) and in the bulk of ∼0.5 μm thick films by grazing incidence small angle X-ray scattering (GISAXS). For a given molecular weight of the brush unstable dispersions at high molecular weight of the matrix indicate an intrinsic incompatibility between polymer-grafted-nanoparticles and homopolymer matrices. This incompatibility is affirmed by a high contact angle between the polymer-grafted-nanoparticles and the high molecular weight matrix as measured by SPM. For unstable dispersions, we measured a decreased glass transition temperature along with a decreased plateau modulus by dynamic mechanical thermal analysis (DMTA) which indicates the formation of a liquid-like layer at the brush-matrix interface. This proves the ability to decouple the structural and mechanical properties from the potential to be swollen with small molecules. It opens a new area of use of these soft nanocomposites as slow release materials with tailored mechanical properties. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/am507572q
  • 2015 • 156 Structural stability of Fe-based topologically close-packed phases
    Ladines, A.N. and Hammerschmidt, T. and Drautz, R.
    Intermetallics 59 59-67 (2015)
    Precipitates of topologically close-packed (TCP) phases play an important role in hardening mechanisms of high-performance steels. We analyze the influence of atomic size, electron count, magnetism and external stress on TCP phase stability in Fe-based binary transition metal alloys. Our density-functional theory calculations of structural stability are complemented by an analysis with an empirical structure map for TCP phases. The structural stability and lattice parameters of the Fe-Nb/Mo/V compounds are in good agreement with experiment. The average magnetic moments follow the Slater-Pauling relation to the average number of valence-electrons and can be rationalized in terms of the electronic density of states. The stabilizing effect of the magnetic energy, estimated by additional non-magnetic calculations, increases as the magnetic moment increases with band filling for the binary systems of Fe and early transition metals. For the case of Fe2Nb, we demonstrate that the influence of magnetism and external stress is sufficiently large to alter the energetic ordering of the closely competing Laves phases C14, C15 and C36. We find that the A15 phase is not stabilized by atomic-size differences, while the stability of C14 is increasing with increasing difference in atomic size. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.12.009
  • 2015 • 155 Suppressing Vertical Displacement of Lithiated Silicon Particles in High Volumetric Capacity Battery Electrodes
    Yu, D.Y.W. and Zhao, M. and Hoster, H.E.
    ChemElectroChem 2 1090-1095 (2015)
    Silicon is a potential high-capacity anode material for lithium-ion batteries. However, large volume changes in the material remains a bottleneck to its commercialization. Many works have been devoted to nanostructured composites with voids to accommodate the volume expansion. Yet, the full capability of silicon cannot be utilized, because these nanostructured electrodes have low volumetric capacities. Herein, we redesign dense silicon electrodes with three times the volumetric capacity of graphite. Insitu electrochemical dilatometry reveals that the electrode thickness change is nonlinear as a function of state of charge and highly affected by the electrode composition. One key problem is the large vertical displacement of the silicon particles during lithiation, which leads to irreversible particle detachment and electrode porosity increase. Better reversibility in electrode thickness changes can be achieved by using polyimide, with a higher modulus and larger ultimate elongation, as the binder, leading to better cycle stability. On the move: Vertical displacement of silicon particles, owing to volume expansion and contraction during charge and discharge in a high volumetric capacity battery electrode, is monitored by using electrochemical dilatometry and suppressed by the use of a polyimide binder. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500133
  • 2015 • 154 The effect of patterned titanium substrates on the properties of silver-doped hydroxyapatite coatings
    Grubova, I.Y. and Surmeneva, M.A. and Ivanova, A.A. and Kravchuk, K. and Prymak, O. and Epple, M. and Buck, V. and Surmenev, R.A.
    Surface and Coatings Technology 276 595-601 (2015)
    This paper reports the effect of substrate nano/micro-structure design on the grain size, mechanical properties and surface wettability of nanostructured radio frequency (RF) magnetron sputter-deposited silver-containing hydroxyapatite (Ag-HA) coatings containing 0.13-0.36. wt.% silver. The results of this study revealed that the Ag-HA coating microstructure could be designed by controlling the pre-treated surface topography of titanium. The nano/micro-patterned surfaces of titanium were prepared by sand-blasting followed by acid-etching. The size of the nano-patterns on the surface of titanium was also affected by the sand-blasting procedure; namely, the lower the pressure was, the larger the size of the nano-structures and the distance between them. The effect of the coating grain size on the surface wettability and physico-mechanical properties of the biocomposites was revealed. The hydrophobic properties were imparted to the rough titanium by a nanostructured Ag-HA coating. Although according to the XRD patterns the coatings were mainly composed of HA, some differences in the morphology were observed. Therefore, the decreased wettability of the Ag-HA coatings could be explained by taking into account the different grain sizes of the films rather than the changes to the surface chemistry. Nanoindentation studies revealed that in the case of the Ag-HA-coated samples, smaller grains resulted in significantly higher nanohardness and Young's modulus. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.surfcoat.2015.06.010
  • 2015 • 153 The evolution of microstructure and mechanical properties of Ti-5Al-5Mo-5V-2Cr-1Fe during ageing
    Ahmed, M. and Li, T. and Casillas, G. and Cairney, J.M. and Wexler, D. and Pereloma, E.V.
    Journal of Alloys and Compounds 629 260-273 (2015)
    The phase transformations and compositional changes occurring during thermo-mechanical processing and subsequent high temperature ageing of Ti-5Al-5Mo-5V-2Cr-1Fe (wt.%) were investigated using scanning transmission electron microscopy (STEM) and atom probe tomography (APT). High resolution STEM revealed nano-sized α (< 10 nm) and athermal ω (∼1-3 nm) formed during accelerated cooling from 800°C and slow heating to an ageing temperature of 650°C. Nuclei of α were found to form heterogeneously in the β matrix as well as at the ω phase. APT revealed pronounced Mo compositional fluctuations in the β matrix. No direct connection was established between Mo-rich or Mo-lean regions and α or ω nuclei. APT also failed to detect the ω phase, which supports theories that it forms by a shuffle mechanism, without any compositional difference from the β phase. Very small α particles, after initial ageing, showed only a minute change in composition with respect to the β matrix, indicative of a displacive-diffusional transformation. With further ageing, growth of the α lamellae was accompanied by compositional changes according to the diffusion rates of β-stabilising elements. Pile-up of the slowest diffusing solutes (Mo, V) at the α/β interface were pronounced in the initial stages of ageing. The best combination of mechanical properties (1200 MPa ultimate tensile strength with 15% total elongation) was recorded after 3.6 ks of ageing. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.005
  • 2015 • 152 Thermo-physical properties of heat-treatable steels in the temperature range relevant for hot-stamping applications
    Kuepferle, J. and Wilzer, J. and Weber, S. and Theisen, W.
    Journal of Materials Science 50 2594-2604 (2015)
    In many industrial processes, the resulting mechanical properties of produced steel parts are directly influenced by the thermo-physical properties, which affect the heat treatment significantly. The quality of application-oriented simulations is strongly dependent on the input quantities, which are often generated by regression analysis or simple extrapolations. The aim of this paper is to demonstrate the influence of the thermo-physical properties on such a process simulation referring to the hot stamping. Hot stamping is an established process in the automotive industry to produce ultra-high strength parts. A typical material used for this application is the low-alloyed steel 22MnB5. The thermal conductivity of this steel was investigated referring to the temperature-dependent microstructural changes during the hot-stamping process, particularly the γ to α′ transformation. In terms of the dynamic measuring method, the specific heat capacity, the thermal expansion coefficient, the density and the thermal diffusivity for the different temperature-dependent microstructures of the steel 22MnB5 were determined. The thermal conductivity for the complete temperature range of the hot-stamping process was generated, referring to measured and extrapolated data. To account for the fast γ–α′ transformation kinetics, a novel characterization and extrapolation method was applied. The heat capacity and the thermal diffusivity have a major impact on the thermal conductivity compared to the subordinated influence of the density. The metastable austenitic condition (T ≥ 900 °C) was compared to the martensitic condition (T ≤ 400 °C). The dependent thermal conductivity is significantly dependent on the crystallographic orientation of the lattice. The face-centred cubic lattice (austenite) has referring to the body-centred cubic lattice (martensite), a proportionally low thermal conductivity. During the transformation from austenite to martensite, the development is not linear but based on complex interactions. The results reveal that the temperature-dependent thermal conductivity has to be considered for reliable process simulations. © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-015-8829-z
  • 2015 • 151 Wet Nanoindentation of the Solid Electrolyte Interphase on Thin Film Si Electrodes
    Kuznetsov, V. and Zinn, A.-H. and Zampardi, G. and Borhani-Haghighi, S. and La Mantia, F. and Ludwig, Al. and Schuhmann, W. and Ventosa, E.
    ACS Applied Materials and Interfaces 7 23554-23563 (2015)
    The solid electrolyte interphase (SEI) film formed at the surface of negative electrodes strongly affects the performance of a Li-ion battery. The mechanical properties of the SEI are of special importance for Si electrodes due to the large volumetric changes of Si upon (de)insertion of Li ions. This manuscript reports the careful determination of the Young's modulus of the SEI formed on a sputtered Si electrode using wet atomic force microscopy (AFM)-nanoindentation. Several key parameters in the determination of the Young's modulus are considered and discussed, e.g., wetness and roughness-thickness ratio of the film and the shape of a nanoindenter. The values of the Young's modulus were determined to be 0.5-10 MPa under the investigated conditions which are in the lower range of those previously reported, i.e., 1 MPa to 10 GPa, pointing out the importance of the conditions of its determination. After multiple electrochemical cycles, the polymeric deposits formed on the surface of the SEI are revealed, by force-volume mapping in liquid using colloidal probes, to extend up to 300 nm into bulk solution. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b06700
  • 2014 • 150 A new possibility of melt cooling in extrusion dies to prevent sagging-effects in thick-walled pipes
    Te Heesen, O. and Wortberg, J.
    AIP Conference Proceedings 1593 133-136 (2014)
    One challenge in the extrusion process of thick-walled pipes is the cooling of the product. Besides the output of the extruder, the line speed is also limited by the efficiency of the cooling line. The cooling time increases according to the wall thickness of the pipe under otherwise equal process conditions. State of the art is the cooling of the outer surface in water tanks or spray-cool-tanks. In addition to that, it is possible to cool the inner surface by air that is sucked through the pipe. Despite these technologies it is problematic to cool down thick walled-products with the right speed. Especially thick-walled pipes show problems by cooling the layers in the middle of the wall. On the one hand an intensive cooling of the outer and inner surface of the pipe entail the formation of shrink holes in the middle of the pipe wall. On the other hand without a quick cooling the melt flow in circumferential direction because of the gravity takes place (sagging-effect). Because of this reason in the presented paper new possibilities of melt cooling in extrusion dies to prevent sagging-effects are given. An aimed cooling of the inbound melt layers inside the extrusion die could prevent the effect of melt flow in circumferential direction after the extrusion die, allows the specification of a specific temperature profile over the radius of the pipe wall and helps to reduce the melt temperature for rising mass throughputs and screw driving speeds of the extruder. It is also thinkable to influence the crystallization process and thereby the mechanical properties of the end-product by an aimed cooling of the inner pipe layers. © 2014 American Institute of Physics.
    view abstractdoi: 10.1063/1.4873749
  • 2014 • 149 Bridge design influences on the pressure conditions in the welding chamber for porthole die extrusion
    Gagliardi, F. and Schwane, M. and Citrea, T. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Key Engineering Materials 622-623 87-94 (2014)
    Porthole die extrusion of lightweight alloys is used for the production of profiles, which may have complex cross section geometries. The mechanical properties of these profiles are deeply affected by the seam welds, which are generated in hollow profiles along the whole length. The seam welds result from the rejoining of the material streams in the welding chamber of the porthole die. The joining phase and hence the seam weld quality are strongly influenced by the temperature and the pressure conditions in the welding chamber. Those process conditions can be adjusted by a proper die design. In this work, the focus lies on the feeder section of the extrusion die, which consists of a set of bridges, whose shapes influence the material entry in the welding chamber. A numerical study was carried out to investigate different bridge shapes with regard to the pressure inside the welding chamber and the punch load. Subsequently, the volume of the bridge was fixed to isolate and better investigate the influence of the shape. It was observed that bridge designs leading to higher flow distortion cause higher pressure decrement along the welding plane and, consequently, degradation of the welding conditions. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 148 Chemical-Mechanical Characterization of the Creep-Resistant Mg-Al-Ca Alloy DieMag422 Containing Barium - Quasistatic and Cyclic Deformation Behavior in Different Defined Corrosion Conditions
    Wittke, P. and Klein, M. and Walther, F.
    Materials Testing 56 16-23 (2014)
    The influence of corrosion on the microstructure and the depending mechanical properties was investigated for the creep-resistant Mg-Al-Ca alloy DieMag422 containing barium. In order to investigate the corrosion behavior, potentio-dynamic polarization measurements and immersion tests were performed in pH7 without and with sodium chloride. Specimens in defined corrosion conditions were investigated by SEM for microstructure-related assessment of corrosion mechanisms. Strength and strain properties of non-corroded and corroded specimens were compared in tensile tests, underlining a significant decrease of tensile strength and fracture strain with increasing corrosion grade. The fatigue behaviour of the DieMag422 alloy in different corrosion conditions was characterized in multiple step and single step tests by means of mechanical stress-strain-hysteresis, temperature and electrical resistance measurements. Load increase tests allow to estimate the endurance limit and to determine the stress amplitude leading to fracture with one specimen. Fatigue results also showed a significant decrease in the estimated endurance limit and the failure stress with increasing corrosion grade. The applied physical measurement techniques can be equivalently used for the characterization of the fatigue behavior and representation of the actual fatigue state. The thermal and electrical materials responses were proportional to cyclic plastic deformation and provide the opportunity to evaluate the actual fatigue state of components under service loading in terms of condition monitoring. © Carl Hanser Verlag.
    view abstractdoi: 10.3139/120.110519
  • 2014 • 147 Composition Dependence of Phase Stability, Deformation Mechanisms, and Mechanical Properties of the CoCrFeMnNi High-Entropy Alloy System
    Tasan, C.C. and Deng, Y. and Pradeep, K.G. and Yao, M.J. and Springer, H. and Raabe, D.
    JOM 66 1993-2001 (2014)
    The proposal of configurational entropy maximization to produce massive solid-solution (SS)-strengthened, single-phase high-entropy alloy (HEA) systems has gained much scientific interest. Although most of this interest focuses on the basic role of configurational entropy in SS formability, setting future research directions also requires the overall property benefits of massive SS strengthening to be carefully investigated. To this end, taking the most promising CoCrFeMnNi HEA system as the starting point, we investigate SS formability, deformation mechanisms, and the achievable mechanical property ranges of different compositions and microstructural states. A comparative assessment of the results with respect to room temperature behavior of binary Fe-Mn alloys reveals only limited benefits of massive SS formation. Nevertheless, the results also clarify that the compositional requirements in this alloy system to stabilize the face-centered cubic (fcc) SS are sufficiently relaxed to allow considering nonequiatomic compositions and exploring improved strength–ductility combinations at reduced alloying costs. © 2014, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1133-6
  • 2014 • 146 CrN/AlN nanolaminate coatings deposited via high power pulsed and middle frequency pulsed magnetron sputtering
    Bagcivan, N. and Bobzin, K. and Ludwig, Al. and Grochla, D. and Brugnara, R.H.
    Thin Solid Films 572 153-160 (2014)
    Nanolaminate coatings based on transition metal nitrides such as CrN, AlN and TiN deposited via physical vapor deposition (PVD) have shown great advantage as protective coatings on tools and components subject to high loads in tribological applications. By varying the individual layer materials and their thicknesses it is possible to optimize the coating properties, e.g. hardness, Young's modulus and thermal stability. One way for further improvement of coating properties is the use of advanced PVD technologies. High power pulsed magnetron sputtering (HPPMS) is an advancement of pulsed magnetron sputtering (MS). The use of HPPMS allows a better control of the energetic bombardment of the substrate due to the higher ionization degree of metallic species. It provides an opportunity to influence chemical and mechanical properties by varying the process parameters. The present work deals with the development of CrN/AlN nanolaminate coatings in an industrial scale unit by using two different PVD technologies. Therefore, HPPMS and mfMS (middle frequency magnetron sputtering) technologies were used. The bilayer period Λ, i.e. the thickness of a CrN/AlN double layer, was varied between 6.2nm and 47.8 nm by varying the rotational speed of the substrate holders. In a second step the highest rotational speed was chosen and further HPPMS CrN/AlN coatings were deposited applying different HPPMS pulse lengths (40, 80, 200 μs) at the same mean cathode power and frequency. Thickness, morphology, roughness and phase composition of the coatings were analyzed by means of scanning electron microscopy (SEM), confocal laser microscopy, and X-ray diffraction (XRD), respectively. The chemical composition was determined using glow discharge optical emission spectroscopy (GDOES). Detailed characterization of the nanolaminate was conducted by transmission electron microscopy (TEM). The hardness and the Young's modulus were analyzed by nanoindentation measurements. The residual stress was determined via Si microcantilever curvature measurements. The phase analysis revealed the formation of h-Cr2N, c-CrN and c-AlN mixed phases for the mfMS CrN/AlN coatings, whereas the HPPMS coatings exhibited only cubic phases (c-CrN, c-AlN). A hardness of 31.0 GPa was measured for the HPPMS coating with a bilayer period of 6.2 nm. The decrease of the HPPMS pulse length at constant mean power leads to a considerable increase of the cathode current on the Cr and Al target associated with an increased ion flux towards the substrate. Furthermore, it was observed that the deposition rate of HPPMS CrN/AlN decreases with shorter pulse lengths, so that a CrN/AlN coating with a bilayer period of 2.9 nm, a high hardness of 40.8 GPa and a high compressive stress (- 4.37 GPa) was achieved using a short pulse length of 40 μs. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2014.06.058
  • 2014 • 145 Designing Heusler nanoprecipitates by elastic misfit stabilization in Fe-Mn maraging steels
    Millán, J. and Sandlöbes, S. and Al-Zubi, A. and Hickel, T. and Choi, P. and Neugebauer, J. and Ponge, D. and Raabe, D.
    Acta Materialia 76 94-105 (2014)
    B2 NiMn and Ni2MnAl Heusler nanoprecipitates are designed via elastic misfit stabilization in Fe-Mn maraging steels by combining transmission electron microscopy (TEM) correlated atom probe tomography (APT) with ab initio simulations. Guided by these predictions, the Al content of the alloys is systematically varied, and the influence of the Al concentration on structure stability, size and distribution of precipitates formed during ageing at 450 °C is studied using scanning electron microscopy-electron backscatter diffraction, TEM and APT. Specifically, the Ni2MnAl Heusler nanoprecipitates exhibit the finest sizes and highest dispersion and hence lead to significant strengthening. The formation of the different types of precipitates and their structure, size, dispersion and effect on the mechanical properties of the alloys are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.05.016
  • 2014 • 144 Determination of robust material qualities and processing conditions for laser sintering of polyamide 12
    Wegner, A. and Mielicki, C. and Grimm, T. and Gronhoff, B. and Witt, G. and Wortberg, J.
    Polymer Engineering and Science 54 1540-1554 (2014)
    Material aging of Polyamide 12 (Laurinlactam) is a very common problem in laser sintering (LS). For stable process conditions, recycled material used in previous processes should be refreshed with 30-50% virgin powder material. However, even by following these refreshing strategies, material quality drops to an insufficient level after several process cycles which leads to poor part quality showing orange peel or poor mechanical properties when processed. In order to avoid this, a quality assurance system has been established to provide recommendations for robust process conditions and material qualities. A detailed study on aging processes in LS comparing two different machines was performed in order to analyze correlations between material quality, process parameters and part properties. Energy input allowing for robust processing conditions should be in a range between 0.325 and 0.42 J/mm3 showing almost identical values for both machines. Optimal material quality ranges was found to be machine specific, while the lower limit lies between 20 and 25 cm 3/ 10 min for both machines used. Additionally, material aging characteristics in an oven and a LS machine were compared, in order to simulate material aging in the LS process by simple experiments in an oven. POLYM. ENG. SCI., 54:1540-1554, 2014. © 2013 Society of Plastics Engineers © 2013 Society of Plastics Engineers.
    view abstractdoi: 10.1002/pen.23696
  • 2014 • 143 Experimental investigation and numerical simulation of the mechanical and thermal behavior of a superelastic shape memory alloy beam during bending
    Ullrich, J. and Schmidt, M. and Schütze, A. and Wieczorek, A. and Frenzel, J. and Eggeler, G. and Seelecke, S.
    ASME 2014 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2014 2 (2014)
    Superelastic Shape Memory Alloys (SMA) are typically used in applications where the martensitic phase transformation is exploited for its reversible, large deformation such as medical applications (e.g. stents). In this work, we focus on the mechanical and thermal behavior of a Nickel-Titanium SMA strip in bending mode. One possible application of this mode is to provide a restoring force when used in joints of SMA wire actuator systems making the need for an antagonistic SMA actuator redundant. In these applications mentioned above, typically only the mechanical properties are of interest while the temperature is considered constant, even though the martensitic phase transformation in SMA is a thermomechanically coupled process. As a part of the DFG (German Research Association) Priority Programme SPP1599 "Ferroic Cooling" which aims at advancing the development of solid state cooling devices, we have an equally large interest for the thermal evolution of Nickel-Titanium SMA during deformation and its induced phase transformation. In this paper we investigate the thermal and the mechanical response of a SMA beam during bending experiments in which the deformation is induced by holding one end of a SMA strip fixed while the other end is subject to a prescribed deflection. Sensors and high speed thermal cameras are used to capture reaction forces, deformations and temperature changes. We compare these experimental results with numerical simulation results obtained from Finite Element simulations where a thermo-mechanically coupled SMA model is implemented into a finite deformation framework. © 2014 by ASME.
    view abstractdoi: 10.1115/SMASIS20147619
  • 2014 • 142 Extending the flexibility in the composite extrusion process
    Dahnke, C. and Pietzka, D. and Haase, M. and Tekkaya, A.E.
    Procedia CIRP 18 33-38 (2014)
    The research in the field of composite extrusion led to a fundamental understanding and characterization of the process in the last years. Based on the gained knowledge, this paper focuses on the possibilities to increase the flexibility of the composite extrusion process i.e. to manufacture profiles with functional properties graded over the length, profiles with improved mechanical properties and profiles with functional properties. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2014.06.103
  • 2014 • 141 Guest-dependent mechanical anisotropy in pillared-layered soft porous crystals - a nanoindentation study
    Henke, S. and Li, W. and Cheetham, A. K.
    Chemical Science 5 2392--2397 (2014)
    Soft porous crystals (SPCs) of the type [Zn-2(L)(2)(dabco)](n) (L _ linear dicarboxylate linker, dabco _ 1,4-diazabicyclo[2.2.2]octane) show exceptional mechanical anisotropy. Single-crystal nanoindentation experiments reveal very large changes of the elastic modulus and hardness triggered by exchange of the guests adsorbed in the porous metal-organic framework. The substantial variations of the mechanical properties as a function of the guest molecules can be explained by the responsive nature of these SPCs. Based on non-specific guest-framework interactions, crucial changes of the network geometry induce a complex and dynamical mechanical behaviour.
    view abstractdoi: 10.1039/c4sc00497c
  • 2014 • 140 High-precision green densities of thick films and their correlation with powder, ink, and film properties
    Mücke, R. and Büchler, O. and Menzler, N.H. and Lindl, B. and Vaßen, R. and Buchkremer, H.P.
    Journal of the European Ceramic Society 34 3897-3916 (2014)
    A precise geometrical method employing optical profilometry for green density measurements of thick films is presented that provides a typical reproducibility of 0.1-0.2% theoretical density (TD) and a measurement uncertainty of 0.2-0.4% TD for layer thicknesses of around 50. μm. The procedure can be applied for all thick films with a dried thickness of 10. μm or greater. In a case study, the green densities of screen-printed zirconia layers were investigated as a function of the starting powders (grain sizes from 0.1 to 0.4. μm), the solid content, the chain length of ethyl cellulose as binder and its concentration, and two different dispersants and their concentration. Rheological ink properties, surface roughness, drying stresses from deflection measurements, the mechanical properties of green films, and the equivalent compaction pressure were measured and correlated with the green density data. Compressive binder forces and lubrication effects dominated the packing of the particles. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jeurceramsoc.2014.04.012
  • 2014 • 139 Impact of nanodiffusion on the stacking fault energy in high-strength steels
    Hickel, T. and Sandlöbes, S. and Marceau, R.K.W. and Dick, A. and Bleskov, I. and Neugebauer, J. and Raabe, D.
    Acta Materialia 75 147-155 (2014)
    A key requirement of modern steels - the combination of high strength and high deformability - can best be achieved by enabling a local adaptation of the microstructure during deformation. A local hardening is most efficiently obtained by a modification of the stacking sequence of atomic layers, resulting in the formation of twins or martensite. Combining ab initio calculations with in situ transmission electron microscopy, we show that the ability of a material to incorporate such stacking faults depends on its overall chemical composition and, importantly, the local composition near the defect, which is controlled by nanodiffusion. Specifically, the role of carbon for the stacking fault energy in high-Mn steels is investigated. Consequences for the long-term mechanical properties and the characterisation of these materials are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.04.062
  • 2014 • 138 Indentation of self-similar indenters: An FEM-assisted energy-based analysis
    Pöhl, F. and Huth, S. and Theisen, W.
    Journal of the Mechanics and Physics of Solids 66 32-41 (2014)
    In this study, an energy-based approach is used to derive general relationships between two independent parameters of the load-displacement curve (C and Wel/Wtot) and the mechanical properties of Ludwik-power law materials (E, K, and n). The approach uses conventional continuum mechanics to describe the elastic and plastic zone including their mean strains and volumes induced by indentation. It does not account for the indentation size effect which is owed to the nucleation of dislocations within the plastic zone, as it is described by the model developed by Nix and Gao (1998). The energy-based approach in combination with FEM simulations give an insight into the complex deformation processes during indentation and the relationships between the material parameters and the indentation results. The results are discussed and interpreted in the context of solving the forward and inverse indentation problem for self-similar indenters. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmps.2014.02.002
  • 2014 • 137 Influence of substrate pre-treatments on residual stresses and tribo-mechanical properties of TiAlN-based PVD coatings
    Sprute, T. and Tillmann, W. and Grisales, D. and Selvadurai, U. and Fischer, G.
    Surface and Coatings Technology 260 369-379 (2014)
    Residual stresses in the substrate and in the PVD coating have a significant influence on the coating adhesion and lifespan of machining as well as forming tools. Therefore, the understanding and control of the system's residual stresses will lead to a better performance of the coated components. Moreover, although investigations were conducted in the field of stress analysis of PVD coatings, they do not focus on interdependencies of residual stresses in the substrate and in the coating. In this investigation, three different metallographically prepared substrates were used. SiC grinding, diamond grinding, and SiC grinding and plasma nitriding preparations were selected, due to the substantial differences in their final residual stress states. Additionally, a Ti/TiAlN multilayer coating and a reference TiAlN monolayer were deposited on each pre-treated substrate. Their initial and final residual stress states were measured by means of X-ray diffraction. In addition to the residual stress analyses, tribo-mechanical tests, such as nano-indentation, ball-on-disc, and scratch tests were performed in order to correlate the results with these residual stress states. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2014.08.075
  • 2014 • 136 Influence of the PM-Processing Route and Nitrogen Content on the Properties of Ni-Free Austenitic Stainless Steel
    Lefor, K. and Walter, M. and Weddeling, A. and Hryha, E. and Huth, S. and Weber, S. and Nyborg, L. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 1154-1167 (2014)
    Ni-free austenitic steels alloyed with Cr and Mn are an alternative to conventional Ni-containing steels. Nitrogen alloying of these steel grades is beneficial for several reasons such as increased strength and corrosion resistance. Low solubility in liquid and δ-ferrite restricts the maximal N-content that can be achieved via conventional metallurgy. Higher contents can be alloyed by powder-metallurgical (PM) production via gas–solid interaction. The performance of sintered parts is determined by appropriate sintering parameters. Three major PM-processing routes, hot isostatic pressing, supersolidus liquid phase sintering (SLPS), and solid-state sintering, were performed to study the influence of PM-processing route and N-content on densification, fracture, and mechanical properties. Sintering routes are designed with the assistance of thermodynamic calculations, differential thermal analysis, and residual gas analysis. Fracture surfaces were studied by X-ray photoelectron spectroscopy, secondary electron microscopy, and energy dispersive X-ray spectroscopy. Tensile tests and X-ray diffraction were performed to study mechanical properties and austenite stability. This study demonstrates that SLPS process reaches high densification of the high-Mn-containing powder material while the desired N-contents were successfully alloyed via gas–solid interaction. Produced specimens show tensile strengths >1000 MPa combined with strain to fracture of 60 pct and thus overcome the other tested production routes as well as conventional stainless austenitic or martensitic grades. © 2014, The Author(s).
    view abstractdoi: 10.1007/s11661-014-2701-7
  • 2014 • 135 Interface properties in lamellar TiAl microstructures from density functional theory
    Kanani, M. and Hartmaier, A. and Janisch, R.
    Intermetallics 54 154-163 (2014)
    The deformability and strength of lamellar two-phase (γ and α2) TiAl alloys strongly depends on the mechanical properties of the different interfaces in such microstructures. We carried out ab-initio density functional theory calculations of interface energy and strength for all known interface variants as well as the corresponding single crystal slip/cleavage planes to obtain a comprehensive database of key mechanical quantities. This data collection can be used for meso-scale simulations of deformation and fracture in TiAl. In spite of the different atomic configurations of the lamellar interfaces and the single crystal planes, the calculated values for the tensile strength are in the same range and can be considered as equal in a meso-scale model. Analysis of generalized stacking fault energy surfaces showed that the shear strength is directional dependent, however, the [112̄] direction is an invariant easy gliding direction in all investigated systems. The probability of different dislocation dissociation reactions as part of a shear deformation mechanism are discussed as well. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.06.001
  • 2014 • 134 Intrinsic acidity of surface sites in calcium silicate hydrates and its implication to their electrokinetic properties
    Churakov, S.V. and Labbez, C. and Pegado, L. and Sulpizi, M.
    Journal of Physical Chemistry C 118 11752-11762 (2014)
    Calcium Silicate Hydrates (C-S-H) are the major hydration products of portland cement paste. The accurate description of acid-base reactions at the surface of C-S-H particles is essential for both understanding the ion sorption equilibrium in cement and prediction of mechanical properties of the hardened cement paste. Ab initio molecular dynamics simulations at the density functional level of theory were applied to calculate intrinsic acidity constants (pK a's) of the relevant -SiOH and -CaOH2 groups on the C-S-H surfaces using a thermodynamic integration technique. Ion sorption equilibrium in C-S-H was modeled applying ab initio calculated pKa's in titrating Grand Canonical Monte Carlo simulations using a coarse-grained model for C-S-H/solution interface in the framework of the Primitive Model for electrolytes. The modeling results were compared with available data from electrophoretic measurements. The model predictions were found to satisfactorily reproduce available experimental data. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp502514a
  • 2014 • 133 Investigation on the effectiveness of chemically synthesized nano cement in controlling the physical and mechanical performances of concrete
    Jo, B.W. and Chakraborty, S. and Kim, K.H.
    Construction and Building Materials 70 1-8 (2014)
    Present investigation deals with the effectiveness of the chemically synthesized nano cement in controlling physical and mechanical performances of concrete. In this investigation, concrete samples were fabricated using variable amounts of aggregates and alkali activator content w.r.t. weight of nano cement. Based on the mechanical properties analyses, it is assessed that chemically synthesized cement is able to produce 43 MPa compressive strength of concrete after 14 days curing instead of 28 days at an optimized amount of aggregates content as well as alkali activator content. Finally, a model has been proposed to explain the overall performances of nano cement based concrete. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.conbuildmat.2014.07.090
  • 2014 • 132 Mechanical properties of Al-Cu-Fe quasicrystalline and crystalline phases: An analogy
    Laplanche, G. and Bonneville, J. and Joulain, A. and Gauthier-Brunet, V. and Dubois, S.
    Intermetallics 50 54-58 (2014)
    The mechanical properties of the ω-Al7Cu2Fe crystalline phase have been investigated over a large temperature range (650-1000 K). Despite of its antinomic structure with the icosahedral Al-Cu-Fe quasicrystalline phase, i.e. periodic vs non-periodic, its mechanical properties are very similar to those of the quasicrystalline phase, which strongly suggest similar deformation mechanisms. Consequently, as for the quasicrystalline structure, we propose that dislocation climb might control the plastic deformation of the ω-phase. However, in the present case, the specificities of the quasicrystalline structure cannot be invoked to justify the predominance of dislocation climb, which questions the role of quasiperiodicity on dislocation mobility. We suggest that this deformation mode certainly results from specific non-planar extensions of the dislocation core. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.02.004
  • 2014 • 131 Mechanical properties of SiLix thin films at different stages of electrochemical Li insertion
    Zinn, A.-H. and Borhani-Haghighi, S. and Ventosa, E. and Pfetzing-Micklich, J. and Wieczorek, N. and Schuhmann, W. and Ludwig, Al.
    Physica Status Solidi (A) Applications and Materials Science 211 2650-2656 (2014)
    The mechanical properties of amorphous Si thin films, lithiated electrochemically to different Si£Li compositions are studied by ex situ nanoindentation. The compositions of the films are adjusted using an electrochemical routine that corrects for the Li consumed by SEI layer growth during initial lithiation. The mechanical properties such as Young's modulus and hardness are derived from nanoindentation. For compositions between Si and SiLi<inf>2.5</inf> the Young's modulus decreases with increasing Li content from ∼160 GPa to ∼8 GPa and the hardness decreases from ∼14 GPa to ∼0.1 GPa. The yield strength values, as deduced from hardness measurements, decrease from ∼5 GPa to 0.05 GPa. AFM imaging is used on the electrochemically cycled films to assess the SEIs impact on the nanomechanical measurements. XPS depth-profiling of the electrochemically cycled sample indicated a Li concentration gradient across the film thickness. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201431303
  • 2014 • 130 Mechanics of sheet-bulk indentation
    Sieczkarek, P. and Isik, K. and Ben Khalifa, N. and Martins, P.A.F. and Tekkaya, A.E.
    Journal of Materials Processing Technology 214 2387-2394 (2014)
    The purpose of this paper is twofold: first, it aims to characterize plastic flow and ductile fracture in sheet-bulk indentation and, second, it proposes a closed-form analytical framework that can be easily applied to estimate the through-thickness pressure and force that needs to be applied by a flat compression punch as a function of the geometry, the mechanical properties of the blanks and the friction along the blank-tool interfaces. The methodology combines experiments with properly designed tool systems, which facilitate or constrain material to flow sideways (in the direction of the length), and analytical developments build upon the upper bound method for upsetting, transition to die filling and die filling of sheet-bulk compression by a flat punch. Experimental work with aluminium EN AW-1050A shows that depending on the geometry of the punch, the physics of sheet-bulk indentation may exclusively involve plastic flow or may result from a combination of plastic flow and fracture to detach surfaces from the neighbouring regions of the blank through controlled crack propagation. Results also show that the mechanics of sheet-bulk indentation can be easily and effectively analyzed by means of sheet-bulk compression under plane strain conditions. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2014.05.018
  • 2014 • 129 PH controlled condensation of polysiloxane networks at the water-air interface
    Wieland, D.C.F. and Degen, P. and Paulus, M. and Schroer, M.A. and Rehage, H. and Tolan, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 455 44-48 (2014)
    Structural and mechanical properties of molecularly thick polysiloxane membranes were studied on different liquid subphases to investigate the impact of the subphase's pH value on the cross-linking process. The lateral structure of these films was studied in-situ by grazing incidence diffraction while torsions pendulum experiments reveal the response of the system to mechanical stress. The results show a hindered cross-linking on acidic subphases. At alkaline and neutral pH conditions the cross-linking process was not effected. The data revealed that the degree of polymerization can be tuned by regulating the subphase's pH value, which opens the opportunity to build complex polysiloxane membranes in a controlled manner. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfa.2014.03.099
  • 2014 • 128 Plasma spray physical vapor deposition of La1-x Sr x Co y Fe1-y O3-δ Thin-film oxygen transport membrane on porous metallic supports
    Jarligo, M.O. and Mauer, G. and Bram, M. and Baumann, S. and Vaßen, R.
    Journal of Thermal Spray Technology 23 213-219 (2014)
    Plasma spray physical vapor deposition (PS-PVD) is a very promising route to manufacture ceramic coatings, combining the efficiency of thermal spray processes and characteristic features of thin PVD coatings. Recently, this technique has been investigated to effectively deposit dense thin films of perovskites particularly with the composition of La0.58Sr 0.4Co0.2Fe0.8O3-δ (LSCF) for application in gas separation membranes. Furthermore, asymmetric type of membranes with porous metallic supports has also attracted research attention due to the advantage of good mechanical properties suitable for use at high temperatures and high permeation rates. In this work, both approaches are combined to manufacture oxygen transport membranes made of gastight LSCF thin film by PS-PVD on porous NiCoCrAlY metallic supports. The deposition of homogenous dense thin film is challenged by the tendency of LSCF to decompose during thermal spray processes, irregular surface profile of the porous metallic substrate and crack and pore-formation in typical ceramic thermal spray coatings. Microstructure formation and coating build-up during PS-PVD as well as the annealing behavior at different temperatures of LSCF thin films were investigated. Finally, measurements of leak rates and oxygen permeation rates at elevated temperatures show promising results for the optimized membranes. © 2013 ASM International.
    view abstractdoi: 10.1007/s11666-013-0004-y
  • 2014 • 127 Plasma Spraying of Ceramics with Particular Difficulties in Processing
    Mauer, G. and Schlegel, N. and Guignard, A. and Jarligo, M.O. and Rezanka, S. and Hospach, A. and Vaßen, R.
    Journal of Thermal Spray Technology 24 30-37 (2014)
    Emerging new applications and growing demands of plasma-sprayed coatings initiate the development of new materials. Regarding ceramics, often complex compositions are employed to achieve advanced material properties, e.g., high thermal stability, low thermal conductivity, high electronic and ionic conductivity as well as specific thermo-mechanical properties and microstructures. Such materials however, often involve particular difficulties in processing by plasma spraying. The inhomogeneous dissociation and evaporation behavior of individual constituents can lead to changes of the chemical composition and the formation of secondary phases in the deposited coatings. Hence, undesired effects on the coating characteristics are encountered. In this work, examples of such challenging materials are investigated, namely pyrochlores applied for thermal barrier coatings as well as perovskites for gas separation membranes. In particular, new plasma spray processes like suspension plasma spraying and plasma spray-physical vapor deposition are considered. In some cases, plasma diagnostics are applied to analyze the processing conditions. © 2014, ASM International.
    view abstractdoi: 10.1007/s11666-014-0149-3
  • 2014 • 126 Precipitation of the α-phase in an ultrafine grained beta-titanium alloy processed by severe plastic deformation
    Li, T. and Kent, D. and Sha, G. and Dargusch, M.S. and Cairney, J.M.
    Materials Science and Engineering A 605 144-150 (2014)
    A fine and uniform distribution of α phase at grain boundaries is expected to improve the mechanical properties and thermal stability of beta Ti alloys. To design high strength alloys, a key factor is the volume fraction of α, which is related to the concentration of the α phase. In this study, α-phase precipitates were characterized in an ultrafine-grained Ti-15Nb-2Mo-2Zr-1Sn (at%) alloy processed by severe plastic deformation in two different ways (hot drawing and cold rolling in conjunction with annealing). A combination of transmission Kikuchi diffraction, transmission electron microscopy and atom-probe tomography revealed that ultra-fine α precipitates precipitate at grain boundaries in hot-drawn material or at sub-grain boundaries in the cold-rolled samples. The Nb concentrations of α phases formed were not those expected for an equilibrium state, which highlights the importance of understanding the chemistry of the α precipitates for engineering microstructures in advanced Ti alloys. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.03.044
  • 2014 • 125 Recycling of aluminum chips by hot extrusion with subsequent cold extrusion
    Haase, M. and Tekkaya, A.E.
    Procedia Engineering 81 652-657 (2014)
    In this paper, the direct conversion of AA6060 aluminum alloy machining chips into finished products by hot extrusion with subsequent cold extrusion is investigated. For hot extrusion, two different types of extrusion dies, a conventional flat-face die and an experimental die, are used. The experimental die combines the process of equal channel angular pressing with the process of hot extrusion in a single die, which increases the strain and pressure affecting the chips during extrusion, both critical factors for achieving sound chip bonding. Subsequently, the chip-based extrudates are machined to fabricate chip-based preforms for the cold extrusion experiments. In order to investigate different processing routes, forward rod extrusion and backward can extrusion trials were conducted. In all steps, cast material was processed similar to the chips as a reference. The results showed that the quality of the chip-based finished parts strongly depends on the bonding quality between the individual chips, determined during the hot extrusion process. © 2014 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2014.10.055
  • 2014 • 124 Temperature dependence of the mechanical properties of equiatomic solid solution alloys with face-centered cubic crystal structures
    Wu, Z. and Bei, H. and Pharr, G.M. and George, E.P.
    Acta Materialia 81 428-441 (2014)
    Compared to decades-old theories of strengthening in dilute solid solutions, the mechanical behavior of concentrated solid solutions is relatively poorly understood. A special subset of these materials includes alloys in which the constituent elements are present in equal atomic proportions, including the high-entropy alloys of recent interest. A unique characteristic of equiatomic alloys is the absence of "solvent" and "solute" atoms, resulting in a breakdown of the textbook picture of dislocations moving through a solvent lattice and encountering discrete solute obstacles. To clarify the mechanical behavior of this interesting new class of materials, we investigate here a family of equiatomic binary, ternary and quaternary alloys based on the elements Fe, Ni, Co, Cr and Mn that were previously shown to be single-phase face-centered cubic (fcc) solid solutions. The alloys were arc-melted, drop-cast, homogenized, cold-rolled and recrystallized to produce equiaxed microstructures with comparable grain sizes. Tensile tests were performed at an engineering strain rate of 10-3 s-1 at temperatures in the range 77-673 K. Unalloyed fcc Ni was processed similarly and tested for comparison. The flow stresses depend to varying degrees on temperature, with some (e.g. NiCoCr, NiCoCrMn and FeNiCoCr) exhibiting yield and ultimate strengths that increase strongly with decreasing temperature, while others (e.g. NiCo and Ni) exhibit very weak temperature dependencies. To better understand this behavior, the temperature dependencies of the yield strength and strain hardening were analyzed separately. Lattice friction appears to be the predominant component of the temperature-dependent yield stress, possibly because the Peierls barrier height decreases with increasing temperature due to a thermally induced increase of dislocation width. In the early stages of plastic flow (5-13% strain, depending on material), the temperature dependence of strain hardening is due mainly to the temperature dependence of the shear modulus. In all the equiatomic alloys, ductility and strength increase with decreasing temperature down to 77 K. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.08.026
  • 2014 • 123 The modeling scheme to evaluate the influence of microstructure features on microcrack formation of DP-steel: The artificial microstructure model and its application to predict the strain hardening behavior
    Vajragupta, N. and Wechsuwanmanee, P. and Lian, J. and Sharaf, M. and Münstermann, S. and Ma, A. and Hartmaier, A. and Bleck, W.
    Computational Materials Science 94 198-213 (2014)
    Due to the existence of constituents with strong distinction in mechanical properties, dual phase steels exhibit remarkably high-energy absorption along with excellent combination of strength and ductility. Furthermore, these constituents also affect deformation and microcrack formation in which various mechanisms can be observed. Thus, a reliable microstructure-based simulation approach for describing these deformations and microcrack initiation is needed. Under this framework of modeling scheme development, several work packages have been carried out. These work packages includes algorithm to generate the artificial microstructure model, a procedure to derive plasticity parameters for each constituent, and characterization of the microcrack formation and initiation criteria determination. However, due to the complexity of topic and in order to describe each work package in detail, this paper focused only on the approach to generate the artificial microstructure model and its application to predict the strain hardening behavior. The approach was based on the quantitative results of metallographic microstructure analysis and their statistical representation. The dual phase steel was first characterized by EBSD analysis to identify individual phase grain size distribution functions. The results were then input into a multiplicatively weighted Voronoi tessellation based algorithm to generate artificial microstructure geometry models. Afterwards, nanoindentation was performed to calibrate crystal plasticity parameters of ferrite and empirical approach based on local chemical composition was used to approximate flow curve of martensite. By assigning the artificial microstructure model with plasticity description of each constituent, strain-hardening behavior of DP-steel was then predicted. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2014.04.011
  • 2014 • 122 The principle of the minimum of the dissipation potential for non-isothermal processes
    Junker, P. and Makowski, J. and Hackl, K.
    Continuum Mechanics and Thermodynamics 26 259-268 (2014)
    In this paper, we contribute to the methodology of material modeling by presenting a potential-based approach for non-isothermal inelastic processes. It is based on the principle of the minimum of the dissipation potential which was used previously only in the isothermal context. In contrast to the principle of maximum dissipation, the presented procedure results in mathematically simplified equations. Due to its variational character, the inclusion of constraints is very simple. After derivation of our method, we use the examples of non-isothermal perfect plasticity and shape memory alloys for demonstration of the validity and performance of the concept. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00161-013-0299-4
  • 2014 • 121 Tribological and mechanical properties of Ti/TiAlN/TiAlCN nanoscale multilayer PVD coatings deposited on AISI H11 hot work tool steel
    Al-Bukhaiti, M.A. and Al-Hatab, K.A. and Tillmann, W. and Hoffmann, F. and Sprute, T.
    Applied Surface Science 318 180-190 (2014)
    A new [Ti/TiAlN/TiAlCN]5 multilayer coatings were deposited onto polished substrate AISI H11 (DIN 1.2343) steel by an industrial magnetron sputtering device. The tribological performance of the coated system was investigated by a ball-on-disk tribometer against 100Cr6 steel and Al2O3 balls. The friction coefficients and specific wear rates were measured at various normal loads (2, 5, 8, and 10 N) and sliding velocities (0.2, 0.4, and 0.8 m/s) in ambient air and dry conditions. The phase structure, composition, wear tracks morphologies, hardness, and film/substrate adhesion of the coatings were characterized by light-microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), 3D-surface analyzer, nanoindentation, and scratch tests. Results showed that the deposited coatings showed low wear rates in the scale of 10-15 m3/N m, low friction coefficients against 100Cr6 and Al2O3 balls in the range of 0.25-0.37, and good hardness in the range of 17-20 GPa. Results also revealed that the friction coefficients and disc wear rates decrease and increase, respectively with the increase in normal load and sliding velocity for both coating/Al2O3 and coating/100Cr6 sliding system. Compared with the uncoated-H11 substrate, the deposited coating exhibited superior tribological and mechanical properties. The dominant wear mechanism was abrasive wear for coating/Al2O3 pair, while for coating/100Cr6 pair, a combination of mild adhesive wear, severe adhesive wear, and abrasive wear (extensive plowing) were the dominant wear mechanisms at different applied normal loads. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2014.03.026
  • 2014 • 120 Uniform pressure electromagnetic actuator - An innovative tool for magnetic pulse welding
    Weddeling, C. and Hahn, M. and Daehn, G.S. and Tekkaya, A.E.
    Procedia CIRP 18 156-161 (2014)
    The uniform pressure electromagnetic actuator (UPEA) is an innovative tool design for electromagnetic forming applications. In this article its suitability for magnetic pulse welding is demonstrated. To facilitate the process design, a simple mathematical model based on analytical equations describing the electromagnetic behavior of the system and the mechanical behavior of the workpieces under impulse loads is presented in this manuscript. The goal of the model is to predict the workpiece velocity considering the input energy, equipment and setup characteristics as well as mechanical properties of the workpiece. To validate the model, experimental analyses with aluminum-to-aluminum joints were conducted. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2014.06.124
  • 2014 • 119 Vacancy-induced toughening in hard single-crystal V0.5Mo 0.5Nx/MgO(0 0 1) thin films
    Kindlund, H. and Sangiovanni, D.G. and Lu, J. and Jensen, J. and Chirita, V. and Birch, J. and Petrov, I. and Greene, J.E. and Hultman, L.
    Acta Materialia 77 394-400 (2014)
    Using a combination of experiments and density functional theory (DFT), we demonstrate the first example of vacancy-induced toughening, in this case for epitaxial pseudobinary NaCl-structure substoichiometric V0.5Mo 0.5Nx alloys, with N concentrations 0.55 ≤ x ≤ 1.03, grown by reactive magnetron sputter deposition. The nanoindentation hardness H(x) increases with increasing vacancy concentration from 17 GPa with x = 1.03 to 26 GPa with x = 0.55, while the elastic modulus E(x) remains essentially constant at 370 GPa. Scanning electron micrographs of indented regions show ductile plastic flow giving rise to material pile-up, rather than cracks as commonly observed for hard, but brittle, transition-metal nitrides. The increase in alloy hardness with an elastic modulus that remains constant with decreasing x, combined with the observed material pile-up around nanoindents, DFT-calculated decrease in shear to bulk moduli ratios, and increased Cauchy pressures (C12-C44), reveals a trend toward vacancy-induced toughening. Moreover, DFT crystal orbital overlap population analyses are consistent with the above results. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.06.025
  • 2013 • 118 A porous pH-stabilized composite material consisting of poly (D,L-lactide), calcium carbonate and gentamicin for bone substitution
    Backhaus, S. and Annen, T. and Epple, M.
    Materialwissenschaft und Werkstofftechnik 44 107-111 (2013)
    Macroporous objects of poly(D,L-lactide), PDLLA, and calcium carbonate were prepared by hot-pressing (0% porosity), gas-foaming (50% porosity), and a combination of gas-foaming and salt leaching (70% porosity). They were loaded with 15 wt% of gentamicin without loss of the mechanical properties. The release of gentamicin occurred over several weeks, making this material suitable as a mechanically stable porous and degradable bone substitution material with antibacterial properties. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300081
  • 2013 • 117 A thermodynamic approach for the development of austenitic steels with a high resistance to hydrogen gas embrittlement
    Martín, M. and Weber, S. and Theisen, W.
    International Journal of Hydrogen Energy 38 14887-14895 (2013)
    The CALPHAD method was employed to assess the austenite stability of model alloys based on the Cr-Mn-Ni-Cu system. Stability was evaluated as the difference in Gibbs free energy between the austenite and ferrite phases. This energy difference represents the chemical driving force for the martensitic transformation and is employed as a design criterion. Six novel alloys featuring a lower driving force compared to the reference material AISI 316L were produced in laboratory. The susceptibility of all alloys to hydrogen gas embrittlement was evaluated by slow strain-rate tensile testing in air and hydrogen gas at 40 MPa and -50 C. The mechanical properties and ductility response of four of the six alloys exhibited an equivalent performance in air and hydrogen. Thermodynamic calculations were in agreement with the amount of α′-martensite formed during testing. Furthermore, a 4.5 wt.% reduction in the nickel content in comparison to 316L promises a cost benefit for the novel materials. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2013.08.133
  • 2013 • 116 Ab initio calculation of traction separation laws for a grain boundary in molybdenum with segregated C impurites
    Tahir, A.M. and Janisch, R. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    We have determined the influence of carbon on mechanical properties such as grain boundary energy, work of separation (WoS) and fracture strength of the Σ5(3 1 0)[0 0 1] symmetrical tilt grain boundary (STGB) in molybdenum with ab initio methods. From our ab initio results, we derived traction-separation laws that can be used in continuum simulations of fracture employing cohesive zones. Our results show that with an increasing number of C atoms at the grain boundary, the energy of the grain boundary is lowered, indicating a strong driving force for segregation. Uni-axial tensile tests of the grain boundary reveal that there is only a small effect of segregated C atoms on the cohesive energy or WoS of the grain boundary, while the strength of the Σ5(3 1 0)[0 0 1] STGB increases by almost 30% for a complete monolayer of C. This increase in strength is accompanied by an increase in grain boundary stiffness and a decrease of the interface excess volume. The characteristic parameters are combined in the concentration-dependent traction-separation laws. A study of the scaling behaviour of the different investigated systems shows that the energy-displacement curves can be well described by the universal binding energy relationship even for different C concentrations. These findings open the way for significant simplification of the calculation of ab initio traction separation laws for grain boundaries with and without impurities. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/7/075005
  • 2013 • 115 Ab initio prediction of the critical thickness of a precipitate
    Sampath, S. and Janisch, R.
    Journal of Physics Condensed Matter 25 (2013)
    Segregation and precipitation of second phases in metals and metallic alloys is an important phenomenon that has a strong influence on the mechanical properties of the material. Models exist that describe the growth of coherent, semi-coherent and incoherent precipitates. One important parameter of these models is the energy of the interface between matrix and precipitate. In this work we apply ab initio density functional theory calculations to obtain this parameter and to understand how it depends on chemical composition and mechanical strain at the interface. Our example is a metastable Mo-C phase, the body-centred tetragonal structure, which exists as a semi-coherent precipitate in body-centred cubic molybdenum. The interface of this precipitate is supposed to change from coherent to semi-coherent during the growth of the precipitate. We predict the critical thickness of the precipitate by calculating the different contributions to a semi-coherent interface energy by means of ab initio density functional theory calculations. The parameters in our model include the elastic strain energy stored in the precipitate, as well as a misfit dislocation energy that depends on the dislocation core width and the dislocation spacing. Our predicted critical thickness agrees well with experimental observations. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/35/355005
  • 2013 • 114 Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-to-austenite reversion
    Springer, H. and Belde, M. and Raabe, D.
    Materials Science and Engineering A 582 235-244 (2013)
    The effect of local martensite-to-austenite reversion on microstructure and mechanical properties was studied with the aim of designing ductile martensitic steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0-5. wt% Mn, 0-2. wt% Si, constant 13.5. wt% Cr and 0.45. wt% C) and seven martensite tempering conditions (300-500. °C, 0-30. min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35. vol%) leading to a martensitic-austenitic TRIP steel with optimum mechanical properties (1.5. GPa ultimate tensile strength and 18% elongation), Si led to brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the austenite, as a result of the composition effects on austenite retention and reversion. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.06.036
  • 2013 • 113 Characterization of crocodile teeth: Correlation of composition, microstructure, and hardness
    Enax, J. and Fabritius, H.-O. and Rack, A. and Prymak, O. and Raabe, D. and Epple, M.
    Journal of Structural Biology 184 155-163 (2013)
    Structure and composition of teeth of the saltwater crocodile Crocodylus porosus were characterized by several high-resolution analytical techniques. X-ray diffraction in combination with elemental analysis and infrared spectroscopy showed that the mineral phase of the teeth is a carbonated calcium-deficient nanocrystalline hydroxyapatite in all three tooth-constituting tissues: Dentin, enamel, and cementum. The fluoride content in the three tissues is very low (<0.1. wt.%) and comparable to that in human teeth. The mineral content of dentin, enamel, and cementum as determined by thermogravimetry is 71.3, 80.5, and 66.8. wt.%, respectively. Synchrotron X-ray microtomography showed the internal structure and allowed to visualize the degree of mineralization in dentin, enamel, and cementum. Virtual sections through the tooth and scanning electron micrographs showed that the enamel layer is comparably thin (100-200 μm). The crystallites in the enamel are oriented perpendicularly to the tooth surface. At the dentin-enamel-junction, the packing density of crystallites decreases, and the crystallites do not display an ordered structure as in the enamel. The microhardness was 0.60 ± 0.05. GPa for dentin, 3.15 ± 0.15. GPa for enamel, 0.26 ± 0.08. GPa for cementum close to the crown, and 0.31 ± 0.04. GPa for cementum close to the root margin. This can be explained with the different degree of mineralization of the different tissue types and is comparable with human teeth. © 2013 Elsevier Inc.
    view abstractdoi: 10.1016/j.jsb.2013.09.018
  • 2013 • 112 Combined experimental and numerical approach for linking microstructure and mechanical properties on different length scales for near γ-TiAl alloys
    Kabir, M.R. and Bartsch, M. and Chernova, L. and Schneider, J. and Kelm, K.
    Materials Science Forum 750 76-79 (2013)
    At room temperature the macroscopic tensile behavior of TiAl alloys is extremely microstructure sensitive. In general the microstructures of TiAl alloys are heterogeneous at micro and meso scale. The materials micromechanisms that occur at different length scale have to be linked for a proper understanding of the macroscopic response. In order to explore those micromechanisms, methodologies combining advanced experimental and computational analysis have been proposed. Linking microstructure and properties using a two-scale numerical model we are able to explain the stress-strain and hardening behavior of this alloy. © (2013) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2013 • 111 Composite extrusion of thin aluminum profiles with high reinforcing volume
    Pietzka, D. and Khalifa, N.B. and Gerke, S. and Tekkaya, E.
    Key Engineering Materials 554-557 801-808 (2013)
    The combination of different materials within aluminum profiles offers significant potential for increasing the mechanical properties as well as the functionality [1]. Direct extrusion using special porthole dies which feed elements in terms of continuous wires through bridges into the aluminum base material flow, was studied to manufacture continuous reinforced profiles. To achieve an essential advantage of the technology for lightweight applications a high reinforcing volume of aluminum profiles is targeted. A comparatively high reinforcing volume can be reached either by a high number of reinforcing elements or through a reduction of the profile wall thickness. A high number of reinforcing elements leads to a small distance between the single elements in the profile cross-section. The paper will show the results of an experimental and numerical analysis carried out to determine the minimum distance between the reinforcing elements as well as the minimum allowable profile thickness. In the trials different arrangements of the elements in the profile cross-section and profile thicknesses were considered. Main parameters which have an influence on the process stability were analyzed and a process window for the manufacture of thin profiles with high reinforcing volume was deduced. Copyright © 2013 Trans Tech Publications Ltd.
    view abstractdoi: 10.4028/
  • 2013 • 110 Effect of die design on the welding quality during solid state recycling of AA6060 chips by hot extrusion
    Güley, V. and Güzel, A. and Jäger, A. and Ben Khalifa, N. and Tekkaya, A.E. and Misiolek, W.Z.
    Materials Science and Engineering A 574 163-175 (2013)
    Solid state recycling of aluminum chips by hot extrusion is a novel processing technique, which utilizes remarkably lower energies compared to conventional recycling by remelting. The mechanical properties of the extruded profiles can be improved by optimizing the effect of extrusion die design on the welding quality of machining chips. The chips were extruded through two dies of different design to produce solid rectangular profiles. One of the dies was a flat-face die, which represents a conventional extrusion die design for production of solid aluminum profiles. The second die was a porthole die typically used for complex hollow and semi hollow aluminum profiles. AA6060 chips were compacted at room temperature into billets and hot-extruded at approximately 500 °C to aluminum profiles. The microstructure and the mechanical properties of the profiles extruded through the flat-face and porthole dies were compared. The extrusion through the porthole die resulted in a much better welding of the chips and revealed more than 80% higher ductility compared to the profiles extruded through a flat-face die. The welding quality of the chips was studied using a two-step analytical approach: a criterion for the breaking of the oxide layers and an index for the welding quality. These analytical approaches were implemented with the help of subroutines in the FEM code, in which the results of the simulations were compared and confirmed by the experimental results. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.03.010
  • 2013 • 109 Fabrication of Borassus fruit lignocellulose fiber/PP composites and comparison with jute, sisal and coir fibers
    Sudhakara, P. and Jagadeesh, D. and Wang, Y. and Venkata Prasad, C. and Devi, A.P.K. and Balakrishnan, G. and Kim, B.S. and Song, J.I.
    Carbohydrate Polymers 98 1002-1010 (2013)
    Novel composites based on borassus fruit fine fiber (BFF) and polypropylene (PP) were fabricated with variable fiber composition (5, 10, 15 and 20 wt%) by injection molding. Maleated PP (MAPP) was also used as compatibilizer at 5 wt% for effective fiber-matrix adhesion. FTIR analysis confirms the evidence of a chemical bonding between the fiber and polymeric matrix through esterification in presence of MAPP. The tensile and flexural properties were found to increase with 15 and 10 wt% fiber loadings respectively, and decreased thereafter. Coir, jute and sisal fiber composites were also fabricated with 15 wt% fiber loading under the same conditions as used for BFF/PP composites. It was found that the mechanical properties of BFF (15 wt%)/PP composites were equivalent to jute/PP, sisal/PP and superior to coir/PP composites. Jute/PP and sisal/PP composites showed higher water absorption than BFF/PP and coir/PP composites. These results have demonstrated that the BFF/PP composites can also be an alternative material for composites applications. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbpol.2013.06.080
  • 2013 • 108 Ferritic stainless steels for high-temperature applications: Stabilization of the microstructure by solid state precipitation of MX carbonitrides
    Nabiran, N. and Weber, S. and Theisen, W.
    High Temperature Materials and Processes 32 563-572 (2013)
    Ferritic heat-resistant steels are commonly used for automotive exhaust systems and have replaced cast iron, the traditional material for this application. Efforts to improve the efficiency of engines, reduce weight, and minimize toxic ingredients by increasing the gas temperature have shifted the requirement for ferritic heat-resistant steels to a higher hot strength. Methods of improving the high-temperature strength are solid-solution strengthening, precipitation hardening, and grain refinement. In this work, the influence of MX precipitates on the high-temperature mechanical properties of three different ferritic Fe-Cr stainless steels was investigated and compared to a reference material. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900 °C for up to 1440 h. The most effective method of increasing the high-temperature strength is to alloy the steel with 2 mass% tungsten. Grain growth during annealing at 900 °C was decelerated by solid-state formation of MX carbonitrides. Microstructural investigations also revealed a slow coarsening rate of the MX precipitates. © [2013] by Walter de Gruyter Berlin Boston 2013.
    view abstractdoi: 10.1515/htmp-2013-0004
  • 2013 • 107 Friction surfacing of a cold work tool steel-Microstructure and sliding wear behavior
    Hanke, S. and Beyer, M. and Dos Santos, J.F. and Fischer, A.
    Wear 308 180-185 (2013)
    Friction surfacing is a solid-state joining process, during which process temperatures below melting, a high cooling rate, and a high degree of deformation lead to a very fine microstructure and exceptional mechanical properties of the coating material. In the presented study, the friction surfacing process was used to apply self-mating layers onto cold work tool steels, which e.g. are used for deep-drawing dies in the automotive industry. Such dies are subject to wear during operation and the repair of the dies by arc-welding includes many process steps besides the final hard surfacing. By the use of friction surfacing, hard tool steel coatings can be generated in one process step before the final machining operation. A martensitic microstructure and high hardness up to 900 HV10 can be reached within the coatings. Boundary lubricated reciprocating sliding wear tests (ball-on-flat) were conducted on cast and hardened material as well as the coatings. The results showed that over a wide range of loading the coated samples perform as well as the original die material, showing tribochemical reactions and very small wear volumes. After more than 500,000 sliding passes, both the coatings' and the original tool material's wear is dominated by surface fatigue. © 2013.
    view abstractdoi: 10.1016/j.wear.2013.06.017
  • 2013 • 106 High resolution imaging for inspection of Laser Beam Melting systems
    Jacobsmuhlen, J.Z. and Kleszczynski, S. and Schneider, D. and Witt, G.
    Conference Record - IEEE Instrumentation and Measurement Technology Conference 707-712 (2013)
    Laser Beam Melting (LBM) allows the fabrication of three-dimensional parts from metallic powder with almost unlimited geometrical complexity and very good mechanical properties. LBM works iteratively: a thin powder layer is deposited onto the build platform which is then melted by a laser according to the desired part geometry. Today, the potential of LBM in application areas such as aerospace or medicine has not yet been exploited due to the lack of process stability and quality management. For that reason, we present a high resolution imaging system for inspection of LBM systems which can be easily integrated into existing machines. A container file stores calibration images and all layer images of one build process (powder and melt result) with corresponding metadata (acquisition and process parameters) for documentation and further analysis. We evaluate the resolving power of our imaging system and show that it is able to inspect the process result on a microscopic scale. Sample images of a part built with varied process parameters are provided, which show that our system can detect topological flaws and is able to inspect the surface quality of built layers. The results can be used for flaw detection and parameter optimization, for example in material qualification. © 2013 IEEE.
    view abstractdoi: 10.1109/I2MTC.2013.6555507
  • 2013 • 105 Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging
    Koyama, M. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Acta Materialia 61 4607-4618 (2013)
    We investigated the hydrogen embrittlement of a Fe-18Mn-1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10-6 s -1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe-Mn-C twining-induced plasticity steel. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.030
  • 2013 • 104 Improvement of the mechanical properties of jute fibre reinforced cement mortar: A statistical approach
    Chakraborty, S. and Kundu, S.P. and Roy, A. and Basak, R.K. and Adhikari, B. and Majumder, S.B.
    Construction and Building Materials 38 776-784 (2013)
    We have demonstrated that the physical characteristics and mechanical properties of cement mortar are significantly improved by the jute fibre reinforcement. Three different processes methodologies were adopted to mix the jute fibre homogeneously in the mortar matrix. By optimising the processing conditions and fibre loading; the cold crushing strength and flexural strength, flexural toughness and the toughness index of the mortar has significantly been increased. Based on the Fourier transformed infrared spectroscopy and thermo-gravimetric analyses a plausible mechanism of the effect of jute reinforcement controlling the physical and mechanical properties of cement mortar have been proposed. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.conbuildmat.2012.09.067
  • 2013 • 103 Influence of bias voltage on residual stresses and tribological properties of TiAlVN-coatings at elevated temperatures
    Tillmann, W. and Sprute, T. and Hoffmann, F. and Chang, Y.-Y. and Tsai, C.-Y.
    Surface and Coatings Technology 231 122-125 (2013)
    The extension of tool life is a crucial goal for heat resistant forming tools. Therefore, the industry is interested to reduce the friction and wear for these tools. The employment of metals, polymeric composites, and ceramics as solid lubricants increases the production as well as maintenance costs. Thus, the thin film technology and especially new self-lubricating coatings will become increasingly important. Titanium aluminum vanadium nitride as a self-lubricating coating has a high potential to improve the tribological behavior of heat resisting tool surfaces and has good mechanical properties such as a high hardness (more than 40GPa). In this study, TiAlVN coatings were deposited on HS6-5-2C high speed steel substrates by using a magnetron sputtering system. After annealing at 650°C, a V2O5 (Magnéli phase) which adds self-lubricating qualities to the coatings could be detected in the TiAlVN layer. Due to the influence of adhesive and cohesive damage processes, resulting from the residual stress behavior in the layer close to the substrate area, it is critical to measure residual stresses in order to increase the wear resistance. In addition to the phase analyses, residual stress measurements were investigated by means of x-ray diffractometry as well. An experimental method, based on the traditional sin2ψ-method and utilizing a grazing-incidence diffraction geometry was used in order to enhance the irradiation volume of thin film samples. This resulted in a higher intensity for high-angle Bragg peaks than for the Bragg-Brentano geometry. Furthermore, the mechanical and tribological properties of the TiAlVN coatings were characterized at elevated temperatures. The required results were provided by a high temperature ball-on-disk device and a nanoindenter. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2012.03.012
  • 2013 • 102 Influence of fiber alignment on creep in directionally solidified NiAl-10Mo in-situ composites
    Seemüller, C. and Heilmaier, M. and Haenschke, T. and Bei, H. and Dlouhy, A. and George, E.P.
    Intermetallics 35 110-115 (2013)
    A NiAl-Mo eutectic having a nominal composition of Ni-45Al-10Mo was directionally solidified in a floating-zone furnace at two different growth rates, 20 and 80 mm/h. At the slower growth rate, the Mo fibers in the composite are well-aligned with the growth direction, whereas at the higher growth rate cellular microstructures are observed. Creep testing at 900°C showed that the minimum creep rate is much higher for cellular than for well-aligned structures. In the cellular case a "soft" cell boundary consisting primarily of binary NiAl surrounding "hard" eutectic cell interiors seems to facilitate cell boundary sliding and therefore results in low creep strength. A microstructure-based composite model is used to explain the effects of fiber alignment on creep resistance. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2012.12.007
  • 2013 • 101 Influence of fine powder feedstock (-10 + 2 μm) on the HVOF spraying characteristics, coating morphology, and properties of WC-CoCr 86-10-4
    Tillmann, W. and Nebel, J. and Piotrowski, W.
    Journal of Thermal Spray Technology 22 242-249 (2013)
    The use of fine feedstock powder can extend the feasibility and scope of HVOF coatings to new fields of applications. Especially for the purpose of near-net-shape coatings, these powders facilitate homogeneous layer morphologies, and smooth coating surfaces. However, the small particle sizes also lead to several challenges. One major issue is the in-flight behavior which is distinctly affected by the low mass and relatively large specific surface of the particles. In this paper, the in-flight and coating characteristics of WC-CoCr 86-10-4 (-10 + 2 μm) were investigated. It was determined that the fine powder feedstock shows a high sensitivity to the gas flow, velocity, and temperature of the spray jet. Because of their low mass inertia, their velocity, for example, is actually influenced by local pressure nodes (shock diamonds) in the supersonic flow. Additionally, the relatively large specific surface of the particles promotes partial overheating and degradation. Nevertheless, the morphological and mechanical properties of the sprayed layer are hardly affected. In fact, the coatings feature a superior surface roughness, porosity, hardness, and wear resistance. © 2012 ASM International.
    view abstractdoi: 10.1007/s11666-012-9832-4
  • 2013 • 100 Massive anisotropic thermal expansion and thermo-responsive breathing in metal-organic frameworks modulated by linker functionalization
    Henke, S. and Schneemann, A. and Fischer, R.A.
    Advanced Functional Materials 23 5990-5996 (2013)
    Functionalized metal-organic frameworks (fu-MOFs) of general formula [Zn2(fu-L)2dabco]n show unprecedentedly large uniaxial positive and negative thermal expansion (fu-L = alkoxy functionalized 1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). The magnitude of the volumetric thermal expansion is more comparable to property of liquid water rather than any crystalline solid-state material. The alkoxy side chains of fu-L are connected to the framework skeleton but nevertheless exhibit large conformational flexibility. Thermally induced motion of these side chains induces extremely large anisotropic framework expansion and eventually triggers reversible solid state phase transitions to drastically expanded structures. The thermo-responsive properties of these hybrid solid-liquid materials are precisely controlled by the choice and combination of fu-Ls and depend on functional moieties and chain lengths. In principle, this combinatorial approach allows for a targeted design of extreme thermo-mechanical properties of MOFs addressing the regime between crystalline solid matter and the liquid state. Extremely large thermal expansion is shown by pillared-layered metal-organic frameworks (MOFs) exhibiting alkoxy-functionalized 1,4-benzenedicarboxylate linkers. At a certain threshold temperature the materials reversibly switch from a narrow pore to large pore form. This unprecedented thermo-mechanical behavior is an intrinsic property of the materials and can be modulated substantially by mixing differently functionalized linkers to obtain mixed linker MOF solid solutions. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201301256
  • 2013 • 99 Mechanical properties of zirconia composite ceramics
    Zhang, Y. and Malzbender, J. and Mack, D.E. and Jarligo, M.O. and Cao, X. and Li, Q. and Vaßen, R. and Stöver, D.
    Ceramics International 39 7595-7603 (2013)
    Composite materials based on 8 wt% yttria partially stabilized zirconia, with additions of gadolinium zirconate, lanthanum lithium hexaaluminate, yttrium aluminum garnet and strontium zirconate were characterized. Samples were fabricated by hot-press sintering at 1550° C. The effect of the secondary phase content on the mechanical properties of the composites was evaluated. Hardness, elastic modulus and fracture toughness of the fabricated composites were determined by means of depth-sensitive indentation testing. The fracture toughness of the samples as determined by the indentation method was found to increase with increasing YSZ content, reaching 3 MPa·m0.5 for samples with 80 wt% YSZ. The fracture toughness appeared to be affected by thermal expansion coefficient mismatch, crack bridging and crack deflection. © 2013 Elsevier Ltd and Techna Group S.r.l.
    view abstractdoi: 10.1016/j.ceramint.2013.03.014
  • 2013 • 98 Micromechanical modelling of damage and failure in dual phase steels
    Lian, J. and Vajragupta, N. and Münstermann, S.
    Key Engineering Materials 554-557 2369-2374 (2013)
    Dual phase (DP) steels consisting of two phases, ferrite and dispersed martensite, offer an attractive combination of strength and stretchability, which is a result of the strong distinctions of these constituents in mechanical properties. However, the damage behaviour in DP steels exhibits a rather complex scenario: voids are generated by the debonding of the hard phase from the matrix and the inner cracking of the hard phase in addition to by inclusions. The target of this study is to describe the initiation and evolution of damage in DP steel and develop a microstructure-based model which is capable of reflecting the underlying damage mechanisms. Both uniaxial and biaxial tensile tests are performed and the subsequent metallographic investigations are executed to reveal the mechanisms of damage initiation and evolution under different stress state condition and attention will be paid on the influence of various microstructural features on the initiation of damage. In finite element (FE) simulations, the microstructural features are taken into account by the representative volume elements (RVE). Different treatments of the constitutive behaviour of each constituent including isotropic hardening rule and crystallographically dependent configuration with crystal plasticity finite element method are investigated. Several numerical aspects are also discussed, such as RVE size, mesh size, element type, and boundary connections. In the end, the study is attempting to achieve a quantitative assessment of the cold formability of the investigated steel in a microscopic level based on microstructure information of material as well as to understand the damage mechanisms under different stress states condition which cause the macroscopic failure during plastic deformation. Copyright © 2013 Trans Tech Publications Ltd.
    view abstractdoi: 10.4028/
  • 2013 • 97 Microstructure, mechanical and biological properties of zirconium alloyed with niobium after severe plastic deformation
    Sharkeev, Y.P. and Eroshenko, A.Y. and Kulyashova, K.S. and Fortuna, S.V. and Suvorov, K.A. and Epple, M. and Prymak, O. and Sokolova, V. and Chernousova, S.
    Materialwissenschaft und Werkstofftechnik 44 198-204 (2013)
    A comparative investigation of microstructure, mechanical and biological properties for zirconium alloyed with niobium in coarse-grained and ultra-fine grained states is presented. The temperature and deformation regimes of multi-stage abc-pressing resulted in ultra-fine grained states with an average size of the structural elements in the range of 0.28-0.55 μm, depending on the accumulated strain during pressing. The increase of the accumulated strain at each stage of pressing increased the uniformity of the structure. The microhardness increased by 50% with increased accumulated strain during the severe plastic deformation. Between the microhardness and the average size of the structural elements, a linear dependence was found, indicating a Hall-Petch relationship. The alloy had a good biocompatibility as shown by an MTT test with osteoblasts (MG-63 cell line). The good mechanical properties (microhardness) of zirconium alloyed with niobium in the ultra-fine grained state make it suitable for medical applications, e. g. as implant material. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300113
  • 2013 • 96 Modeling sample/patient-specific structural and diffusional responses of cartilage using DT-MRI
    Pierce, D.M. and Ricken, T. and Holzapfel, G.A.
    International Journal for Numerical Methods in Biomedical Engineering 29 807-821 (2013)
    We propose a new 3D biphasic constitutive model designed to incorporate structural data on the sample/patient-specific collagen fiber network. The finite strain model focuses on the load-bearing morphology, that is, an incompressible, poroelastic solid matrix, reinforced by an inhomogeneous, dispersed fiber fabric, saturated with an incompressible fluid at constant electrolytic conditions residing in strain-dependent pores of the collagen-proteoglycan solid matrix. In addition, the fiber network of the solid influences the fluid permeability and an intrafibrillar portion that cannot be 'squeezed out' from the tissue. We implement the model into a finite element code. To demonstrate the utility of our proposed modeling approach, we test two hypotheses by simulating an indentation experiment for a human tissue sample. The simulations use ultra-high field diffusion tensor magnetic resonance imaging that was performed on the tissue sample. We test the following hypotheses: (i) the through-thickness structural arrangement of the collagen fiber network adjusts fluid permeation to maintain fluid pressure (Biomech. Model. Mechanobiol. 7:367-378, 2008); and (ii) the inhomogeneity of mechanical properties through the cartilage thickness acts to maintain fluid pressure at the articular surface (J. Biomech. Eng. 125:569-577, 2003). For the tissue sample investigated, both through-thickness inhomogeneities of the collagen fiber distribution and of the material properties serve to influence the interstitial fluid pressure distribution and maintain fluid pressure underneath the indenter at the cartilage surface. Tissue inhomogeneity appears to have a larger effect on fluid pressure retention in this tissue sample and on the advantageous pressure distribution. © 2012 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/cnm.2524
  • 2013 • 95 New tribological strategies for cutting tools following nature
    Rechberger, M. and Paschke, H. and Fischer, A. and Bertling, J.
    Tribology International 63 243-249 (2013)
    The material concept of animal teeth for cutting viscoelastic and abrasive food is completely different to those existing for industrial cutting tools. Biological cutting systems use abrasive wear in order to form sharp cutting edges. This work gives an overview of biological principles and describes a biomimetic approach for designing industrial cutting tools. The developed tools based on nature inspired hierarchic structure and shape show outstanding mechanical properties and provide evidence that self-sharpening effects and high abrasive resistance must not be contradicting. & 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.triboint.2012.09.008
  • 2013 • 94 On the crystallographic anisotropy of nanoindentation in pseudoelastic NiTi
    Pfetzing-Micklich, J. and Somsen, C. and Dlouhy, A. and Begau, C. and Hartmaier, A. and Wagner, M.F.-X. and Eggeler, G.
    Acta Materialia 61 602-616 (2013)
    We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈0 0 1〉, 〈1 0 1〉 and 〈1 1 1〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈0 0 1〉 direction and lowest along the 〈1 1 1〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.09.081
  • 2013 • 93 Phase formation at the interface between a boron alloyed steel substrate and an Al-rich coating
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 226 130-139 (2013)
    Al-base coating (AlSi10Fe3) was applied to a steel substrate (22MnB5) by hot dipping. The coated steel substrates were austenitized at 920. °C for several dwells, and phase formation at the steel/coating interface was investigated by means of ex-situ phase analysis with synchrotron radiation and EBSD. Phase identification by EBSD and XRD confirmed the formation of Al-rich intermetallics during austenitization. Increasing the dwell time led to Fe diffusion into the Al-base coating as well as Al diffusion into the substrate. As a result of the diffusion processes, Al-rich intermetallics in the coating transformed to more Fe-rich intermetallics. Simultaneously, Al diffusion into the substrate changed the microstructure of the steel substrate near the coating interface. Formation of FeAl intermetallics and thus the mechanical properties of the AlSi10Fe3 coating can be influenced by heat treatment. Higher austenitization temperatures and longer dwell times support the formation of more ductile FeAl intermetallics but also lead to grain growth; thus having a negative effect on the mechanical properties of the steel. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2013.03.045
  • 2013 • 92 Polymer modified jute fibre as reinforcing agent controlling the physical and mechanical characteristics of cement mortar
    Chakraborty, S. and Kundu, S.P. and Roy, A. and Adhikari, B. and Majumder, S.B.
    Construction and Building Materials 49 214-222 (2013)
    Polymer modified alkali treated jute fibre as a reinforcing agent, substantially improves the physical and mechanical properties of cement mortar with a mix design cement:sand:fibre:water::1:3:0.01:0.6. The workability of the mortar is found to increase systematically from 155 ± 5 mm (control mortar) to 167 ± 8 mm (0.2050% polymer modified mortar). The density of the mortar is increased from 2092 kg/m3 to 2136 kg/m3 with a concomitant reduction of both water absorption and apparent porosity. Optimal polymer content in emulsion (0.0513%) is found to increase the compressive strength, modulus of rupture and flexural toughness 25%, 28%, 387% respectively as compared to control mortar. Based on the X-ray diffraction and infra-red spectroscopy analyses of the mortar samples a plausible mechanism of the effect of modified jute fibre controlling the physical and mechanical properties of cement mortar has been proposed. © 2013 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.conbuildmat.2013.08.025
  • 2013 • 91 Re effects on phase stability and mechanical properties of Mo SS+Mo3Si+Mo5SiB2 alloys
    Yang, Y. and Bei, H. and Tiley, J. and George, E.P.
    Journal of Alloys and Compounds 556 32-38 (2013)
    In this paper, we investigate the effects of Re additions on the microstructure and mechanical properties of a ternary alloy with the composition Mo-12.5Si-8.5B (at.%). This alloy has a three-phase microstructure consisting of Mo solid-solution (MoSS), Mo3Si, and Mo 5SiB2 and our results show that up to 8.4 at.% Re can be added to it without changing its microstructure or forming any brittle σ phase at 1600 °C. Three-point bend tests using chevron-notched specimens showed that Re did not improve fracture toughness of the three-phase alloy. Nanoindentation performed on the MoSS phase in the three-phase alloy showed that Re increases Young's modulus, but does not lower hardness as in some Mo solid solution alloys. Based on our thermodynamic calculations and microstructural analyses, the lack of a Re softening effect is attributed to the increased Si levels in the Re-containing MoSS phase since Si is known to increase its hardness. This lack of softening is possibly why there is no Re-induced improvement in fracture toughness. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2012.12.047
  • 2013 • 90 The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy
    Otto, F. and Dlouhý, A. and Somsen, C. and Bei, H. and Eggeler, G. and George, E.P.
    Acta Materialia 61 5743-5755 (2013)
    An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4-160 μm. Quasi-static tensile tests at an engineering strain rate of 10-3 s-1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ∼2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (∼20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing ("dynamic Hall-Petch"), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.06.018
  • 2013 • 89 Tribochemical reactions in metal-on-metal hip joints influence wear and corrosion
    Wimmer, M.A. and Mathew, M.T. and Laurent, M.P. and Nagelli, C. and Liao, Y. and Marks, L.D. and Pourzal, R. and Fischer, A. and Jacobs, J.J.
    ASTM Special Technical Publication 1560 STP 292-309 (2013)
    Recent findings indicate the presence of tribochemically gener-ated layers on metal-on-metal (MoM) bearing surfaces. These tribolayers are films of a few-hundred-nanometer thickness and are constituted of carbonaceous material mixed with metal and oxide particles. The purpose of the study was to characterize these tribofilms mechanically and electrochemically. Using a nanoindenter, the local mechanical properties of the tribolayer were measured. On average a hardness of ∼1.0 GPa was determined, which was softer than the underlying metal. The influence of tribomaterial on the electrochemistry of the cobalt-chromium-molybdenum alloy (CoCrMo) was investigated. Bovine calf serum mixture was used as the electrolyte. High- and low-carbon CoCrMo-samples with and without tribolayer were compared using potentiodynamic testing. This corrosive investigation was followed by tribocorrosive tests using a custom made apparatus, where a ceramic ball oscillated against a flat CoCrMo surface. Potential and coefficient of friction were monitored throughout this 100 K cycle test. Electrochemical impedance spectroscopy tests before and after testing were conducted. Weight loss was determined using planimetric analysis. It was found that the tribolayered surface had better corrosion resistance than the corresponding tribolayer-free (polished) surface. The tribolayered surface also exhibited a more noble potential during tribocorrosive testing and demonstrated less wear. High- carbon was the superior alloy compared with low carbon for all surface conditions; however, the differences seemed to equalize in the presence of a tribo- film. There were also differences in tribofilm generation, possibly related to the microstructure of the two alloys. Copyright © 2013 by ASTM International.
    view abstractdoi: 10.1520/STP156020120050
  • 2013 • 88 Vacuum brazing titanium using thin nickel layer deposited by PVD technique
    Elrefaey, A. and Wojarski, L. and Janczak-Rusch, J. and Tillmann, W.
    Materials Science and Engineering A 565 180-186 (2013)
    In this study, the evolution of the interfacial microstructure, hardness distribution, and the joint strength of vacuum brazed commercially pure titanium were evaluated. A thin nickel layer, with different thicknesses, was deposited by PVD technique to serve as the brazing filler metal. Test joints were processed at temperatures of 910°C and 960°C using a soaking time of 15 and 90min. The experimental results showed that sound joints with a good wetting quality as well as lack of pores and cracks can be achieved at a brazing temperature of 960°C. A Ti2Ni intermetallic compound was formed at the interfacial area at a soaking time of 15min and with a deposition rate of 90AH which was detrimental to the joint mechanical properties. Meanwhile, at a soaking time of 90min, intermetallic compound was not detected and the diffusion of nickel was completed at all deposition rates which improve the shear strength of the joints. © 2012 Elsevier B.V..
    view abstractdoi: 10.1016/j.msea.2012.12.028
  • 2012 • 87 A comparison of different experimental methods for investigating the mechanical properties of plane polysiloxane membranes and capsule walls
    Koleva, I. and Rehage, H.
    Soft Matter 8 7672-7682 (2012)
    In this publication we systematically compared the surface rheological properties of different polysiloxane capsule membranes with planar cross-linked films of the same chemical composition. We performed interfacial rheological measurements and different experiments of the capsule deformation in shear and centrifugal fields. In a series of experiments we determined the surface shear and Young's moduli of the polysiloxane membranes. These investigations allowed us to calculate the two-dimensional Poisson ratio of the viscoelastic polysiloxane shells. We also compared the yield values of the capsule deformation Dmax and the maximal shear strain γmax, which describe the limits of the linear viscoelastic response. In the regime of large centrifugal forces, we observed a new capsule bursting mechanism. For small deformations, where the membranes showed linear-viscoelastic properties we found a very good agreement between measurements and theoretical predictions. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2sm25720c
  • 2012 • 86 A micromechanical damage simulation of dual phase steels using XFEM
    Vajragupta, N. and Uthaisangsuk, V. and Schmaling, B. and Münstermann, S. and Hartmaier, A. and Bleck, W.
    Computational Materials Science 54 271-279 (2012)
    As a result of their microstructures being made up by constituents with strong distinctions in mechanical properties, multiphase steels exhibit high energy absorption as well as an excellent combination of strength and ductility. Furthermore, the microstructural composition influences the failure behaviour of such kind of steels because of the occurrence of different fracture mechanisms in parallel. When the failure behaviour of dual phase (DP) steels is investigated, several types of failures are typically observed, such as the ductile failure of ferrite, the brittle failure of martensite and the interface debonding between phases. Hence, a reliable microstructure-based simulation approach must be developed that describes material deformation and failure under any given loading condition. In this work, two different damage mechanics methods were employed to study the interaction between failure modes in DP steels by means of a representative volume element (RVE). In order to consider the characteristics of a real microstructure, all involved phases were modelled with a precise volume fraction. Firstly, the extended finite element method (XFEM) was used to study the damage onset and progression in martensitic regions without prescribing the crack path. Secondly, a damage curve was derived and employed for the ductile ferritic phase. By combining these two damage models in the RVE model on microscopic scale, development of different failures modes in DP steels could be investigated. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2011.10.035
  • 2012 • 85 Characterization of the mechanical properties of technical fibers at extreme strain rates
    Bahners, T. and Schloßer, U. and Gutmann, J.
    Macromolecular Materials and Engineering 297 550-558 (2012)
    Technical textiles can be subject to mechanical stress with strain rates far from the quasi-static conditions of common standardized tests. The objective of the presented work was therefore to study the mechanical properties of technical yarns made of PA 6.6, PA 4.6, and PET under strain rates up to 200 s -1, making use of a "falling weight" apparatus. It can be summarized that the moduli at specific points of the stress-strain-curve increase with the strain rate to values up to 50% higher than the data determined under quasi-static strain. Saturation is observed for strain rates larger than 50 s -1. The analysis of failure morphology by scanning electron microscopy revealed molten material at the ends of broken fibers. This indicates thermal failure of the fibers due to local concentration of energy loading. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mame.201100312
  • 2012 • 84 Chemically modified jute fibre reinforced non-pressure (NP) concrete pipes with improved mechanical properties
    Kundu, S.P. and Chakraborty, S. and Roy, A. and Adhikari, B. and Majumder, S.B.
    Construction and Building Materials 37 841-850 (2012)
    To improve the load bearing capacity of underground sewage pipe, we have formulated a concrete mix using chemically-modified jute fibre (reinforcing agent), polymer latex (surface modifier both for fibre and matrix) and tannin (water reducing admixture). As compared to commercial non-pressure grade pipes (NP3 type), significant strength improvement, under three-edge-bearing test (∼129.4%), is achieved in the pipes made using the modified concrete mix. NP3 pipes made using this modified concrete exhibit similar mechanical properties to that of NP4 pipes resulting an effective reduction of 31.6 wt% of steel used for reinforcement in NP4 pipes. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.conbuildmat.2012.07.082
  • 2012 • 83 Correlation between tribological properties, sp 2/sp 3-ratio and H-content of low-wear diamond-like carbon (DLC) layers
    Vogli, E. and Hoffmann, F. and Bartis, E. and Oehrlein, G.S. and Tillmann, W.
    Materials Science Forum 706-709 2596-2601 (2012)
    It has been established that hardness and density of diamond-like carbon (DLC) layers can be raised by increasing ion energy during deposition, decreasing H-content and by increasing sp 3-fraction. To confirm differences in hydrogen content of hydrogen containing and hydrogen free DLC films deposited at different bias voltages, layers were etched in oxygen atmosphere in a capacitively coupled plasma device. By employing real-time ellipsometry measurements, the Hcontent of the hydrogen containing a-C:H layers were estimated by determining the optical constants n and k (n-real part and k-imaginary part of the refractive index). In addition, DLC layers were analyzed by X-ray photoelectron spectroscopy to estimate the ratio of sp 2- and sp 3- hybridization. The mechanical and tribological properties of the coatings were evaluated by means of nanoindentation and ball-on-disc-tests. Finally correlations between these properties, H-content and sp 3/sp 2-ratio were obtained in an effort to explain different tribological behaviors of DLC-layers. © 2012 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 82 Correlation of process parameters and part properties in laser sintering using response surface modeling
    Wegner, A. and Witt, G.
    Laser Assisted Net Shape Engineering 7 (lane 2012) 39 480--490 (2012)
    Due to the advancements during the last decade, the laser sintering process has achieved a high technical level, allowing for Rapid Manufacturing in some applications. However, processes still show poor repeatability of part quality, process interruptions or defective parts. The knowledge needed to avoid such problems is still insufficient. Literature provides only few detailed correlations between process parameters and part properties. Therefore, an approach using response surface methodology was chosen to correlate part properties with main influencing factors. Aim of the analyses was to predict and to improve part properties based on an enhanced process understanding. (C) 2012 Published by Elsevier B. V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH
    view abstractdoi: 10.1016/j.phpro.2012.10.064
  • 2012 • 81 Crystal chemistry and properties of mullite-type Bi 2M 4O 9: An overview
    Schneider, H. and Fischer, R.X. and Gesing, T.M. and Schreuer, J. and Mühlberg, M.
    International Journal of Materials Research 103 422-429 (2012)
    Bi 2M 4O 9 (M = Al 3+, Ga 3+, Fe 3+) belongs to the family of mullite-type crystal structures. The phases are orthorhombic with the space group Pbam. The backbones of the isostructural phases are edge-connected, mullite-type octahedral chains. The octahedral chains are linked by dimers of M 2O 7 tetrahedral groups and by BiO polyhedra. The Bi 3+ cations in Bi 2M 4O 9 contain stereo-chemically active 6s 2 lone electron pairs (LEPs) which are essential for the stabilization of the structure. Although the octahedral chains of the closely related Bi 2Mn 4O 10 are similar to those of Bi 2M 4O 9, Bi 2 Mn 4O 10 contains dimers of edge-connected, five-fold coordinated pyramids instead of four-fold coordinated tetrahedra. Also the 6s 2 LEPs of Bi 3+ in Bi 2Mn 4O 10 are not stereo-chemically active. Complete and continuous solid solutions exist for Bi 2(Al 1-xFe x) 4O 9 and Bi 2(Ga 1-x Fe x) 4O 9 (x = 0 - 1). Things are more complex in the case of the Bi 2(Fe 1-xMn x) 4O 9+y mixed crystals, where a miscibility gap occurs between x = 0.25 - 0.75. In the Fe-rich mixed crystals most Mn atoms enter the octahedra as Mn 4+, with part of the tetrahedral dimers being replaced by fivefold coordinated polyhedra, whereas in the Mn-rich compound Fe 3+ favorably replaces Mn 3+ in the pyramids. The crystal structure of Bi 2M 4O 9 directly controls its mechanical properties. The stiffnesses of phases are highest parallel to the strongly bonded octahedral chains running parallel to the crystallographic c-axis. Perpendicular to the octahedral chains little anisotropy is observed. The temperature- induced expansion perpendicular to the octahedral chains is probably superimposed by contractions. As a result the c-axis expansion appears as relatively high and does not display its lowest value parallel to c, as could be inferred. Maximally 6% of Bi 3+ is substituted by Sr 2+ in Bi 2Al 4O 9 corresponding to a composition of (Bi 0.94Sr 0.06) 2Al 4O 8.94. Sr 2+ for Bi 3+ substitution is probably associated with formation of vacancies of oxygen atoms bridging the tetrahedral dimers. Hopping of oxygen atoms towards the vacancies should strongly enhance the oxygen conductivity. Actually the conductivity is rather low (σ = 7 . 10 -2 S m -1 at 1073 K, 800 °C). An explanation could be the low thermal stability of Sr-doped Bi 2Al 4O 9, especially in coexistence with liquid Bi 2O 3. Therefore, Bi 2Al 4O 9 single crystals and polycrystalline ceramics both with significant amounts of M2+ doping (M = Ca 2+, Sr 2+) have not been produced yet. Thus the question whether or not M 2+-doped Bi 2M 4O 9 is an oxygen conducting material is still open. © 2012 Carl Hanser Verlag.
    view abstractdoi: 10.3139/146.110716
  • 2012 • 80 Do cement nanotubes exist?
    Manzano, H. and Enyashin, A.N. and Dolado, J.S. and Ayuela, A. and Frenzel, J. and Seifert, G.
    Advanced Materials 24 3239-3245 (2012)
    Using atomistic simulations, this work indicates that cement nanotubes can exist. The chemically compatible nanotubes are constructed from the two main minerals in ordinary Portland cement pastes, namely calcium hydroxide and a calcium silicate hydrate called tobermorite. These results show that such nanotubes are stable and have outstanding mechanical properties, unique characteristics that make them ideally suitable for nanoscale reinforcements of cements. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adma.201103704
  • 2012 • 79 Expected and unexpected plastic behavior at the micron scale: An in situ μlaue tensile study
    Kirchlechner, C. and Imrich, P.J. and Grosinger, W. and Kapp, M.W. and Keckes, J. and Micha, J.S. and Ulrich, O. and Thomas, O. and Labat, S. and Motz, C. and Dehm, G.
    Acta Materialia 60 1252-1258 (2012)
    The study of mechanical properties in micron- and submicron-sized metal crystals raises fundamental questions about the influence of size on different aspects of plasticity. In situ characterization of the microstructure evolution during loading is necessary to understand the physics underlying crystal deformation. In situ μLaue diffraction is able to provide unique statistical information on the evolution of type and density of stored dislocations. Here we show macroscopically expected and unexpected plastic behavior at low strains, observed during in situ μLaue tensile tests on micron-sized, single slip oriented Cu samples. Regardless of the initial behavior, a steady state is reached which qualifies a technical yield criterion at the micron scale. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.10.058
  • 2012 • 78 Frequency response of polymer films made from a precursor colloidal monolayer on a nanomechanical cantilever
    Liu, T. and Pihan, S. and Roth, M. and Retsch, M. and Jonas, U. and Gutmann, J.S. and Koynov, K. and Butt, H.-J. and Berger, R.
    Macromolecules 45 862-871 (2012)
    Nanomechanical cantilevers (NMC) were used for the characterization of the film formation process and the mechanical properties of colloidal monolayers made from polystyrene (PS). Closely packed hexagonal monolayers of colloids with diameters ranging from 400 to 800 nm were prepared at the air-water interface and then transferred in a controlled way on the surface of NMC. The film formation process upon annealing of the monolayer was investigated by measuring the resonance frequency of the NMC (≈12 kHz). Upon heating of non-cross-linked PS colloids, we could identify two transition temperatures. The first transition resulted from the merging of polymer colloids into a film. This transition temperature at 147 ± 3 °C as measured at ≈12 kHz remained constant for subsequent heating cycles. We attributed this transition temperature to the glass transition temperature T g of PS which was confirmed by dynamic mechanical thermal analysis (DMTA) and using the time temperature superposition principle. The second transition temperature (175 ± 3 °C) was associated with the end of the film formation process and was measured only for the first heating cycle. Furthermore, the transition of the colloidal monolayer into a homogeneous film preserved the mass loading on the NMC which allowed determination of the Young's modulus of PS (≈3 GPa) elegantly. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ma202396h
  • 2012 • 77 Friction-stir dissimilar welding of aluminium alloy to high strength steels: Mechanical properties and their relation to microstructure
    Coelho, R.S. and Kostka, A. and dos Santos, J.F. and Kaysser-Pyzalla, A.
    Materials Science and Engineering A 556 175-183 (2012)
    The use of light-weight materials for industrial applications is a driving force for the development of joining techniques. Friction stir welding (FSW) inspired joints of dissimilar materials because it does not involve bulk melting of the basic components. Here, two different grades of high strength steel (HSS), with different microstructures and strengths, were joined to AA6181-T4 Al alloy by FSW. The purpose of this study is to clarify the influence of the distinct HSS base material on the joint efficiency. The joints were produced using the same welding parameter/setup and characterised regarding microstructure and mechanical properties. Both joints could be produced without any defects. Microstructure investigations reveal similar microstructure developments in both joints, although there are differences e.g. in the size and amount of detached steel particles in the aluminium alloy (heat and thermomechanical affected zone). The weld strengths are similar, showing that the joint efficiency depends foremost on the mechanical properties of the heat and the thermomechanical affected zone of the aluminium alloy. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2012.06.076
  • 2012 • 76 High quality extrudates from aluminum chips by new billet compaction and deformation routes
    Misiolek, W.Z. and Haase, M. and Ben Khalifa, N. and Tekkaya, A.E. and Kleiner, M.
    CIRP Annals - Manufacturing Technology 61 239-242 (2012)
    The effects of different billet preparation techniques as well as selection of various deformation routes and their influence on the final mechanical properties in chip extrusion was studied. The AA6060 chips were compacted into billets using various techniques and then extruded through the flat-face, porthole and ECAP dies to create different deformation routes. The microstructures and the mechanical properties of the chip extruded profiles were compared to cast billets extruded through the flat-face die under the same conditions. The proposed technology shows very promising results in terms of energy savings and production of the high quality engineered aluminum profiles. © 2012 CIRP.
    view abstractdoi: 10.1016/j.cirp.2012.03.113
  • 2012 • 75 Impact of heat treatment on the mechanical properties of AISI 304L austenitic stainless steel in high-pressure hydrogen gas
    Weber, S. and Martin, M. and Theisen, W.
    Journal of Materials Science 47 6095-6107 (2012)
    Hydrogen environment embrittlement of metastable austenitic stainless steels is a well-known phenomenon partially related to the formation of straininduced martensite. In the literature, hydrogen environment embrittlement is often discussed on the basis of nominal chemical compositions only and neglects effects of metallurgical production and processing. The aim of this study is to investigate the influence of the d-ferrite volume fraction and grain size on the mechanical properties of a standard grade 1.4307 (AISI 304L) tested in high-pressure hydrogen gas. A negligible influence was found for dferrite volume fractions between 2 and 10 %. This result is explained by the dominating influence of machininginduced a-martensite on the surface of the tensile samples. In contrast, the grain size was found to have a significant effect on hydrogen environment embrittlement. In particular, grain sizes smaller than 50 lm were found to have a higher ductility. The results are discussed with respect to stacking fault energy, formation of strain-induced a-martensite, trapping of hydrogen and microsegregations. The results are of particular interest for the materials selection and development of materials for hydrogen applications. © Springer Science+Business Media, LLC 2012.
    view abstractdoi: 10.1007/s10853-012-6526-8
  • 2012 • 74 Improvement in mechanical properties of jute fibres through mild alkali treatment as demonstrated by utilisation of the Weibull distribution model
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Basak, R.K. and Basu Majumder, S. and Adhikari, B.
    Bioresource Technology 107 222-228 (2012)
    Chemically modified jute fibres are potentially useful as natural reinforcement in composite materials. Jute fibres were treated with 0.25%-1.0% sodium hydroxide (NaOH) solution for 0.5-48. h. The hydrophilicity, surface morphology, crystallinity index, thermal and mechanical characteristics of untreated and alkali treated fibres were studied.The two-parameter Weibull distribution model was applied to deal with the variation in mechanical properties of the natural fibres. Alkali treatment enhanced the tensile strength and elongation at break by 82% and 45%, respectively but decreased the hydrophilicity by 50.5% and the diameter of the fibres by 37%. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.biortech.2011.11.073
  • 2012 • 73 Improving mechanical properties of chip-based aluminum extrudates by integrated extrusion and equal channel angular pressing (iECAP)
    Haase, M. and Ben Khalifa, N. and Tekkaya, A.E. and Misiolek, W.Z.
    Materials Science and Engineering A 539 194-204 (2012)
    In order to improve the mechanical properties of profiles extruded from aluminum chips, a four turn equal channel angular pressing tool was integrated into an extrusion die (iECAP die). AA6060 aluminum alloy turning chips were cold pre-compacted to chip-based billets and hot extruded through the iECAP die on a conventional forward extrusion press. Mechanical properties and microstructure of the chip-based billets extruded through the iECAP die were investigated and compared to those extruded through a conventional flat-face die and a porthole die. To evaluate the performance of the iECAP processed chip-based profiles, conventional cast billets were extruded through the flat-face die as a reference material. To investigate the influence of temperature on mechanical properties and microstructure of chip-based profiles, the extrusion was performed at 450. °C and 550. °C.Tensile tests revealed superior mechanical properties of the chip-based billets extruded through the iECAP die in comparison to chip-based billets extruded through the flat-face and the porthole die as well as to cast billets extruded through the flat-face die. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2012.01.081
  • 2012 • 72 In-Situ TEM Straining Experiments: Recent Progress in Stages and Small-Scale Mechanics
    Dehm, G. and Legros, M. and Kiener, D.
    In-Situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science 227-254 (2012)
    doi: 10.1002/9783527652167.ch10
  • 2012 • 71 Influence of laves phase precipitation and coarsening on high-temperature strength of ferritic stainless steels
    Nabiran, N. and Weber, S. and Theisen, W.
    Steel Research International 83 758-765 (2012)
    Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the high-temperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic FeCr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms - precipitation-hardening, grain refinement, and solid-solution strengthening - were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900°C for up to 1440h. It is shown that, the solid solution effect of Mo and W increases the high-temperature strength about 40%, also after long-term annealing. The contribution of the Laves phase precipitates on the high-temperature strength is rather small due to their rapid coarsening. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201200016
  • 2012 • 70 Influence of surface characteristics on fatigue behaviour of laser sintered plastics
    Blattmeier, M. and Witt, G. and Wortberg, J. and Eggert, J. and Toepker, J.
    Rapid Prototyping Journal 18 161-171 (2012)
    Purpose - The purpose of this paper is to provide macromechanical insight into the fatigue behaviour of laser sintered parts and to understand the influence of the laser sintered surface structure on this behaviour. Design/methodology/approach - A background on the technological maturity of manufacturing processes and the demand for structural and aesthetic properties of laser sintered plastic products is given. As the contribution of surface structure on part quality was the focus, laser sintered specimens with and without surface finishes, as well as injection moulded specimens were used. The latter simply served as a comparison and was not intended to qualify injection moulding. The study comprises the determination of short-term tensile properties, the load increase method for investigating fracture and deformation behaviours, and fatigue crack propagation analysis. Findings - According to the test results, the contribution of laser sintered surface structures to relevant mechanical properties can be neglected. Under dynamic loading conditions, laser sintered specimens achieved a longer lifetime but showed less deformation capabilities in contrast to injection moulded specimens. In general, laser sintered specimens presented considerable resistance to crack initiation and propagation. Research limitations/implications - Because of the long-term approach of the research, the number of tests conducted per lot was limited. Thus, the effects of different process settings and the reproducibility could not be fully analysed. Practical implications - The studied fatigue behaviour of laser sintered specimens has implications for the functional testing of parts or components, for the product and process design as well as for the general compatibility of laser sintering as a manufacturing technology of end-customer products. Originality/value - The value of this paper lies in the better understanding of deformation and fracture behaviours of laser sintered polymers. © 2012 Emerald Group Publishing Limited.
    view abstractdoi: 10.1108/13552541211212140
  • 2012 • 69 Mechanical properties of (20-30)Mn12Cr(0.56-0.7)CN corrosion resistant austenitic TWIP steels
    Mújica Roncery, L. and Weber, S. and Theisen, W.
    Steel Research International 83 307-314 (2012)
    New developed (20-30)Mn12Cr(0.56-0.7)CN TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300 J ISO-V), and impact wear resistance equivalent to that of Hadfield steel. In addition, they exhibit corrosion resistance by passivation in aqueous acidic media. Microstructure examination by SEM and EBSD at different degrees of deformation reveals that twinning takes place and is responsible for the high cold-work hardening of the steels. Stacking fault energy measurement of three different developed steels locates them in the range of 30-40mJm -2, being highly dependent on the N and Mn contents. Measurements carried out with digital image correlation indicate that at room temperature dynamic strain aging or Portevin-LeChatelier effect takes place. Measurements of impact toughness indicate that the steels have ductile to brittle transition at cryogenic temperatures as a consequence of the effect of nitrogen on the deformation mechanisms, resulting in a quasi-cleavage fracture along the {111} planes at -196°C. New Fe-Cr-Mn-C-N TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300J ISO-V), high impact wear resistance, and corrosion resistance by passivation in aqueous acidic media. This work examines the microstructure, stacking fault energy, and dynamic strain aging to understand the tensile behavior and toughness of these materials. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201100316
  • 2012 • 68 Microtomographic analysis of splat formation and layer build-up of a thermally sprayed coating
    Tillmann, W. and Nellesen, J. and Abdulgader, M.
    Journal of Thermal Spray Technology 21 514-521 (2012)
    Thermal spraying is a material processing technique, which is based on the combination of thermal and kinetic energy. The used feedstock is melted in a hot flame. The melt is atomized and accelerated by means of atomization or process gases. As the formed particles hit a pre-treated substrate they are rapidly solidified and consolidate to form splats. The splats pile one-on-top-of-other forming lamellas creating the final coating. In the work presented here a combination of cored wire (WC as filling powder) and massive wire (copper) were simultaneously sprayed using the twin wire arc spraying process. 3D micro tomography was used in order to gain knowledge about splat formation and layer build-up. Due to the high attenuation coefficient of tungsten in comparison with copper and carbon, tungsten-rich particles and splats can easily be spotted in the tomogram of the coating layer. It turns out that besides irregular formed flat splats also ball-shaped particles exist in the coating layer which suggests that the spherical particles impacted on the substrate in an un-molten state. By 3D data processing tungsten-rich particles were visualized to analyze their spatial distributions and to quantify their geometric parameters. This work aims at contributing to the understanding of spraying processes. © ASM International.
    view abstractdoi: 10.1007/s11666-012-9737-2
  • 2012 • 67 Modification of pineapple leaf fibers and graft copolymerization of acrylonitrile onto modified fibers
    Maniruzzaman, M. and Rahman, M.A. and Gafur, M.A. and Fabritius, H. and Raabe, D.
    Journal of Composite Materials 46 79-90 (2012)
    Raw pineapple leaf fibers (PALFs) were chemically modified by scouring, NaOH treatment, and bleaching (NaClO2). The graft copolymerization of synthetic acrylonitrile monomer onto bleached PALFs was carried out in aqueous medium using potassium persulfate (K2S2O8/FeSO4) as a redox initiator. The maximum grafting level at optimum conditions, namely, monomer concentration, initiator concentration, catalyst concentration, reaction time, and temperature have been determined. The main objective of this study is to decrease the amorphous region of lignocellulose in PALFs and improve its hydrophobic nature by incorporation of synthetic polymer of polyacrylonitrile and mechanical properties. The modified and grafted fibers were characterized by Fourier transform infrared spectroscopy, scanning electron microscope, thermogravimetric analysis, and X-ray diffraction study techniques. The moisture content and tensile strength properties were also evaluated for their environmental and mechanical performances. © The Author(s) 2011.
    view abstractdoi: 10.1177/0021998311410486
  • 2012 • 66 New insights into hard phases of CoCrMo metal-on-metal hip replacements
    Liao, Y. and Pourzal, R. and Stemmer, P. and Wimmer, M.A. and Jacobs, J.J. and Fischer, A. and Marks, L.D.
    Journal of the Mechanical Behavior of Biomedical Materials 12 39-49 (2012)
    The microstructural and mechanical properties of the hard phases in CoCrMo prosthetic alloys in both cast and wrought conditions were examined using transmission electron microscopy and nanoindentation. Besides the known carbides of M23C6-type (M=Cr, Mo, Co) and M6C-type which are formed by either eutectic solidification or precipitation, a new mixed-phase hard constituent has been found in the cast alloys, which is composed of ~100nm fine grains. The nanosized grains were identified to be mostly of M23C6 type using nano-beam precession electron diffraction, and the chemical composition varied from grain to grain being either Cr- or Co-rich. In contrast, the carbides within the wrought alloy having the same M23C6 structure were homogeneous, which can be attributed to the repeated heating and deformation steps. Nanoindentation measurements showed that the hardness of the hard phase mixture in the cast specimen was ~15.7GPa, while the M23C6 carbides in the wrought alloy were twice as hard (~30.7GPa). The origin of the nanostructured hard phase mixture was found to be related to slow cooling during casting. Mixed hard phases were produced at a cooling rate of 0.2°C/s, whereas single phase carbides were formed at a cooling rate of 50°C/s. This is consistent with sluggish kinetics and rationalizes different and partly conflicting microstructural results in the literature, and could be a source of variations in the performance of prosthetic devices in-vivo. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2012.03.013
  • 2012 • 65 Novel temperature dependent tensile test of freestanding copper thin film structures
    Smolka, M. and Motz, C. and Detzel, T. and Robl, W. and Griesser, T. and Wimmer, A. and Dehm, G.
    Review of Scientific Instruments 83 (2012)
    The temperature dependent mechanical properties of the metallization of electronic power devices are studied in tensile tests on micron-sized freestanding copper beams at temperatures up to 400 °C. The experiments are performed in situ in a scanning electron microscope. This allows studying the micromechanical processes during the deformation and failure of the sample at different temperatures. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4725529
  • 2012 • 64 Polycrystal model of the mechanical behavior of a Mo-TiC 30 vol.% metal-ceramic composite using a three-dimensional microstructure map obtained by dual beam focused ion beam scanning electron microscopy
    Cédat, D. and Fandeur, O. and Rey, C. and Raabe, D.
    Acta Materialia 60 1623-1632 (2012)
    The mechanical behavior of a Mo-TiC 30 vol.% ceramic-metal composite was investigated over a wide temperature range (25-700 °C). High-energy X-ray tomography was used to reveal percolation of the hard titanium carbide phase through the composite. Using a polycrystal approach for a two-phase material, finite-element simulations were performed on a real three-dimensional (3-D) aggregate of the material. The 3-D microstructure, used as the starting configuration for the predictions, was obtained by serial sectioning in a dual beam focused ion beam scanning electron microscope coupled to an electron backscattered diffraction system. The 3-D aggregate consists of a molybdenum matrix and a percolating TiC skeleton. As for most body-centered cubic (bcc) metals, the molybdenum matrix phase is characterized by a change in plasticity mechanism with temperature. We used a polycrystal model for bcc materials which was extended to two phases (TiC and Mo). The model parameters of the matrix were determined from experiments on pure molydenum. For all temperatures investigated the TiC particles were considered to be brittle. Gradual damage to the TiC particles was treated, based on an accumulative failure law that is approximated by evolution of the apparent particle elastic stiffness. The model enabled us to determine the evolution of the local mechanical fields with deformation and temperature. We showed that a 3-D aggregate representing the actual microstructure of the composite is required to understand the local and global mechanical properties of the composite studied. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.11.055
  • 2012 • 63 Rapid alloy prototyping: Compositional and thermo-mechanical high throughput bulk combinatorial design of structural materials based on the example of 30Mn-1.2C-xAl triplex steels
    Springer, H. and Raabe, D.
    Acta Materialia 60 4950-4959 (2012)
    We introduce a new experimental approach to the compositional and thermo-mechanical design and rapid maturation of bulk structural materials. This method, termed rapid alloy prototyping (RAP), is based on semi-continuous high throughput bulk casting, rolling, heat treatment and sample preparation techniques. 45 Material conditions, i.e. 5 alloys with systematically varied compositions, each modified by 9 different ageing treatments, were produced and investigated within 35 h. This accelerated screening of the tensile, hardness and microstructural properties as a function of chemical and thermo-mechanical parameters allows the highly efficient and knowledge-based design of bulk structural alloys. The efficiency of the approach was demonstrated on a group of Fe-30Mn-1.2C-xAl steels which exhibit a wide spectrum of structural and mechanical characteristics, depending on the respective Al concentration. High amounts of Al addition (>8 wt.%) resulted in pronounced strengthening, while low concentrations (<2 wt.%) led to embrittlement of the material during ageing. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2012.05.017
  • 2012 • 62 Robust mechanical performance of chromium-coated polyethylene terephthalate over a broad range of conditions
    Cordill, M.J. and Taylor, A.A. and Berger, J. and Schmidegg, K. and Dehm, G.
    Philosophical Magazine 92 3346-3362 (2012)
    Mechanical properties of metal films on polymer substrates are normally studied in terms of the fracture and adhesion of the film, while the properties of the polymer substrate and testing conditions are overlooked. Substrate orientation and thickness, as well as strain rate and temperature effects, are examined using Cr films deposited onto polyethylene terephthalate substrates. A faster strain rate affects only the initial fracture strain of the Cr film and not the crack and buckle spacings in the high strain condition. The substrate orientation slightly changes the average crack spacing while the substrate thickness has little effect on the cracking and buckling behaviour. Straining experiments at high temperature increased the average crack spacing and led to a change in buckling mode. The lack of sizeable changes in the mechanical behaviour over the large range of testing procedures leads to a resilient material system for flexible applications. © 2012 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2012.700418
  • 2012 • 61 Synthesis and mechanical properties of organic-inorganic hybrid materials from lignin and polysiloxanes
    Lippach, A.K.W. and Krämer, R. and Hansen, M.R. and Roos, S. and Stöwe, K. and Stommel, M. and Wenz, G. and Maier, W.F.
    ChemSusChem 5 1778-1786 (2012)
    The preparation of silica-containing organic-inorganic hybrid materials composed of kraft lignin, alkoxysilanes, and organic linkers was investigated. 3-Glycidyloxypropyltrimethoxysilane, 3-(triethoxysilyl)propylisocyanate (IPTES), and bis(trimethoxysilyl)hexane were selected as the most promising linkers. The best materials obtained showed improved mechanical and thermal properties compared with lignin itself. The reaction of the hydroxyl groups with IPTES and the sol-gel reaction between the organic linker molecules were studied by attenuated total reflectance FTIR and solid-state 29Si magic-angle spinning NMR spectroscopy. The homogeneous composition was demonstrated by electron microscopy and energy-dispersive X-ray spectroscopy mapping. The mechanical properties were investigated by microindentation and dynamic mechanical thermal analysis. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201200095
  • 2012 • 60 The mechanical shear behavior of Al single crystals and grain boundaries
    Pang, X. and Ahmed, N. and Janisch, R. and Hartmaier, A.
    Journal of Applied Physics 112 (2012)
    To investigate the mechanical shear properties of interfaces in metals, we have determined the γ-surfaces of different special tilt and twist grain boundaries in aluminum by means of ab initio calculations. From the γ-surfaces, we obtained minimum energy paths and barriers, as well as the theoretical shear strength. For the [110] tilt grain boundaries, there is a pronounced easy-sliding direction along the tilt axis. The theoretical shear strength scales with the height of the slip barrier and exhibits a relation with the misorientation angle: the closer the angle to 90°, the higher the shear stress. There is no simple relationship with the periodicity of the grain boundary, i.e., the Σ value or the grain boundary energy. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4736525
  • 2012 • 59 Tuning the size and the optical properties of ZnO mesocrystals synthesized under solvothermal conditions
    Distaso, M. and Segets, D. and Wernet, R. and Taylor, R.K. and Peukert, W.
    Nanoscale 4 864-873 (2012)
    Mesocrystals are a promising class of nanomaterials enabling new optical and mechanical properties due to their three dimensional organization of primary crystallites sharing a common crystallographic orientation. In the present article, the influence of process parameters such as temperature profile and stirring on the primary and secondary size of ZnO mesocrystals synthesized under solvothermal conditions has been investigated. In general, small but noticeable lattice strain is introduced to the particles during the synthesis process. Additionally, with increasing mass transport the fusion of primary crystallites due to coarsening is enhanced. A closer analysis revealed an influence of the polymer chain length on the final particle structure throughout different hierarchical levels. Based on our findings a reaction mechanism with nucleation and growth taking place embedded in poly-N-vinylpyrrolidone (PVP), followed by a polymer-mediated step of oriented aggregation and subsequent coarsening is proposed. In consequence, the careful control throughout all hierarchical levels of particle synthesis allows fine-tuning of the optical properties of ZnO mesocrystals which show a high UV absorption and minimal scattering in the visible region. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1nr11226k
  • 2011 • 58 A preliminary study of bending stiffness alteration in shape changing nitinol plates for fracture fixation
    Olender, G. and Pfeifer, R. and Müller, C.W. and Gösling, T. and Barcikowski, S. and Hurschler, C.
    Annals of Biomedical Engineering 39 1546-1554 (2011)
    Nitinol is a promising biomaterial based on its remarkable shape changing capacity, biocompatibility, and resilient mechanical properties. Until now, very limited applications have been tested for the use of Nitinol plates for fracture fixation in orthopaedics. Newly designed fracture-fixation plates are tested by four-point bending to examine a change in equivalent bending stiffness before and after shape transformation. The goal of stiffness alterable bone plates is to optimize the healing process during osteosynthesis in situ that is customized in time of onset, percent change as well as being performed non-invasively for the patient. The equivalent bending stiffness in plates of varying thicknesses changed before and after shape transformation in the range of 24-73% (p values < 0.05 for all tests). Tests on a Nitinol plate of 3.0 mm increased in stiffness from 0.81 to 0.98 Nm2 (corresponding standard deviation 0.08 and 0.05) and shared a good correlation to results from numerical calculation. The stiffness of the tested fracture-fixation plates can be altered in a consistent matter that would be predicted by determining the change of the cross-sectional area moment of inertia. © 2011 Biomedical Engineering Society.
    view abstractdoi: 10.1007/s10439-011-0257-x
  • 2011 • 57 Analysis of the mechanical properties of an arc sprayed WC-FeCSiMn coating: Compression, bending, and tension behavior
    Tillmann, W. and Nebel, J.
    Journal of Thermal Spray Technology 20 317-327 (2011)
    This paper is concerned with the elastic and plastic forming behavior of arc sprayed WC-FeCSiMn coatings. The mechanical properties were investigated by indentation, bending, and tensile tests. These were performed on coated mild steel substrates as well as spark eroded and ground freestanding coatings with different geometries. The results of the indentation, bending, and tensile tests were evaluated concerning the coating microstructure, element, and pore distribution, as well as the local microhardness. The critical role of pores and inhomogeneities within the sprayed coating was examined in detail. Micro- and macrocracking were investigated by scanning electron microscopy after the indentation and tensile tests. It was figured out that the WC-FeCSiMn coating featured a distinctive brittle behavior. During the bending and tension tests, brittle forced fracture of the layer appeared almost without plastic deformations. A significant difference was determined between the compression and tensile performance of the coating. For instance, the Young's modulus for compression strains was measured to be approximately 60% higher than the tension case. © 2010 ASM International.
    view abstractdoi: 10.1007/s11666-010-9567-z
  • 2011 • 56 Analysis of the mechanical properties of an arc-sprayed WC-FeCSiMn coating: Nanoindentation and simulation
    Tillmann, W. and Klusemann, B. and Nebel, J. and Svendsen, B.
    Journal of Thermal Spray Technology 20 328-335 (2011)
    The characterization of thermal-sprayed coatings is often limited to microstructural analysis to evaluate the coatings morphology. Indentation is commonly used to determine the mechanical properties of different kinds of engineering materials. However, due to the complex structure of thermal-sprayed coatings, few results have been obtained so far. In this article, experimental nanoindentation tests and simulation results are compared. The experimental indentation tests show scattering in the force-deformation data due to the complex structure of the arc-sprayed coating which is investigated by means of an indentation test simulation. Based on the results for single constituent parts of the coating, the Young's modulus as well as further mechanical properties are identified. A general procedure is presented to predict the effective mechanical properties based on the microstructure, porosity, chemical composition, and properties of the coating after thermal spraying. © 2010 ASM International.
    view abstractdoi: 10.1007/s11666-010-9550-8
  • 2011 • 55 Analysis of the plastic anisotropy and pre-yielding of (γ/ α2)-phase titanium aluminide microstructures by crystal plasticity simulation
    Zambaldi, C. and Roters, F. and Raabe, D.
    Intermetallics 19 820-827 (2011)
    The plastic deformation of lamellar microstructures composed of the two phases γ-TiAl and α2-Ti3Al is highly orientation dependent. In this paper we present a homogenized model that takes into account the micromechanical effect of the plate-like morphologies that are often observed in two-phase titanium aluminide alloys. The model is based on crystal elasto-viscoplasticity and 18 deformation systems were implemented that have been identified to govern the plastic flow of the lamellar microstructures. The model is validated against experiments on polysynthetically twinned (PST) crystals and shows good agreement with the data. On a larger length scale, the model is applied to a 64-grain aggregate to investigate the mechanical response of two different kinds of microstructures. Different magnitudes of the kinematic constraints exerted by the densely spaced and highly aligned interfaces are shown to affect the macroscopic flow behavior of the microstructures. The phenomenon of pronounced microplasticity of fully lamellar material as well as the stress variation inside two-phase microstructures are studied quantitatively. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2011.01.012
  • 2011 • 54 Anisotropic mechanical behavior of ultrafine eutectic TiFe cast under non-equilibrium conditions
    Schlieter, A. and Kühn, U. and Eckert, J. and Löser, W. and Gemming, T. and Friák, M. and Neugebauer, J.
    Intermetallics 19 327-335 (2011)
    The effect of solidification conditions on microstructural and mechanical properties of eutectic TiFe alloy cast under different conditions was examined. Samples exhibit different ultrafine eutectic structures (β-Ti(Fe) solid solution + TiFe). Different cooling conditions lead to the evolution of ultrafine eutectic oval-shaped colonies or elongated lamellar colonies with preferred orientation. Isotropic as well as anisotropic mechanical properties were obtained. Alloys exhibit compressive strengths between 2200 and 2700 MPa and plastic strains between 7 and 19 pct. in compression. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.10.012
  • 2011 • 53 Deformation and fracture mechanisms in fine- and ultrafine-grained ferrite/martensite dual-phase steels and the effect of aging
    Calcagnotto, M. and Adachi, Y. and Ponge, D. and Raabe, D.
    Acta Materialia 59 658-670 (2011)
    Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 μm) but with the same martensite content (∼30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2 μm) to aging at 170 °C was investigated. The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elongation and total elongation are less affected. This can be partly explained by the increase in the initial strain-hardening rate. Moreover, the stress/strain partitioning characteristics between ferrite and martensite change due to grain refinement, leading to enhanced martensite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of the improved fracture toughness of martensite. The aging treatment leads to a strong increase in yield strength and improves the uniform and total elongation. These effects are attributed to dislocation locking due to the formation of Cottrell atmospheres and relaxation of internal stresses, as well as to the reduction in the interstitial carbon content in ferrite and tempering effects in martensite. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.10.002
  • 2011 • 52 Determining the elasticity of materials employing quantum-mechanical approaches from the electronic ground state to the limits of materials stability
    Friák, M. and Hickel, T. and Körmann, F. and Udyansky, A. and Dick, A. and Von Pezold, J. and Ma, D. and Kim, O. and Counts, W.A. and Šob, M. and Gebhardt, T. and Music, D. and Schneider, J. and Raabe, D. and Neugebauer, J.
    Steel Research International 82 86-100 (2011)
    Quantum-mechanical (so-called ab initio) calculations have achieved considerable reliability in predicting physical and chemical properties and phenomena. Due to their reliability they are becoming increasingly useful when designing new alloys or revealing the origin of phenomena in existing materials, also because these calculations are able to accurately predict basic material properties without experimental input. Due to the universal validity of fundamental quantum mechanics, not only ground-state properties, but also materials responses to external parameters can reliably be determined. The focus of the present paper is on ab initio approaches to the elasticity of materials. First, the methodology to determine single-crystalline elastic constants and polycrystalline moduli of ordered compounds as well as disordered alloys is introduced. In a second part, the methodology is applied on α-Fe, with a main focus on (i) investigating the influence of magnetism on its elasticity and phase stability and (ii) simulating extreme loading conditions that go up to the theoretical tensile strength limits and beyond. Copyright © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000264
  • 2011 • 51 Development and characterization of novel corrosion-resistant TWIP steels
    Mujica, L. and Weber, S. and Hunold, G. and Theisen, W.
    Steel Research International 82 26-31 (2011)
    Austenitic steels exhibiting twinning induced plasticity (TWIP) are materials with exceptional mechanical properties. In this work, the development of new grades of TWIP steels exhibiting corrosion resistance is presented. The alloy development was supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium to increase nitrogen solubility and provide corrosion resistance, while carbon and nitrogen provide mechanical strength. Diffusion calculations were used in order to predict the extent of micro segregations and additionally to evaluate the effect of diffusion annealing treatments. The material was cast in a laboratory scale with a nominal composition of Fe-20Mn-12Cr-0.25C-0.3N. Diffusion annealing was followed by hot rolling and solution annealing resulting in a fully austenitic microstructure. Tensile tests at room temperature were performed, exhibiting yield strengths of 430 MPa and elongation to fracture of 93%. In addition, not only the mechanical properties but also the weldability was studied, focussing on the characterization of the microstructure of bead on plate welds obtained by laser and TIG welding. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000219
  • 2011 • 50 Effect of large mechanical stress on the magnetic properties of embedded Fe nanoparticles
    Saranu, S. and Selve, S. and Kaiser, U. and Han, L. and Wiedwald, U. and Ziemann, P. and Herr, U.
    Beilstein Journal of Nanotechnology 2 268-275 (2011)
    Magnetic nanoparticles are promising candidates for next generation high density magnetic data storage devices. Data storage requires precise control of the magnetic properties of materials, in which the magnetic anisotropy plays a dominant role. Since the total magneto-crystalline anisotropy energy scales with the particle volume, the storage density in media composed of individual nanoparticles is limited by the onset of superparamagnetism. One solution to overcome this limitation is the use of materials with extremely large magneto-crystalline anisotropy. In this article, we follow an alternative approach by using magneto-elastic interactions to tailor the total effective magnetic anisotropy of the nanoparticles. By applying large biaxial stress to nanoparticles embedded in a non-magnetic film, it is demonstrated that a significant modification of the magnetic properties can be achieved. The stress is applied to the nanoparticles through expansion of the substrate during hydrogen loading. Experimental evidence for stress induced magnetic effects is presented based on temperature-dependent magnetization curves of superparamagnetic Fe particles. The results show the potential of the approach for adjusting the magnetic properties of nanoparticles, which is essential for application in future data storage media. © 2011 Saranu et al.
    view abstractdoi: 10.3762/bjnano.2.31
  • 2011 • 49 Experimental and numerical study on geometrically necessary dislocations and non-homogeneous mechanical properties of the ferrite phase in dual phase steels
    Kadkhodapour, J. and Schmauder, S. and Raabe, D. and Ziaei-Rad, S. and Weber, U. and Calcagnotto, M.
    Acta Materialia 59 4387-4394 (2011)
    The microstructure of dual phase steels can be compared with a composite composed of a matrix of ferrite reinforced by small islands of martensite. This assumption has been used in several attempts to model the mechanical properties of dual phase steels. However, recent measurements show that the properties of the ferrite phase change with distance from the martensite grains. These measurements showed that the grains of the ferrite phase are harder in the vicinity of martensite grains. As a consequence of this local hardening effect, the ferrite phase has to be considered as an inhomogeneous matrix in modeling dual phase steels. This experiment inspired the idea that local hardening is caused by geometrically necessary dislocations. The idea is investigated experimentally and numerically in the present analysis, which for the first time leads to good agreement with experimental observations of the mechanical stress-strain behavior. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.062
  • 2011 • 48 Flow drilling and thread forming of continuously reinforced aluminium extrusionsg
    Engbert, T. and Heymann, T. and Biermann, D. and Zabel, A.
    Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 225 398-407 (2011)
    Light-metal extrusions are widely used as frame-structure elements. Joining these profiles via screw coupling is a challenging task due to the small wall thickness of the extrusions and the missing accessibility within a frame structure. The combination of flow drilling and thread forming offers a possibility to cope with this task. These processing techniques allow the manufacture of stable threads in thin-walled structures with the profile accessible from one side only. Nowadays, aluminium profiles can be continuously reinforced through composite extrusion. Mechanical properties, like increased tensile strength compared with homogeneous profiles, make reinforced profiles preferable for applications such as safety-relevant components. However, the reinforcement can seriously affect machining processes as well as the machining results. Therefore, the flow-drilling operation, the thread-forming operation, and the process results have been analysed in detail with a new, difficult-to-machine material combination, namely steel-wire-reinforced aluminium extrusions. The crucial factor when machining lightweight extrusions are the forces acting perpendicular to the thin walls, so the influence of the reinforcement and the processing parameters on the feed force during flow drilling is presented. To examine the effect of the reinforcement on the thread-forming result and to quantify the benefit of flow drilling, the threads are stressed with a defined tensile load until failure.
    view abstractdoi: 10.1243/2041297510394104
  • 2011 • 47 From nanoparticles to nanocrystalline bulk: Percolation effects in field assisted sintering of silicon nanoparticles
    Schwesig, D. and Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Wiggers, H. and Schmechel, R. and Wolf, D.E.
    Nanotechnology 22 (2011)
    Nanocrystalline bulk materials are desirable for many applications as they combine mechanical strength and specific electronic transport properties. Our bottom-up approach starts with tailored nanoparticles. Compaction and thermal treatment are crucial, but usually the final stage sintering is accompanied by rapid grain growth which spoils nanocrystallinity. For electrically conducting nanoparticles, field activated sintering techniques overcome this problem. Small grain sizes have been maintained in spite of consolidation. Nevertheless, the underlying principles, which are of high practical importance, have not been fully elucidated yet. In this combined experimental and theoretical work, we show how the developing microstructure during sintering correlates with the percolation paths of the current through the powder using highly doped silicon nanoparticles as a model system. It is possible to achieve a nanocrystalline bulk material and a homogeneous microstructure. For this, not only the generation of current paths due to compaction, but also the disintegration due to Joule heating is required. The observed density fluctuations on the micrometer scale are attributed to the heat profile of the simulated powder networks. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/13/135601
  • 2011 • 46 Improvement of NiTi shape memory actuator performance through ultra-fine grained and nanocrystalline microstructures
    Frenzel, J. and Burow, J.A. and Payton, E.J. and Rezanka, S. and Eggeler, G.
    Advanced Engineering Materials 13 256-268 (2011)
    Ultra-fine grain sizes have been shown to enhance some key mechanical and functional properties of engineering materials, including shape memory alloys. While the effect of ultra-fine and nanocrystalline grain sizes on pseudoelastic shape memory materials is well-appreciated in medical device engineering, the effect of such microstructures on actuators has not been sufficiently characterized. In the present work, it is demonstrated that NiTi spring actuators with ultra-fine grained microstructures can be obtained by conventional wire drawing in combination with heat treatments and that the final grain size can be controlled by varying the final annealing temperature. Annealing at 400deg;C for 600s allows for the evolution of microstructures with median grain sizes of about 34nm, while annealing at 600deg;C for the same length of time results in median grain sizes of about 5 μm. It is observed that the grain size strongly affects the elementary processes of the martensitic phase transformation. Small austenite grain sizes inhibit twinning accommodation of transformation strains, such that a higher driving force is required to nucleate martensite. This increase in the martensite nucleation barrier decreases the martensite transformation temperatures such that only partial transformation to martensite is possible upon cooling to room temperature. The incomplete martensitic transformation reduces the exploitable actuator stroke; however, a reduction in grain size is shown to improve the functional stability of the material during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity. NiTi spring actuators with ultra-fine grained and nanocrystalline microstructures can be obtained by conventional wire drawing in combination with heat treatments. Grain size refinements into this range improve the functional stability during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity. The improvement in functional stability comes at the cost of exploitable actuator stroke, however, because very fine grain sizes result in only a partial transformation to martensite upon cooling to room temperature. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000285
  • 2011 • 45 Influence of intermetallic phases and Kirkendall-porosity on the mechanical properties of joints between steel and aluminium alloys
    Springer, H. and Kostka, A. and dos Santos, J.F. and Raabe, D.
    Materials Science and Engineering A 528 4630-4642 (2011)
    The formation of intermetallic reaction layers and their influence on mechanical properties was investigated in friction stir welded joints between a low C steel and both pure Al (99.5wt.%) and Al-5wt.% Si. Characterisation of the steel/Al interface, tensile tests and fractography analysis were performed on samples in the as-welded state and after annealing in the range of 200-600°C for 9-64min. Annealing was performed to obtain reaction layers of distinct thickness and composition. For both Al alloys, the reaction layers grew with parabolic kinetics with the η phase (Al5Fe2) as the dominant component after annealing at 450°C and above. In joints with pure Al, the tensile strength is governed by the formation of Kirkendall-porosity at the reaction layer/Al interface. The tensile strength of joints with Al-5wt.% Si is controlled by the thickness of the η phase (Al5Fe2) layer. The pre-deformation of the base materials, induced by the friction stir welding procedure, was found to have a pronounced effect on the composition and growth kinetics of the reaction layers. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.02.057
  • 2011 • 44 Influence of intermetallic precipitates and heat treatment on the mechanical properties of high-temperature corrosion resistant ferritic steels
    Nabiran, N. and Weber, S. and Theisen, W.
    Procedia Engineering 10 1651-1656 (2011)
    High-temperature corrosion resistant ferritic steels are commonly used in heat exchangers for auxiliary power units (APU), automotive exhaust systems and structural parts of solid oxide fuel cells (SOFC) due to their excellent thermal fatigue resistance. As the process temperatures in these applications are primarily limited by the materials high temperature strength, the main focus of this work is on the improvement of this property by adjustments in the material design. Generally, two mechanisms were used to increase the high temperature strength, solid solution strengthening and precipitation hardening. Due to their large atomic radii and high solubility in α-Fe, W and Mo were used for solid solution strengthening. Furthermore, the content of niobium, which is well known to form Laves phase precipitates, was raised. This led to a higher content of Laves phase precipitates compared to the reference material. The analyses concentrated on the effect of the Laves phase. Strength at elevated temperature was investigated in compression tests at 900°C with respect to the annealing time which was varied between 1h and 1440 h. © 2011 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2011.04.276
  • 2011 • 43 Investigation of forming strategies to set up mechanical properties of parts made by incremental sheet bulk rolling
    Plugge, B. and Schunck, S. and Kwiatkowski, L. and Brosius, A. and Tekkaya, A.E.
    AIP Conference Proceedings 1353 195-200 (2011)
    Load-adapted parts with an increasing number of functions become more and more interesting in order to reduce the weight of all kinds of mechanical constructions. Such parts require varying mechanical properties and towards they are cost-expensive. One approach to reduce the costs is the application of cheap semi-finished parts. To process such parts, especially in thickness direction, bulk-forming operations are requested. This leads to high forming forces. A feasible approach to reduce the forces is the application of incremental forming techniques. In this paper an incremental rolling process is presented. The sequential order of forming operations during incremental rolling allows an individual adjustment of mechanical and the geometrical properties. In the presented study a sheet with a thickness of 2 mm made of mild steel is formed using a roller ball with a diameter of 13 mm. Main objective of the investigation is to manufacture parts with equal values for thickness but different values of the local surface hardness. The investigation is supported by Finite-Element-Analysis (FEA) to determine the distribution of the strains over the part's thickness. The results show that it is possible to manufacture parts with the same thickness and different surface hardness by applying different forming strategies. Keeping the entire process parameters constant but rolling the part alternating at both surfaces will result in a higher hardness on the surface in comparison to a one-sided rolling. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3589514
  • 2011 • 42 Machines and tools for sheet-bulk metal forming
    Merklein, M. and Tekkaya, A.E. and Brosius, A. and Opel, S. and Kwiatkowski, L. and Plugge, B. and Schunck, S.
    Key Engineering Materials 473 91-98 (2011)
    The demand on closely-tolerated and complex functional components in the automotive sector, like e.g. synchronizer rings, leads to the development of a new process-class named "sheet-bulk metal forming". Within this technology bulk metal forming operations are applied on sheet metals. In the following two novel approaches considering machines and tools for sheet-bulk metal forming are presented. The first approach aims on a technology based on rolling, which is suitable for mass production. The second one is an incremental forming solution for low batch production. Both machine concepts allow the application of different forming strategies to manufacture individual tailored semi-finished products in term of a pre-distribution of material. These products feature variable sheet thicknesses and mechanical properties, which can be adapted to their case of application. Depending on the individual batch size, the blanks can be finished to functional parts by subsequent forming processes like deep drawing and upsetting, extrusion or incremental forming. In this paper the case of an incremental tooth-forming is mainly considered. Forming sequences and resulting loads are modeled and calculated by finite elements simulations for all discussed processes to serve as a basis for the design and dimensioning of the machine components and forming tools. © (2011) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2011 • 41 Machining of β-titanium-alloy Ti-10V-2Fe-3Al under cryogenic conditions: Cooling with carbon dioxide snow
    MacHai, C. and Biermann, D.
    Journal of Materials Processing Technology 211 1175-1183 (2011)
    Titanium alloys are widely used in applications that demand a good combination of high strength, good corrosion resistance and low mass. Beta-Titanium alloys offer the highest specific strength among titaniumbased materials. The mechanical properties lead to challenges in machining operations such as high process temperatures, high specific mechanical loads and rapidly increasing tool wear. The high chemical reactivity of titanium leads to rapidly developing flank and notch wear limiting cutting speeds and tool life. Applying industrial gases instead of conventional cooling and lubrication fluids promises increased productivity. In this work, the effectiveness of carbon dioxide snow (CO2) as a coolant for turning Ti-10V-2Fe-3Al is analyzed. The carbon dioxide is provided in a pressurized gas bottle and is fed to the tool tip through holes in the tool holders clamping jaw. Compared to flood emulsion cooling the flank wear was uniform spreaded and tool life was increased by a factor of two even at higher cutting speeds. Tool-life-limiting notch wear and the burr formation at the workpiece have been suppressed. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2011.01.022
  • 2011 • 40 Recrystallization and grain growth in ultrafine-grained materials produced by high pressure torsion
    Khorashadizadeh, A. and Raabe, D. and Winning, M. and Pippan, R.
    Advanced Engineering Materials 13 245-250 (2011)
    Ultrafine-grained (UFG) materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of UFG materials. For this purpose a Cu-0.17 wt%Zr alloy was subjected to high pressure torsion (HPT) with a maximal pressure of 4.8GPa at room temperature. The microstructures of the specimens were characterized using electron back scatter (EBSD) measurements, transmission electron microscopy (TEM), and hardness measurements. During annealing of the samples, dispersoids were formed which improved the thermal stability of the alloy. At higher strain levels the fraction of high angle grain boundaries (HAGBs) increased above 70% of the total grain boundaries. Ultrafine-grained materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of such materials. For this purpose a Cu-0.17 wt%Zr alloy was subjected to high pressure torsion. The microstructures of the specimens were characterized in the deformed state as well as after annealing using EBSD and hardness measurements. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000253
  • 2011 • 39 Robustness and optimal use of design principles of arthropod exoskeletons studied by ab initio-based multiscale simulations
    Nikolov, S. and Fabritius, H. and Petrov, M. and Friák, M. and Lymperakis, L. and Sachs, C. and Raabe, D. and Neugebauer, J.
    Journal of the Mechanical Behavior of Biomedical Materials 4 129-145 (2011)
    Recently, we proposed a hierarchical model for the elastic properties of mineralized lobster cuticle using (i) ab initio calculations for the chitin properties and (ii) hierarchical homogenization performed in a bottom-up order through all length scales. It has been found that the cuticle possesses nearly extremal, excellent mechanical properties in terms of stiffness that strongly depend on the overall mineral content and the specific microstructure of the mineral-protein matrix. In this study, we investigated how the overall cuticle properties changed when there are significant variations in the properties of the constituents (chitin, amorphous calcium carbonate (ACC), proteins), and the volume fractions of key structural elements such as chitin-protein fibers. It was found that the cuticle performance is very robust with respect to variations in the elastic properties of chitin and fiber proteins at a lower hierarchy level. At higher structural levels, variations of design parameters such as the volume fraction of the chitin-protein fibers have a significant influence on the cuticle performance. Furthermore, we observed that among the possible variations in the cuticle ingredients and volume fractions, the experimental data reflect an optimal use of the structural variations regarding the best possible performance for a given composition due to the smart hierarchical organization of the cuticle design. © 2010 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2010.09.015
  • 2011 • 38 Shear stiff, surface modified, mica-like nanoplatelets: A novel filler for polymer nanocomposites
    Schütz, M.R. and Kalo, H. and Lunkenbein, T. and Gröschel, A.H. and Müller, A.H.E. and Wilkie, C.A. and Breu, J.
    Journal of Materials Chemistry 21 12110-12116 (2011)
    Synthesis of polymer nanocomposites with novel shear stiff, mica-like nanoplatelets from a synthetic layered silicate is presented. This novel synthetic clay filler shows high aspect ratios while organophilization may be selectively restricted to external surfaces minimizing the organic content of the filler. The obtained nanocomposite shows superior mechanical, thermal and fire properties as compared to commonly used natural clays. Furthermore, the influence of the blending method on the nanocomposite properties was investigated. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c1jm11443c
  • 2011 • 37 Sliding wear behaviour of diamond-like carbon (DLC) coatings deposited on plasma nitrided steels
    Tillmann, W. and Momeni, S. and Hoffmann, F.
    International Journal of Materials Research 102 1007-1013 (2011)
    A hydrogen-free DLC (diamond-like carbon) coating was deposited with a bias voltage of 150 V on various high and low alloy tool steels to study the effect of the pre-treatment of the steel substrate on the wear behaviour of the DLC coating in sliding contact with uncoated counterparts. The morphology and mechanical properties of the DLC coating as well as the effect of plasma nitriding on the surface roughness and the hardness of the steels were studied in order to perform a correlation with the results of tribology tests. It could be concluded from the results that the plasma nitriding of the high alloy tool steel X210CrW12 leads to a significant decrease in the wear and friction coefficient of the DLC coating. Furthermore, it was found that plasma nitriding of the steel results in a decrease in the wear of uncoated counterparts as well. Finally, the wear mechanisms and failure of DLC coatings deposited on various steels were compared with each other and discussed analytically. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110547
  • 2011 • 36 Supertoughening in B1 transition metal nitride alloys by increased valence electron concentration
    Sangiovanni, D.G. and Hultman, L. and Chirita, V.
    Acta Materialia 59 2121-2134 (2011)
    We use density functional theory calculations to explore the effects of alloying cubic TiN and VN with transition metals M = Nb, Ta, Mo or W at 50% concentrations. The ternary systems obtained are predicted to be supertough, as they are shown to be harder and significantly more ductile compared with reference binary systems. The primary electronic mechanism of this supertoughening effect is shown in a comprehensive electronic structure analysis of these compounds to be the increased valence electron concentration intrinsic to these ternary systems. Our investigations reveal the complex nature of chemical bonding in these compounds, which ultimately explains the observed selective response to stress. The findings presented in this paper thus offer a design route for the synthesis of supertough transition metal nitride alloys via valence electron concentration tuning. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.12.013
  • 2011 • 35 The collagen fibril architecture in the lamina cribrosa and peripapillary sclera predicted by a computational remodeling approach
    Grytz, R. and Meschke, G. and Jonas, J.B.
    Biomechanics and Modeling in Mechanobiology 10 371-382 (2011)
    The biomechanics of the optic nerve head is assumed to play an important role in ganglion cell loss in glaucoma. Organized collagen fibrils form complex networks that introduce strong anisotropic and nonlinear attributes into the constitutive response of the peripapillary sclera (PPS) and lamina cribrosa (LC) dominating the biomechanics of the optic nerve head. The recently presented computational remodeling approach (Grytz and Meschke in Biomech Model Mechanobiol 9:225-235, 2010) was used to predict the micro-architecture in the LC and PPS, and to investigate its impact on intraocular pressure-related deformations. The mechanical properties of the LC and PPS were derived from a microstructure-oriented constitutive model that included the stretch-dependent stiffening and the statistically distributed orientations of the collagen fibrils. Biomechanically induced adaptation of the local micro-architecture was captured by allowing collagen fibrils to be reoriented in response to the intraocular pressure-related loading conditions. In agreement with experimental observations, the remodeling algorithm predicted the existence of an annulus of fibrils around the scleral canal in the PPS, and a predominant radial orientation of fibrils in the periphery of the LC. The peripapillary annulus significantly reduced the intraocular pressure-related expansion of the scleral canal and shielded the LC from high tensile stresses. The radial oriented fibrils in the LC periphery reinforced the LC against transversal shear stresses and reduced LC bending deformations. The numerical approach presents a novel and reasonable biomechanical explanation of the spatial orientation of fibrillar collagen in the optic nerve head. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s10237-010-0240-8
  • 2011 • 34 The evolution of laminates in finite crystal plasticity: A variational approach
    Kochmann, D.M. and Hackl, K.
    Continuum Mechanics and Thermodynamics 23 63-85 (2011)
    The analysis and simulation of microstructures in solids has gained crucial importance, virtue of the influence of all microstructural characteristics on a material's macroscopic, mechanical behavior. In particular, the arrangement of dislocations and other lattice defects to particular structures and patterns on the microscale as well as the resultant inhomogeneous distribution of localized strain results in a highly altered stress-strain response. Energetic models predicting the mechanical properties are commonly based on thermodynamic variational principles. Modeling the material response in finite strain crystal plasticity very often results in a non-convex variational problem so that the minimizing deformation fields are no longer continuous but exhibit small-scale fluctuations related to probability distributions of deformation gradients to be calculated via energy relaxation. This results in fine structures that can be interpreted as the observed microstructures. In this paper, we first review the underlying variational principles for inelastic materials. We then propose an analytical partial relaxation of a Neo-Hookean energy formulation, based on the assumption of a first-order laminate microstructure, thus approximating the relaxed energy by an upper bound of the rank-one-convex hull. The semi-relaxed energy can be employed to investigate elasto-plastic models with a single as well as multiple active slip systems. Based on the minimization of a Lagrange functional (consisting of the sum of energy rate and dissipation potential), we outline an incremental strategy to model the time-continuous evolution of the laminate microstructure, then present a numerical scheme by means of which the microstructure development can be computed, and show numerical results for particular examples in single- and double-slip plasticity. We discuss the influence of hardening and of slip system orientations in the present model. In contrast to many approaches before, we do not minimize a condensed energy functional. Instead, we incrementally solve the evolution equations at each time step and account for the actual microstructural changes during each time step. Results indicate a reduction in energy when compared to those theories based on a condensed energy functional. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s00161-010-0174-5
  • 2010 • 33 A novel tensed mechanism for simulation of maneuvers in wind tunnels
    Bruckmann, T. and Mikelsons, L. and Brandt, T. and Schramm, D. and Pott, A. and Abdel-Maksoud, M.
    Proceedings of the ASME International Design Engineering Technical Conferences and Computers and Information in Engineering Conference 2009, DETC2009 7 17-24 (2010)
    Wind tunnels are a standard tool to evaluate the air flow properties of aerodynamical vehicles in model scale. This is widely used to optimize the design of aircrafts and aircraft components. Additionally, the hydrodynamic properties of marine components like ship hulls or propulsion systems can be predicted. It is desirable to guide the models along defined trajectories during the tests to vary the angle of attack. Parallel wire robots were successfully used to perform airplane maneuvers in wind tunnels due to their good aerodynamical and mechanical properties. Compared to aircraft design, marine models are very heavy (up to 500kg). Thus, the positioning system must be very stiff to avoid vibrations. Additionally, fast maneuvers require powerful drives. Nevertheless, the positioning system should not influence the air flow. In this contribution, a novel design is presented. Additionally, a new realtime capable force distribution calculation method for parallel tensed systems is presented. Copyright © 2009 by ASME.
    view abstractdoi: 10.1115/DETC2009-86718
  • 2010 • 32 A review of crystallographic textures in chemical vapor-deposited diamond films
    Liu, T. and Raabe, D. and Mao, W.-M.
    Signal, Image and Video Processing 4 1-16 (2010)
    Diamond is one of the most important functional materials for film applications due to its extreme physical and mechanical properties, many of which depend on the crystallographic texture. The influence of various deposition parameters matters to the texture formation and evolution during chemical vapor deposition (CVD) of diamond films. In this overview, the texture evolutions are presented in terms of both simulations and experimental observations. The crystallographic textures in diamond are simulated based on the van der Drift growth selection mechanism. The film morphology and textures associated with the growth parameters α (proportional to the ratio of the growth rate along the 〈100〉 direction to that along the 〈111〉 direction) are presented and determined by applying the fastest growth directions. Thick films with variations in substrate temperature, methane concentration, film thickness, and nitrogen addition were analyzed using high-resolution electron back-scattering diffraction (HR-EBSD) as well as X-ray diffraction (XRD), and the fraction variations of fiber textures with these deposition parameters were explained. In conjunction with the focused ion beam (FIB) technique for specimen preparation, the grain orientations in the beginning nucleation zones were studied using HR-EBSD (50 nm step size) in another two sets of thin films deposited with variations in methane concentration and substrate material. The microstructures, textures, and grain boundary character were characterized. Based on the combination of an FIB unit for serial sectioning and HR-EBSD, diamond growth dynamics was observed using a 3D EBSD technique, with which individual diamond grains were investigated in 3D. Microscopic defects were observed in the vicinity of the high-angle grain boundaries by using the transmission electron microscopy (TEM) technique, and the advances of TEM orientation microscopy make it possible to identify the grain orientations in nano-crystalline diamond. © 2010 Higher Education Press and Springer Berlin Heidelberg.
    view abstractdoi: 10.1007/s11760-008-0099-7
  • 2010 • 31 A review on hot stamping
    Karbasian, H. and Tekkaya, A.E.
    Journal of Materials Processing Technology 210 2103-2118 (2010)
    The production of high strength steel components with desired properties by hot stamping (also called press hardening) requires a profound knowledge and control of the forming procedures. In this way, the final part properties become predictable and adjustable on the basis of the different process parameters and their interaction. In addition to parameters of conventional cold forming, thermal and microstructural parameters complicate the description of mechanical phenomena during hot stamping, which are essential for the explanation of all physical phenomena of this forming method. In this article, the state of the art in the thermal, mechanical, microstructural, and technological fields of hot stamping are reviewed. The investigations of all process sequences, from heating of the blank to hot stamping and subsequent further processes, are described. The survey of existing works has revealed several gaps in the fields of forming-dependent phase transformation, continuous flow behavior during the whole process, correlation between mechanical and geometrical part properties, and industrial application of some advanced processes. The review aims at providing an insight into the forming procedure backgrounds and shows the great potential for further investigations and innovation in the field of hot sheet metal forming. © 2010 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2010.07.019
  • 2010 • 30 Ab Initio guided design of bcc ternary Mg-Li-X (X=Ca, Al, Si, Zn, Cu) alloys for ultra-lightweight applications
    Counts, W.A. and Friák, M. and Raabe, D. and Neugebauer, J.
    Advanced Engineering Materials 12 572-576 (2010)
    Ab initio calculations are becoming increasingly important for designing new alloys as these calculations can accurately predict basic structural, mechanical, and functional properties using only the atomic composition as a basis. In this paper, fundamental physical properties (like formation energies and elastic constants) of a set of bcc Mg-Li and Mg-Li-based compounds are calculated using density functional theory (DFT). These DFT-determined properties are in turn used to calculate engineering parameters such as (i) specific Young's modulus (Y/p) or (ii) shear over bulk modulus ratio (G/B) differentiating between brittle and ductile behavior. These parameters are then used to identify those alloys that have optimal mechanical properties for lightweight structural applications. First, in case of the binary Mg-Li system, an Ashby map containing Y/r versus G/B shows that it is not possible to increase Y/r without simultaneously increasing G/B (i.e., brittleness) by changing only the composition of a binary alloy. In an attempt to bypass such a fundamental materials-design limitation, a set of Mg-Li-X ternaries (X=Ca, Al, Si, Cu, Zn) based on stoichiometric Mg-Li with CsCl structure was studied. It is shown that none of the studied ternary solutes is able to simultaneously improve both specific Young's modulus and ductility. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.200900308
  • 2010 • 29 Accurate welding line prediction in extrusion processes
    Kloppenborg, T. and Ben Khalifa, N. and Tekkaya, A.E.
    Key Engineering Materials 424 87-95 (2010)
    In contrast to conventional extrusion processes, where a lot of research is done on in the welding quality, in composite extrusion, research is investigated into the welding line positioning. As a result of the process principle, the reinforcing elements are embedded into the longitudinal welding line. Hence, an undefined material flow inside the welding chamber induces reinforcement deflection, which can lead to reduced mechanical properties, as momentum of inertia. Therefore and to reduce costly experimental investigations, a new method of an automated numerical welding line prediction was developed. The results form HyperXtrude finite element calculations are used for special particle tracing simulations to predict the welding line in the profile cross section accurately. The procedures of segmentation and characteristic extraction are presented to approximate the welding line by cubic spline functions. The method was fully programmed in the Java program language, and works well for all HyperXtrude process models consisting of tetrahedral elements. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2010 • 28 An image morphing method for 3D reconstruction and FE-analysis of pore networks in thermal spray coatings
    Wiederkehr, T. and Klusemann, B. and Gies, D. and Müller, H. and Svendsen, B.
    Computational Materials Science 47 881-889 (2010)
    Using thermal spraying various surface coatings consisting of different material compositions can be manufactured. Besides different solid phases the resulting coating microstructure often contains a non-negligible amount of pores altering their mechanical properties. A common practice to analyze the porosity and composition of a coating is to create cross section images using standard light microscopy equipment or a scanning electron microscope. In this paper a method is presented to construct a three-dimensional multiphase model of the coating from a number of such cross section images by means of an image morphing technique. The resulting model can then be used for visualization purposes or further analysis e.g. within a finite element simulation. The described method has been applied to the construction of a finite element model of a porous coating sample which is used in a compaction simulation to determine its behavior in a rolling process. The required cross section images were obtained using a successive grinding and microscopy procedure. The material behavior of the porous material is modeled by using a modified Johnson-Cook material model formulation for an elasto-viscoplastic material. Comparison of 2D and 3D-simulation results are shown. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2009.11.019
  • 2010 • 27 Anodic oxides on a beta type Nb-Ti alloy and their characterization by electrochemical impedance spectroscopy
    Woldemedhin, M.T. and Raabe, D. and Hassel, A.W.
    Physica Status Solidi (A) Applications and Materials Science 207 812-816 (2010)
    Anodic oxides were grown on the surface of an electropolished (Ti-30at% Nb) beta-titanium (β-Ti) alloy by cyclic voltammetry. The scan rate was 100 mVs -1 between 0 and 8V in increments of lV in an acetate buffer of pH 6.0. Electrochemical impedance spectroscopy was carried out right after each anodic oxide growth increment to study the electronic properties of the oxide/electrolyte interface in a wide frequency range from 100 kHz to 10 MHz with an AC perturbation voltage of 10 mV. A film formation factor of 2.4 nm V -1 was found and a relative permittivity number (dielectric constant) of 42.4 was deter- mined for the oxide film formed. Mott-Schottky analysis on a potentiostatically formed 7 nm thick oxide film was performed to assess the semiconducting properties of the mixed anodic oxide grown on the alloy. A flat band potential of - 0.47 V (standard hydrogen electrode, SHE) was determined, connected to a donor density of 8.2 × 1017cm -3. β-Ti being highly isotropic in terms of mechanical properties should be superior to the stiffer α-Ti compound. Its application, however, requires a passivation behaviour comparable or better than α-Ti which in fact is found. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.200983324
  • 2010 • 26 Brazing of titanium to steel with different filler metals: Analysis and comparison
    Elrefaey, A. and Tillmann, W.
    Journal of Materials Science 45 4332-4338 (2010)
    Evaluations of vacuum brazed commercially pure titanium and low-carbon steel joints using one copper-based alloy (Cu-12Mn-2Ni) and two silver-based braze alloys (Ag-34Cu-2Ti, Ag-27.25Cu-12.5In-1.25Ti) have been studied. Both the interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate the joint quality. The optical and scanning electron microscopic results showed that all the filler metals interact metallurgically with steel and titanium, forming different kinds of intermetallic compounds (IMC) such as CuTi, Cu2Ti, Cu4Ti3, and FeTi. The presence of IMC (interfacial reaction layers) at the interfacial regions strongly affects the shear strength of the joints. Furthermore, it was found that the shear strength of brazed joints and the fracture path strongly depend on the thickness of the IMC. The maximum shear strength of the joints was 113 MPa for the specimen brazed at 750 °C using an Ag-27.25Cu-12.5In-1.25Ti filler alloy. © 2010 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-010-4357-z
  • 2010 • 25 Crystal plasticity modelling and experiments for deriving microstructure-property relationships in γ-TiAl based alloys
    Zambaldi, C. and Raabe, D.
    Journal of Physics: Conference Series 240 (2010)
    Single-crystals of γ-TiAl cannot be grown for the compositions present inside the two-phase γ/α 2-microstructures that show good mechanical properties. Therefore the single crystal constitutive behaviour of γ-TiAl was studied by nanoindentation experiments in single phase regions of these microstructures. The experiments were extensively characterized by a combined experimental approach to clarify the orientation dependent mechanical response during nanoindentation. They further were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behaviour of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. On the length-scale of multi-grain aggregates two kinds of microstructures were investigated. The lamellar microstructure was analyzed in terms of kinematic constraints perpendicular to densely spaced lamellar boundaries which lead to pronounced plastic anisotropy. Secondly, the mechanical behaviour of massively transformed microstructures was modelled by assuming a lower degree of kinematic constraints. This resulted in less plastic anisotropy on a single grain scale and lower compatibility stresses in a 64-grain aggregate. On the macroscopic length scale, the results could possibly explain the pre-yielding of lamellar microstructures. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/240/1/012140
  • 2010 • 24 EBSD study of substructure and texture formation in dual-phase steel sheets for semi-finished products
    Masimov, M. and Peranio, N. and Springub, B. and Roters, F. and Raabe, D.
    Solid State Phenomena 160 251-256 (2010)
    Using SEM/EBSD the substructure and texture evolution in dual phase steels in the first steps of the process chain, i.e. hot rolling, cold rolling, and following annealing were characterized. In order to obtain dual phase steels with high ductility and high tensile strength an industrial process was reproduced by cold rolling of industrially hot rolled steel sheets of a thickness of 3.75 mm with ferrite and pearlite morphology down to a thickness of 1.75 mm and finally annealing at different temperatures. Such technique allows a compilation of ferrite and martensite morphology typical for dual phase steels. Due to the competition between recovery, recrystallization and phase transformation during annealing a variety of ferrite martensite morphologies was produced by promoting one of the mechanisms through the variation of technological parameters such as heating rate, intercritical annealing temperature, annealing time, cooling rate and the final annealing temperature. Annealing induced changes of the mechanical properties were determined by hardness measurements and are discussed on the basis of the results of the substructure investigations. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2010 • 23 Effect of grain refinement to 1μm on strength and toughness of dual-phase steels
    Calcagnotto, M. and Ponge, D. and Raabe, D.
    Materials Science and Engineering A 527 7832-7840 (2010)
    Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4μm) and ultrafine grained (1.2μm) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4μm) reference material. Both yield strength and tensile strength follow a Hall-Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.08.062
  • 2010 • 22 Effects of annealing on the microstructure and the mechanical properties of EB-PVD thermal barrier coatings
    Zotov, N. and Bartsch, M. and Chernova, L. and Schmidt, D.A. and Havenith, M. and Eggeler, G.
    Surface and Coatings Technology 205 452-464 (2010)
    The effects of thermal annealing at 1000°C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4mol% Y2O3 partially stabilized ZrO2 top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2010.07.008
  • 2010 • 21 Evaluation of factors influencing deep cryogenic treatment that affect the properties of tool steels
    Oppenkowski, A. and Weber, S. and Theisen, W.
    Journal of Materials Processing Technology 210 1949-1955 (2010)
    Deep cryogenic treatment (DCT) of tool steels is used as an additive process to conventional heat treatment and usually involves cooling the material to liquid nitrogen temperature (-196 °C). This kind of treatment has been reported to improve the wear resistance of tools. In this study, the Taguchi method was used to identify the main factors of DCT that influence the mechanical properties and the wear resistance of the powder metallurgically produced cold-work tool steel X153CrVMo12 (AISI D2). Factors investigated were the austenitizing temperature, cooling rate, holding time, heating rate, and tempering temperature. In order to study the significance of these factors and the effect of possible two-factor interactions L27(313), an orthogonal array (OA) was applied to conduct several heat treatments, including a single DCT cycle directly after quenching prior to tempering. The results show that the most significant factors influencing the properties of tool steels are the austenitizing and tempering temperatures. In contrast, the parameters of deep cryogenic treatment exhibit a lower level of significance. Further investigations identified a nearly constant wear rate for holding times of up to 24 h. The wear rate reaches a minimum for a longer holding time of 36 h and increases again with further holding. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2010.07.007
  • 2010 • 20 Improvement of press dies used for the production of diamond composites by means of DUPLEX-PVD-coatings
    Tillmann, W. and Vogli, E. and Momeni, S.
    Surface and Coatings Technology 205 1571-1577 (2010)
    In the machining of hard materials such as glass or stone, cemented carbides have been recently replaced by diamond tools, consisting of a metallic carrier, on to which diamond segments are brazed. One of the most economic ways for the production of diamond segments is the cold compaction of the mixture of a metallic powder and diamond particles. Due to a highly abrasive sliding contact between diamond particles and the die walls, the wear rate of the press dies is very high. As a result of a low lifetime of the press dies, they must be replaced in short time periods. To avoid the costly and time-consuming substitution of the press dies, in this work PVD-coatings were deposited on the inner surface of the pre-plasma nitrided press dies (DUPLEX treatment). Thereby, various high and low alloy tool steels were treated by means of plasma nitriding process. Subsequently, a nanocomposite TiAlN coating (nc-TiAlN) was deposited by means of a high ionization magnetron sputtering device on nitrided and non-nitrided steel substrates. The mechanical and tribological properties of these coating systems were studied by means of several standard tests such as nanoindentation, ball-on-disc and scratch test. The most wear resistant coating system was chosen to employ on the inner surface of the press dies. The wear resistance of the press dies developed in this study was tested under real loading condition during compaction of the mixture of diamond particles and cobalt powder. It was revealed that employing plasma nitrided tool, steels coated with nanocomposite TiAlN decreases the wear rate of the press dies up to 76%. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2010.08.048
  • 2010 • 19 Influence on diamonds during the spraying of diamond-bronze abrasive coatings
    Tillmann, W. and Vogli, E. and Nebel, J. and Buck, V. and Reuter, S.
    Journal of Thermal Spray Technology 19 350-357 (2010)
    Detonation spraying provides the opportunity to produce diamond grinding tools for the machining of stone, cement, and concrete. Especially the atmospheric conditions of the spraying process yield in a high production flexibility. However, during detonation spraying, the oxygenic atmosphere as well as the thermal and kinetic energy have an impact on the processed diamond. Despite its importance for the tools' performance, the influence of the spraying process on the superabrasive diamond is predominantly unknown. The potential decrease of the diamond durability and strength due to degradation effects during the production of sprayed diamond-CuSn 85/15 composites has not yet been determined. X-ray diffraction and Raman spectroscopy were used to verify thermally initiated surface reactions of the sprayed diamonds after exposure to the spraying process. Additionally, reference measurements on the degradation of diamonds in oxidizing and inert conditions were carried out to compare the spraying results. Differential thermal and thermogravimetric analyses were employed. To validate the mechanical properties of the diamond superabrasives, friability tests and fracture force tests were performed. It was found that under optimized detonation spraying conditions the thermal and mechanical impact remains low enough to ensure a good reliability of the processed diamonds. The diamond crystal structure endured the spraying process without detectable graphitization or oxidation. Deterioration indicators were not observed in SEM micrographs, x-ray diffraction patterns or Raman spectra. Furthermore, a high durability and strength of the sprayed diamonds were confirmed by mechanical testing. © 2009 ASM International.
    view abstractdoi: 10.1007/s11666-009-9418-y
  • 2010 • 18 Mechanical properties of blood clots - A new test method
    Krasokha, N. and Theisen, W. and Reese, S. and Mordasini, P. and Brekenfeld, C. and Gralla, J. and Slotboom, J. and Schrott, G. and Monstadt, H.
    Materialwissenschaft und Werkstofftechnik 41 1019-1024 (2010)
    A blood clot needs to have the right degree of mechanical, chemical and biological properties to stem the flow of blood and yet to be suitable for lytic enzymes or mechanical thrombectomy so as not to form a thrombotic event. The origin and understanding of these mechanical properties are still not known in detail. Clots are made of a three-dimensional network of fibrin fibers stabilized through ligation with a transglutaminase, factor XIIIa. New methods to achieve information about mechanical properties were established in this work. We performed compressive strength experiments of aged human blood clots. Furthermore after the set up of a new test environment, it was possible to perform tensile strength measurements of aged animal blood clots. Stress strain curves of aged clots were measured and discussed. The viscoelastic properties of the clot material were quantitatively described. This work should make a contribution to a better understanding of the behaviour of aged blood thrombus bulk material and induced mechanical stress. In der Behandlung ischämischer Schlaganfälle sind mechanische, chemische und biologische Eigenschaften von Blutgerinnseln von essentieller Bedeutung und entscheidend über den Erfolg einer Lyse-Therapie bzw. mechanischer Thrombektomie. Die Einflussfaktoren der Gerinnungsbedingungen auf die mechanischen Eigenschaften sind jedoch bisher noch nicht eingehend untersucht. Aus Schrifttum ist bekannt, dass auf mikrostruktureller Ebene die Blutgerinnsel aus Fibrinfasern, die untereinander in Knotenpunkten verbunden sind und in der Gesamtheit ein dreidimensionales Netzwerk bilden, bestehen. Dabei haben besonders die Faserdicke und die Knotenpunktendichte einen direkten Einfluss auf die mechanischen Eigenschaften eines Thrombus. Im Rahmen dieser Arbeit wurden Druck- und Zugversuche an Thrombus-Proben (hergestellt aus humanem bzw. tierischem Blut) durchgeführt und die Messergebnisse hinsichtlich der Gerinnungsbedingungen diskutiert. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201000703
  • 2010 • 17 Microstructural and mechanical study of an Al matrix composite reinforced by Al-Cu-Fe Icosahedral particles
    Laplanche, G. and Joulain, A. and Bonneville, J. and Gauthier-Brunet, V. and Dubois, S. and El Kabir, T.
    Journal of Materials Research 25 957-965 (2010)
    In this study, we produced an Al matrix composite material reinforced by Al-Cu-Fe particles of the icosahedral phase. The composite material was prepared using a hot isostatic pressure technique at T = 673 K and P = 180 MPa. The mechanical properties were investigated by compression tests performed at constant strain rate over the temperature range 290-823 K. The results show a vigorous strengthening effect resulting from the reinforcement particles. Strengthening is attributed to two main contributions arising from load transfer between the Al matrix and the reinforcement particles and from plastic deformation of the Al grains. The present results are compared with those obtained in a previous work on an Al-based composite reinforced by Al-Cu-Fe particles of the ω-tetragonal phase. © 2010 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2010.0118
  • 2010 • 16 Microstructures and mechanical properties of Al-base composite materials reinforced by Al-Cu-Fe particles
    Laplanche, G. and Joulain, A. and Bonneville, J. and Schaller, R. and El Kabir, T.
    Journal of Alloys and Compounds 493 453-460 (2010)
    In this study, we produced four composite materials with Al-based matrix reinforced by Al-Cu-Fe particles initially of the quasicrystalline (QC) phase. The processing route was a gas-pressure infiltration of QC particle preforms by molten commercial Al and Al alloys. The resulting composites were investigated by scanning electron microscopy (SEM) working in the energy dispersive spectroscopy (EDS) mode and by X-ray diffraction (XRD). It is shown that such a synthesis technique leads to the formation of various phases resulting from specific diffusion processes. Compression tests were performed at constant strain rate in the temperature range 290-770 K. The stress-strain curves look similar to those of Al-Cu-Fe poly-quasicrystals and show the yield point, the origin of which is however of very different nature. Composite deformation is recognised to occur through the rupture of a hard phase skeleton and localised plastic deformation in the matrix. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2009.12.124
  • 2010 • 15 Modeling of hot ductility during solidification of steel grades in continuous casting - Part I
    Senk, D. and Stratemeier, S. and Böttger, B. and Göhler, K. and Steinbach, I.
    Advanced Engineering Materials 12 94-100 (2010)
    The present paper gives an overview of the simultaneous research work carried out by RWTH Aachen University and ThyssenKrupp Steel Europe AG. With a combination of sophisticated simulation tools and experimental techniques it is possible to predict the relations between temperature distribution in the mould, solidification velocity, chemical steel composition and, furthermore, the mechanical properties of the steel shell. Simulation results as well as experimentally observed microstructure parameters are used as input data for hot tearing criteria. A critical choice of existing hot tearing criteria based on different approaches, like critical strain and critical strain rate, are applied and developed. The new "damage model" is going to replace a basic approach to determine hot cracking susceptibility in a mechanical FEM strand model for continuous slab casting of ThyssenKrupp Steel Europe AG. Critical strains for hot cracking in continuous casting were investigated by in situ tensile tests for four steel grades with carbon contents in the range of 0.036 and 0.76 wt%. Additionally to modeling, fractography of laboratory and industrial samples was carried out by SEM and EPMA and the results are discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201000021
  • 2010 • 14 Nanoindentation of a pseudoelastic NiTiFe shape memory alloy
    Pfetzing-Micklich, J. and Wagner, M.F.-X. and Zarnetta, R. and Frenzel, J. and Eggeler, G. and Markaki, A.E. and Wheeler, J. and Clyne, T.W.
    Advanced Engineering Materials 12 13-19 (2010)
    Nanoindentation is a suitable tool for characterizing the local mechanical properties of shape memory alloys (SMA) and to study their pseudoelastic behavior. There is a special interest in indenting with different indenter tips (as not all tips are associated with strain states that predominantly induce the martensitic transformation) and in indenting at different temperatures, where different phases are present. In this study, we perform nanoindentation on a ternary NiTiFe SMA with different indenter tips and at various testing temperatures. For nanoindentation with spherical tips, load-displacement hystereses clearly indicate pseudoelastic behavior, whereas indentation with Berkovich tips results in more pronounced plastic deformation. Testing at different temperatures is associated with different volume fractions of austenite, martensite, and R-phase. The corresponding nanoindentation responses differ considerably in terms of pseudoelastic behavior. Best pseudoelastic recovery is found at testing temperatures close to the R-phase start temperature, even though this temperature is below the austenite finish temperature, which is a well-known lower temperature bound for full recovery in macroscopic tests. Our results are discussed considering micromechanical aspects and the interaction between stress-induced phase transformation and dislocation plasticity. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.200900266
  • 2010 • 13 Numerical investigation of room-temperature deformation behavior of a duplex type γtiAl alloy using a multi-scale modeling approach
    Kabir, M.R. and Chernova, L. and Bartsch, M.
    Acta Materialia 58 5834-5847 (2010)
    Room-temperature deformation of a niobium-rich TiAl alloy with duplex microstructure has been numerically investigated. The model links the microstructural features at micro- and meso-scale by the two-level (FE 2) multi-scale approach. The deformation mechanisms of the considered phases were described in the micro-mechanical crystal-plasticity model. Initial material parameters for the model were taken from the literature and validated using tensile experiments at macro-scale. For the niobium-rich TiAl alloy further adaptation of the crystal plasticity parameters is proposed. Based on these model parameters, the influences of the grain orientation, grain size, and texture on the global mechanical behavior have been investigated. The contributions of crystal deformation modes (slips and dislocations in the phases) to the mechanical response are also analyzed. The results enable a quantitative prediction of relationships between microstructure and mechanical behavior on global and local scale, including an assessment of possible crack initiation sites. The model can be used for microstructure optimization to obtain better material properties. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.06.058
  • 2010 • 12 Plastic anisotropy of γ-TiAl revealed by axisymmetric indentation
    Zambaldi, C. and Raabe, D.
    Acta Materialia 58 3516-3530 (2010)
    Single crystals of γ-TiAl cannot be grown in the near-stoichiometric compositions that are present inside two-phase γ / α2-microstructures with attractive mechanical properties. Therefore, the single-crystal constitutive behavior of γ-TiAl was studied by nanoindentation experiments in single-phase regions of these γ / α2-microstructures. The experiments were characterized by orientation microscopy and atomic force microscopy to quantify the orientation-dependent mechanical response during nanoindentation. Further, they were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behavior of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. A convention was defined to unambiguously relate any indentation axis to a crystallographic orientation. Experiments and simulations were combined to study the orientation-dependent surface pile-up. The characteristic pile-up topographies were simulated throughout the unit triangle of γ-TiAl and represented graphically in the newly introduced inverse pole figure of pile-up patterns. Through this approach, easy activation of ordinary dislocation glide in stoichiometric γ-TiAl was confirmed independently from dislocation observation by transmission electron microscopy. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.02.025
  • 2010 • 11 Revealing the design principles of high-performance biological composites using Ab initio and multiscale simulations: The example of lobster cuticle
    Nikolov, S. and Petrov, M. and Lymperakis, L. and Friák, M. and Sachs, C. and Fabritius, H.-O. and Raabe, D. and Neugebauer, J.
    Advanced Materials 22 519-526 (2010)
    Natural materials are hierarchically structured nanocomposites. A bottom-up multiscale approach to model the mechanical response of the chitin-based mineralized cuticle material of Homarus americanus is presented, by combining quantummechanical ab initio calculations with hierarchical homogenization. The simulations show how the mechanical properties are transferred from the atomic scale through a sequence of specifically designed microstructures to realize optimal stiffness. (Figure Presented) © 2010 WILEY-VCH Verlag GmbH & Co. KGaA,.
    view abstractdoi: 10.1002/adma.200902019
  • 2010 • 10 Silver containing sol-gel coatings on polyamide fabrics as antimicrobial finish-description of a technical application process for wash permanent antimicrobial effect
    Mahltig, B. and Textor, T.
    Fibers and Polymers 11 1152-1158 (2010)
    This paper reports on an antimicrobial finishing for polyamide with high washfastness. As antimicrobial agent modified silica sols containing silver components are used as coating agent and are applied to the polyamide fabric by using a semi-industrial procedure. The antimicrobial properties of coated polyamide fabrics are determined against the bacteria E. coli. Significant antimicrobial effects are observed even after 40 washing cycles. The amount of silver on the polyamide fabrics was measured by using ICP-OES. Besides this, samples are investigated by means of UV/Vis-spectroscopy and scanning electron microscopy. Furthermore textile properties as, e. g., air permeability and mechanical properties were measured. Due to high antimicrobial effect and the strong washfastness of this finishing, this reported method could be of high interest for industrial production processes. © 2010 The Korean Fiber Society and Springer Netherlands.
    view abstractdoi: 10.1007/s12221-010-1152-z
  • 2010 • 9 Stimulated deformation of polysiloxane capsules in external electric fieldsa
    Degen, P. and Chen, Z. and Rehage, H.
    Macromolecular Chemistry and Physics 211 434-442 (2010)
    Due to their pronounced viscoelastic properties polymer microcapsules are able to undergo mechanical deformations when they are stimulated by external signals. The aim of our work was to prepare new types of electric switchable microcapsules based on the interfacial polycondensation of octadecyltrichlorosilane (OTS) and to investigate their deformation behavior in externally electric fields. In a series of experiments we explored the influence of different additives on the deformation behavior of the capsules in electric fields as well as on the shear rheological properties of the polysiloxane networks at the planar interface. We could show that the additives had a significant influence on the rheological membrane properties, but we observed only small changes in the deformation of the whole capsules. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/macp.200900452
  • 2010 • 8 Surface modification of press dies used in a powder metallurgical production process
    Tillmann, W. and Vogli, E. and Momeni, S.
    Advanced Engineering Materials 12 1146-1153 (2010)
    Cold compaction in a press die and subsequent sintering of diamond particles, homogeneously distributed in a metallic powder as matrix, is one of the most economic ways for the production of diamond composites, used widely for cutting very hard materials like stone and glass. Owing to the very high hardness of diamonds, the wear of the press die is considerably high and because of a short life time, press tools must be substituted regularly. Recently, through plasma nitriding process and deposition of thin solid films, the wear resistance of the press dies has been significantly increased. This work aims at the investigation of the influence of roughness, friction coefficient, and hardness of the inner surface of various dies, which have been modified in different ways, on the physical properties of the compacted diamond segments. It was evidenced that improving the mechanical and tribological properties of the die surface leads to an increase of the hardness and density of the diamond composites produced. Several novel PVD coatings have been employed in order to improve the wear resistance of the press dies employed for the production of diamond composites. This study aims to investigate the relation between the surface modification of dies and the most important physical properties of the diamond composites compacted. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201000146
  • 2010 • 7 Synthesis and characterization of lamellar and fibre-reinforced NiAl-Mo and NiAl-Cr
    Haenschke, T. and Gali, A. and Heilmaier, M. and Krüger, M. and Bei, H. and George, E.P.
    Journal of Physics: Conference Series 240 (2010)
    Directionally solidified (DS) alloys of the eutectic systems NiAl-10Mo and NiAl-34Cr (at.%) are potential candidates for high-temperature structural applications. Here, these alloys were first arc-melted and drop-cast. Thereafter, they were directionally solidified (DS) at growth rates of 20 and 80 mm/h while rotating at a fixed rotation speed of 60 revolutions per minute. Specimens of the DS alloys were tested in three-point-bending and uniaxial compression to obtain mechanical properties, including the ductile to brittle transition temperature (DBTT). For the NiAl-Cr system DBTT was found to be around 300 °C. Microstructural observations revealed that in the section perpendicular to the growth direction a uniform distribution of fibres was observed. The expected decrease of the fibre diameter with increasing growth rate was not observed. Instead, the fibre diameter slightly increased with increasing crystal growth rates. First compression tests were performed to get insights into the creep behaviour of these fibre-reinforced microstructures. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/240/1/012063
  • 2010 • 6 Synthesis of defect-free single-phase bars of high-melting Laves phases through modified cold crucible levitation melting
    Voß, S. and Stein, F. and Palm, M. and Raabe, D.
    Materials Science and Engineering A 527 7848-7853 (2010)
    Laves phases are the most abundant intermetallic phases. However, their mechanical properties are only poorly understood due to difficulties in producing defect-free samples which are large enough for mechanical testing. By using a modified cold crucible levitation melting technique with in situ heat treatment and subsequent defined cooling, massive and large bars of several hundreds of grams of brittle and high-melting Laves phases were produced. Metallographic investigation revealed single- or near single-phase microstructures and a homogeneous chemical composition within the cast ingots with a diameter of 15. mm and of more than 100. mm length. With these high-quality bars it is now possible to prepare large samples that can be used for obtaining mechanical data of these phases. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.08.066
  • 2010 • 5 Tensile creep measurements of glassy VOC-loaded polymers
    Mueller, F. and Heuwers, B. and Katzenberg, F. and Tiller, J.C. and Sadowski, G.
    Macromolecules 43 8997-9003 (2010)
    The paper describes a new apparatus to measure tensile creep curves of polymer/volatile organic compound (VOC) systems, especially designed for measurements of small VOC loadings in glassy polymers. For the first time creep curves for glassy polymer/VOC systems are recorded. The measurements were performed for the system polystyrene/toluene at different toluene loads up to wtoluene = 0.13 and at temperatures of 30, 50, and 70 °C. It was found that increasing VOC mass fractions qualitatively influence the mechanical properties of a polymer in the same way like increasing temperature does. Since at isothermal conditions these properties are affected by the glass transition of the system, this information for the polystyrene/toluene mixtures was used to modify and to verify the correlation of Kelly and Bueche to predict the glass-transition temperature of polymer/solvent systems. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ma101782d
  • 2010 • 4 The effect of radiation processing and filler morphology on the biomechanical stability of a thermoset polyester composite
    Jayabalan, M. and Shalumon, K.T. and Mitha, M.K. and Ganesan, K. and Epple, M.
    Biomedical Materials 5 (2010)
    The effect of radiation processing and filler morphology on the biodegradation and biomechanical stability of a poly(propylene fumarate)/hydroxyapatite composite was investigated. Radiation processing influenced both cross-linking and biodegradation of the composites. Irradiation with a dose of 3 Mrad resulted in enhanced cross-linking, mechanical properties and a higher storage modulus which are favourable for dimensional stability of the implant. The particle morphology of the added hydroxyapatite in the highly cross-linked state significantly influenced the biomechanical and interfacial stability of the composites. Reorganization of agglomerated hydroxyapatite occurred in the cross-linked polymeric matrix under dynamic mechanical loading under simulated physiological conditions. Such a reorganization may increase the damping characteristics of the composite. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1748-6041/5/2/025009
  • 2010 • 3 The exoskeleton of the American lobster- From texture to anisotropic properties
    Raue, L. and Klein, H. and Raabe, D.
    Solid State Phenomena 160 287-294 (2010)
    The exoskeleton of the crustacean Homarus americanus, the American lobster, is a biological multiphase composite consisting of a crystalline organic matrix (chitin), crystalline biominerals (calcite), amorphous calcium carbonate and proteins. One special structural aspect is the occurrence of pronounced crystallographic orientations and resulting directional anisotropic mechanical properties. The crystallographic textures of chitin and calcite have been measured by wide-angle Bragg diffraction, calculating the Orientation Distribution Function (ODF) from pole figures by using the series expansion method according to Bunge. A general strong relationship can be established between the crystallographic and the resulting mechanical and physical properties. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2010 • 2 Using Ab initio calculations in designing bcc MgLi-X alloys for ultra-lightweight applications
    Counts, W.A. and Friák, M. and Raabe, D. and Neugebauer, J.
    Advanced Engineering Materials 12 1198-1205 (2010)
    Body centered cubic (bcc) Mg-Li-based alloys are a promising light-weight structural material. In order to tailor the Mg-Li composition with respect to specific industrial requirements, systematic materials-design concepts need to be developed and applied. Quantum-mechanical calculations are increasingly employed when designing new alloys as they accurately predict basic thermodynamic, structural, and functional properties using only the atomic composition as input. We have therefore performed a quantum-mechanical study using density functional theory (DFT) to systematically explore fundamental physical properties of a broad set of bcc MgLi-based compounds. These DFT-determined properties are used to calculate engineering parameters such as (i) the specific Young's modulus (Y/ρ) or (ii) the bulk over shear modulus ratio (B/G) which allow differentiating between brittle and ductile behavior. As we have recently shown, it is not possible to increase both specific Young's modulus, as a measure of strength, and B/G ratio, as a proxy for ductility, by changing only the composition in the binary bcc Mg-Li system. In an attempt to bypass such fundamental materials-design limitations, a large set of MgLi-X substitutional ternaries derived from stoichiometric MgLi with CsCl structure are studied. Motivated by the fact that for Mg-Li alloys (i) 3rd row Si and Al and (ii) 4th row Zn are industrially used as alloying elements, we probe the alloying performance of the 3rd (Na, Al, Si, P, S, Cl) and 4th row transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) elements. The studied solutes offer a variety of properties but none is able to simultaneously improve both specific Young's modulus and ductility. Therefore, in order to explore the alloying performance of yet a broader set of solutes, we predict the bulk modulus of MgX and LiX B2-compounds running over 40 different elements. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000225
  • 2010 • 1 Where Does the Lithium Go? - A Study of the Precipitates in the Stir Zone of a Friction Stir Weld in a Li-containing 2xxx Series Al Alloy
    Rao, J.C. and Payton, E.J. and Somsen, C. and Neuking, K. and Eggeler, G. and Kostka, A. and Dos Santos, J.F.
    Advanced Engineering Materials 12 298-303 (2010)
    The main strengthening precipitates of aluminum alloy 2198-T8, which are of the T1 phase, dissolve during friction stir welding, sending many Li atoms into solid solution. The stir zone precipitates are characterized using high-resolution transmission electron microscopy, energy dispersive spectroscopy, and selected area diffraction techniques to begin answering questions about the microstructural evolution and the relationship between microstructure and mechanical properties in friction stir welding of the next generation of lightweight Li-containing Al alloys. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.200900284