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.

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  • 2020 • 295 Ultraviolet/vacuum-ultraviolet emission from a high power magnetron sputtering plasma with an aluminum target
    Iglesias, E.J. and Hecimovic, A. and Mitschker, F. and Fiebrandt, M. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 53 (2020)
    We report the in situ measurement of the ultraviolet/vacuum-ultraviolet (UV/VUV) emission from a plasma produced by high power impulse magnetron sputtering with aluminum target, using argon as background gas. The UV/VUV detection system is based upon the quantification of the re-emitted fluorescence from a sodium salicylate layer that is placed in a housing inside the vacuum chamber, at 11 cm from the center of the cathode. The detector is equipped with filters that allow for differentiating various spectral regions, and with a front collimating tube that provides a spatial resolution ≈ 0.5 cm. Using various views of the plasma, the measured absolutely calibrated photon rates enable to calculate emissivities and irradiances based on a model of the ionization region. We present results that demonstrate that Al+ ions are responsible for most of the VUV irradiance. We also discuss the photoelectric emission due to irradiances on the target ∼ 2 × 1018 s-1 . cm-2 produced by high energy photons from resonance lines of Ar+. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/ab52f8
  • 2020 • 294 High-strength Damascus steel by additive manufacturing
    Kürnsteiner, P. and Wilms, M.B. and Weisheit, A. and Gault, B. and Jägle, E.A. and Raabe, D.
    Nature 582 515-519 (2020)
    Laser additive manufacturing is attractive for the production of complex, three-dimensional parts from metallic powder using a computer-aided design model1–3. The approach enables the digital control of the processing parameters and thus the resulting alloy’s microstructure, for example, by using high cooling rates and cyclic re-heating4–10. We recently showed that this cyclic re-heating, the so-called intrinsic heat treatment, can trigger nickel-aluminium precipitation in an iron–nickel–aluminium alloy in situ during laser additive manufacturing9. Here we report a Fe19Ni5Ti (weight per cent) steel tailor-designed for laser additive manufacturing. This steel is hardened in situ by nickel-titanium nanoprecipitation, and martensite is also formed in situ, starting at a readily accessible temperature of 200 degrees Celsius. Local control of both the nanoprecipitation and the martensitic transformation during the fabrication leads to complex microstructure hierarchies across multiple length scales, from approximately 100-micrometre-thick layers down to nanoscale precipitates. Inspired by ancient Damascus steels11–14—which have hard and soft layers, originally introduced via the folding and forging techniques of skilled blacksmiths—we produced a material consisting of alternating soft and hard layers. Our material has a tensile strength of 1,300 megapascals and 10 per cent elongation, showing superior mechanical properties to those of ancient Damascus steel12. The principles of in situ precipitation strengthening and local microstructure control used here can be applied to a wide range of precipitation-hardened alloys and different additive manufacturing processes. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-020-2409-3
  • 2020 • 293 Acidity enhancement through synergy of penta- and tetra-coordinated aluminum species in amorphous silica networks
    Wang, Z. and Li, T. and Jiang, Y. and Lafon, O. and Liu, Z. and Trébosc, J. and Baiker, A. and Amoureux, J.-P. and Huang, J.
    Nature Communications 11 (2020)
    Amorphous silica-aluminas (ASAs) are widely used in acid-catalyzed C-H activation reactions and biomass conversions in large scale, which can be promoted by increasing the strength of surface Brønsted acid sites (BAS). Here, we demonstrate the first observation on a synergistic effect caused by two neighboring Al centers interacting with the same silanol group in flame-made ASAs with high Al content. The two close Al centers decrease the electron density on the silanol oxygen and thereby enhance its acidity, which is comparable to that of dealuminated zeolites, while ASAs with small or moderate Al contents provide mainly moderate acidity, much lower than that of zeolites. The ASAs with enhanced acidity exhibit outstanding performances in C–H bond activation of benzene and glucose dehydration to 5-hydroxymethylfurfural, simultaneously with an excellent calcination stability and resistance to leaching, and they offer an interesting potential for a wide range of acid and multifunctional catalysis. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-019-13907-7
  • 2019 • 292 Tuning the charge flow between Marcus regimes in an organic thin-film device
    Atxabal, A. and Arnold, T. and Parui, S. and Hutsch, S. and Zuccatti, E. and Llopis, R. and Cinchetti, M. and Casanova, F. and Ortmann, F. and Hueso, L.E.
    Nature Communications 10 (2019)
    Marcus’s theory of electron transfer, initially formulated six decades ago for redox reactions in solution, is now of great importance for very diverse scientific communities. The molecular scale tunability of electronic properties renders organic semiconductor materials in principle an ideal platform to test this theory. However, the demonstration of charge transfer in different Marcus regions requires a precise control over the driving force acting on the charge carriers. Here, we make use of a three-terminal hot-electron molecular transistor, which lets us access unconventional transport regimes. Thanks to the control of the injection energy of hot carriers in the molecular thin film we induce an effective negative differential resistance state that is a direct consequence of the Marcus Inverted Region. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-10114-2
  • 2019 • 291 An insight into using DFT data for Calphad modeling of solid phases in the third generation of Calphad databases, a case study for Al
    Bigdeli, S. and Zhu, L.-F. and Glensk, A. and Grabowski, B. and Lindahl, B. and Hickel, T. and Selleby, M.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 65 79-85 (2019)
    In developing the next generation of Calphad databases, new models are used in which each term contributing to the Gibbs energy has a physical meaning. To continue the development, finite temperature density-functional-theory (DFT) results are used in the present work to discuss and suggest the most applicable and physically based model for Calphad assessments of solid phases above the melting point (the breakpoint for modeling the solid phase in previous assessments). These results are applied to investigate the properties of a solid in the superheated temperature region and to replace the melting temperature as the breakpoint with a more physically based temperature, i.e., where the superheated solid collapses into the liquid. The advantages and limitations of such an approach are presented in terms of a new assessment for unary aluminum. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.calphad.2019.02.008
  • 2019 • 290 Effect of mg and si content in aluminum alloys on friction surfacing processing behavior
    Ehrich, J. and Roos, A. and Hanke, S.
    Minerals, Metals and Materials Series 357-363 (2019)
    Friction surfacing (FS) coating layers are generated through severe plastic deformation (SPD) at elevated temperatures (≈0.8 Tmelt). Alloying elements in metals affect heat generation and dynamic recrystallization kinetics during SPD, and therefore require significant adjustments of FS processing conditions. In this study, custom made Aluminum alloys (AA 6060 with additions of 2 and 3.5 wt% Mg, and 6.6, 10.4 and 14.6 wt% Si) were processed by FS. It was found that for the high-Mg Aluminum alloys especially the rotational speeds require a downward adaption to achieve a steady state process. A higher content of Mg results in a reduced rate of thermal softening and more efficient heat generation. With regard to the plasticization behavior during FS, the high amount of hard phases in the high-Si alloys was expected to cause additional friction and increase heat generation. However, as the Si content increases, the process temperatures decrease. Influences of Mg and Si content on material efficiency and coating dimensions were evaluated and discussed. © 2019, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-030-05864-7_45
  • 2019 • 289 Computed tomography-based characterization of the fatigue behavior and damage development of extruded profiles made from recycled AW6060 aluminum chips
    Koch, A. and Wittke, P. and Walther, F.
    Materials 12 (2019)
    The possibility of producing profiles directly by hot extrusion of aluminum chips, normally considered as scrap, is a promising alternative to the energy-intensive remelting process. It has to be taken into account that the mechanical properties depend on the quality of the weld seams between the chips, which arise during the extrusion process. To estimate the influence of the weld seams, quasistatic and cyclic investigations were performed on chip-based profiles and finally compared with cast-based extruded profiles. In order to gain comprehensive information about the fatigue progress, different measurement techniques like alternating current potential drop (ACPD)-technique, hysteresis measurements, and temperature measurements were used during the fatigue tests. The weld seams and voids were investigated using computed tomography and metallographic techniques. Results show that quasistatic properties of chip-based specimens are only reduced by about 5%, whereas the lifetime is reduced by about a decade. The development of the fatigue cracks, which propagate between the chip boundaries, was characterized by an intermittent testing strategy, where an initiation of two separate cracks was observed. © 2019 by the authors.
    view abstractdoi: 10.3390/ma12152372
  • 2019 • 288 Potential Precursor Alternatives to the Pyrophoric Trimethylaluminium for the Atomic Layer Deposition of Aluminium Oxide
    Mai, L. and Boysen, N. and Zanders, D. and de los Arcos, T. and Mitschker, F. and Mallick, B. and Grundmeier, G. and Awakowicz, P. and Devi, A.
    Chemistry - A European Journal 25 7489-7500 (2019)
    New precursor chemistries for the atomic layer deposition (ALD) of aluminium oxide are reported as potential alternatives to the pyrophoric trimethylaluminium (TMA) which is to date a widely used Al precursor. Combining the high reactivity of aluminium alkyls employing the 3-(dimethylamino)propyl (DMP) ligand with thermally stable amide ligands yielded three new heteroleptic, non-pyrophoric compounds [Al(NMe2)2(DMP)] (2), [Al(NEt2)2(DMP)] (3, BDEADA) and [Al(NiPr2)2(DMP)] (4), which combine the properties of both ligand systems. The compounds were synthesized and thoroughly chemically characterized, showing the intramolecular stabilization of the DMP ligand as well as only reactive Al−C and Al−N bonds, which are the key factors for the thermal stability accompanied by a sufficient reactivity, both being crucial for ALD precursors. Upon rational variation of the amide alkyl chains, tunable and high evaporation rates accompanied by thermal stability were found, as revealed by thermal evaluation. In addition, a new and promising plasma enhanced (PE)ALD process using BDEADA and oxygen plasma in a wide temperature range from 60 to 220 °C is reported and compared to that of a modified variation of the TMA, namely [AlMe2(DMP)] (DMAD). The resulting Al2O3 layers are of high density, smooth, uniform, and of high purity. The applicability of the Al2O3 films as effective gas barrier layers (GBLs) was successfully demonstrated, considering that coating on polyethylene terephthalate (PET) substrates yielded very good oxygen transmission rates (OTR) with an improvement factor of 86 for a 15 nm film by using DMAD and a factor of 25 for a film thickness of just 5 nm by using BDEDA compared to bare PET substrates. All these film attributes are of the same quality as those obtained for the industrial precursor TMA, rendering the new precursors safe and potential alternatives to TMA. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201900475
  • 2019 • 287 On the effects of microstructure on the mechanical properties of open-pore Al–11Zn foams
    Matz, A.M. and Matz, B.S. and Parsa, A.B. and Jost, N. and Eggeler, G.
    Materials Science and Engineering A 759 552-564 (2019)
    The mechanical properties of investment casted open-pore metal foams have been investigated on the example of the binary alloy Al–11Zn. The samples were subjected to different cooling conditions subsequent to casting and to different homogenization and ageing treatments. Variation in cooling was done either by quenching the mold in water or slowly cooling it in air. Homogenization and ageing varied in terms of temperature and time. The effects of the different treatments were investigated through microstructural and mechanical characterization methods. Using TEM, we found that the presence of GP zones and their morphological arrangement are the main factors dominating the mechanical performance. Micro- and nanoindentation testing of single foam struts reveal maximum hardness H when room temperature ageing was applied. Ageing at a temperature of 150 °C results in the lowest H in the present study; that is approximately 2/3 of the hardness achieved when ageing at room temperature. This can also be confirmed by the strength of non-porous bulk material obtained by tensile tests, which further show an increase in ductility up to a factor of 5 due to ageing at elevated temperatures. By compression testing of open-pore Al–11Zn foams, we notice that the presence of the microstructural effects varies in extent as a function of the strain ε. At low strains, we observe differences in mechanical performance to a high extent, becoming less with increasing compaction of the samples until they behave as non-porous bulk material. Based on these findings, we deduce a strong interaction of the structural morphology of the foam and its microstructure that determines the mechanical properties dominated by strength and ductility of the base material. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.05.087
  • 2019 • 286 Simultaneous deep drawing and cold forging of multi-material components: Draw-forging
    Napierala, O. and Dahnke, C. and Tekkaya, A.E.
    CIRP Annals 68 269-272 (2019)
    A new process combining cold forging and deep drawing is introduced for forming multi-material components. The components consist of a cold extruded core with a deep drawn and redrawn cup acting as shell. Process mechanics, failures, and process window are investigated for a steel-aluminium pairing. The joining mechanisms between the steel shell and the aluminium core is of force- and form-fit type. The joining strength is larger than 40% of the shear yield stress of the weakest material. Alternative material pairings, chip-cores, double stepped shafts, and deep drawing with sequential backward cup-extrusion, are explored demonstrating the technological potential of the process. © 2019 CIRP
    view abstractdoi: 10.1016/j.cirp.2019.03.001
  • 2019 • 285 Analytical and experimental bond strength investigation of cold forged composite shafts
    Ossenkemper, S. and Dahnke, C. and Tekkaya, A.E.
    Journal of Materials Processing Technology 264 190-199 (2019)
    Composite cold forging denotes the simultaneous processing of hybrid raw parts by cold forging operations. The objective is the manufacturing of composite components by means of joining by plastic deformation. For lightweight purposes, composite shafts were produced by forward rod extrusion of backward extruded steel cups, into which an aluminum core has been inserted. The final composite shafts possess a wear-resistant outer steel sleeve and a light aluminum core. An analytical model has been developed to predict the strength of the force fit of composite shafts produced by cold forging. Push-out tests were conducted in order to experimentally determine the bond strength for the validation of the model. The analytically estimated bond strengths are in good accordance with the experimental values determined by push-out tests. The bond strength can be increased significantly by structuring the inner surface of the cups for the hybrid raw part to cause a form fit between steel sleeve and aluminum core after forward rod extrusion. Metallurgical bonds are not established between steel and aluminum in the investigated composite cold forged shafts. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2018.09.008
  • 2019 • 284 Strategies for improving the sustainability of structural metals
    Raabe, D. and Tasan, C.C. and Olivetti, E.A.
    Nature 575 64-74 (2019)
    Metallic materials have enabled technological progress over thousands of years. The accelerated demand for structural (that is, load-bearing) alloys in key sectors such as energy, construction, safety and transportation is resulting in predicted production growth rates of up to 200 per cent until 2050. Yet most of these materials require a lot of energy when extracted and manufactured and these processes emit large amounts of greenhouse gases and pollution. Here we review methods of improving the direct sustainability of structural metals, in areas including reduced-carbon-dioxide primary production, recycling, scrap-compatible alloy design, contaminant tolerance of alloys and improved alloy longevity. We discuss the effectiveness and technological readiness of individual measures and also show how novel structural materials enable improved energy efficiency through their reduced mass, higher thermal stability and better mechanical properties than currently available alloys. © 2019, Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-019-1702-5
  • 2019 • 283 Light, strong and cost effective: Martensitic steels based on the Fe – Al – C system
    Springer, H. and Zhang, J.-L. and Szczepaniak, A. and Belde, M. and Gault, B. and Raabe, D.
    Materials Science and Engineering A 762 (2019)
    We introduce a novel alloy design concept for density reduced ultra-high strength steels. It is based on the effects of C to increase martensite strength and the Al-solubility in austenite, in conjunction with Al to increase the martensite start temperature and to reduce density. This alloy combination results in inherently strong but light martensitic microstructures, whose mechanical properties (i.e. strength and ductility) can be readily adjusted over a wide range by applying straightforward and established heat treatments. The concept is validated on an Fe – 8 Al – 1.1C (wt.%) alloy subjected to quench and tempering treatments. The steel exhibits relatively low yield strength (~600 MPa) and reasonable ductility (~ 15% elongation) after hot rolling and soft annealing, but can be brought to a maximum hardness of 62 HRC after quenching and tempering at 250 °C. These attractive mechanical properties are coupled with a low density (6.95 g cm–3) and high elastic stiffness (Young's modulus 199 GPa). These lightweight martensitic steels are cost-effective structural materials for weight-critical applications, promising unprecedented specific strength. Advanced characterisation presented herein allows us to derive the fundamental underpinnings of carbide precipitation and phase transformation, and outline and discuss perspectives for refining the alloy composition and processing parameters. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.138088
  • 2018 • 282 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 • 281 Influence of surface roughness on the shear strength of direct injection molded plastic-aluminum hybrid-parts
    Bonpain, B. and Stommel, M.
    International Journal of Adhesion and Adhesives 82 290-298 (2018)
    The strength of hybrid metal and plastic joints is strongly influenced by the surface roughness of metal. Although many investigations on the change in shear strength of adhesively bonded joints due to roughening have been published, it is not completely understood how different mean roughness indexes Ra of directly joined plastic-metal-parts correlate to the resulting shear strength. This paper describes a schematic roughness - shear strength curve for adhesively bonded specimens and an experimental one for direct injection molded hybrid specimens which is reconciled with the state of the art. Roughening the surface of the metal is realized by grit blasting using fused alumina. After that, the metal is coated by direct injection molded PA 6.6 with 30% short glass fiber and shear strength tests are carried out. It can be concluded that with increasing Ra the shear strength of adhesively bonded specimens increases to a first maximum, then decreases and finally increases again. Direct injection molded samples exhibit a similar trend. The difference is that the second increase is significantly more pronounced, leading to a second maximum which is nearly twice as high as the first one. This difference is explained by the lower strength of the adhesive compared to the plastic and the different pronounced so called surface area effects, notch effects and thickness effects. By increasing Ra they promote cohesive fracture which is detrimental for adhesively bonded but more beneficial for direct injection molded samples. To further explain the experimental results, a FEM model to predict the probability of failure, the expected stress and the failure mode for direct injection molded samples is developed. The FE-analyses confirm the explanatory approach on the joint strength in dependence of the Ra value. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijadhadh.2018.02.003
  • 2018 • 280 Effect of tool wear evolution on chip formation during dry machining of Ti-6Al-4V alloy
    Dargusch, M.S. and Sun, S. and Kim, J.W. and Li, T. and Trimby, P. and Cairney, J.
    International Journal of Machine Tools and Manufacture 126 13-17 (2018)
    The complex microstructure of segmented chips and the changing deformation mechanisms during the machining of the Ti-6Al-4V alloy for a given cutting tool have been explored. Chip geometry and microstructure were investigated for increasing volumes of material removed at a cutting speed at which the tool characteristically develops gradual flank wear. The degree of chip segmentation and deformation mode changed significantly as machining progressed from using a new tool towards a worn tool. Chip formation processes when machining near the end of the cutting tool life is characterised by increasing amounts of twinning formed through both tension and compression. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijmachtools.2017.12.003
  • 2018 • 279 A method for the in-situ study of solid-state joining techniques using synchrotron radiation - observation of phase transformations in Ti-6Al-4V after friction surfacing
    Hanke, S. and Staron, P. and Fischer, T. and Fitseva, V. and dos Santos, J.F.
    Surface and Coatings Technology 335 355-367 (2018)
    The solid-state deposition process Friction Surfacing (FS) was applied to Ti-6Al-4V alloy on portable welding equipment at a high-energy synchrotron beamline. The heat input and coating thickness were altered by varying the deposition speed. X-ray diffraction was carried out in-situ during the deposition process and the cooling of the coated samples. Phase transformations were evaluated and correlated with thermal cycles determined by thermocouples and an infrared camera. SEM investigation of the coating microstructure was conducted to examine the morphology of the α phase. During FS the coating material is severely deformed and dynamically recrystallized in the β phase state at temperatures > 1300 °C. Small changes in the β grain size were observed within the first 2 s after deposition only. Depending on the cooling rate it transforms into different types of α phase during cooling. Phase transformation rates were found to correlate well with the differences in α morphology. The two faster translational speeds showed transformation rates > 45 vol%/s and a partially martensitic microstructure. When a thick coating is deposited at low translational speed, α → β transformation continues for several seconds after deposition, followed by a slow cooling rate resulting in martensite free coatings containing α m from massive transformation. The potential gain and the deficiencies of this complex in-situ study of a technical process, instead of simplified model experiments, for the understanding of fundamental mechanisms involved in FS are discussed. © 2017
    view abstractdoi: 10.1016/j.surfcoat.2017.12.049
  • 2018 • 278 Generation of Frenkel defects above the Debye temperature by proliferation of phonons near the Brillouin zone edge
    Jongmanns, M. and Raj, R. and Wolf, D.E.
    New Journal of Physics 20 (2018)
    A novel, non-radiative mechanism is reported by which Frenkel pairs of vacancies and interstitials are generated in molar concentrations far above thermal equilibrium. This mechanism is demonstrated in molecular dynamics (MD) simulations of an aluminum single crystal with a free surface. They suggest that three conditions must be fulfilled: (i) lattice vibrations near the Brillouin zone edge are being excited, (ii) these vibrations proliferate at a sufficiently high rate, and (iii) the sample temperature is above the Debye temperature (but significantly below the melting point). The simulations employed an EAM potential for Al. We attempt to draw a confluence between our MD simulations and recent experiments on flash sintering of aluminum. The simulation results are also consistent with flash experiments on polycrystals and single crystals of zirconium and titanium oxides where the Debye temperature was discovered to be the lower limit for the onset of the flash. © 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aadd5a
  • 2018 • 277 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 • 276 Deformation induced degradation of hot-dip aluminized steel
    Lemmens, B. and Springer, H. and Peeters, M. and De Graeve, I. and De Strycker, J. and Raabe, D. and Verbeken, K.
    Materials Science and Engineering A 710 385-391 (2018)
    In this work the fracture and corrosion behaviour of hot-dip aluminized steels is investigated in controlled dipping experiments which allowed to separately study the effects of Si in the Al bath (1–10 wt%) and the intermetallic phase thickness (5–30 µm). The addition of Si had no direct influence on the performance of the coating system for similar thickness values of the IMP seam, which in turn showed to be the dominant factor independent from the amount of Si. Thin intermetallic phase seams (< about 10 µm) exhibited more (about 5–10 per 100 µm interfacial length) but smaller cracks with a fishnet pattern on the outer Al-Si coating, which remained intact and interconnected until a tensile deformation of 15–20%. Thicker intermetallic phase seams resulted in less (about 2 per 100 µm interfacial length) but broader cracks perpendicular to the tensile direction, giving rise to a lamellar pattern on the Al-Si coating, which cracks and uncovers the steel already at strains below 10%, and readily flakes off leaving the steel substrate to accelerated corrosion in chloride environments. Our results indicate that the reduction of the intermetallic phase seam thickness remains the main target to improve the performance of hot-dip aluminized coated steel by combining appropriate Si additions with minimized dipping temperatures and times. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.10.094
  • 2018 • 275 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 • 274 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 • 273 Joining by die-less hydroforming of profiles with oval cross section
    Müller, M. and Gies, S. and Tekkaya, A.E.
    Key Engineering Materials 767 KEM 405-412 (2018)
    Joining by die-less hydroforming is used to produce overlap joints by means of hydraulic expansion. Due to a difference in the elastic recovery of the two joining partners an interference pressure p remains at their contact area. Due to the possibility to produce multi-material joints without relying on heat, the process has great potential for joining parts in lightweight applications. Therefore, the process limits were extended so that profiles with non-rotationally symmetric cross-sections can be joined. For this purpose a new tool for profiles with oval cross-section was developed. The inner and the outer joining partner ware made of aluminum 6060 and aluminum 6082 respectively. The influence of the overlap length and different wall thicknesses of the outer joining partner were investigated by numerical simulations and validated by experiments. An upper limit in interference pressure was observed which was also found previously for profiles with circular cross sections. The fluid pressure limit is compared with the analytically calculated value for a configuration with circular tubes under equivalent conditions. The analytical model underestimated the pressure limit. In contrast to circular tubes, the strain distribution of profiles with oval cross sectional shapes is not uniform, which results in superposed bending stresses. Also a difference in stiffness of the inner and outer joining partner leads to a pressure depended contact area which is assumed constant in the analytical model. © 2018 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2018 • 272 In situ analysis of damage evolution in an Al/ Al 2O 3 MMC under tensile load by synchrotron X-ray refraction imaging
    Nellesen, J. and Laquai, R. and Müller, B.R. and Kupsch, A. and Hentschel, M.P. and Anar, N.B. and Soppa, E. and Tillmann, W. and Bruno, G.
    Journal of Materials Science 53 6021-6032 (2018)
    The in situ analysis of the damage evolution in a metal matrix composite (MMC) using synchrotron X-ray refraction radiography (SXRR) is presented. The investigated material is an Al alloy (6061)/10 vol% Al 2O 3 MMC after T6 heat treatment. In an interrupted tensile test the gauge section of dog bone-shaped specimens is imaged in different states of tensile loading. On the basis of the SXRR images, the relative change of the specific surface (proportional to the amount of damage) in the course of tensile loading was analyzed. It could be shown that the damage can be detected by SXRR already at a stage of tensile loading, in which no observation of damage is possible with radiographic absorption-based imaging methods. Moreover, the quantitative analysis of the SXRR images reveals that the amount of damage increases homogeneously by an average of 25% with respect to the initial state. To corroborate the experimental findings, the damage distribution was imaged in 3D after the final tensile loading by synchrotron X-ray refraction computed tomography (SXRCT) and absorption-based synchrotron X-ray computed tomography (SXCT). It could be evidenced that defects and damages cause pronounced indications in the SXRCT images. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s10853-017-1957-x
  • 2018 • 271 Understanding precipitate evolution during friction stir welding of Al-Zn-Mg-Cu alloy through in-situ measurement coupled with simulation
    dos Santos, J.F. and Staron, P. and Fischer, T. and Robson, J.D. and Kostka, A. and Colegrove, P. and Wang, H. and Hilgert, J. and Bergmann, L. and Hütsch, L.L. and Huber, N. and Schreyer, A.
    Acta Materialia 148 163-172 (2018)
    Friction Stir Welding (FSW) imparts both heat and deformation to the metal being joined, producing profound microstructural changes that determine the weld properties. In the case of welding of aerospace aluminium alloys, the most important change is the modification of the size, nature, and fraction of strengthening precipitates. To understand these changes requires the ability to measure the microstructural evolution during the welding process. This paper describes a new tool, the FlexiStir system, a portable friction stir unit designed for use in a high-energy synchrotron beamline that enables in-situ studies of microstructural evolution during FSW. FlexiStir has been used to measure precipitate evolution during FSW of aluminium alloy 7449-TAF and provide time-resolved measurement of precipitate size and volume fraction via small angle X-ray scattering (SAXS). These measurements have been interpreted with the aid of a previously developed microstructural model. The model predictions and SAXS measurements are in good qualitative agreement and demonstrate the complex precipitate transformation, dissolution, and reprecipitation events that occur during welding. © 2018
    view abstractdoi: 10.1016/j.actamat.2018.01.020
  • 2018 • 270 Defect-correlated fatigue assessment of A356-T6 aluminum cast alloy using computed tomography based Kitagawa-Takahashi diagrams
    Tenkamp, J. and Koch, A. and Knorre, S. and Krupp, U. and Michels, W. and Walther, F.
    International Journal of Fatigue 108 25-34 (2018)
    Modern aluminum cast alloys are promising candidates for highly loaded lightweight components due to their good strength-to-weight ratio. To enable a safe design of aluminum cast components, the microstructure characteristics, e.g. porosity, secondary dendrite arm spacing and Si eutectic morphology, have to be taken into account. This study deals with the influence of different porosity characteristics on the fatigue behavior of A356-T6 aluminum cast alloy. For this purpose, cyclic deformation tests have been carried out to study fatigue damage and fatigue crack growth in the high cycle fatigue (HCF) regime up to 107 cycles. The porosity of each batch was quantified using micro-computed tomography. Results show that large porosity dominates the HCF fatigue behavior. The relationship between porosity characteristics and the fatigue limit as well as S-N curve was assessed by Kitagawa-Takahashi diagrams and a fracture mechanic approach. © 2017
    view abstractdoi: 10.1016/j.ijfatigue.2017.11.003
  • 2017 • 269 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 • 268 Investigations of aging behaviour for aluminium powders during an atmosphere simulation of the LBM process
    Bauer, D.M. and Schwarzenböck, E. and Ludwig, I. and Schupp, N. and Palm, F. and Witt, G.
    Powder Metallurgy 1-9 (2017)
    Additive layer manufacturing (ALM) offers for production of parts and components for aeronautical applications potential cost benefits over conventional manufacturing routes. In particular, powder bed processes offer a high degree of design flexibility while enabling weight reduction due to topological optimisation. The quality and properties of the parts are strongly dependent on the powder quality which, in turn, is influenced by handling and storage of the powder. For this reasons an undefined contamination of atomised powder materials by oxygen and hydrogen has to be avoided. Aluminium-silicon powder was aged under atmosphere of different moistures and temperatures for defined duration. The effect of these environments as well as the effect of vacuum drying on the flowability was investigated. The morphology was evaluated by scanning electron microscope. The chemistry including oxygen content of the powder was measured by inductively coupled plasma optical emission spectrometry and hot fusion analysis. © 2017 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute
    view abstractdoi: 10.1080/00325899.2017.1288841
  • 2017 • 267 TiC particle reinforced Ti-6Al-4V friction surfacing coatings
    Belei, C. and Fitseva, V. and dos Santos, J.F. and Alcântara, N.G. and Hanke, S.
    Surface and Coatings Technology 329 163-173 (2017)
    Friction surfacing is a thermo-mechanical process employed to deposit coatings in solid state resorting to friction between a rotating consumable rod and a substrate. The current work focuses on deposition of Ti-6Al-4V composite coatings reinforced with TiC particles on Ti-6Al-4V substrate. Particles were added using holes drilled into the rod tip. Different configurations of hole placements within the rod were correlated with process behavior, coating quality, deposition efficiency and particle distribution within the deposits. Configurations varied in number of holes and their distance to the rod's cross-sectional center. Holes placed near to the rod center increased axial forces during the plastification stage, whereas particles in holes far off the rod center were mainly expelled, not yielding as much effect on the process response. An increase in number of holes amplified the effects of the hole distance. The axial force during plastification stage affected both coating appearance and process efficiency. No full intermixing of coating material and particles during deposition occurred, thereby preventing a uniform distribution of particles throughout the coatings. Particles were mostly deposited along trails, which influenced the behavior of growing grains during recrystallization. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2017.09.050
  • 2017 • 266 Fabrication of Oxide Dispersion Strengthened Bond Coats with Low Al2O3 Content
    Bergholz, J. and Pint, B.A. and Unocic, K.A. and Vaßen, R.
    Journal of Thermal Spray Technology 1-12 (2017)
    Nanoscale oxide dispersions have long been used to increase the oxidation and wear resistance of alloys used as bond coatings in thermal barrier coatings. Their manufacturing via mechanical alloying is often accompanied by difficulties regarding their particle size, homogeneous distribution of the oxide dispersions inside the powder, involving considerable costs, due to cold welding of the powder during milling. A significant improvement in this process can be achieved by the use of process control agent (PCA) to achieve the critical balance between cold welding and fracturing, thereby enhancing the process efficiency. In this investigation, the influence of the organic additive stearic acid on the manufacturing process of Al2O3-doped CoNiCrAlY powder was investigated. Powders were fabricated via mechanical alloying at different milling times and PCA concentrations. The results showed a decrease in particle size, without hindering the homogeneous incorporation of the oxide dispersions. Two powders manufactured with 0.5 and 1.0 wt.% PCA were deposited by high velocity oxygen fuel (HVOF) spraying. Results showed that a higher content of elongated particles in the powder with the higher PCA content led to increased surface roughness, porosity and decreased coating thickness, with areas without embedded oxide particles. © 2017 ASM International
    view abstractdoi: 10.1007/s11666-017-0550-9
  • 2017 • 265 Front Face Flow Drilling of Lightweight Cast Materials
    Biermann, D. and Walther, F. and Hannich, S. and Wittke, P.
    Procedia Engineering 207 956-961 (2017)
    Front face flow drilling, which has been investigated at the Institute of Machining Technology (ISF), TU Dortmund University, represents a new application of the conventional flow drilling process. With this new technique it is possible to form closed holes with diameters, which can exceed the local wall thickness of the profile. By using a subsequent threading operation, it is possible to generate solid joints. In this article investigations regarding the machining of the aluminium cast alloy AlSi10Mg are presented. The feasibility of the front face flow drilling application was analysed for the machining of thin profiles with a wall thickness of tW = 6 mm. Flow drilling tools with a diameter of dFD = 5.4 mm have been used. Feed forces as well as torques were measured during the flow drilling process. In order to generate solid threads, a high quality of the formed holes has to be ensured. To quantify the quality, measurements of the circularity as well as the diameters were carried out. Both aspects can have an influencing effect on the threading operation. Due to the low formability of the used alloys, adjustments of the flow drilling process had to be realised. Besides the variation of process parameters such as peripheral speed and feed velocity, a pre-heating of the tool was conducted to raise the formability of the workpiece material and to increase the process stability. The pre-heating of the tool was accomplished by using a portable induction system with an integrated temperature control. As a result of the investigations, suitable process strategies will be recommended regarding a stable front face flow drilling operation of the lightweight cast alloy AlSi10Mg. Further research regarding front face flow drilling is planned towards an adaptation to magnesium cast alloys. © 2017 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2017.10.858
  • 2017 • 264 Characterization of DC magnetron plasma in Ar/Kr/N2 mixture during deposition of (Cr,Al)N coating
    Bobzin, K. and Bagcivan, N. and Theiß, S. and Brugnara, R. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 50 (2017)
    doi: 10.1088/1361-6463/aa4ea2
  • 2017 • 263 Fundamental study of an industrial reactive HPPMS (Cr,Al)N process
    Bobzin, K. and Brögelmann, T. and Kruppe, N.C. and Engels, M. and Von Keudell, A. and Hecimovic, A. and Ludwig, Al. and Grochla, D. and Banko, L.
    Journal of Applied Physics 122 (2017)
    In this work, a fundamental investigation of an industrial (Cr,Al)N reactive high power pulsed magnetron sputtering (HPPMS) process is presented. The results will be used to improve the coating development for the addressed application, which is the tool coating for plastics processing industry. Substrate-oriented plasma diagnostics and deposition of the (Cr,Al)N coatings were performed for a variation of the HPPMS pulse frequency with values from f = 300 Hz to f = 2000 Hz at constant average power P = 2.5 kW and pulse length ton = 40 μs. The plasma was investigated using an oscilloscope, an intensified charge coupled device camera, phase-resolved optical emission spectroscopy, and an energy-dispersive mass spectrometer. The coating properties were determined by means of scanning electron microscopy, glow discharge optical emission spectroscopy, cantilever stress sensors, nanoindentation, and synchrotron X-ray diffraction. Regarding the plasma properties, it was found that the average energy within the plasma is nearly constant for the frequency variation. In contrast, the metal to gas ion flux ratio is changed from JM/JG = 0.51 to JM/JG = 0.10 for increasing frequency. Regarding the coating properties, a structure refinement as well as lower residual stresses, higher universal hardness, and a changing crystal orientation from (111) to (200) were observed at higher frequencies. By correlating the plasma and coating properties, it can be concluded that the change in the gas ion to metal ion flux ratio results in a competitive crystal growth of the film, which results in changing coating properties. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4990997
  • 2017 • 262 Modeling of Water Adsorption in SAPO-34-Coated Aluminum Foam
    Bruckner, S. and Demmer, T. and Ganswind, M. and Bathen, D.
    Chemie-Ingenieur-Technik 89 757-764 (2017)
    After presenting the most promising continuous adsorption cooling topology for air-conditioning of a passenger cabin, a transient, 3D model is proposed, based on a modified linear driving force approach, to simulate an adsorption process in a SAPO-34-coated open-celled aluminum foam. The simulation is executed in ANSYS CFX 15.0 and the additional model equations are implemented through the CFX expression language. As a result the influence of the pore diameter onto the system simulation and the mass transport limitation is shown and the sensitivity concerning the pressure in the gas phase adsorption is confirmed. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cite.201600035
  • 2017 • 261 High-Throughput Structural and Functional Characterization of the Thin Film Materials System Ni-Co-Al
    Decker, P. and Naujoks, D. and Langenkämper, D. and Somsen, C. and Ludwig, Al.
    ACS Combinatorial Science 19 618-624 (2017)
    High-throughput methods were used to investigate a Ni-Co-Al thin film materials library, which is of interest for structural and functional applications (superalloys, shape memory alloys). X-ray diffraction (XRD) measurements were performed to identify the phase regions of the Ni-Co-Al system in its state after annealing at 600 °C. Optical, electrical, and magneto-optical measurements were performed to map functional properties and confirm XRD results. All results and literature data were used to propose a ternary thin film phase diagram of the Ni-Co-Al thin film system. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscombsci.6b00176
  • 2017 • 260 Modifications of aluminum film caused by micro-plasmoids and plasma spots in the effluent of an argon non-equilibrium plasma jet
    Engelhardt, M. and Ries, S. and Hermanns, P. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 50 (2017)
    A smooth layer of hard aluminium film is deposited onto a glass substrate with a multi-frequency CCP discharge and then treated in the effluent of a non-equilibrium atmospheric pressure plasma jet (N-APPJ) operated with Ar flow. A thin filament is formed in the argon N-APPJ through contraction of a diffuse feather-like discharge. The aluminium surface treated in the effluents of the N-APPJ is significantly modified. Erosion tracks of different forms and micro-balls composed of aluminium are observed on the treated surface. Based on CCD images of active plasma discharge channels, SEM images of the treated surface and current-voltage characteristics, these surface modifications are interpreted as traces of plasma spots and plasmoids. Plasma spots are focused plasma channels, which are characterized by an intense emission in CCD images at the contact point of a plasma channel with the treated metal surface and by deep short tracks on the aluminium surface, observed in SEM images. Plasmoids are plasma objects without contact to any power supply which can produce long, thin and shallow traces, as can be observed on the treated surface using electron microscopy. Based on observed traces and numerous transformations of plasma spots to plasmoids and vice versa, it is supposed that both types of plasma objects are formed by an extremely high axial magnetic field and differ from each other due to the existence or absence of contact to a power supply and the consequential transport of electric current. The reason for the magnetic field at the axis of these plasma objects is possibly a circular current of electron pairs in vortices, which are formed in plasma by the interaction of ionization waves with the substrate surface. The extremely high magnetic field of plasma spots and plasmoids leads to a local destruction of the metal film and top layer of the glass substrate and to an attraction of paramagnetic materials, namely aluminium and oxygen. The magnetic attraction of aluminium is a reason for the extraction of some pieces of metal and the formation of erosion tracks and holes in the metal film. In the absence of metal atomization, the extracted aluminium forms spherical micro-particles, which are distributed over the surface of the treated metal film by the gas flow. A thin (100 nm) gold (diamagnetic) layer on top of the aluminium film surface reduces the erosion rate of plasma spots and plasmoids drastically (more than three orders of magnitude). © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa802f
  • 2017 • 259 Influence of rotational speed on process characteristics in friction surfacing of Ti-6Al-4V
    Fitseva, V. and Hanke, S. and dos Santos, J.F.
    Materials and Manufacturing Processes 32 557-563 (2017)
    Friction surfacing process is employed to deposit metallic coatings, whereby similar and dissimilar material combinations can be realized. The process can be applied as a local repair technology, or the coating material can locally modify the surfaces. One advantage of this process is that the coatings are deposited in solid state without reaching the melting range of materials, thereby avoiding dilution with the substrate. The involved severe plastic deformation under high temperatures alters the microstructure of the coating material, leaving it fully dynamically recrystallized. The current work focuses on deposition of Ti-6Al-4V coatings. For that material, the process parameter rotational speed plays a major role in the material’s response during processing. Two different regimes with a threshold at 2000 min−1exist, upon which the flow behavior of Ti-6Al-4V significantly differs, affecting among others the coating dimensions. Microstructural analysis reveals that the material is deformed in a high temperature β phase, and the high cooling rates (46.4 Ks−1) lead to martensitic transformation. The β grain size differs in the low and high rotational speed regimes. This study shows that metallurgical processes play an important role in friction surfacing, since they influence all relevant process characteristics, including microstructure, material efficiency and process forces. © 2017 Taylor & Francis.
    view abstractdoi: 10.1080/10426914.2016.1257799
  • 2017 • 258 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 • 257 Local photocurrent mapping and cell performance behaviour on a nanometre scale for monolithically interconnected Cu(In,Ga)Se2 solar cells
    Haggui, M. and Reinhold, B. and Andrae, P. and Greiner, D. and Schmid, M. and Fumagalli, P.
    Journal of Microscopy 268 66-72 (2017)
    The local efficiency of lamellar shaped Cu(In,Ga)Se2 solar cells has been investigated using scanning near-field optical microscopy (SNOM). Topographic and photocurrent measurements have been performed simultaneously with a 100 nm tip aperture. The lamellar shaped solar cell with monolithic interconnects (P scribe) has been investigated on a nanometre scale for the first time at different regions using SNOM. It was found that, the cell region between P1 and P2 significantly contributes to the solar cells overall photocurrent generation. The photocurrent produced depends locally on the sample topography and it is concluded that it is mainly due to roughness changes of the ZnO:Al/i-ZnO top electrode. Regions lying under large grains of ZnO produce significantly less current than regions under small granules. The observed photocurrent features were allocated primarily to the ZnO:Al/i-ZnO top electrode. They were found to be independent of the wavelength of the light used (532 nm and 633 nm). © 2017 The Authors Journal of Microscopy © 2017 Royal Microscopical Society
    view abstractdoi: 10.1111/jmi.12587
  • 2017 • 256 Comparative study of severe plastic deformation at elevated temperatures of two aluminium alloys during friction surfacing
    Hanke, S. and dos Santos, J.F.
    Journal of Materials Processing Technology 247 257-267 (2017)
    Aluminium alloys 5083 and 6082 were deposited by Friction Surfacing (FS) under the same process conditions. Process characteristics including torque and forces, temperatures and the deposit microstructure were compared. The observed differences are discussed with regard to material strength, thermal softening rate and recrystallization mechanisms. AA 6082 plasticises faster, reaching ≈30 K higher temperatures, thicker and wider coatings and a higher material efficiency. The specific energy required for plastification is in the same order of magnitude as the activation energy for self-diffusion, emphasising the influence of dynamic recrystallization (DRX) mechanisms. A tendency for lower grain size and larger variations in grain boundary misorientation observed for AA 5083 points towards a shift in the steady-state DRX balance towards dislocation generation, due to the higher Mg content of this alloy. This corresponds to the lower process speeds required for AA 5083. AA 6082 may undergo more localized shear because of its high thermal softening rate and additional loss of strength through dissolution of Mg2Si with increasing temperature. This may contribute to a higher energy and material efficiency for plastification and deposition of AA 6082 by FS. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2017.04.021
  • 2017 • 255 Precipitation of T1 and θ' phase in Al-4Cu-1Li-0.25Mn during age hardening: Microstructural investigation and phase-field simulation
    Häusler, I. and Schwarze, C. and Bilal, M.U. and Ramirez, D.V. and Hetaba, W.d and Kamachali, R.D. and Skrotzki, B.
    Materials 10 (2017)
    Experimental and phase field studies of age hardening response of a high purity Al-4Cu-1Li-0.25Mn-alloy (mass %) during isothermal aging are conducted. In the experiments, two hardening phases are identified: the tetragonal θ' (Al2Cu) phase and the hexagonal T1 (Al2CuLi) phase. Both are plate shaped and of nm size. They are analyzed with respect to the development of their size, number density and volume fraction during aging by applying different analysis techniques in TEM in combination with quantitative microstructural analysis. 3D phase-field simulations of formation and growth of θ' phase are performed in which the full interfacial, chemical and elastic energy contributions are taken into account. 2D simulations of T1 phase are also investigated using multi-component diffusion without elasticity. This is a first step toward a complex phase-field study of T1 phase in the ternary alloy. The comparison between experimental and simulated data shows similar trends. The still unsaturated volume fraction indicates that the precipitates are in the growth stage and that the coarsening/ripening stage has not yet been reached. © 2017 by the authors.
    view abstractdoi: 10.3390/ma10020117
  • 2017 • 254 Microstructural evolution and solid state dewetting of epitaxial Al thin films on sapphire (α-Al2O3)
    Hieke, S.W. and Breitbach, B. and Dehm, G. and Scheu, C.
    Acta Materialia 133 356-366 (2017)
    Solid state dewetting can be used for targeted patterning, but also causes degradation or failure of thin film devices. In this work the temperature-induced changes of a tetracrystalline model system with inhibited surface diffusion are studied. This is accomplished by growing Al thin films by molecular beam epitaxy on single crystalline (0001) oriented sapphire substrates. The as-deposited Al films form two orientation relationships (OR I and OR II) both subdivided in two twin-related growth variants leading to a tetracrystalline microstructure. Two processes evolve during annealing at 600 °C. Grain growth and texture evolution towards OR II occur in addition to the formation of drum-like voids in the Al film covered by a thin membrane. The surface oxide suppresses Al surface diffusion and in contrast to classical solid state dewetting interface and grain boundary diffusion dominate. High energy grain boundaries were identified as initial points of the void formation. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.05.026
  • 2017 • 253 Annealing induced void formation in epitaxial Al thin films on sapphire (α-Al2O3)
    Hieke, S.W. and Dehm, G. and Scheu, C.
    Acta Materialia 140 355-365 (2017)
    In this work faceted voids are studied which were induced by solid state dewetting at 600 °C of tetracrystalline Al thin films covered with a native oxide layer. Hexagonally shaped voids are observed in a few locations where Al is uniformly redistributed to the surrounding thin film. Although faceted, the majority of the voids exhibit irregular shapes caused by pinning of distinct sides of the retracting Al thin film. The two different Al|void shapes (hexagonal or irregular) are investigated by site-specific cross-sectional transmission electron microscopy (TEM) analysis. The TEM studies reveal Al|void regions with and without rims and ridges. The presence of rims and ridges is explained by a discontinuous void formation process caused by pinning of the retracting Al film. During annealing, crystallization and a thickness increase of the surface oxide, which is still continuously covering the Al thin film as well as the void, occurs. The surface scale undergoes a phase transformation from the amorphous state to γ-Al2O3, which is confirmed locally on the nanometer scale using scanning TEM techniques including electron energy loss near-edge structure investigations. Spherical aberration corrected atomic column resolved scanning TEM revealed a cube-on-cube orientation relationship between the Al thin film and the γ-Al2O3 surface oxide. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.08.050
  • 2017 • 252 Investigation on the influence of surface roughness on the moment coefficient in a rotor-stator cavity with centripet al through-flow
    Hu, B. and Brillert, D. and Dohmen, H.J. and Benra, F.-K.
    American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM 1A-2017 (2017)
    In radial pumps and turbines, the leakage flow (centripet al through-flow) is quite common from the outer radius of the impeller to the impeller eye, which has major impact on frictional torque. There is still an uncertainty on the impact of surface roughness on the moment coefficient. Part of the 2D Daily&Nece diagram where the flow is categorized into four regimes is extended into 3D with the third axis of through-flow coefficient by distinguishing the profiles of tangential velocity. After classifying the flow regimes with respect to the centripet al through-flow, two correlations are developed to predict the impact of the axial gap, the global Reynolds number, the through-flow coefficient and the surface roughness on the moment coefficient according to the experimental results for both regime III and regime IV. Using the proposed equations for the moment coefficient, the influence of the centripet al throughflow and surface roughness can be better considered when designing radial pumps and turbines. © Copyright 2017 ASME.
    view abstractdoi: 10.1115/FEDSM2017-69018
  • 2017 • 251 Ab initio modelling of solute segregation energies to a general grain boundary
    Huber, L. and Grabowski, B. and Militzer, M. and Neugebauer, J. and Rottler, J.
    Acta Materialia 132 138-148 (2017)
    We apply a quantum mechanical/molecular mechanical (QM/MM) multiscale approach to calculate the segregation energies of Mg and Pb to two kinds of grain boundaries in Al. The first boundary, a symmetric (310)[001] Σ5 tilt boundary, is also tractable using traditional QM calculations, and serves as a validation for the QM/MM method. The second boundary is a general, low-symmetry tilt boundary that is completely inaccessible to pure QM calculations. QM/MM results for both of these boundaries are used to evaluate the accuracy of empirical (EAM) potentials for the Al-Mg and Al-Pb alloy systems. Based on these results we develop a physical model for the segregation energy based on elastic interaction and bond breaking terms. Both MM calculations with the EAM potentials and the model work quantitatively well for describing Mg-GB interaction across a wide range of local environments. For Pb, MM performance is weaker and the model provides only qualitative insight, demonstrating the utility of a QM/MM approach. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.04.024
  • 2017 • 250 Comparison of deep drawability of AA5754-H22 and AA6061-T6 aluminum alloys for automotive applications
    Ipekoglu, M. and Erbas, O. and Ul Hassan, H.
    Materialpruefung/Materials Testing 59 1003-1008 (2017)
    Being one of the most commonly applied sheet metal forming processes in automotive industry, deep drawing technologies are challenged by the concerns of global warming for higher fuel economy requirements in the recent years. To reduce the weight of the vehicles in order to obtain fuel economy, lighter and safer materials are used in automotive industry. Aluminum alloys, due to their low density compared to steels, are an important group of materials, in particular for lightweight construction of vehicles. In this study, two different aluminum alloys, namely AA5754-H22 and AA6061-T6, are evaluated for their potential use in nonload bearing applications in commercial vehicles by comparing their deep drawability characteristics. For this purpose, they are characterized by uniaxial tensile and Nakajima tests. In the next step, the deep drawing experiments are performed under different levels of blankholder force. The distance of the cup center to the edge of the die is also evaluated to investigate the formability of the designed cup for commercial use and to obtain the required product quality. © Carl Hanser Verlag, München.
    view abstractdoi: 10.3139/120.111100
  • 2017 • 249 Massive nanoprecipitation in an Fe-19Ni-xAl maraging steel triggered by the intrinsic heat treatment during laser metal deposition
    Kürnsteiner, P. and Wilms, M.B. and Weisheit, A. and Barriobero-Vila, P. and Jägle, E.A. and Raabe, D.
    Acta Materialia 129 52-60 (2017)
    Due to the layer-by-layer build-up of additively manufactured parts, the deposited material experiences a cyclic re-heating in the form of a sequence of temperature pulses. In the current work, this “intrinsic heat treatment (IHT)” was exploited to induce the precipitation of NiAl nanoparticles in an Fe-19Ni-xAl (at%) model maraging steel, a system known for rapid clustering. We used Laser Metal Deposition (LMD) to synthesize compositionally graded specimens. This allowed for the efficient screening of effects associated with varying Al contents ranging from 0 to 25 at% and for identifying promising concentrations for further studies. Based on the existence of the desired martensitic matrix, an upper bound for the Al concentration of 15 at% was defined. Owing to the presence of NiAl precipitates as observed by Atom Probe Tomography (APT), a lower bound of 3–5 at% Al was established. Within this concentration window, increasing the Al concentration gave rise to an increase in hardness by 225 HV due to an exceptionally high number density of 1025 NiAl precipitates per m3, as measured by APT. This work demonstrates the possibility of exploiting the IHT of the LMD process for the production of samples that are precipitation strengthened during the additive manufacturing process without need for any further heat treatment. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.02.069
  • 2017 • 248 Composite targets in HiPIMS plasmas: Correlation of in-vacuum XPS characterization and optical plasma diagnostics
    Layes, V. and Monje, S. and Corbella, C. and Schulz-Von der Gathen, V. and Von Keudell, A. and De Los Arcos, T.
    Journal of Applied Physics 121 (2017)
    In-vacuum characterization of magnetron targets after High Power Impulse Magnetron Sputtering (HiPIMS) has been performed by X-ray photoelectron spectroscopy (XPS). Al-Cr composite targets (circular, 50 mm diameter) mounted in two different geometries were investigated: an Al target with a small Cr disk embedded at the racetrack position and a Cr target with a small Al disk embedded at the racetrack position. The HiPIMS discharge and the target surface composition were characterized in parallel for low, intermediate, and high power conditions, thus covering both the Ar-dominated and the metal-dominated HiPIMS regimes. The HiPIMS plasma was investigated using optical emission spectroscopy and fast imaging using a CCD camera; the spatially resolved XPS surface characterization was performed after in-vacuum transfer of the magnetron target to the XPS chamber. This parallel evaluation showed that (i) target redeposition of sputtered species was markedly more effective for Cr atoms than for Al atoms; (ii) oxidation at the target racetrack was observed even though the discharge ran in pure Ar gas without O2 admixture, the oxidation depended on the discharge power and target composition; and (iii) a bright emission spot fixed on top of the inserted Cr disk appeared for high power conditions. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4977820
  • 2017 • 247 Stress-modulated tilt actuator for tunable optical prisms
    Leopold, S. and Paetz, D. and Sinzinger, S. and Hoffmann, M.
    Sensors and Actuators, A: Physical 266 328-337 (2017)
    A tunable optical prism MOEMS based on the deformation of a liquid droplet is presented. An aluminum-nitride membrane is tilted by a novel type of thermo-mechanical actuator. The actuation principle is based on a thermo-mechanical modulation of the intrinsic stress in aluminum-nitride beams. Based on an analytical model, the key parameters of the actuator are optimized. Furthermore, the influence of the intrinsic stress on the actuator properties is investigated. These dependencies and the model are verified by mechanical characterization of samples. Operation in air and with ambient fluid has been confirmed. An image shift of 30 mm is found in a microscopic setup which corresponds to 19 % of the field of view. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.sna.2017.09.021
  • 2017 • 246 Unearthing [3-(Dimethylamino)propyl]aluminium(III) Complexes as Novel Atomic Layer Deposition (ALD) Precursors for Al2O3: Synthesis, Characterization and ALD Process Development
    Mai, L. and Gebhard, M. and de los Arcos, T. and Giner, I. and Mitschker, F. and Winter, M. and Parala, H. and Awakowicz, P. and Grundmeier, G. and Devi, A.
    Chemistry - A European Journal 23 10768-10772 (2017)
    Identification and synthesis of intramolecularly donor-stabilized aluminium(III) complexes, which contain a 3-(dimethylamino)propyl (DMP) ligand, as novel atomic layer deposition (ALD) precursors has enabled the development of new and promising ALD processes for Al2O3 thin films at low temperatures. Key for this promising outcome is the nature of the ligand combination that leads to heteroleptic Al complexes encompassing optimal volatility, thermal stability and reactivity. The first ever example of the application of this family of Al precursors for ALD is reported here. The process shows typical ALD like growth characteristics yielding homogeneous, smooth and high purity Al2O3 thin films that are comparable to Al2O3 layers grown by well-established, but highly pyrophoric, trimethylaluminium (TMA)-based ALD processes. This is a significant development based on the fact that these compounds are non-pyrophoric in nature and therefore should be considered as an alternative to the industrial TMA-based Al2O3 ALD process used in many technological fields of application. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201702939
  • 2017 • 245 Correlative plasma-surface model for metastable Cr-Al-N: Frenkel pair formation and influence of the stress state on the elastic properties
    Music, D. and Banko, L. and Ruess, H. and Engels, M. and Hecimovic, A. and Grochla, D. and Rogalla, D. and Brögelmann, T. and Ludwig, Al. and Von Keudell, A. and Bobzin, K. and Schneider, J.M.
    Journal of Applied Physics 121 (2017)
    Correlatively employing density functional theory and experiments congregated around high power pulsed magnetron sputtering, a plasma-surface model for metastable Cr0.8Al0.2N (space group Fm 3 m) is developed. This plasma-surface model relates plasma energetics with film composition, crystal structure, mass density, stress state, and elastic properties. It is predicted that N Frenkel pairs form during Cr0.8Al0.2N growth due to high-energy ion irradiation, yielding a mass density of 5.69 g cm-3 at room temperature and Young's modulus of 358-130 GPa in the temperature range of 50-700 K for the stress-free state and about 150 GPa larger values for the compressive stress of 4 GPa. Our measurements are consistent with the quantum mechanical predictions within 5% for the mass density and 3% for Young's modulus. The hypothesis of a stress-induced Young's modulus change may at least in part explain the spread in the reported elasticity data ranging from 250 to 420 GPa. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4985172
  • 2017 • 244 Effect of Al3+ modification on cobalt ferrite and its impact on the magnetoelectric effect in BCZT–CFO multiferroic composites
    Naveed-Ul-Haq, M. and Shvartsman, V.V. and Constantinescu, G. and Trivedi, H. and Salamon, S. and Landers, J. and Wende, H. and Lupascu, D.C.
    Journal of Materials Science 1-12 (2017)
    One of the methods to enhance the functional properties of two-phase multiferroic magnetoelectrics is to increase magnetostriction of the ferrite phase. Al3+-modified cobalt ferrite Co(Al0.5Fe1.5)O4 shows better magnetostriction than unmodified cobalt ferrite. It is used in combination with (Ba,Ca)(Zr,Ti)O3 which has very good piezoelectric properties, to form a multiferroic composite. The composite shows good magnetoelectric characteristics, both macroscopically (converse magnetoelectric coefficient of 11 ps/m) and microscopically. Al3+ proves to be the best non-magnetic dopant to enhance magnetostriction in CoFe2O4 and thus the magnetoelectric coefficient. © 2017 Springer Science+Business Media, LLC
    view abstractdoi: 10.1007/s10853-017-1444-4
  • 2017 • 243 Unusual application of aluminium-doped ZnO thin film developed by metalorganic chemical vapour deposition for surface temperature sensor
    Nebatti, A. and Pflitsch, C. and Atakan, B.
    Thin Solid Films 636 532-536 (2017)
    A relatively new promising method for surface temperature measurement is the use of thermographic phosphors. For this purpose, the temperature-dependent photoluminescence (PL) properties of aluminium-doped ZnO thin films were studied. The films have been successfully deposited on substrate of Si(100)-orientation by metalorganic chemical vapour deposition (MOCVD) method. For the use of the films as temperature sensors, the Photoluminescence (PL) properties are most important. Consequently, the emission peaks are observed in the undoped and Al-doped films deposited at 550 °C and annealed at 900 °C for 2 h after ultraviolet laser excitation (355 nm). The results show that with increasing temperature the PL intensity is quenched for the Al-doped ZnO film (n(Al)/n(Zn) = 0.051). As a result, the area under the spectrum changes significantly with temperature, making it useful for temperature evaluation. Al-doped ZnO films can be used as temperature sensors within the range of room temperature to 300 °C. Beyond this range the spectrum is no longer sensitive to temperature change.
    view abstractdoi: 10.1016/j.tsf.2017.07.002
  • 2017 • 242 Ammonia Decomposition and Synthesis over Multinary Magnesioferrites: Promotional Effect of Ga on Fe Catalysts for the Decomposition Reaction
    Ortega, K. F. and Rein, D. and Luttmann, C. and Heese, J. and Ozcan, F. and Heidelmann, M. and Folke, J. and Kahler, K. and Schlogl, R. and Behrens, M.
    Chemcatchem 9 659--671 (2017)
    Magnesioferrite (MgFe2O4)-derived Mesoporous spinels of the type MgFeM3+O4 with M3+=Fe, Al, and Ga obtained upon calcination of hydrotalcite-like compounds were investigated in the ammonia decomposition reaction at 1bar and the synthesis of ammonia at 90bar. The corresponding precursors were synthesized by co-precipitation at 50 degrees C and constant pH of 10.5. N-2 physisorption, PXRD, HR-TEM, H-2-TPR, and NH3-TPD were applied in order to obtain information about the textural, (micro-)structural, solid-state kinetics in reducing atmosphere, and adsorption properties of the samples. While phase-pure layered double hydroxides (LDHs) were obtained for Al and Ga, magnesioferrite as the desired oxide phase and a low fraction of magnetite were formed besides the targeted precursor phase during co-precipitation in the presence of Fe2+ and Fe3+ species. Reduction of the binary and ternary magnesioferrites occurs via two consecutive reactions. Only the second stage is shifted towards higher temperatures after incorporation of Al and Ga. The latter element boosts the catalytic decomposition of ammonia, yielding a 2-fold and 5-fold higher conversion at 500 degrees C compared to the samples containing Fe3+ and Al3+ species, respectively. Insitu XRD measurements showed that this unprecedented promotional effect is related to the generation of (Fe, Ga)Fe3N. This phase, however, is detrimental for the synthesis of ammonia at elevated pressures in which the binary system outperforms the ternary spinels, yielding 30% of the activity obtained with a highly promoted Fe-based industrial catalyst.
    view abstractdoi: 10.1002/cctc.201601355
  • 2017 • 241 Microstructure evolution in refill friction stir spot weld of a dissimilar Al–Mg alloy to Zn-coated steel
    Suhuddin, U.F.H. and Fischer, V. and Kostka, A. and dos Santos, J.F.
    Science and Technology of Welding and Joining 1-8 (2017)
    In the present study, dissimilar welds of an Al–Mg–Mn alloy and a Zn-coated high-strength low-alloy steel were welded by refill friction stir spot welding. The maximum shear load recorded was approximately 7.8 kN, obtained from the weld produced with a 1600 rev min−1 tool rotational speed. Microstructural analyses showed the formation of a solid–liquid structure of an Al solid solution in Mg–Al-rich Zn liquid, which gives rise to the formation of Zn-rich Al region and microfissuring in some regions during welding. Exposure of steel surface to Mg–Al-rich Zn liquid led to the formation of Fe2Al5 and Fe4Al13 intermetallics. The presence of defective Zn-rich Al regions and Fe–Al intermetallics at the faying surface affects the weld strength. © 2017 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute
    view abstractdoi: 10.1080/13621718.2017.1300744
  • 2017 • 240 Doubling of the magnetic energy product in ferromagnetic nanowires at ambient temperature by capping their tips with an antiferromagnet
    Wang, F.Z. and Salikhov, R. and Spasova, M. and Liébana-Viñas, S. and Bran, C. and Chen, Y.-S. and Vazquez, M. and Farle, M. and Wiedwald, U.
    Nanotechnology 28 (2017)
    We present an approach to prepare free-standing tips of micrometer-long nanowires electrodeposited in anodic aluminum oxide nanopores. Such open tips can be further processed, e.g. for vertical interconnects of functional layers or for tailoring the magnetization reversal of ferromagnetic nanowires. The magnetic switching of nanowires is usually initiated by vortex or domain formation at the nanowire tips. We show that coating the tips of Fe30Co70 nanowires (diameter 40 nm, length 16 μm) with thin antiferromagnetic Fe50Mn50 capping layers (thickness ≈10 nm) leads to magnetic hardening with a more than doubled energy product at ambient temperature. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/aa77b7
  • 2017 • 239 Microstructure and mechanical properties of the heat-affected zone in laser-welded/brazed steel 22MnB5–AA6016 aluminum/AZ31 magnesium alloy
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W.
    Journal of Materials Processing Technology 247 11-18 (2017)
    The martensitic microstructure of the steel 22MnB5 was tempered during laser welding/brazing. The strength of the HAZ greatly decreased from 1500 MPa to 800–1100 MPa, depending on the heat input. The lowest strength always occurred in the area with the highest heat input directly beside the welding zone. The strength of the aluminum alloy was slightly reduced from 233 MPa to 212 MPa. The strongest decrease in the strength did occur in the area with a critical temperature range of 400–500 °C due to the coarsening of GP zones. The short heat input in the laser welding/brazing process did not lead to a significant change in the material strength and microstructure in the HAZ of the AZ31 magnesium alloy. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2017.04.008
  • 2017 • 238 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 • 237 Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction
    Yang, F. and Sliozberg, K. and Sinev, I. and Antoni, H. and Bähr, A. and Ollegott, K. and Xia, W. and Masa, J. and Grünert, W. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemSusChem 10 156-165 (2017)
    Co-based layered double hydroxide (LDH) catalysts with Fe and Al contents in the range of 15 to 45 at % were synthesized by an efficient coprecipitation method. In these catalysts, Fe3+ or Al3+ ions play an essential role as trivalent species to stabilize the LDH structure. The obtained catalysts were characterized by a comprehensive combination of surface- and bulk-sensitive techniques and were evaluated for the oxygen evolution reaction (OER) on rotating disk electrodes. The OER activity decreased upon increasing the Al content for the Co- and Al-based LDH catalysts, whereas a synergistic effect in Co- and Fe-based LDHs was observed, which resulted in an optimal Fe content of 35 at %. This catalyst was spray-coated on Ni foam electrodes and showed very good stability in a flow-through cell with a potential of approximately 1.53 V at 10 mA cm−2 in 1 m KOH for at least 48 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601272
  • 2017 • 236 Designing duplex, ultrafine-grained Fe-Mn-Al-C steels by tuning phase transformation and recrystallization kinetics
    Zhang, J. and Raabe, D. and Tasan, C.C.
    Acta Materialia 141 374-387 (2017)
    A novel, lightweight Fe-25.7Mn-10.6Al-1.2C (wt.%) steel is designed by exploiting the concurrent progress of primary recrystallization and phase transformation, in order to produce an ultrafine-grained, duplex microstructure. The microstructure consists of recrystallized austenite grains surrounded by submicron-sized ferrite grains, and recovered austenite regions with preferential nano-κ-carbide precipitation. This partially recrystallized duplex microstructure demonstrates excellent strength-ductility combinations, e.g. a yield strength of 1251 MPa, an ultimate tensile strength of 1387 MPa, and a total elongation of 43%, arising from the composite response by virtue of diverging constituent strength and strain hardening behaviors. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.09.026
  • 2016 • 235 Efficient liquid metallurgy synthesis of Fe-TiB2 high modulus steels via in-situ reduction of titanium oxides
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 97 357-363 (2016)
    We studied the in-situ reduction of Ti oxides by Al as an alternative and cost effective route for the liquid metallurgical synthesis of low density, high stiffness steels (high modulus steels) containing about 10 vol.% TiB2. TiO2, TiO1.83 and TiO were inserted via iron tubes into Fe-B melts, with Al either premixed with the oxide powders or liquid in the melt. Depending on Ti oxide type and location of the redox partner Al, greatly differing reaction kinetics, slag formation and corresponding microstructures of the high modulus steels were observed. TiO1.83 and TiO premixed with Al showed the highest TiB2 yield in the cast steel and are thus favourable candidates for the cost effective production of high modulus steels. Based on our findings, a novel synthesis process is proposed, based on filling wire injection into a continuous casting process, allowing the utilisation of the additionally formed oxide particles for the further improvement of the property profile of high modulus steels. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2016.02.076
  • 2016 • 234 Molecular structure of diethylaminoalane in the solid state: An X-ray powder diffraction, DFT calculation and Raman spectroscopy study
    Bernert, T. and Ley, M.B. and Ruiz-Fuertes, J. and Fischer, M. and Felderhoff, M. and Weidenthaler, C.
    Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials 72 232-240 (2016)
    The crystal structure of diethylaminoalane, [H2Al - N(C2H5)2]2, was determined by X-ray powder diffraction in conjunction with DFT calculations. Diethylaminoalane crystallizes in the monoclinic space group P21/c with a = 7.4020 (2), b = 12.9663 (3), c = 7.2878 (2) Å and β = 90.660 (2)° at 293 K. The crystal structure was confirmed by DFT calculations and Raman spectroscopy. The molecular structure of diethylaminoalane consists of dimers of [H2Al - N(CH2CH3)2] in which an Al2N2 four-membered ring is formed by a center of inversion. Such an arrangement of the aminoalane moieties in the crystal structure is well known for this class of compound, as shown by the comparison with ethylmethylaminoalane and diisopropylaminoalane.The crystal structure of diethylaminoalane, [H2Al - N(C2H5)2]2, was determined by X-ray powder diffraction, geometry optimization by density functional theory (DFT) and Raman spectroscopy. The DFT calculations were validated by calculating the ground state structures of two known aminoalanes while the Raman spectrum of diethylaminoalane was measured and compared to the simulated ones. Furthermore, the crystal structure of diethylaminoalane is compared with chemically and structurally similar compounds. © International Union of Crystallography, 2016.
    view abstractdoi: 10.1107/S2052520616000093
  • 2016 • 233 High-temperature stability and interfacial reactions of Ti and TiN thin films on Al2O3 and ZrO2
    Brust, S. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 307 47-55 (2016)
    Metallic thin films are used in many applications to modify ceramic surfaces. However, during subsequent processing, chemical interactions may change the properties of the coating. In addition, differences in thermal expansion can lead to delamination of the coating. In this study, titanium and titanium nitride thin films were deposited via physical and chemical vapor deposition (PVD and CVD, respectively) on alumina- and yttria-stabilised zirconia substrates, before being heat-treated at 1200 °C or 1500 °C in static argon atmosphere and analysed via SEM, EDS and XRD to investigate the effect of temperature on the thin films. It was shown that the chemical interactions between TiN and both Al2O3 and ZrO2 are weak. However, partial delamination of the TiN coating on alumina was observed after both annealing temperatures. The TiN coating on zirconia remained adherent. In contrast, the Ti coatings underwent a transformation to cubic TiO on both oxide substrates. This was due to partial reduction of the ZrO2 to ZrO2 − x and dissolution of the Al2O3, which leads to a Ti3Al0.9O1.1 interlayer. The TiO coating which formed remained adherent on the alumina at both annealing temperatures, but delaminated from the ZrO2 substrate after annealing at 1500 °C. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.08.060
  • 2016 • 232 Revising secondary electron yields of ion-sputtered metal oxides
    Corbella, C. and Marcak, A. and de los Arcos, T. and von Keudell, A.
    Journal of Physics D-applied Physics 49 16LT01 (2016)
    The emission of secondary electrons (SE) during sputtering of Al and Ti foils by argon ions in an oxygen background has been measured in a particle beam reactor equipped with a SE-collector. This experiment mimics the process of reactive magnetron sputtering. Quantified beams of argon ions with energies between 500 eV and 2000 eV were employed, while simultaneously molecular oxygen fluxes impinged on the surface and caused oxidation. The measured secondary electron emission coefficients (gamma) ranged from approximately 0.1 (for clean aluminium and titanium) to 1.2 and 0.6 (in the case of aluminium oxide and titanium oxide, respectively). The increase of gamma is compared to SE measurements based on the modelling of magnetron plasmas. Moreover, the energy distributions of the emitted SE have been measured by varying the retarding potential of the SE-collector, which allows the monitoring of the oxidation state from the position of the Auger peaks. The origin of the observed SE yields based on the emission of low-and high-energy electrons generated on the oxide surface is discussed.
    view abstractdoi: 10.1088/0022-3727/49/16/16LT01
  • 2016 • 231 Are Mo2BC nanocrystalline coatings damage resistant? Insights from comparative tension experiments
    Djaziri, S. and Gleich, S. and Bolvardi, H. and Kirchlechner, C. and Hans, M. and Scheu, C. and Schneider, J.M. and Dehm, G.
    Surface and Coatings Technology 289 213-218 (2016)
    Mo2BC nanocrystalline coatings were deposited on Cu substrates to compare their mechanical performance with bench-mark TiAlN, and pure Mo, Al and Al2O3 reference coatings. The Mo2BC coatings were characterized by X-ray diffraction and transmission electron microscopy to analyze the microstructure. In order to study the damage behavior, the coatings were subjected to uniaxial tensile loading and the crack spacing with increasing strain was monitored using optical and scanning electron microscopy. Based on crack density measurements, the Mo2BC coatings were found to be significantly less prone to cracking than the bench-mark TiAlN coatings. The higher resistance to cracking arises from the electronic structure of the Mo2BC nanolaminates, which imparts moderate ductility to the deformation behavior. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.02.010
  • 2016 • 230 Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO3/SrTiO3 Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
    Eiteneer, D. and Pálsson, G.K. and Nemšák, S. and Gray, A.X. and Kaiser, A.M. and Son, J. and LeBeau, J. and Conti, G. and Greer, A.A. and Keqi, A. and Rattanachata, A. and Saw, A.Y. and Bostwick, A. and Rotenberg, E. and Gulli...
    Journal of Electron Spectroscopy and Related Phenomena 211 70-81 (2016)
    LaNiO3 (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO3 (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La0.7Sr0.3MnO3/SrTiO3 superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.elspec.2016.04.008
  • 2016 • 229 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 • 228 Lewis acid-base adducts of group 13 elements: Synthesis, structure and reactivity toward benzaldehyde
    Ganesamoorthy, C. and Matthias, M. and Bläser, D. and Wölper, C. and Schulz, S.
    Dalton Transactions 45 11437-11444 (2016)
    Lewis acid-base adducts [LGa-M(C6F5)3] (M = B 1, Al 2, Ga 3) were prepared by the reaction of gallanediyl LGa {L = HC[C(Me)N(2,6-i-Pr2C6H3)]2} with the Lewis acids M(C6F5)3 (M = B, Al, Ga). Benzaldehyde reacts with [LGa-M(C6F5)3] (M = B 1, Al 2) at room temperature with the insertion and formation of [LGa(C6F5){CH(Ph)(OB(C6F5)2)}] (4) and the zwitterionic species [LGa(C6F5){CH(Ph)(OAl(C6F5)2)}] (5), respectively, which was found to decompose at 80 °C with the formation of {(C6F5)2Al(OCH2Ph)}2 (6). Any attempts to isolate the insertion complex of [LGa-Ga(C6F5)3] with benzaldehyde failed and only {(C6F5)2Ga(OCH2Ph)}2 (7) was isolated at elevated temperatures. 2-5 and 7 were structurally characterized by heteronuclear NMR spectroscopy and single crystal X-ray diffraction. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c6dt01688j
  • 2016 • 227 Analytical approach for magnetic pulse welding of sheet connections
    Hahn, M. and Weddeling, C. and Lueg-Althoff, J. and Tekkaya, A.E.
    Journal of Materials Processing Technology 230 131-142 (2016)
    An analytical model to calculate the acting forming pressure in magnetic pulse welding by determining the magnetic field strength between the flyer sheet and a one-turn coil was presented. By neglecting plastic deformation of the flyer, the model allows to calculate the transient velocity and displacement behavior, too. The electromagnetic acceleration of 5000-series aluminum alloy sheets was investigated under various experimental parameters. Utilizing Photon Doppler Velocimetry revealed that the analytical model appropriately describes the influence of current amplitude, coil geometry, and, especially, discharge frequency on the velocity-displacement curve of the flyer and hence on the impact velocity. The model introduced was applied to compute the impact velocity for the welding of long lap joints of 5000-series aluminum alloy sheets and 6000-series aluminum alloy hollow profiles. Through peel tests it was shown that the weld strength at least complied with the strength of the weaker base material as failure always happened in the flyer sheet. The wavy interface pattern typical for impact welding was identified with the help of metallography. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2015.11.021
  • 2016 • 226 Vaporizing foil actuator welding as a competing technology to magnetic pulse welding
    Hahn, M. and Weddeling, C. and Taber, G. and Vivek, A. and Daehn, G.S. and Tekkaya, A.E.
    Journal of Materials Processing Technology 230 8-20 (2016)
    Photonic Doppler velocimetry was applied to compare magnetic pulse welding and vaporizing foil actuator welding against each other in the form of lap joints made of 5000 series aluminum alloy sheets under identical experimental conditions which are: charging energies of the pulse generator, specimen geometry, initial distances between flyer and target plate. Impact velocities resulting from rapidly vaporizing aluminum foils were up to three times higher than those of purely electromagnetically accelerated flyer plates. No magnetic pulse welds were achieved, while every vaporizing foil experiment yielded a strong weld in that failure always occurred in the joining partners instead of in the weld seam during tensile tests. An analytical model to calculate the transient flyer velocity is presented and compared to the measurements. The average deviation between model and experiment is about 11% with regard to the impact velocity. Hence, the model may be used for the process design of collision welds generated by vaporizing foil actuators. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2015.11.010
  • 2016 • 225 Advancements in the manufacturing of dies for hot aluminum extrusion with conformal cooling channels
    Hölker, R. and Tekkaya, A.E.
    International Journal of Advanced Manufacturing Technology 83 1209-1220 (2016)
    To prevent an overheating of the workpiece material and to increase the productivity in hot aluminum extrusion, the application of extrusion dies with conformal cooling channels manufactured additively by selective laser melting is known. Since, to date, the additive manufacturing processes are often accompanied with higher manufacturing time and costs in comparison to conventional subtractive methods, a new concept for a hybrid extrusion die is presented. Here, the large volume but geometrically simple die part, the die bridge, is manufactured conventionally by subtractive methods, and the smaller part with geometrical complexity, the tip of the mandrel, is built-up on it additively by laser melting. A further novelty of the developed die is the isolated feeding of the coolant up to the target area, close to die bearings, where the cooling shall be localized. Numerical and experimental investigations revealed that the profile’s exit temperature can be reduced locally and controlled which leads only to a moderate increase of the extrusion force. The experimental results show that the hybrid tools withstand the high mechanical and thermal loads which occur during hot aluminum extrusion. © 2015, Springer-Verlag London.
    view abstractdoi: 10.1007/s00170-015-7647-4
  • 2016 • 224 Influence of process parameters on the quality of aluminium alloy en AW 7075 using Selective Laser Melting (SLM)
    Kaufmann, N. and Imran, M. and Wischeropp, T.M. and Emmelmann, C. and Siddique, S. and Walther, F.
    Physics Procedia 83 918-926 (2016)
    Selective laser melting (SLM) is an additive manufacturing process, forming the desired geometry by selective layer fusion of powder material. Unlike conventional manufacturing processes, highly complex parts can be manufactured with high accuracy and little post processing. Currently, different steel, aluminium, titanium and nickel-based alloys have been successfully processed; however, high strength aluminium alloy EN AW 7075 has not been processed with satisfying quality. The main focus of the investigation is to develop the SLM process for the wide used aluminium alloy EN AW 7075. Before process development, the gas-atomized powder material was characterized in terms of statistical distribution: size and shape. A wide range of process parameters were selected to optimize the process in terms of optimum volume density. The investigations resulted in a relative density of over 99%. However, all laser-melted parts exhibit hot cracks which typically appear in aluminium alloy EN AW 7075 during the welding process. Furthermore the influence of processing parameters on the chemical composition of the selected alloy was determined. © 2016 The Authors.
    view abstractdoi: 10.1016/j.phpro.2016.08.096
  • 2016 • 223 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 • 222 Atom probe tomography of intermetallic phases and interfaces formed in dissimilar joining between Al alloys and steel
    Lemmens, B. and Springer, H. and Duarte, M.J. and De Graeve, I. and De Strycker, J. and Raabe, D. and Verbeken, K.
    Materials Characterization 120 268-272 (2016)
    While Si additions to Al are widely used to reduce the thickness of the brittle intermetallic seam formed at the interface during joining of Al alloys to steel, the underlying mechanisms are not clarified yet. The developed approach for the site specific atom probe tomography analysis revealed Si enrichments at grain and phase boundaries between the θ (Fe4Al13) and η (Fe2Al5) phase, up to about ten times that of the concentration in Al. The increase in Si concentration could play an important role for the growth kinetics of the intermetallic phases formed for example in hot-dip aluminizing of steel. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.matchar.2016.09.008
  • 2016 • 221 Importance of inclusion of the effect of s electrons into bond-order potentials for transition bcc metals with d-band mediated bonding
    Lin, Y.-S. and Mrovec, M. and Vitek, V.
    Modelling and Simulation in Materials Science and Engineering 24 (2016)
    In bond-order potentials (BOPs) for transition metals only the bonding mediated by the d electrons is included explicitly and the covalent part of the cohesive energy is evaluated using Slater-Koster dd bond integrals. However, the effect of s electrons with orbitals centered on atoms neighboring the corresponding dd bond is not necessarily negligible. As shown in Nguyen-Manh et al (2000 Phys. Rev. Lett. 85 4136) this can be taken into account via screening of the dd bond integrals. In a recent paper (Lin et al 2014 Model. Simul. Mater. Sci. Eng. 22 034002) the dd bond integrals were determined using a projection scheme utilizing atomic orbitals that give the best representation of the electronic wave functions in the calculations based on the density functional theory (DFT) (Madsen et al 2011 Phys. Rev. B 83 4119) and it was inferred that in this case the effect of s electrons was already included. In this paper we analyze this hypothesis by comparing studies employing BOPs with both unscreened and screened dd bond integrals. In all cases results are compared with calculations based on DFT and/or experiments. Studies of structures alternate to the bcc lattice, transformation paths that connect the bcc structure with fcc, simple cubic (sc), body centered tetragonal (bct) and hcp structures via continuously distorted configurations and calculations of γ-surfaces were all found to be insensitive to the screening of bond integrals. On the other hand, when the bond integrals are screened, formation energies of vacancies are improved and calculated phonon dispersion spectra reproduce the experimentally observed ones much better. Most importantly, dislocation core structure and dislocation glide are significantly different without and with screening of dd bond integrals. The latter lead to a much better agreement with available experiments. These findings suggest that the effect of s electrons on dd bonds, emulated by the screening of corresponding bond integrals, is the least significant when the lattice is distorted away from the ideal bcc structure homogeneously even if such distortion is large. On the other hand, when the distortion is local and inhomogeneous the impact of screening of the dd bond integrals is significant. In the studies presented in this paper such local inhomogeneities occur when phonons propagate through the lattice, at point defects and in the cores of dislocations. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/24/8/085001
  • 2016 • 220 Quaternary Al-Cu-Mg-Si Q Phase: Sample Preparation, Heat Capacity Measurement and First-Principles Calculations
    Löffler, A. and Zendegani, A. and Gröbner, J. and Hampl, M. and Schmid-Fetzer, R. and Engelhardt, H. and Rettenmayr, M. and Körmann, F. and Hickel, T. and Neugebauer, J.
    Journal of Phase Equilibria and Diffusion 37 119-126 (2016)
    The quaternary Q phase is an important precipitate phase in the Al-Cu-Mg-Si alloy system and its accurate thermodynamic description is crucial for further tailoring this material class for light-weight structural applications. In order to achieve an improved thermochemical parameter set of this phase, we used a combination of experimental measurements and first-principles calculations, which was focussed on the heat capacity. Its accurate experimental determination required the preparation of pure samples of Q phase and sophisticated calorimetric measurements. On the theoretical side, a simultaneous treatment of lattice vibrations within the quasiharmonic approximation, electronic excitations, and configuration entropy within the compound energy formalism were required to achieve a complete description of the heat capacity. The evaluation demonstrates the high predictive power of the first-principles as well as the Calphad modeling. © 2015, ASM International.
    view abstractdoi: 10.1007/s11669-015-0426-y
  • 2016 • 219 Toughness enhancement in highly NbN-alloyed Ti-Al-N hard coatings
    Mikula, M. and Plašienka, D. and Sangiovanni, D.G. and Sahul, M. and Roch, T. and Truchlý, M. and Gregor, M. and Čaplovič, L. and Plecenik, A. and Kúš, P.
    Acta Materialia 121 59-67 (2016)
    Obtaining high hardness combined with enhanced toughness represents one of the current challenges in material design of hard ceramic protective coatings. In this work, we combine experimental and ab initio density functional theory (DFT) analysis of the mechanical properties of Ti-Al-Nb-N coatings to validate the results of previous theoretical investigations predicting enhanced toughness in TiAlN-based systems highly alloyed (&gt;25 at. %) with nitrides of pentavalent VB group elements Nb, Ta, and V. As-deposited Ti1-x-yAlxNbyN coatings (y = 0 ÷ 0.61) exhibit single phase cubic sodium chloride (B1) structure identified as TiAl(Nb)N solid solutions. The highest hardness, ∼32.5 ± 2 GPa, and the highest Young's modulus, ∼442 GPa, are obtained in Nb-free Ti0.46Al0.54N exhibiting pronounced 111 growth-orientation. Additions of Nb in the coatings promote texture evolution toward 200. Nanoindentation measurements demonstrate that alloying TiAlN with NbN yields significantly decreased elastic stiffness, from 442 to ∼358 ÷ 389 GPa, while the hardness remains approximately constant (between 28 ± 2 and 31 ± 3 GPa) for all Nb contents. DFT calculations and electronic structure analyses reveal that alloying dramatically reduces shear resistances due to enhanced d-d second-neighbor metallic bonding while retaining strong metal-N bonds which change from being primarily ionic (TiAlN) to more covalent (TiAlNbN) in nature. Overall, Nb substitutions are found to improve ductility of TiAlN-based alloys at the cost of slight losses in hardness, equating to enhanced toughness. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.08.084
  • 2016 • 218 Low cycle fatigue in aluminum single and bi-crystals: On the influence of crystal orientation
    Nellessen, J. and Sandlöbes, S. and Raabe, D.
    Materials Science and Engineering A 668 166-179 (2016)
    Aluminum single crystals with three different double-slip orientations and two aluminum bi-crystals - one with a high-angle grain boundary and one with a low-angle grain boundary - were cyclically deformed up to 100 cycles under constant displacement control. The distribution of the local strain and the local strain amplitudes was captured by in-situ digital image correlation (DIC). Dislocation structure analysis was performed by electron channeling contrast imaging (ECCI) and the evolution of local misorientations was recorded by high resolution electron backscatter diffraction (EBSD). The DIC results show a homogeneous strain amplitude distribution in the single crystals while the measured strain amplitude in the low-angle grain boundary bi-crystal sample differs significantly. ECCI observations reveal the presence of dislocation cells elongated along the trace of the primary {111} slip plane in all investigated crystals and the formation of deformation bands parallel to the trace of {110} planes. Deformation bands (DB) were observed in all samples but their frequency and misorientation with respect to the matrix was found to sensitively depend on the crystal orientation and the local strain amplitude. Our results on the bi-crystals show that the grain orientation mainly determines the local stresses and therefore also the formation of the associated dislocation structures rather than the grain boundary character. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.05.054
  • 2016 • 217 Development of a post-synthetic method for tuning the Al content of OSDA-free Beta as a catalyst for conversion of methanol to olefins
    Otomo, R. and Müller, U. and Feyen, M. and Yilmaz, B. and Meng, X. and Xiao, F.-S. and Gies, H. and Bao, X. and Zhang, W. and De Vos, D. and Yokoi, T.
    Catalysis Science and Technology 6 713-721 (2016)
    Zeolites synthesized without any organic structure-directing agent (OSDA) have several advantages over conventional zeolites synthesized with OSDAs. Their Al-rich compositions, however, are sometimes not suitable for applications as catalysts. In the present study, post-synthetic modification was performed using an Al-rich Beta zeolite synthesized without any OSDAs (designated as "Beta(OF)") to obtain high-silica Beta zeolites. We have successfully developed a facile post-synthetic method for tuning the Al content of Beta(OF) with the ∗BEA-type structure retained by calcination at &gt;750 °C followed by acid treatment. Solid-state 29Si and 27Al MAS NMR analyses revealed that during calcination, framework Al atoms were isomorphously substituted with Si atoms to form high-silica frameworks and concomitant extra-framework Al species. The stability of the obtained frameworks against acid treatment was evaluated in terms of the framework Al content, finding that the framework with the Si/Al ratio higher than 12 is well stabilized enough for acid treatment. Thus, dealuminated Beta(OF) zeolites with high-silica compositions were found to be effective catalysts for the methanol-to-olefin (MTO) reaction; particularly, the Beta zeolite with the Si/Al ratio of 112 maintained the conversion of methanol over 90% with selectivity to C2-C4 olefins over 70% even at 40 hours on stream at WHSV = 3.2 h-1. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5cy00944h
  • 2016 • 216 Elemental partitioning, lattice misfit and creep behaviour of Cr containing gammaprime strengthened Co base superalloys
    Povstugar, I. and Zenk, C.H. and Li, R. and Choi, P.-P. and Neumeier, S. and Dolotko, O. and Hoelzel, M. and Göken, M. and Raabe, D.
    Materials Science and Technology (United Kingdom) 32 220-225 (2016)
    Novel Cr containing Co-Al-W base superalloys were studied by atom probe tomography and neutron diffraction. Cr is found to predominantly partition to the gamma matrix and decrease partitioning of W to gammaprime. Furthermore, Cr significantly enhances the gammaprime volume fraction, decreases the gamma/gammaprime lattice misfit and deteriorates the creep resistance. Addition of Ni to the Cr containing alloys affects partitioning of W and Al, further decreases the lattice misfit and results in the formation of irregularly shaped precipitates. Al, W and Cr tend to occupy the ‘B'sublattice in the gammaprime-A3B phase (L12 type), while Co and Ni reside in the ‘A' sublattice. © 2016 Institute of Materials.
    view abstractdoi: 10.1179/1743284715Y.0000000112
  • 2016 • 215 Shock-tube and plug-flow reactor study of the oxidation of fuel-rich CH4/O2 mixtures enhanced with additives
    Sen, F. and Shu, B. and Kasper, T. and Herzler, J. and Welz, O. and Fikri, M. and Atakan, B. and Schulz, C.
    Combustion and Flame 169 307-320 (2016)
    Partial oxidation of hydrocarbons under well-controlled conditions opens a path to higher-value chemicals from natural gas with small exergy losses if the chemical conversion proceeds in an internal combustion engine as a polygeneration process (Gossler et al., 2015). For the relevant reaction conditions, kinetics models are not sufficiently validated due to the atypical reaction conditions, e.g., high equivalence ratios and pressures. The purpose of this study is to obtain experimental validation data for chemical reaction mechanisms that can be used to predict polygeneration processes in practical applications. In case of methane these processes proceed under fuel-rich conditions and yield primarily syngas (CO/H2). In this study, the partial oxidation of methane was investigated for an equivalence ratio of φ=2 in a shock-tube and a plug-flow reactor (PFR) in order to cover a wide temperature range. Time-resolved CO mole fractions were measured in shock-heated mixtures between 1600 and 2100K at ~1bar. Good agreement was found between the experiment and the models (Yasunaga et al., 2010; Burke et al., 2015; Zhao et al., 2008). Stable reaction products were monitored by time-of-flight mass spectrometry between 532 and 992K at 6bar in a tubular flow reactor at reaction times &gt;4s. The influence of dimethyl ether (DME) and n-heptane addition on methane reactivity and conversion was investigated. The additives significantly lower the initial reaction temperature by producing significant amounts of OH. The results were compared to simulations and serve as validation data for the development of reaction mechanisms for these atypical reaction conditions. Good agreement was found between the experiment and the models for most of species. © 2016 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2016.03.030
  • 2016 • 214 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 • 213 Design and operation of an aluminium alloy tank using doped Na3AlH6 in kg scale for hydrogen storage
    Urbanczyk, R. and Peinecke, K. and Meggouh, M. and Minne, P. and Peil, S. and Bathen, D. and Felderhoff, M.
    Journal of Power Sources 324 589-597 (2016)
    In this publication the authors present an aluminium alloy tank for hydrogen storage using 1921 g of Na3AlH6 doped with 4 mol% of TiCl3 and 8 mol% of activated carbon. The tank and the heat exchangers are manufactured by extrusion moulding of Al-Mg-Si based alloys. EN AW 6082 T6 alloy is used for the tank and a specifically developed alloy with a composition similar to EN AW 6060 T6 is used for the heat exchangers. The three heat exchangers have a corrugated profile to enhance the surface area for heat transfer. The doped complex hydride Na3AlH6 is densified to a powder density of 0.62 g cm−3. The hydrogenation experiments are carried out at 2.5 MPa. During one of the dehydrogenation experiments approximately 38 g of hydrogen is released, accounting for gravimetric hydrogen density of 2.0 mass-%. With this tank 15 hydrogenation and 16 dehydrogenation tests are carried out. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2016.05.102
  • 2016 • 212 Cyclic degradation in bamboo-like Fe-Mn-Al-Ni shape memory alloys - The role of grain orientation
    Vollmer, M. and Krooß, P. and Kriegel, M.J. and Klemm, V. and Somsen, C. and Ozcan, H. and Karaman, I. and Weidner, A. and Rafaja, D. and Biermann, H. and Niendorf, T.
    Scripta Materialia 114 156-160 (2016)
    In the present study the cyclic deformation behavior within differently oriented grains in Fe-34.8Mn-13.5Al-7.4Ni (at.%) shape memory polycrystals featuring a bamboo-like structure was investigated. In cyclic tensile tests up to 50 cycles, the degree of degradation in pseudoelasticity was evaluated and contributing elementary mechanisms are discussed. The results reveal rapid cyclic degradation in the bamboo-like samples. The unexpected stabilization of parent phase in reverse transformed areas and the proceeding activation of new martensite variants in subsequent cycles were found to be the prevailing degradation mechanisms. Dislocation activity is found to be the most detrimental factor. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2015.12.007
  • 2016 • 211 Removal of oxides and brittle coating constituents at the surface of coated hot-forming 22MnB5 steel for a laser welding process with aluminum alloys
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W.
    Surface and Coatings Technology 285 153-160 (2016)
    The surface of a press-hardened steel 22MnB5 coated with Al-base (AlSi10Fe3) and Zn-base (ZnNi10) was conditioned by a pulse laser and by sandblasting to remove undesirable oxides and brittle phases. Oxides formed on coating surfaces counteract the wettability of welding filler during a welding or brazing process. Furthermore, welding and brazing joints of 22MnB5 coated with aluminum alloys failed along the brittle intermetallic phases in the coating under a low mechanical load. Treated 22MnB5 surfaces were analyzed microscopically, and the phase compositions were investigated by synchrotron diffraction measurements. It was found that brittle phases could be locally removed by laser ablation; however, high laser energies led to remelting and oxidation of the coating surface. In contrast, sandblasting homogenously removed oxides and brittle intermetallic phases. Surface-treated 22MnB5 steel sheets were joined to AA6016 aluminum sheets by laser welding, and the strength of the weldment was determined by tensile tests. The measured mechanical strength of the aluminum/steel joints was 210-230. MPa. Failure of the weldments under tensile loading occurred within the aluminum sheet, away from the steel surface/welding filler interface if brittle coating components and oxides were removed homogenously. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2015.11.037
  • 2016 • 210 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 • 209 Improvement of catalytic activity over Cu–Fe modified Al-rich Beta catalyst for the selective catalytic reduction of NOx with NH3
    Xu, L. and Shi, C. and Chen, B. and Zhao, Q. and Zhu, Y. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Kolb, U. and Feyen, M. and Maurer, S. and Moini, A. and Müller, U. and Zhang, W.
    Microporous and Mesoporous Materials 236 211-217 (2016)
    Copper and iron bimetal modified Al-rich Beta zeolites from template-free synthesis were prepared for selective catalytic reduction (SCR) of NOx with NH3 in exhaust gas streams. Comparing to the Cu-based and Fe-based mono-component Beta catalysts, Cu(3.0)-Fe(1.3)-Beta bi-component catalyst shows better low-temperature activity and wider reaction-temperature window. Over 80% of NO conversion can be achieved at the temperature region of 125–500 °C. Due to the synergistic effect of copper and iron evidenced by XRD, UV–Vis–NIR, EPR and XPS measurements, the dispersion state of active components as well as the ratio of Cu2+/Cu+ and Fe3+/Fe2+ were improved over Cu(3.0)-Fe(1.3)-Beta. Isolated Cu2+ and Fe3+ ions which located at the exchange sites could be the active species at the low-temperature region, while FeOx cluster species may be more important to the high-temperature activity. During the test of sulfur resistance, Fe-containing samples including Cu(3.0)-Fe(1.3)-Beta and Fe(2.7)-Beta-4 present better performance compared to Cu(4.1)-Beta-4. Deactivation of Cu-based catalyst is attributed to the easier deposition of sulfates over the surface according to the results of TGA coupled with TPD experiments. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2016.08.042
  • 2016 • 208 Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel
    Yao, M.J. and Dey, P. and Seol, J.-B. and Choi, P. and Herbig, M. and Marceau, R.K.W. and Hickel, T. and Neugebauer, J. and Raabe, D.
    Acta Materialia 106 229-238 (2016)
    We report on the investigation of the off-stoichiometry and site-occupancy of κ-carbide precipitates within an austenitic (γ), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the κ-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L′12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the κ/γ mismatch, which facilitates occupation of Al sites in κ-carbide by Mn atoms (Mnγ Al anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.01.007
  • 2016 • 207 Determination of the young modulus of Ti-TiAl3 metallic intermetallic laminate composites by nano-indentation
    Yener, T. and Güler, S. and Siddique, S. and Walther, F. and Zeytin, S.
    Acta Physica Polonica A 129 604-606 (2016)
    Nano-indentation is an important technique to determine the Young modulus of multiphase materials where normal tensile tests are not appropriate. In this work, Ti-TiAl3 metallic-intermetallic laminate composites have been fabricated successfully in open atmosphere using commercial purity Al and Ti foils with 250 μm and 500 μm initial thicknesses, respectively. Sintering process was performed at 700 °C under 2 MPa pressure for 7.5 h. Mechanical properties including the Young modulus were determined after manufacturing. The Young moduli of metallic and intermetallic phases were determined as 89 GPa and 140 GPa, respectively. Microstructure analyses showed that aluminum foil was almost consumed by forming a titanium aluminide intermetallic compound. Titanium aluminides grow up through spherical shaped islands and metallic-intermetallic interface is a wavy form in Ti-Al system. Thus, the final microstructure consists of alternating layers of intermetallic compound and unreacted Ti metal. Microstructure and phase characterizations were performed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Hardness of test samples was determined as 600 HV for intermetallic zone and 130 HV for metallic zone by the Vickers indentation method.
    view abstractdoi: 10.12693/APhysPolA.129.604
  • 2015 • 206 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 • 205 Coprecipitation: An excellent tool for the synthesis of supported metal catalysts - From the understanding of the well known recipes to new materials
    Behrens, M.
    Catalysis Today 246 46-54 (2015)
    Constant-pH co-precipitation is a standard synthesis technique for catalyst precursors. The general steps of this synthesis route are described in this work using the successfully applied industrial synthesis of the Cu/ZnO/(Al2O3) catalyst for methanol synthesis as an example. Therein, co-precipitation leads to well-defined and crystalline precursor compound with a mixed cationic lattice that contains all metal species of the final catalyst. The anions are thermally decomposed to give the mixed oxides and the noblest component, in this current case copper, finally segregates on a nano-metric level to yield supported and uniform metal nanoparticles. Recent examples of the application of this synthesis concept for supported catalysts are reported with an emphasis on the layered double hydroxide precursor (Cu,Zn,Al; Ni,Mg,Al; Pd,Mg,Al; Pd,Mg,Ga). This precursor material is very versatile and can lead to highly loaded base metal as well as to mono- and bi-metallic highly dispersed noble metal catalysts. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2014.07.050
  • 2015 • 204 The crystallographic template effect assisting the formation of stable α-Al2O3 during low temperature oxidation of Fe-Al alloys
    Brito, P. and Pinto, H. and Kostka, A.
    Corrosion Science (2015)
    The role of thermally grown α-Fe2O3 on the nucleation of α-Al2O3 during oxidation of binary Fe-Al alloys with 15 and 26 at.%Al at 700°C was investigated. Surface morphology of the oxide scales indicated direct nucleation of α-Al2O3 preferentially instead of conversion from metastable Al2O3 polymorphs. Oxide scale development over time was also monitored by use of synchrotron X-ray diffraction and Raman spectroscopy. The results showed that the α-Fe2O3 crystal lattice decreases in volume as oxidation progresses, which was found to be consistent with an Al3+ enrichment of α-Fe2O3 as confirmed by the change in relative intensity of α-Fe2O3 Raman peaks. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.corsci.2016.01.007
  • 2015 • 203 New Au-Cu-Al thin film shape memory alloys with tunable functional properties and high thermal stability
    Buenconsejo, P.J.S. and Ludwig, Al.
    Acta Materialia 85 378-386 (2015)
    An Au-Cu-Al thin film materials library prepared by combinatorial sputter-deposition was characterized by high-throughput experimentation in order to identify and assess new shape memory alloys (SMAs) in this alloy system. Automated resistance measurements during thermal cycling between -20 and 250 °C revealed a wide composition range that undergoes reversible phase transformations with martensite transformation start temperatures, reverse transformation finish temperatures and transformation hysteresis ranging from -15 to 149 °C, 5 to 185 °C and 8 to 60 K, respectively. High-throughput X-ray diffraction analysis of the materials library confirmed that the phase-transforming compositions can be attributed to the existence of the β-AuCuAl parent phase and its martensite product. The formation of large amount of phases based on face-centered cubic (Au-Cu), Al-Cu and Al-Au is responsible for limiting the range of phase-transforming compositions. Selected alloys in this system show excellent thermal cyclic stability of the phase transformation. The functional properties of these alloys, combined with the inherent properties of Au-based alloys, i.e. aesthetic value, oxidation and corrosion resistance, makes them attractive as smart materials for a wide range of applications, including applications as SMAs for elevated temperatures in harsh environment. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.11.035
  • 2015 • 202 Comparison of two models for anisotropic hardening and yield surface evolution in bcc sheet steels
    Clausmeyer, T. and Svendsen, B.
    European Journal of Mechanics, A/Solids 54 120-131 (2015)
    The purpose of the current work is the investigation and comparison of aspects of the material behavior predicted by two models for anisotropic, and in particular cross, hardening in bcc sheet steels subject to non-proportional loading. The first model is the modified form (Wang et al., 2008) of that due to Teodosiu and Hu (1995, 1998). In this (modified) Teodosiu-Hu model (THM), cross hardening is assumed to affect the yield stress and the saturation value of the back stress. The second model is due to Levkovitch and Svendsen (2007) and Noman et al. (2010). In the Levkovitch-Svendsen model (LSM), cross hardening is assumed to affect the flow anisotropy. As clearly demonstrated in a number of works applying the THM (e.g., Boers et al., 2010; Bouvier et al., 2005, 2003; Hiwatashi et al., 1997; Li et al., 2003; Thuillier et al., 2010; Wang et al., 2008) and the LSM (e.g., Clausmeyer et al., 2014, 2011b; Noman et al., 2010), both of these are capable of predicting the effect of cross hardening on the stress-deformation behavior observed experimentally in sheet steels. As shown in the current work, however, these two models differ significantly in other aspects, in particular with respect to the development of the yield stress, the back stress, and the yield surface. For example, the THM predicts no change in the shape of the yield surface upon change of loading path, in contrast to the LSM and crystal plasticity modeling of bcc sheet steels (Peeters et al., 2002). On the other hand, the LSM predicts no hardening stagnation after cross hardening as observed in experiments, in contrast to the THM. Examples are given. © 2015 Elsevier Masson SAS. All rights reserved.
    view abstractdoi: 10.1016/j.euromechsol.2015.05.016
  • 2015 • 201 Investigations for the Embedding of Functional Elements in the Composite Extrusion Process
    Dahnke, C. and Hilbring, J. and Kloppenborg, T. and Haase, M. and Tekkaya, A.E.
    Materials Today: Proceedings 2 4763-4770 (2015)
    In addition to high strength steel elements, composite extrusion offers the possibility to manufacture functional aluminum profiles by the embedding of isolated electric conductors. However, due to the lower strength and lower thermal stability, the embedding of such elements into the metal matrix by the usage of modified porthole dies is challenging. The length of the welding chamber and the pressure inside the welding chamber are the most important criteria for the successful production of a functional profile. In order to overcome the occurring challenges, the investigation of an adapted die design is shown in this paper. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matpr.2015.10.010
  • 2015 • 200 Insights into the structural, electronic, and magnetic properties of Fe2−xTixO3/Fe2O3 thin films with x = 0.44 grown on Al2O3 (0001)
    Dennenwaldt, T. and Lübbe, M. and Winklhofer, M. and Müller, A. and Döblinger, M. and Nabi, H.S. and Gandman, M. and Cohen-Hyams, T. and Kaplan, W.D. and Moritz, W. and Pentcheva, R. and Scheu, C.
    Journal of Materials Science 50 122-137 (2015)
    The interface between hematite (α-Fe2 IIIO3) and ilmenite (FeIITiO3), a weak ferrimagnet and an antiferromagnet, respectively, has been suggested to be strongly ferrimagnetic due to the formation of a mixed valence layer of Fe2+/Fe3+ (1:1 ratio) caused by compensation of charge mismatch at the chemically abrupt boundary. Here, we report for the first time direct experimental evidence for a chemically distinct layer emerging at heterointerfaces in the hematite—Ti-doped-hematite system. Using molecular beam epitaxy, we have grown thin films (~25 nm thickness) of α-Fe2O3 on α-Al2O3 (0001) substrates, which were capped with a ~25 nm thick Fe2−xTixO3 layer (x = 0.44). An additional 3 nm cap of α-Fe2O3 was deposited on top. The films were structurally characterized in situ with surface X-ray diffraction, which showed a partial low index orientation relationship between film and substrate in terms of the [0001] axis and revealed two predominant domains with (Formula presented.) one with (Formula presented.) and a twin domain with (Formula presented.). Electron energy loss spectroscopy profiles across the Fe2−xTixO3/Fe2O3 interface show that Fe2+/Fe3+ ratios peak right at the interface. This strongly suggests the formation of a chemically distinct interface layer, which might also be magnetically distinct as indicated by the observed magnetic enhancement in the Fe2−xTixO3/α-Fe2O3/Al2O3 system compared to the pure α-Fe2O3/Al2O3 system. © 2014, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-014-8572-x
  • 2015 • 199 Environmental assessment of solid state recycling routes for aluminium alloys: Can solid state processes significantly reduce the environmental impact of aluminium recycling?
    Duflou, J.R. and Tekkaya, A.E. and Haase, M. and Welo, T. and Vanmeensel, K. and Kellens, K. and Dewulf, W. and Paraskevas, D.
    CIRP Annals - Manufacturing Technology 64 37-40 (2015)
    Solid state recycling techniques allow the manufacture of high density aluminium alloy parts directly from production scrap. In this paper the environmental impacts associated with 'meltless' scrap processing routes based on three different techniques, namely hot extrusion, screw extrusion and spark plasma sintering (SPS), are compared with the corresponding remelting route as reference. Analysis of the obtained results allows clear conclusions on the perspectives offered by solid state recycling for systematic environmental impact reduction of aluminium recycling with material and energy savings as most important influencing factors. An overall impact reduction with a factor 2 for the SPS route and 3-4 for the extrusion routes is found to be realistic. © 2015 CIRP.
    view abstractdoi: 10.1016/j.cirp.2015.04.051
  • 2015 • 198 Friction surfacing of Ti-6Al-4V: Process characteristics and deposition behaviour at various rotational speeds
    Fitseva, V. and Krohn, H. and Hanke, S. and dos Santos, J.F.
    Surface and Coatings Technology 278 56-63 (2015)
    By the process of friction surfacing, coatings are generated from metallic materials at temperatures below their melting range. The high degree of deformation while depositing leads to grain refinement in the microstructure, which has a positive effect on the mechanical properties of the layer. The applicability of the process has been described for a large number of materials. The deposition of Ti-6Al-4V has been reported in one publication but was not systematically studied. Therefore, the main aims of the present work are to define the process parameter fields for the deposition of Ti-6Al-4V leading to flash and defect free coatings and associate them with geometric features of the deposited layer.This investigation has shown that Ti-6Al-4V coatings can be effectively deposited onto a Ti-6Al-4V substrate by friction surfacing. A wide range of process parameters was established in which coatings of high quality have been obtained. The consumption rate control has been implemented as an efficient mode for the deposition of Ti-6Al-4V coatings. Temperature measurements at the coating interface have been accomplished showing that the coating material has been deformed in the β-phase. Furthermore, the homogeneity of the coating surface has been established to be a function of the rotational speed. The coatings exhibited a defect-free bond at the interface with the substrate. Two process parameter ranges with respect to the flash formation have been established. One of them enables flash-free coatings and the other generates coatings with flash formation on the retreating side, which can be controlled by the rotational and deposition speeds. Moreover, an increase in the rotational speed has been shown to lead to an increase in the coating thickness and width as well as an increase in the deposition efficiency up to 39 %. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2015.07.039
  • 2015 • 197 Thermoelectric properties of pulsed current sintered nanocrystalline Al-doped ZnO by chemical vapour synthesis
    Gautam, D. and Engenhorst, M. and Schilling, C. and Schierning, G. and Schmechel, R. and Winterer, M.
    Journal of Materials Chemistry A 3 189-197 (2015)
    ZnO is a promising n-type oxide thermoelectric material, which is stable in air at elevated temperatures. In the present study, we report the bottom-up approach to create Al-doped ZnO nanocomposites from nanopowders, which are prepared by chemical vapour synthesis. With our synthesis route, we are able to create highly doped Al-containing ZnO nanocomposites that exhibit bulk-like electrical conductivity. Moreover, the impact of the microstructure of the nanocomposites on their thermal conductivity is enormous, with a value of 1.0 W m-1 K-1 for 1% Al-ZnO at room temperature, which is one of the lowest values reported, to date, on ZnO nanocomposites. The optimization of the Al-doping and microstructure with respect to the transport properties of bulk Al-ZnO nanocomposites leads to a zT value of about 0.24 at 950 K, underlining the potential of our technique. This journal is © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4ta04355c
  • 2015 • 196 In Situ X-ray Diffraction Study of Co-Al Nanocomposites as Catalysts for Ammonia Decomposition
    Gu, Y.-Q. and Fu, X.-P. and Du, P.-P. and Gu, D. and Jin, Z. and Huang, Y.-Y. and Si, R. and Zheng, L.-Q. and Song, Q.-S. and Jia, C.-J. and Weidenthaler, C.
    Journal of Physical Chemistry C 119 17102-17110 (2015)
    Co-Al nanocomposite materials as active and stable catalysts for ammonia decomposition have been synthesized by a one-pot evaporation-induced self-assembly method. The catalysts were characterized by various techniques including powder X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), N<inf>2</inf> adsorption/desorption, and transmission/scanning electron microscopy (TEM/SEM). Especially, in situ XRD under catalytic reaction conditions was performed, and metallic Co with a cubic structure was identified to be most probably the active crystalline phase for the decomposition of ammonia; also, contribution of CoO to the catalytic activity cannot be excluded. Most importantly, the introduction of alumina can significantly suppress the agglomeration of the active metallic Co phase and thus maintain the high activity of the cobalt catalyst. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b02932
  • 2015 • 195 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 • 194 Influence of Temperature and Tempering Conditions on Thermal Conductivity of Hot Work Tool Steels for Hot Stamping Applications
    Hafenstein, S. and Werner, E. and Wilzer, J. and Theisen, W. and Weber, S. and Sunderkötter, C. and Bachmann, M.
    Steel Research International 86 1628-1635 (2015)
    The knowledge of thermal conductivity is essential for improving and designing tools for hot working applications like hot stamping and high-pressure aluminum die casting. This study investigates the influence of alloying composition and heat treatment on thermal conductivity of two different hot work tool steels in the temperature range between 20 and 500 °C. Thermal conductivity was determined with an indirect measurement by using the dynamic method. The thermal conductivity of the two tool steels was found to be dependent on the amount of alloying elements, heat treatment condition, and operating temperature. In the regime of hot stamping applications, i.e for service temperatures below 200°C, thermal conductivity increases with temperature for both steels irrespective of their heat treatment condition. In applications in which tools are subjected to temperatures above 200°C (such as high-pressure die casting operations), thermal conductivity of the steels decreases as tool temperature increases. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201400597
  • 2015 • 193 Measurements of nanoscale TiO2 and Al2O3 in industrial workplace environments – Methodology and results
    Kaminski, H. and Beyer, M. and Fissan, H. and Asbach, C. and Kuhlbusch, T.A.J.
    Aerosol and Air Quality Research 15 129-141 (2015)
    The possible release of engineered nanomaterials was investigated based on a previously developed but now refined methodology. Data from altogether eight industrial work areas in production plants of nanostructured TiO2 and Al2O3 particles were obtained and used to test the methodology and to derive a first assessment of possible exposure of workers. Particle size distributions were determined in work area environments with concurrent measurements at a comparison site. Data from the comparison site were used to estimate the particle background level in the work area and distinguish it from potentially released nanomaterial. The analysis is based on the comparison of time resolved data from the work area and the comparison site as well as data determined during periods with and without work activities in the work area. The data analysis method introduced delivers size-resolved information on the potential nanoparticle exposure of workers. A significant release of particles in the size range 100–562 nm was observed in the work area of bagging aluminum oxide and is stemming from damaging or overfilling of bags, and the necessary activities during the cleaning of the work area. The maximum particle diameter of these particles was around 340 nm. At all other investigated locations no significant releases of particles in the size range 100–562 nm were determined. Also, no significant release of particles < 100 nm was observed in all work areas. The average PM10 exposure during the work activities varied from 48 to 1,330 µg/m3 in the different work areas. The maximum concentrations of aluminum were 118 µg/m3 and 58 µg/m3 for PM10 and PM1, respectively, during the bagging of Al2O3 in small bags. In comparison, the maximum concentrations of titanium were 550 µg/m3 and 434 µg/m3 for PM10 and PM1, respectively, during the bagging of TiO2 and indicate a significant release of coarser particles. © Taiwan Association for Aerosol Research.
    view abstractdoi: 10.4209/aaqr.2014.03.0065
  • 2015 • 192 Metal-organic CVD of Y2O3 Thin Films using Yttrium tris-amidinates
    Karle, S. and Dang, V.-S. and Prenzel, M. and Rogalla, D. and Becker, H.-W. and Devi, A.
    Chemical Vapor Deposition 21 335-342 (2015)
    Thin films of Y2O3 are deposited on Si(100) and Al2O3 (0001) substrates via metal-organic (MO)CVD for the first time using two closely related yttrium tris-amidinate compounds as precursors in the presence of oxygen in the temperature range 400-700 °C. The structural, morphological, and compositional features of the films are investigated in detail. At deposition temperatures of 500 °C and higher both the precursors yield polycrystalline Y2O3 thin films in the cubic phase. The compositional analysis revealed the formation of nearly stoichiometric Y2O3. The optical band gaps are estimated using UV-Vis spectroscopy. Preliminary electrical measurements are performed in the form of a metal oxide semiconductor (MOS) structure of Al/Y2O3/p-Si/Ag. Leakage currents and dielectric constants are also determined. Y2O3 thin films are grown by MOCVD at 400-700 °C using yttrium amidinate presursors in the presence of oxygen. The films, which are polycrystalline in case of deposition temperatures &gt; 400 °C, are dense and exhibit good purity and homogeneity. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201507189
  • 2015 • 191 High-Throughput Investigation of the Oxidation and Phase Constitution of Thin-Film Ni-Al-Cr Materials Libraries
    König, D. and Eberling, C. and Kieschnick, M. and Virtanen, S. and Ludwig, Al.
    Advanced Engineering Materials 17 1365-1373 (2015)
    Thin-film materials libraries of the intermetallic model system Ni-Al-Cr were fabricated and their oxidation behavior was studied by compositional, optical, electrical, and structural high-throughput characterization methods. The study reveals the compositional regions of the binary and ternary compositions which withstand longest to annealing in air (up to 700 C), and are, therefore, resistant to oxidation and delamination under these conditions. A complete ternary thin-film phase diagram for the Ni-Al-Cr system in its state after 9 h annealing in air at 500 C was determined. Optical high-throughput characterization is shown to be valid for rapid identification of oxidizing phases. Generally, the initially metallic phases show different oxidation behavior in air. We find that the ternary compositions are more resistant to oxidation than the binary phases. Compositions around Ni<inf>25</inf>Al<inf>12.5</inf>Cr<inf>62.5</inf> were found to show very good oxidation resistance. These results were supported by additional information from corresponding electrical and optical property investigations. The presented high-throughput approach is generic for the efficient study of multinary thin-film materials in harsh environments. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201500023
  • 2015 • 190 Solubility and ordering of Ti, Ta, Mo and W on the Al sublattice in L12-Co3Al
    Koßmann, J. and Hammerschmidt, T. and Maisel, S. and Müller, S. and Drautz, R.
    Intermetallics 64 44-50 (2015)
    Co-Al-W-based alloys are promising new materials for high-temperature applications. They owe their high-temperature strength to hardening by ternary L1<inf>2</inf>-Co<inf>3</inf>(Al<inf>1-x</inf>W<inf>x</inf>) precipitates, which may form even though binary Co<inf>3</inf>Al is not stable. In the current work, density functional theory calculations are performed to study the solubility and ordering of the transition metals W, Mo, Ti, and Ta at the Al sublattice in L1<inf>2</inf>-Co<inf>3</inf>Al. The sublattice disorder is modelled with a newly parametrised cluster expansion and compared to results using special quasi-random structures. Our results for W and Mo show that the mixing energy exhibits a minimum at approximately x = 0.7. However, the computed small values of the mixing energies indicate that W and Mo atoms are fully disordered with the Al atoms already at low temperatures. For Ti and Ta we find no sizeable driving force for ordering with the Al atoms. The computed solubilities on the Al sublattice obtained are in the range of 40-80 meV/atom for W and Mo and less than 25 meV/atom for Ti and Ta. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2015.04.009
  • 2015 • 189 Homogeneity and composition of AlInGaN: A multiprobe nanostructure study
    Krause, F.F. and Ahl, J.-P. and Tytko, D. and Choi, P.-P. and Egoavil, R. and Schowalter, M. and Mehrtens, T. and Müller-Caspary, K. and Verbeeck, J. and Raabe, D. and Hertkorn, J. and Engl, K. and Rosenauer, A.
    Ultramicroscopy 156 29-36 (2015)
    The electronic properties of quaternary AlInGaN devices significantly depend on the homogeneity of the alloy. The identification of compositional fluctuations or verification of random-alloy distribution is hence of grave importance. Here, a comprehensive multiprobe study of composition and compositional homogeneity is presented, investigating AlInGaN layers with indium concentrations ranging from 0 to 17. at% and aluminium concentrations between 0 and 39 at% employing high-angle annular dark field scanning electron microscopy (HAADF STEM), energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT). EDX mappings reveal distributions of local concentrations which are in good agreement with random alloy atomic distributions. This was hence investigated with HAADF STEM by comparison with theoretical random alloy expectations using statistical tests. To validate the performance of these tests, HAADF STEM image simulations were carried out for the case of a random-alloy distribution of atoms and for the case of In-rich clusters with nanometer dimensions. The investigated samples, which were grown by metal-organic vapor phase epitaxy (MOVPE), were thereby found to be homogeneous on this nanometer scale. Analysis of reconstructions obtained from APT measurements yielded matching results. Though HAADF STEM only allows for the reduction of possible combinations of indium and aluminium concentrations to the proximity of isolines in the two-dimensional composition space. The observed ranges of composition are in good agreement with the EDX and APT results within the respective precisions. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.04.012
  • 2015 • 188 Ternary and quaternary Cr or Ga-containing ex-LDH catalysts - Influence of the additional oxides onto the microstructure and activity of Cu/ZnAl2O4 catalysts
    Kühl, S. and Schumann, J. and Kasatkin, I. and Hävecker, M. and Schlögl, R. and Behrens, M.
    Catalysis Today 246 92-100 (2015)
    The stepwise substitution of Al by Cr and Ga leads to quaternary LDH precursors for Cu/ZnM2O4 (M = Al, Ga, Cr) catalysts. With the substitution of Al by Cr the interaction of the Cu phase with the oxide matrix is gradually weakened, which is caused by the participation of the chromium oxide phase in the redox processes during catalyst preparation. Such reactive Cr oxide matrix is less efficient than the inert Al oxide matrix in stabilizing the special microstructure of Cu/ZnM2O4 catalysts. These weakened interactions led to a lowering of the Cu particle embedment, coinciding with a pronounced Cu crystallite growth during reduction. Both effects partially compensate each other and a maximum in Cu surface area is observed for intermediate Cr contents. In the Ga-substituted catalysts, two distinct Cu species were found for high Ga contents. This is attributed to the presence of partially crystalline spinel and the resulting different strength of interface interaction of the CuO phase with the crystalline and the amorphous oxide. After reduction Cu catalysts with similar average Cu particle sizes as well as Cu surface areas were obtained. In both sample series, the catalytic activity in methanol synthesis does not scale with the Cu surface area and the experiments show that a strong interaction to the oxide is necessary to gain stability and activity of the Cu phase. Al substitution thus confirms that interface interactions between Cu and the oxide seem to beneficially affect the activity of the Cu particles and the optimal catalyst requires a compromise of exposed surface and interface. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2014.08.029
  • 2015 • 187 Composite Extrusion of Aluminum with Discontinuous Eccentric Reinforcements
    Levin, E. and Haase, M. and Foydl, A. and Tekkaya, A.E.
    Materials Today: Proceedings 2 4758-4762 (2015)
    The demand for lightweight components can be seen as a long-term objective of the automotive industry, motivated by ecological, economic and political reasons. The combination of multiple materials can be used to fabricate load-adapted products with the potential for lightweight applications. A promising approach for the production of such components is composite hot extrusion. In this process, high strength material is embedded into an aluminum matrix, in order to gain a local improvement of the mechanical properties of the produced component, while retaining the weight advantage of the light alloy. In this paper, the influence of the initial position of the reinforcements within the billet on their final position in the profile is investigated. © 2015 Elsevier Ltd. All rights.
    view abstractdoi: 10.1016/j.matpr.2015.10.009
  • 2015 • 186 Open volume defects and magnetic phase transition in Fe60Al40 transition metal aluminide
    Liedke, M.O. and Anwand, W. and Bali, R. and Cornelius, S. and Butterling, M. and Trinh, T.T. and Wagner, A. and Salamon, S. and Walecki, D. and Smekhova, A. and Wende, H. and Potzger, K.
    Journal of Applied Physics 117 (2015)
    Magnetic phase transition in the Fe<inf>60</inf>Al<inf>40</inf> transition metal aluminide from the ferromagnetic disordered A2-phase to the paramagnetic ordered B2-phase as a function of annealing up to 1000 °C has been investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation, and Mössbauer spectroscopy. The positron annihilation spectroscopy has been performed in-situ sequentially after each annealing step at the Apparatus for In-situ Defect Analysis that is a unique tool combining positron annihilation spectroscopy with temperature treatment, material evaporation, ion irradiation, and sheet resistance measurement techniques. The overall goal was to investigate the importance of the open volume defects onto the magnetic phase transition. No evidence of variation in the vacancy concentration in matching the magnetic phase transition temperature range (400-600 °C) has been found, whereas higher temperatures showed an increase in the vacancy concentration. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4919014
  • 2015 • 185 The thermal stability of intermetallic compounds in an as-cast SX co-base superalloy
    Lopez-Galilea, I. and Zenk, C. and Neumeier, S. and Huth, S. and Theisen, W. and Göken, M.
    Advanced Engineering Materials 17 741-747 (2015)
    The nature and stability of intermetallic phases in a multi-component γ/γ′ Co-base superalloy is investigated. At least three kinds of unwanted intermetallic phases form due to the segregation of Al, Ta, Ti, Si, and Hf during casting in the interdendritic areas in addition to γ and γ′. One of the intermetallic phases that shows a blocky shape, contains high content of Ta, Hf, and Ti and has been identified as a topologically close-packed Laves phase. A B2 phase appears as relatively big pools and in some cases contains needle-shaped precipitates with high content of Al and Ti. The thermal stability of these intermetallic phases is studied in this work under defined heat treatments. The needle-shaped precipitates already dissolve at 1000°C, whereas the B2 phase is dissolved only at higher temperatures of 1200°C. Small amounts of Laves phases remained stable during aging at 1200°C for 25h. The prediction of phases as well as their stability is also checked by the CALPHAD method. The TCNi5 database, containing the description of Co<inf>3</inf>(Al,W), predicts the presence of the observed intermetallic phases; however, the predicted main transformation temperatures for these phases differ from the experimentally obtained values. This work studies the nature and stability of three intermetallic phases - Laves, B2 and needle-shaped precipitates - in a multi-component γ/γ′ Co-base superalloy, named ERBO-Co0. These phases are generated during casting at the interdendritic areas. Experimental characterization of intermetallic phases is contrasted with thermodynamic calculations, which predicts the presence of the identified phases. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201400249
  • 2015 • 184 Effects of boron on the fracture behavior and ductility of cast Ti-6Al-4V alloys
    Luan, J.H. and Jiao, Z.B. and Heatherly, L. and George, E.P. and Chen, G. and Liu, C.T.
    Scripta Materialia 100 90-93 (2015)
    Minor amounts of boron additions have been found to greatly enhance the ductility of cast Ti-6Al-4V alloys, which was considered to be due to the grain-size refinement. In this paper, we report our interesting finding that the beneficial effect of boron on the ductility of the cast titanium alloys is due not only to the grain-size refinement but the enhancement of the prior-β grain-boundary cohesion by boron segregation at the grain boundaries, as evidenced by Auger electron microscopy. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2014.12.022
  • 2015 • 183 Ab initio study of compositional trends in solid solution strengthening in metals with low Peierls stresses
    Ma, D. and Friák, M. and Von Pezold, J. and Neugebauer, J. and Raabe, D.
    Acta Materialia 98 367-376 (2015)
    Abstract We identify and analyze general trends governing solid solution strengthening in binary alloys containing solutes across the Periodic table using quantum-mechanical calculations. Here we present calculations for the model system of Al binary solid solutions. The identified trends originate from an approximately parabolic dependence of two strengthening parameters to quantitatively predict the solid solution strengthening effect, i.e. the volume and slip misfit parameters. The volume misfit parameter shows a minimum (concave-up behavior) as a function of the solute element group number in the periodic table, whereas the slip misfit parameter shows a maximum (concave-down behavior). By analyzing reported data, a similar trend is also found in Ni and Mg (basal slip) binary systems. Hence, these two strengthening parameters are strongly anti-correlated, which can be understood in terms of the Fermi level shift in the framework of free electron model. The chemical trends identified in this study enable a rapid and efficient identification of the solutes that provide optimum solid-solution strengthening. The approach described here may thus serve as basis for ab initio guided metallurgical materials design. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.054
  • 2015 • 182 Computationally efficient and quantitatively accurate multiscale simulation of solid-solution strengthening by ab initio calculation
    Ma, D. and Friák, M. and Von Pezold, J. and Raabe, D. and Neugebauer, J.
    Acta Materialia 85 53-66 (2015)
    We propose an approach for the computationally efficient and quantitatively accurate prediction of solid-solution strengthening. It combines the 2-D Peierls-Nabarro model and a recently developed solid-solution strengthening model. Solid-solution strengthening is examined with Al-Mg and Al-Li as representative alloy systems, demonstrating a good agreement between theory and experiments within the temperature range in which the dislocation motion is overdamped. Through a parametric study, two guideline maps of the misfit parameters against (i) the critical resolved shear stress, τ0, at 0 K and (ii) the energy barrier, ΔEb, against dislocation motion in a solid solution with randomly distributed solute atoms are created. With these two guideline maps, τ0 at finite temperatures is predicted for other Al binary systems, and compared with available experiments, achieving good agreement. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.10.044
  • 2015 • 181 Quantitative chemical-structure evaluation using atom probe tomography: Short-range order analysis of Fe-Al
    Marceau, R.K.W. and Ceguerra, A.V. and Breen, A.J. and Raabe, D. and Ringer, S.P.
    Ultramicroscopy 157 12-20 (2015)
    Short-range-order (SRO) has been quantitatively evaluated in an Fe-18Al (at%) alloy using atom probe tomography (APT) data and by calculation of the generalised multicomponent short-range order (GM-SRO) parameters, which have been determined by shell-based analysis of the three-dimensional atomic positions. The accuracy of this method with respect to limited detector efficiency and spatial resolution is tested against simulated D0<inf>3</inf> ordered data. Whilst there is minimal adverse effect from limited atom probe instrument detector efficiency, the combination of this with imperfect spatial resolution has the effect of making the data appear more randomised. The value of lattice rectification of the experimental APT data prior to GM-SRO analysis is demonstrated through improved information sensitivity. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.05.001
  • 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 Divorced Eutectic Solidification of Mg-Al Alloys
    Monas, A. and Shchyglo, O. and Kim, S.-J. and Yim, C.D. and Höche, D. and Steinbach, I.
    JOM 67 1805-1811 (2015)
    We present simulations of the nucleation and equiaxed dendritic growth of the primary hexagonal close-packed α-Mg phase followed by the nucleation of the β-phase in interdendritic regions. A zoomed-in region of a melt channel under eutectic conditions is investigated and compared with experiments. The presented simulations allow prediction of the final properties of an alloy based on process parameters. The obtained results give insight into the solidification processes governing the microstructure formation of Mg-Al alloys, allowing their targeted design for different applications. © 2015, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-015-1418-4
  • 2015 • 178 Dual-scale phase-field simulation of Mg-Al alloy solidification
    Monas, A. and Shchyglo, O. and Höche, D. and Tegeler, M. and Steinbach, I.
    IOP Conference Series: Materials Science and Engineering 84 (2015)
    Phase-field simulations of the nucleation and growth of primary α-Mg phase as well as secondary, β-phase of a Mg-Al alloy are presented. The nucleation model for α- and β-Mg phases is based on the "free growth model" by Greer et al.. After the α-Mg phase solidification we study a divorced eutectic growth of α- and β-Mg phases in a zoomed in melt channel between α-phase dendrites. The simulated cooling curves and final microstructures of α-grains are compared with experiments. In order to further enhance the resolution of the interdendritic region a high-performance computing approach has been used allowing significant simulation speed gain when using supercomputing facilities. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/84/1/012069
  • 2015 • 177 Exploration of ternary subsystems of superalloys by high-throughput thin film experimentation: Optical and electrical data of the Co-Al-W system
    Naujoks, D. and Koenig, D. and Ludwig, Al.
    Materials Research Society Symposium Proceedings 1760 145-150 (2015)
    The complete ternary system Co-Al-W was fabricated as a thin film materials library by combinatorial magnetron sputtering. The materials library was investigated using high-throughput characterization methods such as optical measurements as well as automated resistance screening. The obtained data indicate possible phase regions and compositional regions which show early surface oxidation. The demonstrated approach illustrates that using high-throughput measurement methods provides a fast access to data of relatively unexplored materials systems. The gained data provides a valuable basis for further in-depth studies of the investigated materials systems. © 2015 Materials Research Society.
    view abstractdoi: 10.1557/opl.2015.56
  • 2015 • 176 Using the acetylacetonates of zinc and aluminium for the Metalorganic Chemical Vapour Deposition of aluminium doped zinc oxide films
    Nebatti, A. and Pflitsch, C. and Curdts, B. and Atakan, B.
    Materials Science in Semiconductor Processing 39 467-475 (2015)
    Metalorganic Chemical Vapour Deposition is a promising method for the growth of thin aluminium doped zinc oxide films (ZnO:Al), a material with potential application as transparent conducting oxide (TCO), e.g. for the use as front electrode in solar cells. For the low-cost deposition, the choice of the precursors is extremely important. Here we present the deposition of quite homogeneous films from the acetylacetonates of zinc and aluminium that are rather cheap, commercially available and easy to handle. A user-made CVD-reactor activating the deposition process by the light of halogen lamps was used for film deposition. Well-ordered films with an aluminium content between 0 and 8% were grown on borosilicate glass and Si(100). On both types of substrate, the films are crystalline and show a preferred orientation along the (002)-direction. The 0.3 to 0.5 μm thick films are highly transparent in the visible region. The best films show a low electric resistivity between 2.4 and 8 mΩ cm. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.mssp.2015.05.053
  • 2015 • 175 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 • 174 The influence of stacking fault energy on the microstructural and strain-hardening evolution of Fe-Mn-Al-Si steels during tensile deformation
    Pierce, D.T. and Jiménez, J.A. and Bentley, J. and Raabe, D. and Wittig, J.E.
    Acta Materialia 100 178-190 (2015)
    Understanding the relationship between the stacking-fault energy (SFE), deformation mechanisms, and strain-hardening behavior is important for alloying and design of high-Mn austenitic transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The present study investigates the influence of SFE on the microstructural and strain-hardening evolution of three TRIP/TWIP alloys (Fe-22/25/28Mn-3Al-3Si wt.%). The SFE is increased by systemically increasing the Mn content from 22 to 28 wt.%. The Fe-22Mn-3Al-3Si alloy, with a SFE of 15 mJ m-2, deforms by planar dislocation glide and strain-induced ε<inf>hcp</inf>-/α<inf>bcc</inf>-martensite formation which occurs from the onset of plastic deformation, resulting in improved work-hardening at low strains but lower total elongation. With an increased SFE of 21 mJ m-2 in the Fe-25Mn-3Al-3Si alloy, both mechanical twinning and ε<inf>hcp</inf>-martensite formation are activated during deformation, and result in the largest elongation of the three alloys. A SFE of 39 mJ m-2 enables significant dislocation cross slip and suppresses ε<inf>hcp</inf>-martensite formation, causing reduced work-hardening during the early stages of deformation in the Fe-28Mn-3Al-3Si alloy while mechanical twinning begins to enhance the strain-hardening after approximately 10% strain. The increase in SFE from 15 to 39 mJ m-2 results in significant changes in the deformation mechanisms and, at low strains, decreased work-hardening, but has a relatively small influence on strength and ductility. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.08.030
  • 2015 • 173 From high-entropy alloys to high-entropy steels
    Raabe, D. and Tasan, C.C. and Springer, H. and Bausch, M.
    Steel Research International 86 1127-1138 (2015)
    Inspired by high-entropy alloys, we study the design of steels that are based on high configurational entropy for stabilizing a single-phase solid solution matrix. The focus is placed on the system Fe-Mn-Al-Si-C but we also present trends in the alloy system Fe-Mn-Al-C. Unlike in conventional high-entropy alloys, where five or more equiatomically proportioned components are used, we exploit the flat configurational entropy plateau in transition metal mixtures, stabilizing solid solutions also for lean, non-equiatomic compositions. This renders the high-entropy alloying concept, where none of the elements prevails, into a class of Fe-based materials which we refer to as high-entropy steels. A point that has received little attention in high-entropy alloys is the use of interstitial elements. Here, we address the role of C in face-centered cubic solid solution phases. High-entropy steels reveal excellent mechanical properties, namely, very high ductility and toughness; excellent high rate and low-temperature ductility; high strength of up to 1 GPa; up to 17% reduced mass density; and very high strain hardening. The microstructure stability can be tuned by adjusting the stacking fault energy. This enables to exploit deformation effects such as the TRIP, TWIP, or precipitation determined mechanisms. We present a class of massive solid solution steels with high configurational entropy. Focus is placed on the system Fe-Mn-Al-Si-C, i.e., considering also C interstitials. By exploiting the flat configurational entropy plateau in metal mixtures, solid solutions of lean, non-equiatomic compositions can be stabilized. This renders the high-entropy alloying concept, where none of the elements prevails, into high-entropy steels. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201500133
  • 2015 • 172 Vegetable oil-based polybenzoxazine derivatives coatings on Zn-Mg-Al alloy coated steel
    Raicopol, M. and Bălănucă, B. and Sliozberg, K. and Schlüter, B. and Gârea, S.A. and Chira, N. and Schuhmann, W. and Andronescu, C.
    Corrosion Science 100 386-395 (2015)
    Based on environmentally friendly (bio-based) precursor materials a new class of benzoxazine derivatives was synthesized using phenolated high oleic sunflower oil as phenol component and either aniline or 1,6-diaminohexane as amine components. Hydrophobic and dense poly(benzoxazine) coatings on Zn-Mg-Al alloy coated steel were obtained after spin-coating or air-brush type spray coating by crosslinking during a heat treatment step. The poly(benzoxazine)-coated ZM-steel samples showed an anodic shift of the open circuit potential as well as the break-through potential. Using an automatic scanning droplet cell the impact of the polymer film thickness on corrosion protection was evaluated. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.corsci.2015.08.018
  • 2015 • 171 Joining tube to tube sheet for coil wound heat exchangers by hybrid friction diffusion bonding
    Roos, A. and Alba, D.R. and Hanke, S. and Wimmer, G. and Dos Santos, J.F.
    American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP 6A-2015 (2015)
    Coil-wound heat exchangers (CWHE) for low temperature applications such as the liquefaction of natural gas (LNG) are often made of aluminium alloys. The fabrication of these aluminium coil-wound heat exchangers holds several challenges, one of which is joining the tubes to the tube sheet. For this specific task, conventional joining technologies such as laser beam welding (LBW) or tungsten inert gas (TIG) welding cannot be easily performed in fully-mechanised mode or are not cost-effective. A joint project between the Helmholtz-Zentrum Geesthacht (HZG) and LINDE Engineering aims at the development of a new solid state joining process, Hybrid Friction Diffusion Bonding (HFDB), to fabricate tube-to-tubesheet connections for aluminium coil-wound heat exchangers. In the present study, the HFDB process has been developed to industrial maturity and the quality of the joints has been demonstrated by gas leak tightness tests and tensile pull-out tests. The joints meet the requirements for industrial application. Furthermore, the thermal field development in the weld area and the applied process forces have been monitored and correlated to process parameters. The microstructure of the joint has been investigated, and dynamic recrystallization is assumed to be the primary grain refinement mechanism in the thermomechanically affected zone. Copyright © 2015 by ASME.
    view abstractdoi: 10.1115/PVP201545064
  • 2015 • 170 Mullite: Crystal Structure and Related Properties
    Schneider, H. and Fischer, R.X. and Schreuer, J.
    Journal of the American Ceramic Society 98 2948-2967 (2015)
    Mullite is certainly one of the most important oxide materials for both conventional and advanced ceramics. Mullite belongs to the compositional series of orthorhombic aluminosilicates with the general composition Al2(Al2+2xSi2-2x)O10-x. Main members are sillimanite (x = 0), stoichiometric 3/2-mullite (x = 0.25), 2/1-mullite (x = 0.40), and the SiO2-free phase ι-alumina (x = 1, crystal structure not known). This study gives an overview on the present state of research regarding single crystal mullite. Following a short introduction, the second part of the review focuses on the crystal structure of mullite. In particular, the characteristic mullite-type structural backbone of parallel chains consisting of edge-sharing MO6 octahedra and their specific cross-linkage by TO4 tetrahedra is explained in detail, the role of cation disorder and structural oxygen vacancies is addressed, and the possibility of cation substitution on different sites is discussed. The third part of the study deals with physical properties being relevant for technical applications of mullite and includes mechanical properties (e.g., elasticity, compressibility, strength, toughness, creep), thermal properties (e.g., thermal expansion, heat capacity, atomic diffusion, thermal conductivity), electrical conductivity, and optical properties. Special emphasis is put on structure-property relationships which allow for interpretation of corresponding experimental data and offer in turn the possibility to tailor new mullite materials with improved properties. Finally, the reported anomalies and discontinuities in the evolution of certain physical properties with temperature are summarized and critically discussed. © 2015 The American Ceramic Society.
    view abstractdoi: 10.1111/jace.13817
  • 2015 • 169 Promoting strong metal support interaction: Doping ZnO for enhanced activity of Cu/ZnO:M (M = Al, Ga, Mg) catalysts
    Schumann, J. and Eichelbaum, M. and Lunkenbein, T. and Thomas, N. and Álvarez Galván, M.C. and Schlögl, R. and Behrens, M.
    ACS Catalysis 5 3260-3270 (2015)
    The promoting effect of Al, Ga, and Mg on the support in Cu/ZnO catalysts for methanol synthesis has been investigated. Different unpromoted and promoted ZnO supports were synthesized and impregnated with Cu metal in a subsequent step. All materials, supports, and calcined and activated catalysts were characterized by various methods, including contactless (microwave) conductivity measurements under different gas atmospheres. Small amounts of promoters were found to exhibit a significant influence on the properties of the oxide support, concerning textural as well as electronic properties. We found correlations between the conductivity of the ZnO support and the activity of the catalyst in the reverse water-gas shift reaction (rWGS) as well as in methanol synthesis. In rWGS the activation energy and reaction order in H<inf>2</inf> are decreased upon promotion of the ZnO support with the trivalent promoters Al3+ and Ga3+, indicating an electronic promotion. In methanol synthesis, results point to a structural promotion by Al3+ and Ga3+. A detrimental effect of Mg2+ doping was observed in both reactions. This effect is discussed in the context of the reducibility of ZnO under reaction conditions, which can be tuned by the promoter in different ways. The reducibility is seen as a critical property for the dynamic metal support interaction of the Cu/ZnO system. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b00188
  • 2015 • 168 Fe-cr-al containing oxide semiconductors as potential solar water-splitting materials
    Sliozberg, K. and Stein, H.S. and Khare, C. and Parkinson, B.A. and Ludwig, Al. and Schuhmann, W.
    ACS Applied Materials and Interfaces 7 4883-4889 (2015)
    A high-throughput thin film materials library for Fe-Cr-Al-O was obtained by reactive magnetron cosputtering and analyzed with automated EDX and XRD to elucidate compositional and structural properties. An automated optical scanning droplet cell was then used to perform photoelectrochemical measurements of 289 compositions on the library, including electrochemical stability, potentiodynamic photocurrents and photocurrent spectroscopy. The photocurrent onset and open circuit potentials of two semiconductor compositions (n-type semiconducting: Fe51Cr47Al2Ox, p-type semiconducting Fe36.5Cr55.5Al8Ox) are favorable for water splitting. Cathodic photocurrents are observed at 1.0 V vs RHE for the p-type material exhibiting an open circuit potential of 0.85 V vs RHE. The n-type material shows an onset of photocurrents at 0.75 V and an open circuit potential of 0.6 V. The p-type material showed a bandgap of 1.55 eV, while the n-type material showed a bandgap of 1.97 eV. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/am508946e
  • 2015 • 167 On the role of zinc on the formation and growth of intermetallic phases during interdiffusion between steel and aluminium alloys
    Springer, H. and Szczepaniak, A. and Raabe, D.
    Acta Materialia 96 203-211 (2015)
    The effect of Zn - both within Al and as a coating on steel - on the intermetallic phase formation and growth was systematically studied in controlled experiments, simulating the interfacial reactions taking place in dissimilar solid/solid and solid/liquid joining procedures. Independent from the reaction temperature, the addition of 1.05 at.% Zn (2.5 wt.%) to Al had no effect on the reaction layers' build-up with the η phase (Al<inf>5</inf>Fe<inf>2</inf>) as the dominant component, but accelerated their parabolic growth up to a factor of 13. While Zn-coatings on steel were found to be beneficial for the regular and even formation of intermetallic reaction zones in solid/liquid joining procedures, their role in solid-state processes was found to be more complex and, if no countermeasures are taken, extremely detrimental to the joint properties. Possible reasons for the Zn-induced growth acceleration are discussed, as well as consequences for possible optimisation steps for reducing harmful effects of Zn in dissimilar joints between Al alloys and steel. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.06.028
  • 2015 • 166 Isothermal aging of a γ'-strengthened Co-Al-W alloy coated with vacuum plasma-sprayed MCrAlY bond coats
    Terberger, P.J. and Sebold, D. and Webler, R. and Ziener, M. and Neumeier, S. and Klein, L. and Virtanen, S. and Göken, M. and Vaßen, R.
    Surface and Coatings Technology 276 360-367 (2015)
    Cobalt-based superalloys with a γ/γ' microstructure were discovered in 2006 and are currently being investigated as an alternative to nickel-based superalloys for high-temperature, high-load applications in gas turbine blades. They promise a better castability combined with a similar creep strength. Superalloy turbine blades are commonly coated with oxidation resistant bond coats. For this reason their compatibility needs to be studied. Co-9Al-9W specimens with a γ/γ' microstructure were coated with either a nickel-based or cobalt-based MCrAlY bond coat using vacuum plasma spraying. After aging at 900. °C in air for up to 500. h no decomposition of the γ' phase was found in the bulk superalloy. The interdiffusion zone shows several different W-rich topologically close-packed phases arising from the dissolution of the γ' phase in this region. The W-rich phases are identified to be μ phase for both bond coats and R phase for the nickel-based bond coat only. Their total volume is higher for the nickel-based bond coat. Therefore the cobalt-based bond coat is better suited for the Co-based superalloy substrate. Room temperature hardness and Young's modulus were measured using nanoindentation in the initial state and after heat treatment. A significantly higher Young's modulus was found for the cobalt-based bond coat. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.surfcoat.2015.06.048
  • 2015 • 165 The influence of different pre-treatments of current collectors and variation of the binders on the performance of Li4Ti5O12 anodes for lithium ion batteries
    Wennig, S. and Langklotz, U. and Prinz, G.M. and Schmidt, A. and Oberschachtsiek, B. and Lorke, A. and Heinzel, A.
    Journal of Applied Electrochemistry 45 1043-1055 (2015)
    In order to optimize the electron transfer between the Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf>-based active mass and the current collector, the surface of aluminum foil was modified either by alkaline etching or by a carbon coating. The as-modified aluminum foils were coated with an active mass of Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> mixed with polyvinylidene fluoride, sodium carboxymethyl cellulose, or polyacrylic acid as binders. Untreated aluminum and copper foils served as reference current collectors. The corrosion reactions of aluminum foil with the applied binder solutions were studied and the electrode structure has been analyzed, depending on the binder. Finally, the electrochemical performance of the prepared electrodes was investigated. Based on these measurements, conclusions concerning the electrical contact between the different current collectors and the active masses were drawn. The energy density of the Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> electrodes cast on carbon-coated aluminum foils was significantly increased, compared to the corresponding electrodes with a copper current collector. © 2015, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s10800-015-0878-0
  • 2015 • 164 Laser beam welding of aluminum to Al-base coated high-strength steel 22MnB5
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W. and Vollertsen, F.
    Journal of Materials Processing Technology 217 88-95 (2015)
    The microstructure of aluminum-Al-coated steel laser beam welding joints was analyzed with respect to the welding energy. Quantitative and qualitative analysis of the welding microstructure were used to measure the weld width as well as the thickness of the resulting intermetallic layer at the 22MnB5/aluminum interface in relation to the welding parameters. Weldability of Al-coated steel could be improved by removing brittle coating particles and oxides on the steel surface by sandblasting. Adhesion of aluminum filler material to the 22MnB5 steel sheet could be enhanced by inductive preheating of the steel surface during laser welding. This produced welded 22MnB5/aluminum joints that exhibited a linear mechanical resistance of 220 MPa and which failed away from the brittle intermetallic layer on the aluminum side under a tensile load. The shear strength of the intermetallic layer on the 22MnB5/aluminum interface was evaluated to 74 ± 21 MPa. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2014.10.026
  • 2015 • 163 Integration of plasmonic Ag nanoparticles as a back reflector in ultra-thin Cu(In,Ga)Se2 solar cells
    Yin, G. and Steigert, A. and Andrae, P. and Goebelt, M. and Latzel, M. and Manley, P. and Lauermann, I. and Christiansen, S. and Schmid, M.
    Applied Surface Science 355 800-804 (2015)
    Integration of plasmonic Ag nanoparticles as a back reflector in ultra-thin Cu(In,Ga)Se2 (CIGSe) solar cells is investigated. X-ray photoelectron spectroscopy results show that Ag nanoparticles underneath a Sn:In2O3 back contact could not be thermally passivated even at a low substrate temperature of 440 °C during CIGSe deposition. It is shown that a 50 nm thick Al2O3 film prepared by atomic layer deposition is able to block the diffusion of Ag, clearing the thermal obstacle in utilizing Ag nanoparticles as a back reflector in ultra-thin CIGSe solar cells. Via 3-D finite element optical simulation, it is proved that the Ag nanoparticles show the potential to contribute the effective absorption in CIGSe solar cells. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.07.195
  • 2015 • 162 Formability prediction of AL7020 with experimental and numerical failure criteria
    Yue, Z.M. and Badreddine, H. and Dang, T. and Saanouni, K. and Tekkaya, A.E.
    Journal of Materials Processing Technology 218 80-88 (2015)
    The formability of aluminum alloy sheet metal is a key issue in its design, analysis and operation of manufacturing processes. The conventional forming limit diagram (FLD) which evaluates the principal strains at failure is often used to quantify the formability limits. Due to the sensitivity of the FLD to the strain paths, the forming limit stress diagram (FLSD) based on principal stresses is shown to be more efficient. In contrast, a fully coupled behavior-damage models are recently proposed, which allow to predict the strain localization and the failure occurrence based on appropriates fully coupled constitutive equations describing the main physical phenomena involved. In this work a fully coupled constitutive equations taking into account the mixed nonlinear isotropic and kinematic hardenings fully coupled with the isotropic ductile damage is used. The microcracks closure is added to affect the equivalent plastic strain at fracture in a large range of stress states. Various tests are conducted to test the formability of aluminum alloy AL7020. The three different methods (FLD, FLSD and the fully coupled model) are identified and validated separately. With the help of Nakazima tests and cross-section deep drawing tests, the quality of the three failure criteria for AL7020 are compared and demonstrated with the investigations of the simulation results. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2014.11.034
  • 2014 • 161 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 • 160 Nanoscale understanding of bond formation during cold welding of aluminum and steel
    Altin, A. and Wohletz, S. and Krieger, W. and Kostka, A. and Groche, P. and Erbe, A.
    Advanced Materials Research 966-967 445-452 (2014)
    Cold welding, e.g. by cold forging, is a smart manufacturing technology, enabling novel multi material designs. A material combination, which is particularly attractive for manufacturing, though challenging to handle in a cold welding process, is steel and aluminum. We investigate the bond formation between cold forged C 15 (mainly primary heat treated) and AW 6082. Analysis starts with numerical simulations using the finite element analysis (FEA) to identify optimum conditions for bond formation. The bond strength was determined by tensile tests from samples eroded from the cold-welded specimen. Best performing samples showed a maximum tensile strength of ~200 MPa with ductile failure in the AW 6082. Transmission electron microscopy (TEM) inspection of the bonded area between aluminum and steel show a reaction layer consisting of iron and aluminum of few nm thickness throughout the sample. The formation of such a reaction layer is hypothesized to be crucial for bond formation. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 159 Manufacturing of steel-reinforced aluminum parts by co-extrusion and subsequent forging
    Behrens, B.-A. and Tekkaya, A.E. and Kosch, K.-G. and Foydl, A. and Kammler, M. and Jäger, A.
    Key Engineering Materials 585 149-156 (2014)
    The processes of manufacturing continuously and discontinuously steel-reinforced aluminum profiles by means of co-extrusion and subsequent forging were examined. In the co-extrusion and subsequent forging of discontinuously reinforced parts, influences of the reinforcing elements on forming behavior and material bonding for both processes were investigated. It was shown that forming temperature as well as ram speed have no influence on joining quality and forming behavior of the reinforcing elements in the co-extrusion of continuously reinforced profiles. The analyses of the joining zone between the composite partners revealed that a good connection of the two materials could be achieved. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 158 Transmission electron microscopy and ferromagnetic resonance investigations of tunnel magnetic junctions using Co2MnGe Heusler alloys as magnetic electrodes
    Belmeguenai, M. and Genevois, C. and Zighem, F. and Roussigné, Y. and Chérif, S.M. and Westerholt, K. and El Bahoui, A. and Fnidiki, A. and Moch, P.
    Thin Solid Films 551 163-170 (2014)
    High resolution transmission electron microscopy, nano-beam electronic diffraction, energy dispersive X-rays scanning spectroscopy, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) techniques are used in view of comparing (static and dynamic) magnetic and structural properties of Co 2MnGe(13 nm)/Al2O3(3 nm)/Co(13 nm) tunnel magnetic junctions (TMJs), deposited on various single crystalline substrates (a-plane sapphire, MgO(100) and Si(111)). They allow for providing a correlation between these magnetic properties and the fine structure investigated at atomic scale. The Al2O3 tunnel barrier is always amorphous and contains a large concentration of Co atoms, which, however, is significantly reduced when using a sapphire substrate. The Co layer is polycrystalline and shows larger grains for films grown on a sapphire substrate. The VSM investigation reveals in-plane anisotropy only for samples grown on a sapphire substrate. The FMR spectra of the TMJs are compared to the obtained ones with a single Co and Co2MnGe films of identical thickness deposited on a sapphire substrate. As expected, two distinct modes are detected in the TMJs while only one mode is observed in each single film. For the TMJ grown on a sapphire substrate, the FMR behavior does not significantly differ from the superposition of the individual spectra of the single films, allowing for a conclusion that the exchange coupling between the two magnetic layers is too small to give rise to observable shifts. For TMJs grown on a Si or on a MgO substrate, the resonance spectra reveal one mode which is nearly identical to the obtained one in the single Co film, while the other observed resonance shows a considerably smaller intensity and cannot be described using the magnetic parameters appropriate to the single Co2MnGe film. The large Co concentration in the Al2O3 interlayer prevents for a simple interpretation of the observed spectra when using Si or MgO substrates. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2013.11.090
  • 2014 • 157 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 • 156 Correlation of band gap position with composition in high-k films
    Ding, L. and Friedrich, M. and Fronk, M. and Gordan, O.D. and Zahn, D.R.T. and Chen, L. and Zhang, D.W. and Cobet, C. and Esser, N.
    Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures 32 (2014)
    Two series of ultrathin high-k samples (mixed layers Hf<inf>x</inf>Al <inf>y</inf>O<inf>z</inf> and bilayers HfO<inf>2</inf> on Al<inf>2</inf>O <inf>3</inf>) prepared by atomic layer deposition were investigated using spectroscopic ellipsometry in the energy range of 0.7-10.0 eV. The (effective) optical gap of both mixed layer and bilayer structures can be tuned by the film composition. The optical gap of mixed layers is linearly dependent on the Hf fraction from 5.77 (±0.02) eV for pure HfO<inf>2</inf> to 6.71 (±0.02) eV for pure Al<inf>2</inf>O<inf>3</inf>. The effective absorption gap of bilayers measured in reflection geometry is lower than that of mixed layers with a comparable Hf fraction due to the dominant effect of the top layer. An increase in film thickness as well as a decrease in refractive index and gap energy was observed after sample storage for two months in atmosphere. The aging effect is likely due to further oxidation of the oxygen deficient high-k films caused by the oxygen diffusion from air into the films. © 2014 American Vacuum Society.
    view abstractdoi: 10.1116/1.4866399
  • 2014 • 155 Tailored beam shaping for laser spot joining of highly conductive thin foils
    Funck, K. and Nett, R. and Ostendorf, A.
    Physics Procedia 56 750-758 (2014)
    Laser spot joining of thin metallic foils in the order of 100 micrometer and below has a number of interesting applications in electronic industry. However, high thermal conductivity and thermal expansion of common materials largely prohibit spot joints with a sufficiently large contact area needed to satisfy mechanical and electrical requirements. Of the numerous possibilities to positively influence the process of such joints we investigate using a pulsed Nd:YAG laser to generate spot joints of thin foils in combination with a beam shaping optic to tailor the temperature profile during laser spot joining of thin foils. This allows for increased contact area, stabilized process behavior and offers the potential for joining ultra thin foils far below 100 μm. Different configurations are examined, results are presented and discussed, mainly in terms of their general impact on the micro joining process. © 2014 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.phpro.2014.08.082
  • 2014 • 154 Sequential Bi-C bond activation reactions of BiEt3via insertion reactions of RE {R = HC[C(Me)N(2,6-i-Pr2C6H3)]2; E = Al, Ga, In}
    Ganesamoorthy, C. and Bläser, D. and Wölper, C. and Schulz, S.
    Chemical Communications 50 12382-12384 (2014)
    Two of the Bi-C bonds of BiEt3 are sequentially activated by mono-valent RM {R = HC[C(Me)N(2,6-i-Pr2C6H3)]2; M = Al, Ga, In}. The first Bi-C bond activation leads to the formation of insertion complexes, [RMEt(BiEt2)] (M = Al 1; Ga 2; In 3), whereas the consecutive second activation proceeds through a reductive elimination of RMEt2 (M = Al 4, Ga 5), elemental Bi and BiEt3. © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cc05028b
  • 2014 • 153 High strength and ductile low density austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides
    Gutierrez-Urrutia, I. and Raabe, D.
    Materials Science and Technology (United Kingdom) 30 1099-1104 (2014)
    We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered k-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L912 ordered precipitates (so called k-carbides) resulting in high strength (yield stress ∼ 1.0 GPa) and ductile (elongation to fracture 7sim; 30%) steels. Novel insights into dislocation-particle interactions in a Fe- 30.5Mn-8.0Al-1.2C (wt-%) steel strengthened by nanoscale k-carbides are discussed. © 2014 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/1743284714Y.0000000515
  • 2014 • 152 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 • 151 Vacancy strengthening in Fe3Al iron aluminides
    Hasemann, G. and Schneibel, J.H. and Krüger, M. and George, E.P.
    Intermetallics 54 95-103 (2014)
    The room temperature strength of FeAl alloys can be increased significantly by freezing in the high thermal vacancy concentrations present at elevated temperatures. In contrast, because of their lower thermal vacancy concentrations, vacancy strengthening in quenched Fe3Al alloys is believed to be much smaller and has not received much attention to date. In the present work, the influence of annealing time and quench temperature on the room temperature strength of extruded and recrystallized Fe3Al alloys is evaluated. For aluminum concentrations between 28 and 32 at% and quench temperatures between 400 and 900 °C both the magnitude and the kinetics of strengthening are found to be consistent with reported values for the thermal vacancy concentrations and vacancy migration rates. To assess the potential contributions of other strengthening mechanisms, appropriate heat treatments will need to be designed in follow-on studies that alter microstructural features relevant to those mechanisms while maintaining a constant vacancy concentration. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.05.013
  • 2014 • 150 Effect of retained beta layer on slip transmission in Ti-6Al-2Zr-1Mo-1V near alpha titanium alloy during tensile deformation at room temperature
    He, D. and Zhu, J. and Zaefferer, S. and Raabe, D.
    Materials and Design 56 937-942 (2014)
    Slip is the main plastic deformation mechanism in titanium alloys at room temperature. This is especially so for near alpha titanium alloy like Ti-6Al-2Zr-1Mo-1V, which contains low beta stabilizing and high aluminum (alpha stabilizing) element additions. The effects of retained beta layers on slip transmission across α/β interfaces in Ti-6Al-2Zr-1Mo-1V during tensile deformation have been studied in the current work. High resolution scanning electron microscopy (HR-SEM) and electron backscatter diffraction (EBSD) techniques were used to study the deformation microstructure. The results indicate that the full Burgers crystal orientation relationship (OR) between the α and the thin retained β phase layers facilitates slip transition but is not the necessary requirement/restriction. Some novel slip trace morphologies that are characterized by deflection and bifurcation (fork-like morphology) are revealed in the retained β layers between two abutting α grains. The possible reasons for these different slip transmission patterns are analyzed by EBSD results and a schematic model is proposed. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2013.12.018
  • 2014 • 149 Increased productivity in hot aluminum extrusion by using extrusion dies with inner cooling channels manufactured by rapid tooling
    Hölker, R. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Key Engineering Materials 611-612 981-988 (2014)
    The influence of local inner die cooling on the heat balance in hot aluminum extrusion was investigated. For the manufacturing of the die with cooling channels close to the forming zone, the layer-laminated manufacturing method was applied. The new tooling technology was applied in order to decrease the profile's exit temperature and to avoid thermally induced surface defects with the aim to raise the productivity in hot aluminum extrusion processes. Numerical and experimental investigations revealed that, while maintaining the exit temperature of the extrudate, a distinct increase of the production speed up to 300 % can be realized, while the extrusion force increases only slightly. An effect on the profiles microstructure was also detected. By applying die cooling, grain coarsening can be significantly limited or even be avoided. © 2014 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 148 First approach for thermodynamic modelling of the high temperature oxidation behaviour of ternary γ'-strengthened Co-Al-W superalloys
    Klein, L. and Zendegani, A. and Palumbo, M. and Fries, S.G. and Virtanen, S.
    Corrosion Science 89 1-5 (2014)
    In the present work, thermodynamic modelling of the high temperature oxidation behaviour of a γ'-strengthened Co-base superalloy is presented. The ternary Co-9Al-9W alloy (values in at%) was isothermally oxidised for 500h at 800 and 900°C in air. Results reveal that the calculated oxide layer sequence (Thermo-Calc, TCNI6) is in good agreement with the formed oxide scales on the alloy surface. Furthermore, prediction of the influence of oxygen partial pressure on Al2O3 formation is presented. The modelling results indicate pathways for alloy development or possible pre-oxidation surface treatments for improved oxidation resistance of the material. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.corsci.2014.08.016
  • 2014 • 147 Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe-Mn-Al-C light weight austenitic steel
    Koyama, M. and Springer, H. and Merzlikin, S.V. and Tsuzaki, K. and Akiyama, E. and Raabe, D.
    International Journal of Hydrogen Energy 39 4634-4646 (2014)
    Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2013.12.171
  • 2014 • 146 On the preparation and structure of caesium aluminium tetrahydride
    Krech, D. and Zibrowius, B. and Weidenthaler, C. and Felderhoff, M.
    European Journal of Inorganic Chemistry 2014 5683-5688 (2014)
    A new tetragonal phase of CsAlH4 was observed after the precipitation of CsAlH4 from a diglyme solution with an inert solvent. This new phase and the previously described orthorhombic phase were characterized by a combination of Xray powder diffraction analysis and 27Al and 133Cs solid-state NMR spectroscopy. The transformation of the tetragonal CsAlH4 phase into the orthorhombic CsAlH4 phase can be induced by thermal treatment, whereas the opposite process can be stimulated by mechanical treatment. The phase transformation processes are almost completely reversible and can be performed several times without any observable decomposition of CsAlH4. The structure of the tetragonal CsAlH4 phase (space group I41/a) was solved from X-ray powder diffraction data, and the lattice parameters were determined to be a = 5.6732(4) and c = 14.2795(11) A. © 2014 Wiley-VCH Verlag GmbH & Co.
    view abstractdoi: 10.1002/ejic.201402629
  • 2014 • 145 Cu-based catalyst resulting from a Cu,Zn,Al hydrotalcite-like compound: A microstructural, thermoanalytical, and in situ XAS study
    Kühl, S. and Tarasov, A. and Zander, S. and Kasatkin, I. and Behrens, M.
    Chemistry - A European Journal 20 3782-3792 (2014)
    A Cu-based methanol synthesis catalyst was obtained from a phase pure Cu,Zn,Al hydrotalcite-like precursor, which was prepared by co-precipitation. This sample was intrinsically more active than a conventionally prepared Cu/ZnO/Al2O3 catalyst. Upon thermal decomposition in air, the [(Cu0.5Zn0.17Al0.33)(OH) 2(CO3)0.17]×mH2O precursor is transferred into a carbonate-modified, amorphous mixed oxide. The calcined catalyst can be described as well-dispersed "CuO" within ZnAl 2O4 still containing stabilizing carbonate with a strong interaction of Cu2+ ions with the Zn-Al matrix. The reduction of this material was carefully analyzed by complementary temperature-programmed reduction (TPR) and near-edge X-ray absorption fine structure (NEXAFS) measurements. The results fully describe the reduction mechanism with a kinetic model that can be used to predict the oxidation state of Cu at given reduction conditions. The reaction proceeds in two steps through a kinetically stabilized CuI intermediate. With reduction, a nanostructured catalyst evolves with metallic Cu particles dispersed in a ZnAl2O4 spinel-like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7 nm) of this catalyst are partially embedded leading to lower absolute activity in comparison with a catalyst comprised of less-embedded particles. Interestingly, the exposed Cu surface area exhibits a superior intrinsic activity, which is related to a positive effect of the interface contact of Cu and its surroundings. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201302599
  • 2014 • 144 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 • 143 Role and evolution of nanoparticle structure and chemical state during the oxidation of NO over size- and shape-controlled Pt/γ-Al2O 3 catalysts under operando conditions
    Lira, E. and Merte, L.R. and Behafarid, F. and Ono, L.K. and Zhang, L. and Roldan Cuenya, B.
    ACS Catalysis 4 1875-1884 (2014)
    The structure and chemical state of size-selected Pt nanoparticles (NPs) supported on γ-Al2O3 were studied during the oxidation of NO using X-ray absorption near-edge spectroscopy and extended X-ray absorption fine-structure spectroscopy measurements under operando conditions. The data revealed the formation of PtOx species in the course of the reaction that remained present at the maximum temperature studied, 350 °C. The PtOx species were found in all samples, but those with the smallest NPs showed the highest degree of oxidation. Moreover, NO-induced nanoparticle redispersion was observed at temperatures below 150 °C for all catalysts studied. Catalytic tests showed activity toward the oxidation of NO for all samples. Nevertheless, the catalyst with the smallest NPs was found to be the least active, which is explained by a more extensive formation of PtOx species in this catalyst and their detrimental contribution to the oxidation of NO. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cs500137r
  • 2014 • 142 Mn2+ cation-directed ionothermal synthesis of an open-framework fluorinated aluminium phosphite-phosphate
    Liu, H. and Tian, Z.-J. and Gies, H. and Wei, Y. and Marler, B. and Wang, L. and Wang, Y.-S. and Li, D.-W.
    RSC Advances 4 29310-29313 (2014)
    An open-framework fluorinated aluminium phosphite-phosphate, H 3.2Mn3.4[C6N2H11] 2{Al12(HPO3)15.0(HPO 4)3.0F12}·14H2O (DNL-2), was ionothermally synthesized by employing the in situ released Mn2+ cations as structure-directing agent. © 2014 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ra05350h
  • 2014 • 141 Zr-based conversion layer on Zn-Al-Mg alloy coated steel sheets: Insights into the formation mechanism
    Lostak, T. and Maljusch, A. and Klink, B. and Krebs, S. and Kimpel, M. and Flock, J. and Schulz, S. and Schuhmann, W.
    Electrochimica Acta 137 65-74 (2014)
    Zr-based conversion layers are considered as environmentally friendly alternatives replacing trication phosphatation in the automotive industry. Based on excellent electronic barrier properties they provide an effective corrosion protection of the metallic substrate. In this work, thin protective layers were grown on novel Zn-Al-Mg alloy coated steel sheets by increasing the local pH-value at the sample surface leading to deposition of a Zr-based conversion layer. For this purpose Zn-Al-Mg alloy (ZM) coated steel sheets were treated in an aqueous model conversion solution containing well-defined amounts of hexafluorozirconic acid (H2ZrF6) and characterized after different immersion times with SKPFM and field emission SEM (FE-SEM)/EDX techniques. A deposition mechanism of Zr-based conversion coatings on microstructural heterogeneous Zn-Al-Mg alloy surfaces was proposed. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2014.05.163
  • 2014 • 140 Shape-dependent catalytic oxidation of 2-butanol over Pt nanoparticles supported on γ-Al2O3
    Mistry, H. and Behafarid, F. and Zhou, E. and Ono, L.K. and Zhang, L. and Roldan Cuenya, B.
    ACS Catalysis 4 109-115 (2014)
    This study illustrates the effect of nanoparticle (NP) shape on the reactivity of size-selected Pt/γ-Al2O3 nanocatalysts for 2-butanol oxidation. Nanoparticles similar in size [transmission electron microscopy (TEM) diameter of ∼1 nm] but with different shapes were prepared via encapsulation in inverse micelles. The NP shape was resolved by combining information extracted from extended X-ray absorption fine structure spectroscopy (EXAFS) data, TEM, and modeling. A correlation was observed between the average first nearest neighbor coordination number of atoms at the NP surface and their catalytic activity. In particular, the NPs with the largest number of weakly coordinated surface atoms (i.e., edges and corners) were found to be the least active for the total oxidation of 2-butanol. This result highlights that not only size but also shape control must be achieved to tailor the catalytic properties of nanoscale materials. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400888n
  • 2014 • 139 Tool design induced anisotropic flow behavior of hot extruded aluminum profiles
    Ossenkemper, S. and Haase, M. and Soyarslan, C. and Jäger, A. and Tekkaya, A.E.
    Key Engineering Materials 585 131-138 (2014)
    The hot extrusion process may lead to frequently observed textures in the profiles, like fiber structures in longitudinal direction. Aluminum profiles (AA6060) were extruded with different tool types (flat-face dies and modified porthole dies). Compression tests of cylindrical specimens, which were machined out of these profiles likewise in extrusion direction, were conducted to examine possible effects of the in-plane anisotropy in lateral direction. A hardness distribution over the cross section of the specimens was measured. It was found that dependent on tool design and profile geometry, the specimens developed preferred lateral flow directions during upsetting. Simulations of the upsetting test, with assigned Hill parameters to consider anisotropy of the material, showed, that this anisotropy, not the local hardness nonuniformity, is the main reason for the detected plastic flow properties. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 138 Elemental partitioning and mechanical properties of Ti- and Ta-containing Co-Al-W-base superalloys studied by atom probe tomography and nanoindentation
    Povstugar, I. and Choi, P.-P. and Neumeier, S. and Bauer, A. and Zenk, C.H. and Göken, M. and Raabe, D.
    Acta Materialia 78 78-85 (2014)
    Elemental partitioning and hardness in Ti- and Ta-containing Co-base superalloys, strengthened by γ′-Co3(Al, W) precipitates, have been studied by local measurements. Using atom probe tomography, we detect strong partitioning of W (partitioning coefficients from 2.4 to 3.4) and only slight partitioning of Al (partitioning coefficients ≤1.1) to the γ′-Co3(Al, W) phase. Al segregates to the γ/γ′ phase boundaries, whereas W is depleted at the γ side of the boundaries after aging at 900 °C and slow air cooling. This kind of Al segregation and W depletion is much less pronounced when water quenching is applied. As a result, these effects are considered to be absent at high temperatures and therefore should not influence the creep properties. Ti and Ta additions are found to strongly partition to the γ′ phase and greatly increase the γ′ volume fraction. Our results indicate that the alloying elements Al, W, Ti and Ta all occupy the B sublattice of the A 3B structure (L12 type) and affect the partitioning behavior of each other. Nanoindentation measurements show that Ta also increases the hardness of the γ′ phase, while the hardness of the γ channels remains nearly constant in all alloys. The change in hardness of the γ′ phase can be ascribed to the substitution of Al and W atoms by Ti and/or Ta. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.06.020
  • 2014 • 137 Alloy Design, Combinatorial Synthesis, and Microstructure–Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels
    Raabe, D. and Springer, H. and Gutierrez-Urrutia, I. and Roters, F. and Bausch, M. and Seol, J.-B. and Koyama, M. and Choi, P.-P. and Tsuzaki, K.
    JOM 66 1845-1856 (2014)
    We present recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here, two steel types are addressed. The first one is a class of low-density twinning-induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with 25–30 wt.% Mn and <4–5 wt.% Al or even <8 wt.% Al when naturally aged. The second one is a class of κ-carbide strengthened austenitic steels with even higher Al content. Here, κ-carbides form either at 500–600°C or even during quenching for >10 wt.% Al. Three topics are addressed in more detail, namely, the combinatorial bulk high-throughput design of a wide range of corresponding alloy variants, the development of microstructure–property relations for such steels, and their susceptibility to hydrogen embrittlement. © 2014, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1032-x
  • 2014 • 136 Growth mechanism of Al2Cu precipitates during in situ TEM heating of a HPT deformed Al-3wt.%Cu alloy
    Rashkova, B. and Faller, M. and Pippan, R. and Dehm, G.
    Journal of Alloys and Compounds 600 43-50 (2014)
    The microstructural evolution of Al2Cu precipitates in an ultrafine-grained Al-3wt.% Cu model alloy produced by high-pressure torsion (HPT) was studied by in situ transmission electron microscopy (TEM). The precipitation growth was systematically investigated by isothermal heating experiments in the temperature range of 120 C to 170 C. The experimental data is analysed with respect of the diffusion kinetics and activation energy to determine the most prominent diffusion path: lattice or grain boundary diffusion. The results imply that grain boundary diffusion is the relevant mechanism for Al2Cu growth in the HPT deformed material. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2014.02.090
  • 2014 • 135 Aluminum-doped ZnO nanoparticles: Gas-phase synthesis and dopant location
    Schilling, C. and Zähres, M. and Mayer, C. and Winterer, M.
    Journal of Nanoparticle Research 16 (2014)
    Aluminum-doped ZnO (AZO) nanoparticles are studied widely as transparent conducting alternatives for indium tin oxide. However, the properties of AZO vary in different investigations not only with the amount of dopant and the particle size, but also with other parameters such as synthesis method and conditions. Hence, AZO nanoparticles, synthesized in the gas phase, were investigated to study the influence of the synthesis parameters dopant level, reactor temperature and residence time in the reaction zone on the particle characteristics. The local structure of the dopant in semiconductors determines whether the doping is functional, i.e., whether mobile charge carriers are generated. Therefore, information obtained from 27Al solid-state NMR spectroscopy, X-ray diffraction, photoluminescence and UV-Vis spectroscopy was used to understand how the local structure influences particles characteristics and how the local structure itself can be influenced by the synthesis parameters. In addition to AZO particles of different Al content, pure ZnO, Al2O3, ZnAl2O4 and core-shell particles of ZnO and Al2O3 were synthesized for comparison and aid to a deeper understanding of the formation of AZO nanoparticles in the gas phase. © 2014 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-014-2506-z
  • 2014 • 134 Columnar-Structured Mg-Al-Spinel Thermal Barrier Coatings (TBCs) by Suspension Plasma Spraying (SPS)
    Schlegel, N. and Ebert, S. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 24 144-151 (2014)
    The suspension plasma spraying (SPS) process has been developed to permit the feeding of sub-micrometer-sized powder into the plasma plume. In contrast to electron beam-physical vapor deposition and plasma spray-physical vapor deposition, SPS enables the cost-efficient deposition of columnar-structured coatings. Due to their strain tolerance, these coatings play an important role in the field of thermal barrier coatings (TBCs). In addition to the cost-efficient process, attention was turned to the TBC material. Nowadays, yttria partially stabilized zirconia (YSZ) is used as standard TBC material. However, its long-term application at temperatures higher than 1200 °C is problematic. At these high temperatures, phase transitions and sintering effects lead to the degradation of the TBC system. To overcome those deficits of YSZ, Mg-Al-spinel was chosen as TBC material. Even though it has a lower melting point (~2135 °C) and a higher thermal conductivity (~2.5 W/m/K) than YSZ, Mg-Al-spinel provides phase stability at high temperatures in contrast to YSZ. The Mg-Al-spinel deposition by SPS resulted in columnar-structured coatings, which have been tested for their thermal cycling lifetime. Furthermore, the influence of substrate cooling during the spraying process on thermal cycling behavior, phase composition, and stoichiometry of the Mg-Al-spinel has been investigated. © 2014, ASM International.
    view abstractdoi: 10.1007/s11666-014-0138-6
  • 2014 • 133 Synthesis and characterisation of a highly active Cu/ZnO: Al catalyst
    Schumann, J. and Lunkenbein, T. and Tarasov, A. and Thomas, N. and Schlögl, R. and Behrens, M.
    ChemCatChem 6 2889–2897 (2014)
    We report the application of an optimised synthesis protocol of a Cu/ZnO:Al catalyst. The different stages of synthesis are all well-characterised by using various methods with regard to the (micro-)structural, textural, solid-state kinetic, defect and surface properties. The low amount of the Al promoter (3%) influences but does not generally change the phase evolution known for binary Cu/ZnO catalysts. Its main function seems to be the introduction of defect sites in ZnO by doping. These sites as well as the large Cu surface area are responsible for the large N2O chemisorption capacity. Under reducing conditions, the Al promoter, just as Zn, is found enriched at the surface suggesting an active role in the strong metal-support interaction between Cu and ZnO:Al. The different stages of the synthesis are comprehensively analysed and found to be highly reproducible in the 100g scale. The resulting catalyst is characterised by a uniform elemental distribution, small Cu particles (8nm), a porous texture (pore size of approximately 25nm), high specific surface area (approximately 120m2g-1), a high amount of defects in the Cu phase and synergetic Cu-ZnO interaction. A high and stable performance was found in methanol synthesis. We wish to establish this complex but well-studied material as a benchmark system for Cu-based catalysts. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201402278
  • 2014 • 132 Composite hot extrusion of functional elements
    Schwane, M. and Dahnke, C. and Haase, M. and Ben Khalifa, N. and Tekkaya, A. E.
    Advanced Materials Research 1018 223-228 (2014)
    The composite extrusion process, which is investigated at the Institute of Forming Technology and Lightweight Construction (IUL), allows the combination of different materials within an aluminium profile. In contrast to the useage of reinforcing elements, this paper focuses on the embedding of functional elements, such as isolated electric conductors. Results of the experimental and numerical investigations are presented within this paper. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 131 Coherent properties of single rare-earth spin qubits
    Siyushev, P. and Xia, K. and Reuter, R. and Jamali, M. and Zhao, N. and Yang, N. and Duan, C. and Kukharchyk, N. and Wieck, A.D. and Kolesov, R. and Wrachtrup, J.
    Nature Communications 5 (2014)
    Rare-earth-doped crystals are excellent hardware for quantum storage of photons. Additional functionality of these materials is added by their waveguiding properties allowing for on-chip photonic networks. However, detection and coherent properties of rare-earth single-spin qubits have not been demonstrated so far. Here we present experimental results on high-fidelity optical initialization, effcient coherent manipulation and optical readout of a single-electron spin of Ce 3+ ion in a yttrium aluminium garnet crystal. Under dynamic decoupling, spin coherence lifetime reaches T 2 =2 ms and is almost limited by the measured spin-lattice relaxation time T 1 =4.5 ms. Strong hyperfine coupling to aluminium nuclear spins suggests that cerium electron spins can be exploited as an interface between photons and long-lived nuclear spin memory. Combined with high brightness of Ce 3+ emission and a possibility of creating photonic circuits out of the host material, this makes cerium spins an interesting option for integrated quantum photonics. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms4895
  • 2014 • 130 Experimental and numerical analysis of dry shearing of aluminum 6082
    Steinbach, F. and Chen, L. and Güner, A. and Tekkaya, A. E.
    Advanced Materials Research 1018 261-268 (2014)
    In shearing and blanking lubrication is widely used. For future production one aim is to decrease the use of oil or to do without such lubricants. For this purpose it is interesting, if there is an influence of using lubricants on the cut edge quality. Therefore numerical analysis and experiments are carried out with a blanking tool, installed in a servo press, to analyze the cut edge quality of shearing of aluminum 6082-T6. Using FEA with Lemaitre’s damage model, different friction coefficients for modelling different kinds of lubrication are used for modelling a process of punching a circular hole with a punch of 16 mm diameter. In the experiments the cut edge quality and the maximum shearing force do not differ significantly. With a friction coefficient of 0.2 it is possible to give the right cut edge quality by using the damage model of Lemaitre. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 129 Rapid identification of areas of interest in thin film materials libraries by combining electrical, optical, X-ray diffraction, and mechanical high-throughput measurements: A case study for the system ni-al
    Thienhaus, S. and Naujoks, D. and Pfetzing-Micklich, J. and König, D. and Ludwig, Al.
    ACS Combinatorial Science 16 686-694 (2014)
    The efficient identification of compositional areas of interest in thin film materials systems fabricated by combinatorial deposition methods is essential in combinatorial materials science. We use a combination of compositional screening by EDX together with high-throughput measurements of electrical and optical properties of thin film libraries to determine efficiently the areas of interest in a materials system. Areas of interest are compositions which show distinctive properties. The crystallinity of the thus determined areas is identified by X-ray diffraction. Additionally, by using automated nanoindentation across the materials library, mechanical data of the thin films can be obtained which complements the identification of areas of interest. The feasibility of this approach is demonstrated by using a Ni-Al thin film library as a reference system. The obtained results promise that this approach can be used for the case of ternary and higher order systems. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/co5000757
  • 2014 • 128 Development and characterization of B4C reinforced detonation-sprayed Al coatings
    Tillmann, W. and Hollingsworth, P.S. and Fischer, G. and Nellesen, J. and Beckmann, F.
    Journal of Thermal Spray Technology 23 289-295 (2014)
    Because of their excellent properties aluminum and its alloys cover a wide range of applications especially in the lightweight construction sector. In order to reach a higher strength and wear resistance metal matrix composites (MMCs) are used. Typically Al MMCs are manufactured by casting or extrusion processes. The disadvantage of these production routes is a cost-intensive and time-consuming finishing in terms of grinding and milling. The technique of thermal spraying provides the possibility to coat aluminum parts with MMCs close to their final shape. In addition to the shape accuracy the ductility and toughness of the coated parts are generally higher compared to extruded or casted parts. This study describes the development of detonation-sprayed boron carbide reinforced aluminum coatings on aluminum (EN AW 5754) substrates. The optimization of the coatings was focused on a homogeneous coating structure, a low coating porosity, a high deposition efficiency, a high number of embedded carbides, and a small percentage of oxides. In continuous tensile tests the influence of the MMC coating on the tensile strength was determined. Furthermore, the tensile strength was investigated in a discontinuous tensile test step by step. The different stages of deformation were analyzed by using micro computed tomography. This method enables the observation of tensile specimens in 3D, and consequently the site and moment of crack formation. © 2013 ASM International.
    view abstractdoi: 10.1007/s11666-013-0036-3
  • 2014 • 127 Aluminium alloy based hydrogen storage tank operated with sodium aluminium hexahydride Na3AlH6
    Urbanczyk, R. and Peinecke, K. and Felderhoff, M. and Hauschild, K. and Kersten, W. and Peil, S. and Bathen, D.
    International Journal of Hydrogen Energy 39 17118-17128 (2014)
    Here we present the development of an aluminium alloy based hydrogen storage tank, charged with Ti-doped sodium aluminium hexahydride Na3AlH6. This hydride has a theoretical hydrogen storage capacity of 3 mass-% and can be operated at lower pressure compared to sodium alanate NaAlH4. The tank was made of aluminium alloy EN AW 6082 T6. The heat transfer was realised through an oil flow in a bayonet heat exchanger, manufactured by extrusion moulding from aluminium alloy EN AW 6060 T6. Na3AlH6 is prepared from 4 mol-% TiCl3 doped sodium aluminium tetrahydride NaAlH4 by addition of two moles of sodium hydride NaH in ball milling process. The hydrogen storage tank was filled with 213 g of doped Na3AlH6 in dehydrogenated state. Maximum of 3.6 g (1.7 mass-% of the hydride mass) of hydrogen was released from the hydride at approximately 450 K and the same hydrogen mass was consumed at 2.5 MPa hydrogenation pressure. 45 cycle tests (rehydrogenation and dehydrogenation) were carried out without any failure of the tank or its components. Operation of the tank under real conditions indicated the possibility for applications with stationary HT-PEM fuel cell systems. © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2014.08.101
  • 2014 • 126 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 • 125 Formation of intermetallic phases in Al-coated hot-stamped 22MnB5 sheets in terms of coating thickness and Si content
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 246 17-25 (2014)
    AlSiFe coatings with differing thicknesses and Si contents were applied to steel sheets by hot dipping. The steel sheets were austenitized at TAUS=920°C for different dwell times and then quenched in water. Phase formation as a function of coating thickness and Si content at the steel substrate/coating interface was investigated by ex-situ phase analysis with synchrotron radiation and by electron backscatter diffraction (EBSD). X-ray diffraction (XRD) and EBSD investigations confirmed the formation of AlFe-rich intermetallics at the steel/coating interface as a result of a strong diffusion of the elements Al and Fe. Within the first minute, Fe diffusion into the partially melted Al-base coatings promotes the formation of intermetallics of type Al8Fe2Si, Al13Fe4, and Al5Fe2. After the coating has transformed completely into Al-Fe intermetallics, Al diffusion into the steel substrate becomes more pronounced, thus reducing the Al content in the Al-Fe intermetallics and promoting formation of the phases of type Al2Fe and AlFe in the coating and formation of an Al-rich bcc layer in the steel substrate. The transformation kinetics of the resulting Al-, Fe-rich intermetallics are influenced by the coating thickness and the chemical composition of the Al-base coating. On the one hand, faster saturation of Fe in the Al-base coating is promoted by a shorter diffusion path and therefore by a thinner coating thickness. Otherwise, Si influences the diffusivity of the elements Al and Fe in the Al-, Fe-rich intermetallics and promotes the formation of Si-richer intermetallics, which then act as nuclei for Fe-richer intermetallics. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2014.02.056
  • 2014 • 124 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 • 123 Corrosion fatigue behaviour of creep-resistant magnesium alloy mg-4al-2ba-2ca
    Wittke, P. and Klein, M. and Walther, F.
    Procedia Engineering 74 78-83 (2014)
    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 a 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. © 2014 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2014.06.228
  • 2014 • 122 TLP brazing of aluminum to steel using PVD-deposited interlayer
    Wojarski, L. and Tillmann, W.
    Welding in the World 58 673-680 (2014)
    The demand for hybrid material concepts is steadily growing, and especially dissimilar joints between aluminum and steel are, due to their wide dissemination, of major importance. The main obstacle for the fabrication of aluminum-steel joints using thermal processes is the embrittlement of the fusion area. In order to prevent direct contact between aluminum and iron and thus to suppress the formation of brittle iron aluminides, 3-μm thick diffusion barrier coatings, consisting of Ni or Ti, were applied onto the steel surface. Pure copper with a thickness of 3 and 6 μm, respectively, was used as a filler material, and the samples were brazed in a TLP process in a vacuum at 580 °C at varying dwell times (10...50 min). The samples brazed with Ni diffusion barriers showed a considerable formation of Fe2Al 5 even at low dwell times. Furthermore, additional complex ternary phase bands have generated due to the existence of diffusion barrier elements and were detected in the interfacial area. The application of Ti showed a significant decrease of iron aluminides, and no Fe2Al5 could be detected at low dwell times, resulting in a shear strength of 42 MPa for the optimized parameters. © 2014 International Institute of Welding.
    view abstractdoi: 10.1007/s40194-014-0143-x
  • 2014 • 121 Enhancement of low-temperature activity over Cu-exchanged zeolite beta from organotemplate-free synthesis for the selective catalytic reduction of NOx with NH3 in exhaust gas streams
    Xu, L. and Shi, C. and Zhang, Z. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Feyen, M. and Maurer, S. and Yilmaz, B. and Müller, U. and Zhang, W.
    Microporous and Mesoporous Materials 200 304-310 (2014)
    A series of Cu-exchanged Al-rich Beta zeolites from organotemplate-free synthesis was prepared and investigated for selective catalytic reduction (SCR) of NO<inf>x</inf> with NH<inf>3</inf> in exhaust gas streams. In comparison to conventional Cu-Beta zeolite with Si/Al ratio of 19, Cu-Beta zeolite with Si/Al ratio of 4 is a superior low-temperature NH<inf>3</inf>-SCR catalyst. Very high NO conversion (>95%) can be achieved at temperatures as low as 150 to ∼400 °C. XRD, UV-Vis-NIR and NH<inf>3</inf>-TPD measurements show that more isolated Cu2+ ions are present at the exchange sites of Al-rich Beta zeolite. The combination of CO-FTIR and H<inf>2</inf>-TPR analysis demonstrates that Cu2+ ions could be reduced more readily on the Al-rich Beta than on the conventional Beta probably due to the proximity of the isolated Cu2+ ions. These can be correlated to the enhancement of NO conversion at lower temperatures over Cu-exchanged Al-rich Beta zeolite. © 2014 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2014.04.034
  • 2014 • 120 Enhanced superplasticity in an Al-alloyed multicomponent Mn-Si-Cr-C steel
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Choi, P. and Tasan, C.C. and Raabe, D.
    Acta Materialia 63 232-244 (2014)
    Excellent superplasticity (elongation ∼720%) is observed in a novel multi-component (Mn-S-Cr-Al alloyed) ultrahigh carbon steel during tensile testing at a strain rate of 2 × 10-3 s-1 and a temperature of 1053 K (just above the equilibrium austenite-pearlite transformation temperature). In order to understand superplasticity in this material and its strong Al dependence, the deformation-induced microstructure evolution is characterized at various length scales down to atomic resolution, using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, energy-dispersive X-ray spectroscopy and atom probe tomography. The results reveal that 1 wt.% Al addition influences various microprocesses during deformation, e.g. it impedes Ostwald ripening of carbides, carbide dissolution, austenite nucleation and growth and void growth. As a result, the size of the austenite grains and voids remains relatively fine (< 10 μm) during superplastic deformation, and fine-grained superplasticity is enabled without premature failure. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.10.034
  • 2013 • 119 Performance improvement of nanocatalysts by promoter-induced defects in the support material: Methanol synthesis over Cu/ZnO:Al
    Behrens, M. and Zander, S. and Kurr, P. and Jacobsen, N. and Senker, J. and Koch, G. and Ressler, T. and Fischer, R.W. and Schlögl, R.
    Journal of the American Chemical Society 135 6061-6068 (2013)
    Addition of small amounts of promoters to solid catalysts can cause pronounced improvement in the catalytic properties. For the complex catalysts employed in industrial processes, the fate and mode of operation of promoters is often not well understood, which hinders a more rational optimization of these important materials. Herein we show for the example of the industrial Cu/ZnO/Al2O3 catalyst for methanol synthesis how structure-performance relationships can deliver such insights and shed light on the role of the Al promoter in this system. We were able to discriminate a structural effect and an electronic promoting effect, identify the relevant Al species as a dopant in ZnO, and determine the optimal Al content of improved Cu/ZnO:Al catalysts. By analogy to Ga- and Cr-promoted samples, we conclude that there is a general effect of promoter-induced defects in ZnO on the metal-support interactions and propose the relevance of this promotion mechanism for other metal/oxide catalysts also. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja310456f
  • 2013 • 118 Compressive behavior of Ti3AlC2 and Ti 3Al0.8Sn0.2C2 MAX phases at room temperature
    Bei, G.-P. and Laplanche, G. and Gauthier-Brunet, V. and Bonneville, J. and Dubois, S.
    Journal of the American Ceramic Society 96 567-576 (2013)
    In this study, we report on the compressive behavior of Ti 3AlC2 and Ti3Al0.8Sn 0.2C2 MAX phases at room temperature. We found that these two phases could be classified as Kinking Nonlinear Elastic (KNE) solids. The cyclic compressive stress-strain loops for Ti3AlC2 and Ti3Al0.8Sn0.2C2 are typical hysteretic and fully reversible. At failure, both compositions fracture in shear with maximum stresses of 545 MPa for Ti3AlC2 and 839 MPa for Ti3Al0.8Sn0.2C2. Consequently, the macroshear stresses for failure, τc, are 185 MPa and 242 MPa for Ti3AlC2 and Ti3Al0.8Sn 0.2C2, respectively. In addition to the grain size effects, the presence of a ductile TixAly intermetallic distributed in the grain boundaries plays an important role in the enhancement of the ultimate compressive and macroshear stresses for Ti3Al 0.8Sn0.2C2. SEM observations reveal that these two MAX phases exhibit crack deflections, intragranular fractures, kink band formation and delaminations, grain push-in and pull-out. © 2012 The American Ceramic Society.
    view abstractdoi: 10.1111/jace.12092
  • 2013 • 117 Grain size evolution simulation in aluminium alloys AA 6082 and AA 7020 during hot forward extrusion process
    Foydl, A. and Segatori, A. and Ben Khalifa, N. and Donati, L. and Brosius, A. and Tomesani, L. and Tekkaya, A.E.
    Materials Science and Technology (United Kingdom) 29 100-110 (2013)
    The present paper investigates the grain size evolution in aluminium alloys AA 6082 and AA 7020 during hot forward extrusion process. The aim of the present work is the definition and implementation of a predictive algorithm that is able to compute the evolution of the grain shape during the process within the finite element method code Deform. Extrusion experiments were performed at two levels: at reduced scale for investigating and identifying the predictive equations and at industrial scale for validating the developed algorithm. At small scale extrusion, a complete factorial plan was performed for two alloys at three different temperatures, three extrusion ratios and two ram speeds: the discards and extrudates from the experiments were quenched immediately in order to avoid any potential recrystallisation, hence allowing measurements of transitional processing steps. At the industrial scale, instead, the 7020 alloy was extruded with two different die designs, thus producing a 20 mm diameter round bar under different extrusion ratios and strain paths. Finite element simulations were initially validated over visioplastic investigations in order to establish an accurate computation of the material flow, then experimental and numerical results were coupled, thus allowing the definition of the grain evolution model that was successfully integrated and validated on industrial scale trials. © 2013 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/1743284712Y.0000000132
  • 2013 • 116 Diffusive and massive phase transformations in Ti-Al-Nb alloys-Modelling and experiments
    Gamsjäger, E. and Liu, Y. and Rester, M. and Puschnig, P. and Draxl, C. and Clemens, H. and Dehm, G. and Fischer, F.D.
    Intermetallics 38 126-138 (2013)
    The thermodynamic properties of the Ti-Al-Nb system are obtained from recently published thermodynamic assessments. Based on these data the phase boundaries of the (α-Ti + γ-TiAl) two phase region are calculated by utilizing the CALPHAD approach and are compared to those, obtained by ab-initio calculations. It is found that the ab-initio phase boundaries deviate significantly from those based on the CALPHAD fit to experimental data which can be rationalized by the lack of vibrational entropy contributions in the present approach. Consequently a thermodynamic description based on the CALPHAD approach is used to further investigate the kinetics of the massive α → γm phase transformation in the Ti-Al-Nb system by means of a recently developed thick-interface model. Simulation of the transformation kinetics results in a massive transformation in the single-phase region only. However, very thin mole fraction spikes are obtained due to comparatively high interface velocities. It is likely that these spikes cannot be fully developed in experiments meaning that diffusion processes are partly suppressed (quasi-diffusionless transformation). A massive transformation in the two-phase region would then be possible. The theoretical predictions are compared to experimental studies performed on a Ti-45Al-5Nb alloy (composition in atomic percent). The alloy is heat treated slightly above the α-transus temperature and subsequently oil quenched to room temperature to generate γm-α2 interfaces. Energy-dispersive X-ray spectroscopy measurements were performed across γm- α2 interfaces in a scanning transmission electron microscope to search for chemical spikes. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.03.001
  • 2013 • 115 Time- and space-resolved high-throughput characterization of stresses during sputtering and thermal processing of Al-Cr-N thin films
    Grochla, D. and Siegel, A. and Hamann, S. and Buenconsejo, P.J.S. and Kieschnick, M. and Brunken, H. and König, D. and Ludwig, Al.
    Journal of Physics D: Applied Physics 46 (2013)
    (Al100-xCrx)N thin-film materials libraries (x = 31-79 at%) were fabricated on micro-machined cantilever arrays, in order to simultaneously investigate the evolution of stresses during film growth as well as during thermal processing by analysing the changes in cantilever curvature. The issue of the dependence of stress in the growing films on composition, at comparable film thicknesses, was investigated. Among the various experimental parameters studied, it was found that the applied substrate bias has the strongest influence on stress evolution and microstructure formation. The compositions of the films, as well as the applied substrate bias, have a pronounced effect on the lattice parameter and the coherence length. For example, applying a substrate bias in general leads to compressive residual stress, increases the lattice parameter and decreases the coherence length. Moreover, bias can change the film texture from [1 1 1] orientation to [2 0 0]. Further detailed analysis using x-ray diffraction and transmission electron microscopy clearly revealed the presence of a [1 1 1] highly textured face centred cubic (B1 type) Al-Cr-N phase in the as-deposited state as well as the coexistence of the hexagonal [1 1 0] textured Cr2N phase, which forms in the Cr-rich region. These results show that the combinatorial approach provides insight into how stresses and compositions are related to phases and microstructures of different Al-Cr-N compositions fabricated in the form of materials libraries. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/8/084011
  • 2013 • 114 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 • 113 Influence of Al content and precipitation state on the mechanical behavior of austenitic high-Mn low-density steels
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 68 343-347 (2013)
    We investigate the strain hardening of two austenitic high-Mn low density steels, namely, Fe-30.5Mn-2.1Al-1.2C and Fe-30.5Mn-8.0Al-1.2C (wt.%), containing different precipitation states. The strain hardening of the alloy with low Al content is attributed to dislocation and twin substructures. The precipitation of intergranular M3C-type carbides strongly influences the fracture mode. We associate the strain hardening behavior of the alloy with high Al content to the precipitation of shearable nanosized κ-carbides and their role in the development of planar dislocation substructures.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.08.038
  • 2013 • 112 Influence of vacuum heat treatment parameters on the surface composition of MCrAlY coatings
    Keller, I. and Naumenko, D. and Quadakkers, W.J. and Vaßen, R. and Singheiser, L.
    Surface and Coatings Technology 215 24-29 (2013)
    In the present study the synergistic effect of vacuum heat treatment pressure and coating Y and O content on the MCrAlY surface oxide is investigated. For this purpose, free standing MCrAlY coatings with two different Y contents were exposed at 1100°C for times between 1 and 5h in vacuum at pressures of < 10-3Pa and 10-2Pa, and in Argon gas at 10-1Pa. The surface scale composition and morphology were analysed with a variety of analysis techniques. It is shown that the composition at the MCrAlY coating surface is mainly governed by two competing processes: Cr evaporation (β-NiAl formation) and Y oxidation (internal and external). The latter reaction has been observed to depend strongly on the Y reservoir in the coating. It can be observed that the chromium evaporation enhances for reducing pressure. More Y and less O in the coating result in enhanced Y rich oxide formation (Y2O3 and YAP; Yttrium Aluminium Perovskite) and decreased chromium evaporation. On rough surfaces β-NiAl enrichment in hills and yttria formation in valleys can be found. Additionally no alumina formation can be observed under all studied vacuum heat treatment conditions. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2012.09.066
  • 2013 • 111 Al-induced faceting of Si(113)
    Klein, C. and Heidmann, I. and Nabbefeld, T. and Speckmann, M. and Schmidt, T. and Meyer zu Heringdorf, F.-J. and Falta, J. and Horn-Von Hoegen, M.
    Surface Science 618 109-114 (2013)
    Adsorption of Al on a Si(113) substrate at elevated temperatures causes a faceting transition of the initially flat surface. The (113) surface decomposes into a quasi-periodic sequence of Al terminated (115)- and (112)-facets. The resulting surface morphology is characterized in-situ by reciprocal space maps obtained with in-situ spot profile analyzing low-energy electron diffraction and ex-situ atomic force microscopy. The periodicity length of the faceted surface increases with adsorption temperature from 7 nm at 650 C to 80 nm at 800 C. The stability of the Al terminated Si(112) surface is the driving force for the faceting transition. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.08.007
  • 2013 • 110 A solid-solution approach to mixed-metal metal-organic frameworks - Detailed characterization of local structures, defects and breathing behaviour of Al/V frameworks
    Kozachuk, O. and Meilikhov, M. and Yusenko, K. and Schneemann, A. and Jee, B. and Kuttatheyil, A.V. and Bertmer, M. and Sternemann, C. and Pöppl, A. and Fischer, R.A.
    European Journal of Inorganic Chemistry 4546-4557 (2013)
    The doping of [Al(OH)L]n [L = 1,4-benzenedicarboxylate (bdc) or 1,4-naphthalenedicarboxylate (ndc)] with vanadium ions yields crystalline porous mixed-metal solid-solution metal-organic frameworks (MOFs) of general formula [(AlOH)1-x(VO)xL]n (x can be varied in the whole range from 0 to 1). Several characterization methods, including powder X-ray diffraction (PXRD), electron paramagnetic resonance (EPR), solid-state NMR and FTIR spectroscopy, strongly support the effective incorporation of vanadium cations. The Al/V-doped MOFs are isostructural to the parent monometallic MOFs and show a characteristic uniform dependence of the cell parameters on the metal ratios. Detailed spectroscopic investigation provided evidence that the introduced species are fairly well ordered. Interestingly, for low amounts of doped vanadium for both activated and as-synthesized Al/V phases, the EPR results revealed the presence of vanadyl units as local defects in pseudo-octahedral or square-pyramidal environments, which are different from those in the parent MIL-47(V). This observation matches the nonlinear response of the adsorption properties on variation of the composition. Remarkably, the presence of such mixed Al/V chains strongly affects the breathing behaviour of the materials. Both CO2 sorption and in situ PXRD studies validated a gradual change from highly flexible (with easily induced phase transitions) to totally rigid structures upon increasing vanadium content. Systematic studies on the doping of [Al(OH)L]n [L = 1,4-benzenedicarboxylate (bdc) or 1,4-naphthalenedicarboxylate (ndc)] metal-organic frameworks (MOFs) with V ions to obtain [(AlOH)1-x(VO)xL]n (0 < x < 1) solid-solution materials are reported. Detailed characterization allowed us to examine the correlations between compositions and the structures and properties of the final mixed-metal materials. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ejic.201300591
  • 2013 • 109 Ion-induced oxidation of aluminum during reactive magnetron sputtering
    Kreiter, O. and Grosse-Kreul, S. and Corbella, C. and von Keudell, A.
    Journal of Applied Physics 113 143303 (2013)
    Particle beam experiments were conducted in an ultra-high-vacuum vessel to mimic target poisoning during reactive magnetron sputtering of aluminum. Aluminum targets were exposed to quantified beams of argon ions, oxygen atoms and molecules, and aluminum vapour. The growth and etch rates were measured in situ by means of an Al-coated quartz crystal microbalance. The chemical state of the target surface was monitored in-situ by real-time Fourier transform infrared spectroscopy. The surface processes were modelled through a set of balance equations providing sputter yields and sticking coefficients. The results indicate that the oxygen uptake of the aluminum surface is enhanced by a factor 1 to 2 by knock-on implantation and that the deposition of aluminum is not affected by the oxidation state of the surface. (C) 2013 American Institute of Physics. []
    view abstractdoi: 10.1063/1.4799052
  • 2013 • 108 Joining of aluminium sheets by combined solid state and TLP bonding processes
    Lee, E. and Quintana, O. and Indacochea, J.E. and Wojarski, L. and Pfeiffer, J. and Tillmann, W.
    Science and Technology of Welding and Joining 18 98-102 (2013)
    Our effort is to develop an innovative process to join Al sheets at moderate temperatures (200-300uC) by accumulative roll bonding (ARB) combined with transient liquid phase bonding utilising Ga. Solid state joining of Al is limited by the surface Al2O3 that hampers the Al atom diffusion despite the intimate contact between the metal sheets. Mechanical brushing of the Al2O3 improves bonding. Our approach uses ARB to break the brittle Al2O3 by roll bonding two sheets. Strips of 1100-Al were cut, cleaned coated with a thin film of Ga and then preheated before rolling. The Ga coated Al strips roll bonded at 200uC showed greatest strengths, but processing at 300uC resulted in lower strengths with evidence of liquid metal embrittlement. The undamaged Al2O3 prevents liquid Ga from contact with the Al metal. © 2013 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/1362171812Y.0000000087
  • 2013 • 107 Ab initio identified design principles of solid-solution strengthening in Al
    Ma, D. and Friák, M. and Pezold, J.V. and Raabe, D. and Neugebauer, J.
    Science and Technology of Advanced Materials 14 (2013)
    Solid-solution strengthening in six Al-X binary systems is investigated using first-principle methods. The volumetric mismatch parameter and the solubility enthalpy per solute were calculated. We derive three rules for designing solid-solution strengthened alloys: (i) the solubility enthalpy per solute is related to the volumetric mismatch by a power law; (ii) for each annealing temperature, there exists an optimal solute-volume mismatch to achieve maximum strength; and (iii) the strengthening potential of high volumetric mismatch solutes is severely limited by their low solubility. Our results thus show that the thermodynamic properties of the system (here Al-X alloys) set clear upper bounds to the achievable strengthening effects owing to the reduced solubility with increasing volume mismatch. © 2013 National Institute for Materials Science.
    view abstractdoi: 10.1088/1468-6996/14/2/025001
  • 2013 • 106 Development of a stable high-aluminum austenitic stainless steel for hydrogen applications
    Martin, M. and Weber, S. and Theisen, W. and Michler, T. and Naumann, J.
    International Journal of Hydrogen Energy 38 5989-6001 (2013)
    A novel high-aluminum austenitic stainless steel has been produced in the laboratory with the aim of developing a lean-alloyed material with a high resistance to hydrogen environment embrittlement. The susceptibility to hydrogen environment embrittlement was evaluated by means of tensile tests at a slow strain rate in pure hydrogen gas at a pressure of 40 MPa and a temperature of -50 C. Under these conditions, the yield strength, tensile strength and elongation to rupture are not affected by hydrogen in comparison to companion tests carried out in air. Moreover, a very high ductility in hydrogen is evidenced by a reduction of area of 70% in the high-pressure and low-temperature hydrogen environment. The lean degree of alloying is reflected in the molybdenum-free character of the material and a nickel content of 8.0 wt.%. With regard to the alloy concept, a combination of high-carbon, high-manganese, and high-aluminum contents confer an extremely high stability against the formation of strain-induced martensite. This aspect was investigated by means of in-situ magnetic measurements and ex-situ X-ray diffraction. The overall performance of the novel alloy was compared with two reference materials, 304L and 316L austenitic stainless steels, both industrially produced. Its capability of maintaining a fully austenitic structure during tensile testing has been identified as a key aspect to avoid hydrogen environment embrittlement. Copyright © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2013.02.127
  • 2013 • 105 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 • 104 Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography
    Pradeep, K.G. and Wanderka, N. and Choi, P. and Banhart, J. and Murty, B.S. and Raabe, D.
    Acta Materialia 61 4696-4706 (2013)
    We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc-bcc solid solution but instead a composite of bcc structured Ni-Al-, Cr-Fe- and Fe-Cr-based regions and of fcc Cu-Zn-based regions. The Cu-Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr-Fe phase that was inherited from the hot compacted state. The Ni-Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20-30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.059
  • 2013 • 103 Formation of crystalline gamma-Al2O3 induced by variable substrate biasing during reactive magnetron sputtering
    Prenzel, M. and Kortmann, A. and von Keudell, A. and Nahif, F. and Schneider, J. M. and Shihab, M. and Brinkmann, R. P.
    Journal of Physics D-applied Physics 46 084004 (2013)
    Reactive magnetron sputtering is a widely used technique to deposit various materials such as oxides and nitrides with a superior control of morphology and stoichiometry. The adjustment of the film properties at a given substrate temperature is believed to be affected by the average energy < E > per incorporated atom during film growth, which is controlled by the ion-to-neutral ratio in the film forming growth flux and the energy of the incident ions. This concept is tested for alumina growth in an rf-magnetron discharge by keeping < E >, the average energy of the incident ions E-ions, and the ion-to-neutral flux ratio constant, but varying only the energy distribution of the incident ions (ion energy distribution-IED). The influence of the IED on film growth is monitored by observing the transition of the films between x-ray amorphous Al2O3 to gamma-Al2O3. The results reveal that the substrate temperature necessary for the transition to gamma-crystalline films can be lowered by almost 100 degrees C, when the maximum energy of the incident ions is kept at 100 eV, while maintaining the energy per incorporated atom at 11 eV. This result is compared with TRIM calculations for the collision cascades of impacting ions.
    view abstractdoi: 10.1088/0022-3727/46/8/084004
  • 2013 • 102 Modeling approach for the determination of material flow and welding conditions in porthole die extrusion with gas pocket formation
    Schwane, M. and Gagliardi, F. and Jäger, A. and Ben Khalifa, N. and Tekkaya, A.E.
    Key Engineering Materials 554-557 787-793 (2013)
    The material flow in porthole dies is of crucial importance with regard to the seam weld quality in aluminum extrusion. Thus, experimental as well as numerical investigations on the effect of die geometry on the material flow were conducted. The experimental tests were performed on a 10 MN laboratory extrusion press. During the experimental trials, the extrusion ratio was varied by means of exchangeable die plates. Since the modular die allows removal of the aluminum in the welding chamber as well as in the feeders after the process, the material flow could be inspected in detail. The experimental results were used to improve the accuracy of FEA simulations, which were also conducted by commercial software. An attempt was made to improve the result quality of Eulerian FEA model regarding the simulation of an extrusion process with a gas pocket in the welding chamber. The influence of the modeling approach on the predicted material flow and on the contact pressure was analyzed and finally linked to the seam weld quality. Copyright © 2013 Trans Tech Publications Ltd.
    view abstractdoi: 10.4028/
  • 2013 • 101 Direct evidence for the formation of ordered carbides in a ferrite-based low-density Fe-Mn-Al-C alloy studied by transmission electron microscopy and atom probe tomography
    Seol, J.-B. and Raabe, D. and Choi, P. and Park, H.-S. and Kwak, J.-H. and Park, C.-G.
    Scripta Materialia 68 348-353 (2013)
    We study the structure and chemical composition of the κ-carbide formed as a result of isothermal transformation in an Fe-3.0Mn-5.5Al-0.3C alloy using transmission electron microscopy and atom probe tomography. Both methods reveal the evolution of κ-particle morphology as well as the partitioning of solutes. We propose that the κ-phase is formed by a eutectoid reaction associated with nucleation growth. The nucleation of κ-carbide is controlled by both the ordering of Al partitioned to austenite and the carbon diffusion at elevated temperatures.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.08.013
  • 2013 • 100 Target implantation and redeposition processes during high-power impulse magnetron sputtering of aluminum
    Will, A. and de los Arcos, T. and Corbella, C. and Hecimovic, A. and Machura, P. D. and Winter, J. and von Keudell, A.
    Journal of Physics D-applied Physics 46 084009 (2013)
    The processes of argon retention by the target and redeposition of target material were investigated by x-ray photoelectron spectroscopy as a function of radial position for different plasma conditions in high-power impulse magnetron sputtering of aluminum targets. Significant differences in Ar radial concentration profiles were observed for different discharge conditions. Inside the racetrack area, Ar ion flux-dominated implantation is compensated by radiation-enhanced diffusion loss terms. Outside the racetrack, the role of ion implantation is diminished, and Ar retention by the target may stem from a balance between gettering by redeposited Al and ion-induced Ar desorption.
    view abstractdoi: 10.1088/0022-3727/46/8/084009
  • 2013 • 99 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
  • 2012 • 98 Thermal aspects in deep hole drilling of aluminium cast alloy using twist drills and MQL
    Biermann, D. and Iovkov, I. and Blum, H. and Rademacher, A. and Taebi, K. and Suttmeier, F.T. and Klein, N.
    Procedia CIRP 3 245-250 (2012)
    The deep hole drilling process with solid carbide twist drills is an efficient alternative to the classic single-lip deep hole drilling, due to the generally higher feed rates possible and the consequently higher productivity. Furthermore the minimum quantity lubrication (MQL) can be applied, in order to reduce the production costs and implement an environmentally friendly process. Because of the significantly reduced cooling performance when using MQL, a higher heat loading results for the tool and the workpiece. This paper presents the investigations of the temperature distribution in the workpiece and the heat balance of the deep hole drilling process. © 2012 The Authors.
    view abstractdoi: 10.1016/j.procir.2012.07.043
  • 2012 • 97 In situ study of γ-TiAl lamellae formation in supersaturated α 2-Ti 3Al grains
    Cha, L. and Schmoelzer, T. and Zhang, Z. and Mayer, S. and Clemens, H. and Staron, P. and Dehm, G.
    Advanced Engineering Materials 14 299-303 (2012)
    In situ heating transmission electron microscopy (TEM) was used to investigate the initial stage of γ-TiAl lamellae formation in an intermetallic Ti-45Al-7.5Nb alloy (in at.%). The material was heat treated and quenched in a non-equilibrium state to consist mainly of supersaturated, ordered α 2-Ti 3Al grains. Subsequently, specimens were annealed inside a TEM up to 750 °C. The in situ TEM study revealed that ultra-fine γ-TiAl laths precipitate in the α 2-matrix at ≈730 °C which exhibit the classical Blackburn orientation relationship, i.e. (0001)α 2//(111)γ and [$112̄0] α 2//< 110]γ. The microstructural development observed in the in situ TEM experiment is compared to results from conventional ex situ TEM studies. In order to investigate the precipitation behavior of the γ-phase with a complementary method, in situ high energy X-ray diffraction experiments were performed which confirmed the finding that γ-laths start to precipitate at ≈730 °C from the supersaturated α 2- matrix. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201100272
  • 2012 • 96 Lightweight construction by means of profiles
    Chatti, S. and Pietzka, D. and Khalifa, N.B. and Jäger, A. and Selvaggio, A. and Tekkaya, A.E.
    Key Engineering Materials 504-506 369-374 (2012)
    This paper shows some product and process developments at the Institute of Forming Technology and Lightweight Construction of the TU Dortmund University supporting the lightweight construction. It presents the manufacturing of lightweight profiles by hot extrusion and their benefits as well as their design, material, and manufacturing potential for lightweight construction. Examples of process extensions in hot extrusion like curved profile extrusion, twisted profile extrusion and manufacturing of functional graded profiles and profiles with variable crosssections during extrusion are shown. These procedures allow a flexible change of the profile geometry or contour in longitudinal axis and, therefore, support the shape lightweight construction. Other extensions like composite profile extrusion and energy efficient extrusion of profiles from scrap materials like chips support the material lightweight construction. The manufacturing and use of these profiles allow the realisation of diverse lightweight construction principles and promise to become a pillar of lightweight construction in future. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2012 • 95 Microwave-hydrothermal synthesis and characterization of nanostructured copper substituted ZnM2O4 (M = Al, Ga) spinels as precursors for thermally stable Cu catalysts
    Conrad, F. and Massue, C. and Kühl, S. and Kunkes, E. and Girgsdies, F. and Kasatkin, I. and Zhang, B. and Friedrich, M. and Luo, Y. and Armbrüster, M. and Patzke, G.R. and Behrens, M.
    Nanoscale 4 2018-2028 (2012)
    Nanostructured Cu<inf>x</inf>Zn<inf>1-x</inf>Al<inf>2</inf>O<inf>4</inf> with a Cu:Zn ratio of: has been prepared by a microwave-assisted hydrothermal synthesis at 150°C and used as a precursor for Cu/ZnO/Al<inf>2</inf>O <inf>3</inf>-based catalysts. The spinel nanoparticles exhibit an average size of approximately 5 nm and a high specific surface area (above 250 m2 g-1). Cu nanoparticles of an average size of 3.3 nm can be formed by reduction of the spinel precursor in hydrogen and the accessible metallic Cu(0) surface area of the reduced catalyst was 8 m2 g-1. The catalytic performance of the material in CO<inf>2</inf> hydrogenation and methanol steam reforming was compared with conventionally prepared Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> reference catalysts. The observed lower performance of the spinel-based samples is attributed to a lack of synergetic interaction of the Cu nanoparticles with ZnO due to the incorporation of Zn 2+ in the stable spinel lattice. Despite its lower performance, however, the nanostructured nature of the spinel catalyst was stable after thermal treatment up to 500°C in contrast to other Cu-based catalysts. Furthermore, a large fraction of the re-oxidized copper migrates back into the spinel upon calcination of the reduced catalyst, thereby enabling a regeneration of sintered catalysts after prolonged usage at high temperatures. Similarly prepared samples with Ga instead of Al exhibit a more crystalline catalyst with a spinel particle size around 20 nm. The slightly decreased Cu(0) surface area of 3.2 m2 g-1 due to less copper incorporation is not a significant drawback for the methanol steam reforming. © The Royal Society of Chemistry 2012.
    view abstractdoi: 10.1039/c2nr11804a
  • 2012 • 94 Effect of porthole design and welding chamber dimensions on material flow and weld deformability of extruded aluminium profiles
    Donati, L. and Ben Khalifa, N. and Tomesani, L. and Tekkaya, A.E.
    Key Engineering Materials 504-506 523-528 (2012)
    Aim of the work is to investigate different strategies in balancing material flow during direct extrusion through porthole dies. Two AA6082 hollow profiles were simultaneously extruded by a single die with different portholes extrusion ratio, dissimilar welding chambers and different bearing lengths. A strict process control was realized by measuring thermal conditions in the die by means of 6 thermocouples and on the profiles by a self calibrating pyrometer for aluminum alloy applications. Several billets were extruded at different ram speeds (2 to 7 mm/sec) and the effect of die design on surface quality, profile lengths and thermal field was recorded. The profiles were then sectioned and the position of the seam welds in the profiles identified and compared also with the profiles tip. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2012 • 93 Manufacturing of steel-reinforced aluminum products by combining hot extrusion and closed-die forging
    Foydl, A. and Pfeiffer, I. and Kammler, M. and Pietzka, D. and Matthias, T. and Jäger, A. and Tekkaya, A.E. and Behrens, B.-A.
    Key Engineering Materials 504-506 481-486 (2012)
    Aluminum extrudates reinforced with steel elements represent an innovative material concept for lightweight structures. Investigations on the process chain of extrusion and subsequent die forging to produce steel-reinforced aluminum parts were carried out. For the investigation the aluminum alloys AA6060 and AA6082 were used as workpiece material. Regarding the extrusion step the position of reinforcement elements with different shapes and their adhesion to the matrix in discontinuously-reinforced, semi-finished aluminum profiles was in focus. Sometimes defects were formed near the reinforcement elements and the nature of such defects was characterized. For the investigations on the forging step extruded profiles reinforced with steel wires were used. Finite element analyses were carried out in order to predict the position of the wires in the forged parts depending on their initial position in the extrusions. Furthermore, the flow behavior of the wires inside the forging part was investigated. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2012 • 92 Strontium doping in mullite-type bismuth aluminate: A vacancy investigation using neutrons, photons and electrons
    Gesing, T.M. and Schowalter, M. and Weidenthaler, C. and Murshed, M.M. and Nénert, G. and Mendive, C.B. and Curti, M. and Rosenauer, A. and Buhl, J.-C. and Schneider, H. and Fischer, R.X.
    Journal of Materials Chemistry 22 18814-18823 (2012)
    We report on strontium doped dibismuth-nonaoxoaluminate(iii) produced at 1023 K. Partial substitution of bismuth by strontium in the structure yields oxygen vacancies for charge balance. Introducing oxygen vacancies rearranged the associated Al<inf>2</inf>O<inf>7</inf> double-tetrahedra forming "Al <inf>3</inf>O<inf>10</inf>" tri-clusters which were identified by multi-quantum 27Al MAS NMR. Both STEM-EDX and XPS showed homogeneous distribution of strontium in the bulk and on the surface, respectively. Moreover, XPS confirms the valence state of bismuth after doping. The orientations of bismuth 6s2 lone electron pairs were calculated using DFT methods. The amount of strontium in the crystal structure was further confirmed from the decomposition product SrAl<inf>12</inf>O<inf>19</inf> formed during the temperature-dependent X-ray powder diffraction. The structural proof was carried out by refining the structure of (Bi<inf>0.94</inf>Sr <inf>0.06</inf>)<inf>2</inf>Al<inf>4</inf>O<inf>8.94</inf> from powder neutron and X-ray diffraction data. Rietveld refinements clearly showed the under occupation of one oxygen site and the shift of two aluminum atoms from the double-tetrahedra to two tri-cluster sites. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2jm33208f
  • 2012 • 91 One-pot synthesis of polyhedron-like hollow aluminosilicate with mesoporous shells
    Gu, X. and Tao, H. and Schmidt, W. and Lu, G. and Wang, Y.
    Journal of Materials Chemistry 22 2473-2477 (2012)
    Hollow mesoporous aluminosilicate particles have been synthesized through a one-pot synthesis without using any cavity-forming template. The hollow particles have mesoporous shells with surface areas up to 1300 m 2 g -1. The thickness of the shell can be adjusted from about 30 nm to 50 nm by tuning the molar Si/Al ratio and/or variation of the time of hydrothermal treatment. EDX analysis (element mapping) revealed a certain zoning of aluminum in the amorphous aluminosilicate particles investigated. There is an enrichment of Al on the outside of the particles close to the surface. The presence of aluminum appeared to be the key factor for the inhomogeneous dissolution of the aluminosilicate (dissolution of silicon-enriched cores) that leads to the formation of hollow particles. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1jm13400k
  • 2012 • 90 Multistage strain hardening through dislocation substructure and twinning in a high strength and ductile weight-reduced Fe-Mn-Al-C steel
    Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 60 5791-5802 (2012)
    We investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe-30.5Mn-2.1Al-1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. The alloy exhibits a superior combination of strength and ductility (ultimate tensile strength of 1.6 GPa and elongation to failure of 55%) due to the multiple-stage strain hardening. We explain this behavior in terms of dislocation substructure refinement and subsequent activation of deformation twinning. The early hardening stage is fully determined by the size of the dislocation substructure, namely, Taylor lattices, cell blocks and dislocation cells. The high carbon content in solid solution has a pronounced effect on the evolving dislocation substructure. We attribute this effect to the reduction of the dislocation cross-slip frequency by solute carbon. With increasing applied stress, the cross-slip frequency increases. This results in a gradual transition from planar (Taylor lattices) to wavy (cells, cell blocks) dislocation configurations. The size of such dislocation substructures scales inversely with the applied resolved stress. We do not observe the so-called microband-induced plasticity effect. In the present case, due to texture effects, microbanding is not favored during tensile deformation and, hence, has no effect on strain hardening. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.018
  • 2012 • 89 A new method for determining dynamic grain structure evolution during hot aluminum extrusion
    Güzel, A. and Jäger, A. and Parvizian, F. and Lambers, H.-G. and Tekkaya, A.E. and Svendsen, B. and Maier, H.J.
    Journal of Materials Processing Technology 212 323-330 (2012)
    In this paper, a new method for analyzing the microstructure evolution of aluminum during deformation at elevated temperatures by extrusion is presented, which is entirely separated from secondary restoration effects viz. static recrystallization and grain growth. In order to observe the development of grains and their orientation under severe plastic deformation, a small-scale forward extrusion setup was designed which allows quenching the extrusion butt together with the die and the container immediately after extrusion to preserve the grain structure evolved during the deformation. The forming path and the forming history of a selected material point were calculated by numerical simulation. The evolution of the microstructure along the forming path was analyzed using electron backscatter diffraction. A database for the development of physically based phenomenological models for predicting and simulating the evolution of microstructure during the hot deformation of EN AW-6082 alloy is provided. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2011.09.018
  • 2012 • 88 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 • 87 Dependence of the yield stress of Fe3Al on heat treatment
    Hasemann, G. and Schneibel, J.H. and George, E.P.
    Intermetallics 21 56-61 (2012)
    The room-temperature yield strength of quenched FeAl-based iron aluminides depends strongly on the temperature from which the quench occurs. There is evidence that Fe3Al-based iron aluminides show similar behavior, albeit not as pronounced. The purpose of this work was to carry out quenching and annealing experiments to clearly demonstrate this effect, as well as to study its kinetics. Room-temperature compression tests were performed using cast Fe-28at%Al and Fe-30at%Al after quenching from temperatures in the range 300-1000 °C. Kinetic studies were carried out to assess the reduction of the yield stress by subsequent annealing for various times at relatively low temperatures. Results from the present study are compared with data available in the literature and explanations for the dependence of room-temperature mechanical behavior on annealing history are evaluated. It is concluded that room-temperature strengthening in Fe3Al-based iron aluminides is consistent with behavior expected for quenched-in thermal vacancies and this could be an important strengthening mechanism in Fe3Al. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2011.09.005
  • 2012 • 86 New concepts for cooling of extrusion dies manufactured by rapid tooling
    Hölker, R. and Jäger, A. and Khalifa, N.B. and Tekkaya, A.E.
    Key Engineering Materials 491 223-232 (2012)
    To prevent local overheating of the workpiece material in hot aluminum extrusion the influence of die cooling was investigated. Numerical simulations of extrusion revealed an advantage of the die bearing cooling, which can be accomplished by locating the cooling channels close to the die/bearing surface. Since the fabrication of especially geometric complex cooling channels located near the die surface is not possible by conventional manufacturing technologies, the technology of rapid tooling was introduced into hot aluminum extrusion and experimentally tested. Cooling channels near to the bearings show promising results allowing extensions of extrusion limits, especially the extrusion speed and therefore productivity. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2012 • 85 Low cycle fatigue damage mechanism of the lightweight alloy Al2024
    Khan, S. and Wilde, F. and Beckmann, F. and Mosler, J.
    International Journal of Fatigue 38 92-99 (2012)
    Detection of cracks in Al2024 T351 specimens subjected to low cycle fatigue loading by a certain non-destructive inspection technique is demonstrated. In the experimental phase of the study, notched round specimens were fatigue loaded. The tests were performed at different constant strain amplitudes at room temperature. For identifying the crack initiation loci, the specimens were removed from the testing machine after a certain number of cycles and were non-destructively inspected via X-ray technique. Pictures were taken successively while incrementally turning the sample. The re-constructed data were visualized via software (VGStudio MAX 2.1) to obtain a 3D image of the specimen, showing all the details of its inner structure. By taking "virtual" slices from the data, quantification of microstructural properties was done using classical methods. This allowed verifying some frequently mentioned statements concerning the low cycle fatigue behavior of high-strength aluminum alloys. Furthermore, new findings related to the tri-axiality dependence on the resulting fracture process and those related to damage initiation caused by decohesion were also discovered. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2011.11.009
  • 2012 • 84 Experimental and numerical lifetime assessment of Al 2024 sheet
    Khan, S. and Kintzel, O. and Mosler, J.
    International Journal of Fatigue 37 112-122 (2012)
    In the present paper, a thorough analysis of the low-cycle fatigue behavior of flat sheets of aluminum Al 2024-T351 is given. For that purpose, material characterization is combined with material modeling. The experimental analyses include monotonic and cyclic loading tests at high stress levels. For the assessment of microstructural characteristics, advanced imaging technology is used to reveal, e.g. crack initiation loci and particle sizing. The numerical simulation is done using a novel ductile-brittle damage model. Thereby, the model parameters are optimized by means of an inverse parameter identification strategy which, overall, leads to a very good agreement between experimentally observed and computationally predicted data. For demonstrating the prediction capability of the novel coupled model also for complex engineering problems, a certain stringer assembly, as used in fuselage parts of airplanes, is analyzed. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2011.09.010
  • 2012 • 83 Growth optimization and characterization of lattice-matched Al 0.82In 0.18N optical confinement layer for edge emitting nitride laser diodes
    Kim-Chauveau, H. and Frayssinet, E. and Damilano, B. and De Mierry, P. and Bodiou, L. and Nguyen, L. and Vennéguès, P. and Chauveau, J.-M. and Cordier, Y. and Duboz, J.Y. and Charash, R. and Vajpeyi, A. and Lamy, J.-M. and Akhte...
    Journal of Crystal Growth 338 20-29 (2012)
    We present the growth optimization and the doping by the metal organic chemical vapor deposition of lattice-matched Al 0.82In 0.18N bottom optical confinement layers for edge emitting laser diodes. Due to the increasing size and density of V-shaped defects in Al 1-xIn xN with increasing thickness, we have designed an Al 1-xIn xN/GaN multilayer structure by optimizing the growth and thickness of the GaN interlayer. The Al 1-xIn xN and GaN interlayers in the multilayer structure were both doped using the same SiH 4 flow, while the Si levels in both layers were found to be significantly different by SIMS. The optimized 8×(Al 0.82In 0.18N/GaN=54/6 nm) multilayer structures grown on free-standing GaN substrates were characterized by high resolution X-ray diffraction, atomic force microscopy and transmission electron microscopy, along with the in-situ measurements of stress evolution during growth. Finally, lasing was obtained from the UV (394 nm) to blue (436 nm) wavelengths, in electrically injected, edge-emitting, cleaved-facet laser diodes with 480 nm thick Si-doped Al 1-xIn xN/GaN multilayers as bottom waveguide claddings. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jcrysgro.2011.10.016
  • 2012 • 82 A DFT study of formation energies of Fe-Zn-Al intermetallics and solutes
    Klaver, T.P.C. and Madsen, G.K.H. and Drautz, R.
    Intermetallics 31 137-144 (2012)
    We report Density Functional Theory results on FeAl and FeZn intermetallics and Fe, Zn and Al solute atoms. The formation energies of fully relaxed intermetallic geometries were determined, as well as solution energies of the three elements in host lattices of the other two elements. Since it is know that the outcome of the magnetic states of some FeAl intermetallics and Fe solutes in Al depends on subtle details of how the calculations are carried out, we have determined many of our results with two different parameterisations, PBE and PBEsol, so see how the parameterisation influences the results. The relaxed intermetallic geometries are in good agreement with experimental results, with PBEsol calculations resulting in slightly smaller geometries than PBE calculations (0.7-2.1%). Intermetallic formation energies fall within ranges of experimental results where available, and are in excellent or reasonable agreement with other DFT results, except for the FeAl 2 phase. For this phase a structure revision was recently suggested and the heat of formation of the newly suggested structure is 0.1 eV/atom lower than for the long-accepted structure. The formation energies of Fe aluminides are an order of magnitude more negative than those of FeZn intermetallics. Most of the calculated magnetic states of the intermetallics are at odds with experimental results. However, the intermetallic formation energies are often not strongly affected by this. Fe/Al solute systems have the most negative solution energies. All other solution energies are positive and smaller in absolute value than the Fe/Al solution energies. Solution energies were all some tenths of eV. Where comparisons could be made, calculated and experimental results differed by some hundredths of eV. The magnetic moment found on an Fe solute in Al is at odds with experimental results. As with FeAl, the outcome of the magnetic state subtly depends on the details of how calculations were performed and has little energetic effect. Lattice relaxation around solute atoms is mostly in agreement with simple atomic size considerations. The slight relaxation of Al neighbours away from a Zn solute is at odds with this pattern, and also with experimental results. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2012.06.017
  • 2012 • 81 Protective ability of hybrid nano-composite coatings with cerium sulphate as inhibitor against corrosion of AA2024 aluminium alloy
    Kozhukharov, S. and Kozhukharov, V. and Schem, M. and Aslan, M. and Wittmar, M. and Wittmar, A. and Veith, M.
    Progress in Organic Coatings 73 95-103 (2012)
    The corrosion protective ability of hybrid oxy silane nano-composite coatings deposited on AA2024 by sol-gel technique was studied. The coatings are developed as an environmentally friendly alternative of the toxic chromium containing coatings on aluminium. A cerium salt, Ce2(SO 4)3, was used as inhibitor of the corrosion process. Two methods were applied to introduce the salt in the hybrid matrix: directly in the matrix, or by porous Al2O3 nano-particles preliminary loaded by the salt. Atomic force microscopy (AFM) was used to evaluate the superficial morphology of the coatings, while their layer structure was studied by means of scanning electron microscopy (SEM). Linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) were used for assessment of the barrier ability. The hybrid matrix was found to possess remarkable barrier ability which was preserved even after prolonged exposure of the coatings to a model corrosive medium of 0.05 M NaCl. In all cases, the cerium salt involved either directly or by Al2O3 nano-particles proved to deteriorate the protective properties of the coatings and to accelerate pitting nucleation. The experimental results have shown that cerium sulphate, introduced in the by the both manners in the hybrid matrix did not efficiently inhibit the corrosion of AA2024, unlike the reported inhibiting properties of other cerium salts. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.porgcoat.2011.09.005
  • 2012 • 80 On the presence of work-hardened zones around fibers in a short-fiber-reinforced Al metal matrix composite
    Kurumlu, D. and Payton, E.J. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 60 6051-6064 (2012)
    Dislocation densities are investigated in a short-fiber-reinforced Al-11 wt.% Zn-0.2 wt.% Mg metal matrix composite (MMC) with a special focus on regions near the fiber-matrix interfaces. Clear microstructural evidence is provided for the formation of work-hardened zones (WHZs) around fibers during creep using transmission electron microscopy (TEM). The dislocation densities in the WHZs are higher after creep than after squeeze casting, where the plastic strains associated with the thermal stresses that build up during solidification also result in an increased dislocation density close to fibers. The effect of heating and cooling on the dislocation substructure is also considered. The results are discussed in light of previous findings and provide microstructural evidence for the presence of WHZs as predicted by the Dlouhy model of MMC creep. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.042
  • 2012 • 79 High-temperature strength and damage evolution in short fiber reinforced aluminum alloys studied by miniature creep testing and synchrotron microtomography
    Kurumlu, D. and Payton, E.J. and Young, M.L. and Schöbel, M. and Requena, G. and Eggeler, G.
    Acta Materialia 60 67-78 (2012)
    The creep behavior of a squeeze-cast, short fiber reinforced Al metal matrix composite (MMC), consisting of an Al-11 wt.% Zn-0.2 wt.% Mg alloy reinforced with 15 vol.% Al 2O 3 Saffil® short fibers is investigated using miniature creep specimens. The small dimensions of the miniature creep specimens permit them to be machined from regions of an MMC block with different microstructures, thus allowing the effect of grain size and fiber texture on creep to be investigated on a more local level than is possible using conventional specimen geometries. The miniature creep specimens are subjected to uniaxial tensile stresses ranging from 3 to 40 MPa at temperatures between 573 and 623 K. It is shown that tests performed using the miniature creep specimen geometry are in good agreement with results previously obtained with standard creep specimens. Through interrupted creep experiments, it is observed that the creep back flow that occurs after unloading increases with increasing accumulated plastic strain. In the as-cast MMC, synchrotron microtomography reveals a fine distribution of pores whose spatial density increases with the spatial density of the fibers. The presence of fractured fibers in the crept MMC is also revealed. Some of the regions between fractured fiber fragments appear to be filled with matrix material, while others are voided. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.09.022
  • 2012 • 78 Spark plasma sintering synthesis and mechanical spectroscopy of the ω-Al 0.7Cu 0.2Fe 0.1 phase
    Laplanche, G. and Gadaud, P. and Bonneville, J. and Joulain, A. and Gauthier-Brunet, V. and Dubois, S. and Jay, F.
    Journal of Materials Science 47 169-175 (2012)
    Starting from a mixture of Al-Cu-Fe quasicrystalline (QC) particles and Al powder, a fully dense and almost Al-Cu-Fe ω single-phase alloy was produced by spark plasma sintering. This technique allows synthesising large samples with sizes suitable for mechanical spectroscopy experiments. Mechanical spectroscopy was selected because it is a relevant tool for detecting the presence of structural defects at both nano and microscopic scales. Young's moduli were measured in the 15 kHz range as a function of temperature by the resonant frequency method. Young's moduli behave similarly for typical metals and exhibit values that are comparable to those of the Al-Cu-Fe QC phase. The damping coefficient Q -1 was determined at various temperatures between room temperature and 840 K over a large frequency range, i.e. between 10 -3 and 10 Hz. The results suggest that solid friction effects do occur. In addition, a relaxation peak is observed in the intermediate temperature range. © 2011 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-011-5784-1
  • 2012 • 77 Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al-Si-based alloys
    Li, J.H. and Zarif, M.Z. and Dehm, G. and Schumacher, P.
    Philosophical Magazine 92 3789-3805 (2012)
    The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al-5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al-5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al5 FeSi intermetallic was also observed, depending on subsequent controlled cooling. © 2012 Copyright Taylor and Francis Group, LLC.
    view abstractdoi: 10.1080/14786435.2012.687840
  • 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 Numerical and experimental investigation on lap shear fracture of Al/CFRP laminates
    Naghipour, P. and Schulze, K. and Hausmann, J. and Bartsch, M.
    Composites Science and Technology 72 1718-1724 (2012)
    This paper presents a new approach to numerically investigate the lap shear fracture of a hybrid laminate made of Carbon Fibre Reinforced Plastic (CFRP) and metal foil plies (e.g. aluminium), validated by corresponding experiments. The numerical Finite Element (FE) model of the hybrid laminate, subjected to lap shear fracture, is composed of five laminas with alternating metal/CFRP layers with cohesive elements lying within Al/CFRP interface. In the FE model, individual CFRP laminas are assumed as an orthotropic homogenized continuum under plane stress, and aluminium facesheets are modelled as an elastic-plastic continuum. The Al/CFRP interface is represented via quadratic cohesive elements, the constitutive law of which is an exponentially decaying law representing the degrading behaviour of the interface (implemented as user element in ABAQUS). The numerical model captures the experimentally obtained results with minimal error, and predicts the failure modes successfully. The influence of specimen geometry (e.g. overlap length, total length, and total width) on lap shear fracture response is analyzed in detail in this study, too, in order to confirm the specimen design for the test, as there is still no corresponding test standard for hybrid laminates. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.compscitech.2012.07.012
  • 2012 • 74 Thermodynamic and physical properties of FeAl and Fe 3Al: An atomistic study by EAM simulation
    Ouyang, Y. and Tong, X. and Li, C. and Chen, H. and Tao, X. and Hickel, T. and Du, Y.
    Physica B: Condensed Matter 407 4530-4536 (2012)
    With this work we present a newly developed potential for the Fe-Al system, which is based on the analytical embedded atom method (EAM) with long range atomic interactions. The potential yields for the two most relevant phases B2-FeAl and D0 3-Fe 3Al lattice constants, elastic constants, as well as bulk and point defect formation enthalpies, which are in good agreement with experimental and other theoretical data. In addition, the phonon dispersions for B2-FeAl and D0 3-Fe 3Al show a good agreement with available experiments. The calculated lattice constants and formation enthalpy for disordered Fe-Al alloys are in good agreement with experimental data or other theoretical calculations. This indicates that the present EAM potentials of Fe-Al system is suitable for atomistic simulations of structural and kinetic properties for the Fe-Al system. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.physb.2012.08.025
  • 2012 • 73 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 • 72 Atomic scale effects of alloying, partitioning, solute drag and austempering on the mechanical properties of high-carbon bainitic-austenitic TRIP steels
    Seol, J.-B. and Raabe, D. and Choi, P.-P. and Im, Y.-R. and Park, C.-G.
    Acta Materialia 60 6183-6199 (2012)
    Understanding alloying and thermal processing at an atomic scale is essential for the optimal design of high-carbon (0.71 wt.%) bainitic-austenitic transformation-induced plasticity (TRIP) steels. We investigate the influence of the austempering temperature, chemical composition (especially the Si:Al ratio) and partitioning on the nanostructure and mechanical behavior of these steels by atom probe tomography. The effects of the austempering temperature and of Si and Al on the compositional gradients across the phase boundaries between retained austenite and bainitic ferrite are studied. We observe that controlling these parameters (i.e. Si, Al content and austempering temperature) can be used to tune the stability of the retained austenite and hence the mechanical behavior of these steels. We also study the atomic scale redistribution of Mn and Si at the bainitic ferrite/austenite interface. The observations suggest that either para-equilibrium or local equilibrium-negligible partitioning conditions prevail depending on the Si:Al ratio during bainite transformation. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.064
  • 2012 • 71 Development of new processes for welding of thermal Al-Cu solar absorbers using diode lasers
    Springer, A. and Kallage, P. and Hustedt, M. and Barcikowski, S. and Wesling, V. and Haferkamp, H.
    Journal of Laser Applications 24 (2012)
    Flat-plate solar collectors are favorable devices to generate heat from the energy of the Sun. The central part of a collector, the solar absorber, is a front-coated aluminum sheet with a copper tube fixed on the back side in order to transport heated liquid. Nowadays, the absorber is often fabricated by laser welding with two flashlamp-pumped neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers which provide high peak pulse powers. A disadvantage of this procedure is the high electrical energy consumption. Research on new laser welding processes is aimed to reduce the amount of energy required. Diode lasers allow processing with higher efficiency, due to a better absorption behavior of the aluminum sheet. Accordingly, two different processes have been developed. The first one, using two laser sources in pulsed mode, is similar to the industrial process, but enables an extension of the spot welds in order to increase the joint strength. In contrast, the second process requires only one laser source to weld the tube on both sides, using a scanner system which is integrated in a newly developed innovative laser processing head. By adopting these processes, significant energy savings during the production of solar absorbers can be achieved, ensuring high process reliability at the same time. © 2012 Laser Institute of America.
    view abstractdoi: 10.2351/1.4736588
  • 2012 • 70 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 • 69 On the correlation between thermal cycle and formation of intermetallic phases at the interface of laser-welded aluminum-steel overlap joints
    Szczepaniak, A. and Fan, J. and Kostka, A. and Raabe, D.
    Advanced Engineering Materials 14 464-472 (2012)
    A laser beam welding process via heat conduction was applied to join DC01 steel with aluminum (Al) in overlap configuration without filler wire. The effect of the applied laser power (1.7, 1.8, 2.1, and 2.4 kW) on the formation and evolution of the interfaces between steel and Al was analyzed. Two intermetallic compounds were found at the interface, namely, one adjacent to the steel layer (Al 5Fe 2) and one close to the solidified Al (Al 13Fe 4). The thickness of the intermetallic reaction layer increases with laser power, while the morphology of its individual components evolves due to differences in accumulated thermal cycles. Correlations between simulations and measurements show that the peak temperature has significantly stronger influence on the thickness of the intermetallic reaction layer than cooling time and the integral of temperature over the time. Shear/tensile strength tests reveal that all the specimens fail in the Al heat affected zone. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200075
  • 2012 • 68 Co-templating ionothermal synthesis and structure characterization of two new 2D layered aluminophosphates
    Wei, Y. and Marler, B. and Zhang, L. and Tian, Z. and Graetsch, H. and Gies, H.
    Dalton Transactions 41 12408-12415 (2012)
    For the first time, the co-templating ionothermal methodology was used in the preparation of layered aluminophosphate materials. With the addition of either 1,2-ethylenediamine or 1,6-hexanediamine to the ionic liquid 1-ethyl-3-methyl imidazolium chloride, two new 2D layered aluminophosphates RUB-A1 [Al 3P 4O 16][NH 3CH 2CH 2NH 3] 0.5[C 6N 2H 11] 2 and RUB-A2 [Al 3P 4O 16][NH 3(CH 2) 6NH 3][NH 3(CH 2) 6NH 2] 0.5[C 6N 2H 11] 0.5[H 2O] have been synthesized ionothermally by co-templating. The structure of RUB-A1 has been determined from single-crystal X-ray diffraction data using direct methods, while the structure of RUB-A2 has been solved ab initio from powder X-ray diffraction data with limited resolution using direct-space methods. Both of these two compounds have a 2D layered structure consisting of macroanionic sheets of composition [Al 3P 4O 16] 3- stacked in an AAAA sequence. The inorganic layers are built up from alternatively vertex-sharing [AlO 4]- and [PO 3(O)]-tetrahedral units forming a 4.6.8 and a 4.6.12 network for RUB-A1 and RUB-A2, respectively. The layer topology of RUB-A1 is closely related to the previously known 4.6.8-layer topology but with a different sequence of phosphoryl group orientation. Combining the results of structure analysis with the NMR, chemical analysis and TG-DTA experiments, we show that both the ionic liquid cation and the protonated diamines are located in the interlayer space and together direct the formation of these two structures. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2dt31150j
  • 2012 • 67 Simple preparation routes for corrosion protection hybrid sol-gel coatings on AA 2024
    Wittmar, A. and Caparrotti, H. and Wittmar, M. and Veith, M.
    Surface and Interface Analysis 44 70-77 (2012)
    In recent years, many hybrid inorganic-organic systems have been proposed in order to replace the traditional conversion coatings on metals like aluminum, and some results have been promising. However, many proposed solutions are based on complicated processes which are not easy to be adapted to industrial scale. The aim of this study was to establish a simple process leading to the production of highly efficient corrosion protective hybrid sol-gel coating systems for the aluminum alloys as replacement for the highly hazardous conventional chromate conversion coatings. Hybrid coatings have been realized by means of the sol-gel process. CeO 2 and ZnO have been introduced as dispersions of nanoparticles in the system and used as corrosion inhibitors. The aim of this work was to obtain pore-free coatings with increased barrier properties using nanoparticles that possess the double function of pore fillers and corrosion inhibitors. The proposed processes led to coating materials with good adherence to the aluminum substrate and an extremely long life in the accelerated neutral salt spray test according to DIN ISO 9227. Electrochemical impedance spectroscopy approves these results by high impedance values in the low-frequency region of the Bode plot. © 2011 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.3771
  • 2011 • 66 Understanding the complexity of a catalyst synthesis: Co-precipitation of mixed Cu,Zn,Al hydroxycarbonate precursors for Cu/ZnO/Al2O 3 catalysts investigated by titration experiments
    Behrens, M. and Brennecke, D. and Girgsdies, F. and Kißner, S. and Trunschke, A. and Nasrudin, N. and Zakaria, S. and Idris, N.F. and Hamid, S.B.A. and Kniep, B. and Fischer, R. and Busser, W. and Muhler, M. and Schlögl, R.
    Applied Catalysis A: General 392 93-102 (2011)
    Co-precipitation of Cu,Zn,(Al) precursor materials is the traditional way of synthesizing Cu/ZnO/(Al2O3) catalysts for industrial methanol synthesis. This process has been investigated by titration experiments of nitrate and formate solutions. It was found that the solidification of the single components proceeds sequentially in case of nitrates: Cu2+ is precipitated at pH 3 and Zn2+ (as well as Al3+) near pH 5. This behavior prevents a homogeneous distribution of all metal species in the initial precipitate upon gradual increase of pH and requires application of the constant pH micro-droplet method. This effect is less pronounced if formate instead of nitrate is used as counter ion. This can be explained by the strong modification of the hydrolysis chemistry of the metal ions due to the presence of formate anions, which act as ligands and buffer. A formate-derived Cu/ZnO/Al2O3 catalyst was more active in methanol synthesis compared to a nitrate-derived sample although the same crystallographic phases were present in the precursor after co-precipitation and ageing. The effect of precipitation temperature was studied for the binary CuZn nitrate model system. Increasing the temperature of co-precipitation above 50 °C leads to down-shift of the precipitation pH of Zn2+ by a full unit. Thus, in warm solutions more acidic conditions can be used for complete co-precipitation, while in cold solutions, some Zn2+ may remain dissolved in the mother liquor at the same precipitation pH. The higher limit of temperature is given by the tendency of the initial Cu precipitate towards formation of CuO by oxolation. On the basis of these considerations, the empirically determined optimal pH and temperature conditions of the industrially applied synthesis can be rationalized. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2010.10.031
  • 2011 • 65 Microstructure evolution and mechanical properties of an intermetallic Ti-43.5Al-4Nb-1Mo-0.1B alloy after ageing below the eutectoid temperature
    Cha, L. and Clemens, H. and Dehm, G.
    International Journal of Materials Research 102 703-708 (2011)
    Intermetallic γ-TiAl based alloys with a chemical composition of Ti-(42-45)Al-(3-5)Nb-(0.1-2)Mo-(0.1-0.2)B (in atom percent) are termed TNM ™ alloys. They exhibit several distinct characteristics, including excellent hot-workability and balanced mechanical properties. In this study, the relationship between microstructure and mechanical behavior in a Ti-43.5Al-4Nb-1Mo-0.1B alloy after two different heat treatments was investigated. One of the analyzed microstructures consisted of lamellar γ-TiAl/α2-Ti3Al colonies with a small volume fraction of globular γ-TiAl and β0-TiAl grains at their grain boundaries, whereas the second microstructure basically exhibited the same arrangement of the microstructural constituents, but a fraction of the lamellar colonies was altered by a cellular reaction. The prevailing microstructures have been analyzed by means of scanning electron microscopy and transmission electron microscopy. Macro-and micro-hardness measurements as well as room temperature tensile tests have revealed that the sample with both cellular and lamellar features show lower yield stress and hardness than the ones exhibiting undisturbed lamellar microstructures. The strength and hardness properties are primarily connected to the lamellar spacing within the colonies, where strength increases with decreasing lamellar spacing. The appearance of a cellular reaction leads to a refinement of the lamellar colonies which in turn influences positively the plastic fracture strain at room temperature. © Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110526
  • 2011 • 64 In-situ TEM heating study of the γ lamellae formation inside the α2 matrix of a Ti-45Al-7.5Nb alloy
    Cha, L. and Clemens, H. and Dehm, G. and Zhang, Z.
    Advanced Materials Research 146-147 1365-1368 (2011)
    In-situ heating transmission electron microscopy (TEM) was employed to investigate the initial stage of lamellae formation in a high Nb containing γ-TiAl based alloy. A Ti-45Al-7.5Nb alloy (at %), which was heat treated and quenched in a non-equilibrium state such that the matrix consists of ordered α2 grains, was annealed inside a TEM up to 750 °C. The in-situ TEM study reveals that γ laths precipitate in the α2 matrix at ∼750 °C possessing the classical Blackburn orientation relationship, i.e. (0001)α2 // (111)γ and [11-20]α2 // < 110]γ. The microstructure of the in-situ TEM experiment is compared to results from ex-situ heating and subsequent TEM studies.
    view abstractdoi: 10.4028/
  • 2011 • 63 Thermodynamics and molecular dynamics investigation of a possible new critical size for surface and inner cohesive energy of Al nanoparticles
    Chamaani, A. and Marzbanrad, E. and Rahimipour, M. R. and Yaghmaee, M. S. and Aghaei, A. and Kamachali, R. D. and Behnamian, Y.
    Journal of Nanoparticle Research 13 6059--6067 (2011)
    In this study, the authors first review the previously developed, thermodynamics-based theory for size dependency of the cohesion energy of free-standing spherically shaped Al nanoparticles. Then, this model is extrapolated to the cubic and truncated octahedron Al nanoparticle shapes. A series of computations for Al nanoparticles with these two new shapes are presented for particles in the range of 1-100 nm. The thermodynamics computational results reveal that there is a second critical size around 1.62 and 1 nm for cubes and truncated octahedrons, respectively. Below this critical size, particles behave as if they consisted only of surface-energy-state atoms. A molecular dynamics simulation is used to verify this second critical size for Al nanoparticles in the range of 1-5 nm. MD simulation for cube and truncated octahedron shapes shows the second critical point to be around 1.63 and 1.14 nm, respectively. According to the modeling and simulation results, this second critical size seems to be a material property characteristic rather than a shape-dependent feature.
    view abstractdoi: 10.1007/s11051-011-0258-6
  • 2011 • 62 Thermal stability of TiAIN/CrN multilayer coatings studied by atom probe tomography
    Choi, P.-P. and Povstugar, I. and Ahn, J.-P. and Kostka, A. and Raabe, D.
    Ultramicroscopy 111 518-523 (2011)
    This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 °C. At 700 °C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000 °C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.11.012
  • 2011 • 61 Creep in directionally solidified NiAl-Mo eutectics
    Dudová, M. and Kuchařová, K. and Barták, T. and Bei, H. and George, E.P. and Somsen, C. and Dlouhý, A.
    Scripta Materialia 65 699-702 (2011)
    A directionally solidified NiAl-Mo eutectic and an NiAl intermetallic, having respective nominal compositions Ni-45.5Al-9Mo and Ni-45.2Al (at.%), were loaded in compression at 1073 and 1173 K. Formidable strengthening by regularly distributed Mo fibres (average diameter 600 nm, volume fraction 14%) was observed. The fibres can support compression stresses transferred from the plastically deforming matrix up to a critical stress of the order of 2.5 GPa, at which point they yield. Microstructural evidence is provided for the dislocation-mediated stress transfer from the NiAl to the Mo phase. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2011.07.019
  • 2011 • 60 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 • 59 Co-extrusion of discontinuously, non-centric steel-reinforced aluminum
    Foydl, A. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    AIP Conference Proceedings 1353 443-448 (2011)
    The process of manufacturing discontinuously non-centric steel reinforced aluminum by means of co-extrusion has been examined. By this process semi-finished reinforced profiles can be fabricated for further treatment through forging techniques. Therefore, steel reinforcement elements consisting of E295GC were inserted into conventional aluminum billets and co-extruded into two different solid profiles; a rectangle one by an extrusion ratio of 10.1:1 and a round one by 4.8:1. The used aluminum alloy is EN AW-6060. The billet temperature as well as the ram speed were varied to investigate their influence on the position of the reinforcement elements inside the strand. The measurement was done by a video measurement system, called Optomess A250, after milling off the strand. The distances between the elements in longitudinal direction were nearly constant, apart from the rear part of the strand. The same was observed for the distance of the steel elements to the profile edge. This due to the inhomogeneous material flow in the transverse weld, related to the billet-to-billet extrusion. The rotation of the reinforcement elements occurs because the elements flow nearby the shear zone. Further, micrographs were made to investigate the embedding situation and the grain size distribution. The embedding of the reinforcement elements were good in the solid round profile, but in the rectangle profile were found some kind of air pocket. The grain size of the aluminum alloy close to the steel elements is much smaller than in the other parts of the solid round profile. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3589555
  • 2011 • 58 The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys
    Gebhardt, T. and Music, D. and Ekholm, M. and Abrikosov, I.A. and Vitos, L. and Dick, A. and Hickel, T. and Neugebauer, J. and Schneider, J.M.
    Journal of Physics Condensed Matter 23 (2011)
    We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Néel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/24/246003
  • 2011 • 57 The effect of extrusion ratio and material flow on the mechanical properties of aluminum profiles solid state recycled from 6060 aluminum alloy chips
    Güley, V. and Ben Khalifa, N. and Tekkaya, A.E.
    AIP Conference Proceedings 1353 1609-1614 (2011)
    This paper presents the effect of the extrusion ratio (ER) on the mechanical properties of aluminum profiles solid state recycled from 6060 aluminum chips by hot extrusion. The chips were extruded through three different dies. The tensile test results and microstructures were discussed in comparison with the conventional profiles extruded with the same dies under the same extrusion conditions. The conventional profiles extruded with these three different ERs revealed almost the same mechanical properties. The profiles extruded from chips with an ER of 68:1 showed an approx. 20% higher strength and ductility compared to the profiles of an ER of 34:1, while the exerted pressure was not sufficient for welding the chips in case of an ER of 10:1. It was also shown that, by adjusting the material flow through the die, it was possible to recycle aluminum chips even at ERs as low as 10:1. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3589746
  • 2011 • 56 Thermo-mechanical processing of aluminum profiles by integrated electromagnetic compression subsequent to hot extrusion
    Jäger, A. and Risch, D. and Tekkaya, A.E.
    Journal of Materials Processing Technology 211 936-943 (2011)
    In this paper, a strategy for the thermo-mechanical processing of aluminum profiles by subsequent electromagnetic forming and heat treatment is given. A tool coil for electromagnetic compression was positioned behind the die exit and coaxially to the extrudate in order to reduce the workpiece cross section locally. Additionally, a counter die in the shape of a mandrel was mounted to the mandrel of a porthole extrusion die which extended into the tool coil. Besides achieving a more defined geometry in comparison to a free forming operation, by this also the geometrical complexity of locally compressed areas can be achieved. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2010.06.016
  • 2011 • 55 Carbon nanowalls deposited by inductively coupled plasma enhanced chemical vapor deposition using aluminum acetylacetonate as precursor
    Jain, H.G. and Karacuban, H. and Krix, D. and Becker, H.-W. and Nienhaus, H. and Buck, V.
    Carbon 49 4987-4995 (2011)
    Well aligned carbon nanowalls, a few nanometers thick, were fabricated by continuous flow of aluminum acetylacetonate (Al (acac)3) without a catalyst, and independent of substrate material. The nanowalls were grown on Si, and steel substrates using inductively coupled plasma-enhanced chemical vapor deposition. Deposition parameters like flow of argon gas and substrate temperature were correlated with the growth of carbon nanowalls. For a high flow of argon carrier gas, an increased amount of aluminum in the film and a reduced lateral size of the carbon walls were found. The aluminum is present inside the carbon nanowall matrix in the form of well crystallized nanosized Al4C3 precipitates. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2011.07.002
  • 2011 • 54 Newest developments on the manufacture of helical profiles by hot extrusion
    Khalifa, N.B. and Tekkaya, A.E.
    ASME 2011 International Manufacturing Science and Engineering Conference, MSEC 2011 1 459-463 (2011)
    The paper presents a new innovative direct extrusion process, Helical Profile Extrusion (HPE), which increases the flexibility of aluminum profile manufacturing processes. The application fields of such profiles can be seen in screw rotors for compressors and pumps. The investigations concentrate on experimental and numerical analyses by 3D-FEM simulations to analyze the influence of friction on the material flow in the extrusion die in order to find out the optimal parameters with reference to the twisting angle and contour accuracy. By means of FEM, the profile shape could be optimized by modifying the die design. The numerical results were validated by experiments. For these investigations, a common aluminum alloy AA6060 was used. The accuracy of the profile contour could be improved significantly. However, increasing the twist angle is limited due to geometrical aspects. Copyright © 2010 by ASME.
    view abstractdoi: 10.1115/MSEC2011-50126
  • 2011 • 53 Ultrathin metal oxidation for vacuum monitoring device applications
    Mäder, S. and Haas, T. and Kunze, U. and Doll, T.
    Physica Status Solidi (A) Applications and Materials Science 208 1223-1228 (2011)
    The oxide growth on thin metal films at room temperature has been investigated in terms of resistance change during oxidation. These data have been interpreted using the extended Cabrera-Mott theory of oxidation by Boggio. The resulting oxide thickness as well as the oxidation kinetics was found to depend on pressure. According to this dependence, oxidation of ultrathin metal films can be applied for monitoring the vacuum quality inside an evacuated environment. The performance of aluminum and copper sensing layers are compared with respect to sensor lifetime and response. Furthermore, the theoretically evaluated and resistively measured oxide thicknesses are verified by TEM studies. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201000921
  • 2011 • 52 Precipitation kinetics study of Al - Zr - X(Sc or Ti) alloys by phase field simulations and atom probe tomography
    Masquelier, N. and Zapolsky, H. and Lefebvre, W. and Fries, S.G. and Patte, R. and Pareige, P.
    Solid State Phenomena 172-174 869-874 (2011)
    Phase field modeling of precipitation kinetics in Al - Zr - Sc and Al - Zr - Ti ternary alloys has been performed. The free energy was evaluated using the Thermo-calc data. Our simulations showed that L12 precipitates in Al - Zr - Sc alloy consists of Sc rich zone of in core and Zirconium rich zone at the precipitate / matrix interface. In Al - Zr - Ti system, Al3 (Zr-Ti) precipitates are homogeneous and no segregation is observed. Phase-field simulation results are compared with 3D APT data. © (2011) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2011 • 51 Atomically smooth p-doped silicon nanowires catalyzed by aluminum at low temperature
    Moutanabbir, O. and Senz, S. and Scholz, R. and Alexe, M. and Kim, Y. and Pippel, E. and Wang, Y. and Wiethoff, C. and Nabbefeld, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    ACS Nano 5 1313-1320 (2011)
    Silicon nanowires (SiNWs) are powerful nanotechnological building blocks. To date, a variety of metals have been used to synthesize high-density epitaxial SiNWs through metal-catalyzed vapor phase epitaxy. Understanding the impact of the catalyst on the intrinsic properties of SiNWs is critical for precise manipulation of the emerging SiNW-based devices. Here we demonstrate that SiNWs synthesized at low-temperature by ultrahigh vacuum chemical vapor deposition using Al as a catalyst present distinct morphological properties. In particular, these nanowires are atomically smooth in contrast to rough {112}-type sidewalls characteristic of the intensively investigated Au-catalyzed SiNWs. We show that the stabilizing effect of Al plays the key role in the observed nanowire surface morphology. In fact, unlike Au which induces (111) and (113) facets on the nanowire sidewall surface, Al revokes the reconstruction along the [1̄1̄2] direction leading to equivalent adjacent step edges and flat surfaces. Our finding sets the lower limit of the Al surface density on the nanowire sidewalls at ∼2 atom/nm2. Additionally, despite using temperatures of ca. 110-170 K below the eutectic point, we found that the incorporation of Al into the growing nanowires is sufficient to induce an effective p-type doping of SiNWs. These results demonstrate that the catalyst plays a crucial role is shaping the structural and electrical properties of SiNWs. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nn1030274
  • 2011 • 50 Dislocation plasticity of Al film on polyimide investigated by cross-sectional in situ transmission electron microscopy straining
    Oh, S.H. and Rentenberger, C. and Im, J. and Motz, C. and Kiener, D. and Karnthaler, H.-P. and Dehm, G.
    Scripta Materialia 65 456-459 (2011)
    Tensile straining of a cross-sectional Al/polyimide was performed in a transmission electron microscope. The tensile deformation of Al was accomplished mainly by dislocations emitted from the film surface, which glide and impinge on the Al/polyimide interface. During further straining the interfacial dislocations disappeared, indicating dislocation core spreading, whereas threading dislocations moved towards the film surface. While the Al/polyimide interface remains flat and becomes depleted of dislocations, the Al surface becomes increasingly rough accompanied by a noticeable increase in dislocation density. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2011.06.001
  • 2011 • 49 Modeling of dynamic microstructure evolution of en AW-6082 alloy during hot forward extrusion
    Parvizian, F. and Güzel, A. and Jäger, A. and Lambers, H.-G. and Svendsen, B. and Tekkaya, A.E. and Maier, H.J.
    Computational Materials Science 50 1520-1525 (2011)
    The aim of this work is to present briefly a model for predicting and simulating the evolution of microstructure, in particular the evolution of grains, during hot forming processes of aluminum alloy EN AW-6082 and give a comparison with the experimental results. The model is a physically motivated phenomenological model based on internal state dependent variables. The microstructure evolution is a temperature dependent process and is simulated in a fully coupled thermo-mechanical process by help of Finite Element software Abaqus. The results are compared and verified with experimental results obtained by EBSD measurement of a small-scale extrusion process established for scientific purposes. The simulation results are in reasonable agreement with experiment. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2010.12.009
  • 2011 • 48 Investigation and prediction of grain texture evolution in AA6082
    Segatori, A. and Foydl, A. and Donati, L. and Khalifa, N.B. and Brosius, A. and Tomesani, L. and Tekkaya, A.E.
    AIP Conference Proceedings 1353 449-454 (2011)
    Extrusion applications require a strict control of the mechanical proprieties of the extrudates, in particular when undergoing severe loading conditions like in the transportation sector. Profile mechanical properties directly depend on its microstructure and texture, which are the result of multiple mechanisms based on precipitation mechanism or on grain shape evolution (grain refinement, recrystallizations, recovery and grain growth). In this direction, predicting the final profile microstructure under specific process parameters in the die design stage is of great relevance. The present study involved experimental activity on grain size measurements of profile and butt during interrupted direct extrusion of an AA6082 round profile. The grain size measurements were coupled with simulation results in order to regress analytical models based on effective strain, strain rate and temperature. Finally, the developed model was implemented in the numerical code by means of a subroutine that can be used as microstructure prediction tool. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3589556
  • 2011 • 47 Preliminary study on calcium aluminosilicate glass as a potential host matrix for radioactive 90Sr-An approach based on natural analogue study
    Sengupta, P. and Fanara, S. and Chakraborty, S.
    Journal of Hazardous Materials 190 229-239 (2011)
    Given the environmental-, safety- and security risks associated with sealed radioactive sources it is important to identify suitable host matrices for 90Sr that is used for various peaceful applications. As SrO promotes phase separation within borosilicate melt, aluminosilicate bulk compositions belonging to anorthite-wollastonite-gehlenite stability field are studied in this work. Tests for their homogeneity, microstructural characteristics and resistance to phase separation narrowed the choice down to the composition CAS11 (CaO=35wt%, Al2O3=20wt%, SiO2=45wt%). We find that up to 30wt% SrO can be loaded in this glass without phase separation (into Ca, Sr-rich and Sr-poor, Si-rich domains). Leaching behaviour of the glasses differs depending on the content and distribution of Sr. In general, the elemental leach rates determined from conventional PCT experimental procedure yield values better than 10-7gcm-2day-1 for both CAS11 base glass as well as SrO doped glass. It was noted that leach rates calculated on the basis of Ca2+ and Sr2+ were of the same order and bit higher compared to those calculated on the basis of Si4+ and Al3+. During accelerated leaching tests, zeolite and zeolite+epidote were found to have developed on CAS11 base glass and SrO doped glasses respectively. The Sr bulk diffusion coefficients is found to vary from ~10-15 to 10-13cm2/s at temperature intervals as high as 725-850°C. Based on the experimental observations, it is suggested that CAS11 glass can be used as host matrix of 90Sr for various applications of radioactive Sr-pencils. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.jhazmat.2011.03.031
  • 2011 • 46 Highly dispersed MoO 3/Al 2O 3 shell-core composites synthesized by CVD of Mo(CO) 6 under atmospheric pressure
    Shi, G. and Franzke, T. and Xia, W. and Sanchez, M.D. and Muhler, M.
    Chemical Vapor Deposition 17 162-169 (2011)
    MoO 3/γ-Al 2O 3 composites are synthesized by CVD under atmospheric pressure using Mo(CO) 6 as the precursor and porous γ-Al 2O 3 particles in a horizontal, rotating, hot-wall reactor, which is also used for calcination in air. The composites are characterized by N 2 physisorption, atomic absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and laser Raman spectroscopy (LRS). The synthesized samples exhibit excellent porosity, even at high Mo loadings. A much higher Mo yield is achieved when applying sublimation-adsorption in static air instead of using flowing N 2. A high degree of Mo dispersion on alumina is confirmed by XRD, LRS, and TEM; with a Mo surface density as high as 5.2 atoms nm -2, the sample is X-ray amorphous, there are no polymeric molybdate species detectable by LRS, and the island size of the molybdate species is about 1 nm according to TEM. The XPS analysis shows that exclusively Mo VI species are present on all synthesized samples. Thus, the applied rotating, hot-wall reactor achieves efficient mixing and homogeneous deposition. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201106909
  • 2011 • 45 On the formation and growth of intermetallic phases during interdiffusion between low-carbon steel and aluminum alloys
    Springer, H. and Kostka, A. and Payton, E.J. and Raabe, D. and Kaysser-Pyzalla, A. and Eggeler, G.
    Acta Materialia 59 1586-1600 (2011)
    The formation of intermetallic reaction layers was investigated for interdiffusion between a low-carbon steel and commercially pure aluminum (99.99%) and between a low-carbon steel and an aluminum-silicon alloy (Al-5 wt.% Si). Solid/solid, solid/semi-solid and solid/liquid diffusion couples were produced at both 600 and 675 °C. The total width of the reaction layer is governed mainly by the parabolic diffusion-controlled growth of the η phase (Al5Fe2), which exhibits orientation-dependent growth kinetics. The addition of Si to Al, which is known to decelerate reaction layer growth in interdiffusion experiments with Al melts, was found to accelerate the reaction layer growth in solid/semi-solid interdiffusion experiments. This phenomenon is discussed in light of previous atomistic explanations and the apparent activation energy calculated for the growth of the η phase (Al 5Fe2). © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.11.023
  • 2011 • 44 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 • 43 Shape-selective synthesis of methylamines over the RRO zeolite Al-RUB-41
    Tijsebaert, B. and Yilmaz, B. and Müller, U. and Gies, H. and Zhang, W. and Bao, X. and Xiao, F.-S. and Tatsumi, T. and De Vos, D.
    Journal of Catalysis 278 246-252 (2011)
    Aluminum was incorporated into the layered silicate RUB-39, which is transformed by calcination into RUB-41. This new zeolite with RRO topology contains 8- and 10-ring pores, and the acid sites in the aluminated material catalyze the synthesis of methylamines, in particular mono- and dimethylamine, by amination of methanol. Owing to the shape-selective catalytic properties of (H)Al-RUB-41, low selectivity to the thermodynamically favored trimethylamine product is obtained in comparison with results on RUB-39 or non-shape-selective materials. Both activity and selectivity are highest for RUB-41 catalysts with a high Si to Al ratio. Silylation reduces the number of unselective sites and results in a further suppression of trimethylamine formation. The introduction of acidity in the intact RUB-41 structure is supported by Al-MAS NMR and NH 3-TPD data. Additional characterization by XRD and SEM is provided. © 2010 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2010.12.010
  • 2011 • 42 Compositional dependence of the compressive yield strength of Fe-Nb(-Al) and Co-Nb Laves phases
    Voß, S. and Palm, M. and Stein, F. and Raabe, D.
    Materials Research Society Symposium Proceedings 1295 311-316 (2011)
    Large, defect-free single-phase samples of the hexagonal C14 NbFe 2 and Nb(Fe,Al) 2, and the cubic C15 NbCo 2 Laves phases have been produced by a modified levitation melting technique. The compressive strength of NbFe 2 and NbCo 2 has been determined in dependence on the Nb content, that of Nb(Fe,Al) 2 in dependence on the Al content. The binary phases did not show either a maximum (defect softening) or minimum (defect hardening) in strength when the Nb content was varied. Instead, for both phases an increase of the compressive strength with increasing Nb content is observed. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.459
  • 2011 • 41 Powder metallurgy processing and compressive properties of Ti3AlC2/Al composites
    Wang, W.J. and Gauthier-Brunet, V. and Bei, G.P. and Laplanche, G. and Bonneville, J. and Joulain, A. and Dubois, S.
    Materials Science and Engineering A 530 168-173 (2011)
    Al-matrix material composites are produced from pure Al and 40vol.% Ti3AlC2 powders using hot isostatic pressing technique. It is demonstrated that the nanocrystallized-Ti3AlC2 agglomerates, uniformly distributed in the Al matrix, form a hard continuous skeleton. The mechanical properties of the composites are evaluated over the temperature range of 20-500°C by performing compression tests at constant strain rate. The monotonic temperature dependence of the proof stress at 0.2% plastic strain suggests that the same thermally activated mechanism controls the composite plastic deformation over the entire temperature range. The yield stress of the composite, about twice as high as that of the Al matrix in the investigated temperature range proves that Ti3AlC2 particles constitute efficient reinforcement particles for Al matrix. SEM observations indicate that plastic deformation of 40Ti3AlC2/60Al composite takes place in the Al matrix while Ti3AlC2 particle agglomerates undergo substantial fracture. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.09.068
  • 2011 • 40 Influence of groove characteristics on strength of form-fit joints
    Weddeling, C. and Woodward, S.T. and Marré, M. and Nellesen, J. and Psyk, V. and Tekkaya, A.E. and Tillmann, W.
    Journal of Materials Processing Technology 211 925-935 (2011)
    Joining by electromagnetic forming (EMF) is an innovative method to connect, e.g. extruded aluminum profiles to lightweight frame structures without heating or penetrating the profile. This article describes the joining of extruded aluminum profiles by electromagnetic forming, taking into account the process characteristics and the joint design. Forming is initiated by a magnetic impulse of high energy density, such that material with high electrical conductivity is deformed by Lorentz forces. With this high-speed forming process high-strength joints can be manufactured as interference fits, form fits, impact welded joints or a combination of these types. The focus of this paper is on form-fit joints of extruded aluminum profiles for lightweight frame structures. Based on fundamental technological investigations the parameters which specifically affect the strength of the joints were identified and analyzed. Throughout these experiments the groove geometry was varied size and shape wise. The influence of the acting magnetic pressure and the charging energy on the joint strength was also analyzed for the various groove geometries. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2010.08.004
  • 2011 • 39 The influence of the inhibitor particle sizes to the corrosion properties of hybrid sol-gel coatings
    Wittmar, A. and Wittmar, M. and Caparrotti, H. and Veith, M.
    Journal of Sol-Gel Science and Technology 59 621-628 (2011)
    The influence of the inhibitor particle size (nano and micro cerium dioxide) embedded in several hybrid sol-gel coating systems for the corrosion protection of aluminium AA2024 alloy was studied, as well as the influence of other parameters like the inhibitor loading level and the method of reticulation. The properties of the obtained coatings were evaluated by means of transmission electron microscopy, accelerated salt spray test and electrochemical impedance spectroscopy. All the varied parameters proved to have an important influence on the corrosion mechanism, and an improvement induced by the use of nanometric inhibitors in comparison with the micrometric ones was demonstrated. In the case of the strong reticulated matrix, an inhomogeneous dispersion of the inhibiting species (micro inhibitor) favours the tension formation, making the coating more vulnerable to the corrosion attack. For the same doping level it was observed that the reticulation with 1-methylimidazole (MI) leads to a slightly better corrosion protection. Concomitantly, it was shown that high loadings with inhibitors have an adverse effect on the corrosion protection. © Springer Science+Business Media, LLC 2011.
    view abstractdoi: 10.1007/s10971-011-2536-2
  • 2011 • 38 Phase-field simulation of diffusion couples in the Ni-Al system
    Zhang, L. and Steinbach, I. and Du, Y.
    International Journal of Materials Research 102 371-380 (2011)
    By linking thermodynamic and atomic mobility databases with two-dimensional phase-field simulation, the evolution of interdiffusion microstructures in a series of Ni-Al diffusion couples associated with the γ, γ', and b-phases was studied. The formation and subsequent growth of the γ'- phase layer in β/γ and γ' + β/γ diffusion couples reproduced the experimental observations well. Moreover, the effect of coherent strain on the γ - γ' microstructural evolution, as well as that of an external compressive force on the γ + γ'/γ + γ' diffusion couple, was investigated. The phase-field simulated concentration profiles of some of the Ni-Al diffusion couples were also compared with the corresponding experimental data and the results of one-dimensional DICTRA (DIffusion Controlled TRAnsformations) simulations. A discussion of the rafting direction was also made by comprehensively comparing the phase-field simulations with the predicted results from an elastic model. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110493
  • 2010 • 37 Phase-pure Cu,Zn,Al hydrotalcite-like materials as precursors for copper rich Cu/ZnO/Al2O3 catalysts
    Behrens, M. and Kasatkin, I. and Kühl, S. and Weinberg, G.
    Chemistry of Materials 22 386-397 (2010)
    A series of hydrotalcite-like (htl) compounds of the general composition (Cu,Zn)1-xAlx(OH)2-(CO3) x/2 · m H2O was prepared with a fixed Cu:Zn ratio of 70:30. Phase pure samples could be obtained for 0.3 ≤ x ≥ 0.4. The htl precursors thermally decompose in multiple steps. After dehydration and dehydroxylation amorphous materials were obtained at 330 °C. Phase segregation during this mild calcination was only observed for samples with a Zn: Al ratio deviating strongly from 1:2. A mechanism for this low-temperature segregation process basing on the preformation of the ZnAl2O 4 phase within the amorphous material is proposed. Samples with Zn: Al ratios near 1:2 form an amorphous carbonate-modified mixed oxide "(CuO)x(ZnAl2O4)y" of homogeneous microstructure. Crystallization occurs upon carbonate decomposition at temperatures higher than 500 °C. Despite the small size of the Cu nanoparticles (around 7 nm) formed upon reduction, the accessible Cu surface area is below 5 m2g-1. This can be explained by the unfavorable microstructure of the resulting Cu/ZnAl2O4 catalyst: The Cu particles are to a large extent embedded in a compact oxide matrix. The applicability as Cu/ZnO/Al2O3 catalysts and the role of htl precursor phases in course of industrial catalyst preparation are discussed. © 2009 American Chemical Society.
    view abstractdoi: 10.1021/cm9029165
  • 2010 • 36 Al-matrix composite materials reinforced by Al-Cu-Fe particles
    Bonneville, J. and Laplanche, G. and Joulain, A. and Gauthier-Brunet, V. and Dubois, S.
    Journal of Physics: Conference Series 240 (2010)
    Al-matrix material composites were produced using hot isostatic pressing technique, starting with pure Al and icosahedral (i) Al-Cu-Fe powders. Depending on the processing temperature, the final reinforcement particles are either still of the initial i-phase or transformed into the tetragonal ω-Al0 0.70Cu0.20Fe0.10 crystalline phase. Compression tests performed in the temperature range 293K - 823K on the two types of composite, i.e. Al/i and Al/ω, indicate that the flow stress of both composites is strongly temperature dependent and exhibit distinct regimes with increasing temperature. Differences exist between the two composites, in particul ar in yield stress values. In the low temperatureregime (T ≤ 570K), the yield stress of the Al/ω composite is nearly 75% higher than that of the Al/i composite, while for T &gt; 570K both composites exhibit similar yield stress values. The results are interpreted in terms of load transfer contribution between the matrix and the reinforcement particles and elementary dislocation mechanisms in the Al matrix. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/240/1/012013
  • 2010 • 35 Friction model selection in FEM simulations of aluminium extrusion
    Donati, L. and Tomesani, L. and Schikorra, M. and Ben Khalifa, N. and Tekkaya, A.E.
    International Journal of Surface Science and Engineering 4 27-41 (2010)
    Visioplastic analyses with the rod technique were performed in the extrusion of AA6060 alloy at different processing conditions in order to measure the friction effect at the billet-container interface. During the trials an accurate monitoring of the relevant process variables such as punch force and temperatures was performed in order to validate FEM simulations. Different FE codes were used to carry out the simulations: Deform, HyperXtrude, and Superform. A particular attention was given on evaluating the several coefficients of the available friction models by comparing the FEM results with experimental results. Copyright © 2010 Inderscience Enterprises Ltd.
    view abstractdoi: 10.1504/IJSURFSE.2010.029627
  • 2010 • 34 Sol-gel deposition of multiply doped thermographic phosphor coatings Al2O3:(Cr3+, M3+) (M = Dy, Tm) for wide range surface temperature measurement application
    Eckert, C. and Pflitsch, C. and Atakan, B.
    Progress in Organic Coatings 68 126-129 (2010)
    A promising method of measuring surface temperatures in harsh environments is the use of thermographic phosphor coatings. There, the surface temperature is evaluated from the phosphorescence decay lifetime following a pulsed laser or flash lamp light excitation. Depending on the used dopant, single doped M3+:α-Al2O3 (M = Cr, Dy, Tm) emit at 694 nm (Cr3+), 488 nm (Dy3+), 584 nm (Dy3+), and 459 nm (Tm3+), respectively. However, the accessible temperature range with a single dopant is limited: for the Cr3+-transition from 293 K up to 900 K, and for the Dy3+ and Tm3+-transitions both from 1073 K up to 1473 K. In the present study a new approach is followed to extend these limitations by co-doping two dopants using the sol-gel method and dip coating of α-Al2O3 thin films. For that application (Dy3+ + Cr3+) co-doped thin α-Al2O3 films and (Tm3+ + Cr3+) co-doped α-Al2O3 films with thicknesses of 4-6 μm were prepared, and the temperature-dependent luminescence properties (emission spectra and lifetimes) were analysed after pulsed laser excitation in the UV (355 nm). The phosphorescence lifetime as a function of temperature were measured between 293 K and 1473 K. A considerably extended range for surface temperature evaluation was established following this new approach by combining different dopants on the molecular level. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.porgcoat.2009.08.021
  • 2010 • 33 Can local hot spots induce α2/γ lamellae during incomplete massive transformation of γ-TiAl alloys?
    Fischer, F.D. and Cha, L. and Dehm, G. and Clemens, H.
    Intermetallics 18 972-976 (2010)
    Oil quenching is applied to a Ti-45Al-7.5b alloy (in at%) in order to investigate the occurrence of local "hot spots" during rapid cooling from the single α-phase region. After quenching the microstructure consists of large α2-Ti3Al grains and a small volume fraction of massively transformed γm-TiAl particles. The majority of the γm-particles show a featureless α2/γm interface, whereas in the immediate vicinity of some γm-particles lamellar α2/γ-structures exist, exhibiting a Blackburn orientation relationship. The extension into the α2-parent grain is in the range of 1-10 μm. In this study it is investigated, if the diffusion-controlled formation of the γ-laths is triggered by the release of latent heat during the α → γm transformation, which leads to a local increase in temperature by a hot spot effect. A theoretical model is presented, which describes the temperature conditions around the hot spot. The model predicts that the life time of the hot spot is much too short to generate γ-laths observed with a length of 1-10 μm. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.01.017
  • 2010 • 32 Study of nanometer-scaled lamellar microstructure in a Ti-45Al-7.5Nb alloy - Experiments and modeling
    Fischer, F.D. and Waitz, T. and Scheu, Ch. and Cha, L. and Dehm, G. and Antretter, T. and Clemens, H.
    Intermetallics 18 509-517 (2010)
    Quenching of Ti-45 at%Al-7.5 at%Nb from the single α-phase region to room temperature followed by aging below the eutectoid temperature leads to the precipitation of ultra-fine γ-TiAl lamellae. In addition to an extensive experimental program, reported by Cha et al. in Intermetallics 16 (2008) 868-875, in this work a micromechanical and thermodynamical model is presented for the formation of γ-TiAl lamellae within the α2-Ti3Al parent phase. A global transformation condition allows to predict a thickness to length ratio in accordance with experimental observations. Furthermore, a local transformation condition offers the basis for a kinetic law. The modeling concept can be applied to similar problems of combined diffusive and displacive phase transformations. © 2009 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2009.09.012
  • 2010 • 31 An assessment of the grain structure evolution during hot forward extrusion of aluminum alloy 7020
    Foydl, A. and Ben Khalifa, N. and Brosius, A. and Tekkaya, A.E.
    Key Engineering Materials 424 35-41 (2010)
    The current investigation is concerned with the grain structure evolution in an Al-Zn alloy (EN AW-7020) during the hot forward extrusion process. In order to analyze that, a miniature hot forward extrusion setup was designed which allows the quenching of the extrusion butt immediately after extrusion. In order to gain a better understanding of the process, the shape of the deformed grains was analyzed and the process was simulated. The shape of these grains was indentified in two directions in the different grain zones, e.g. dead metal zone and shear zone. The FE simulations showing the different grain zones were also illustrated. Simulation results and the micrographs were quite promising to find parameters for simulation models in order to predict grain sizes with the method presented in the current research work. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2010 • 30 Combined ab initio and experimental study of structural and elastic properties of Fe3Al-based ternaries
    Friák, M. and Deges, J. and Krein, R. and Frommeyer, G. and Neugebauer, J.
    Intermetallics 18 1310-1315 (2010)
    A combined theoretical and experimental study of thermodynamical, structural, and elastic properties of Fe3Al-based ternary alloys is presented. The theoretical part is based on a scale-bridging, multi-disciplinary combination of (i) thermodynamic aspects of the site preference and (ii) elastic stiffness data for substitutional ternary elements in Fe3Al single crystals, as determined by parameter-free first-principles calculations, and (iii) Hershey's homogenization model for the polycrystalline aggregates within the frame of linear elasticity theory. The approach was employed in order to explore the relation between chemical composition and both structural and elastic properties of Fe3Al ternary alloys containing the selected substituents (Ti, V, W, Cr and Si). The ab initio calculations employ density-functional theory (DFT) and the generalized gradient approximation (GGA). The determined elastic constants are used to calculate the elastic moduli, such as the Young's and bulk modulus. The theoretical results are compared to both literature data and novel impulse excitation measurements. Specifically, for Fe3Al-Ti alloys with low to medium Ti concentrations, an unexpected non-linear compositional dependence of the polycrystalline Young's modulus was found experimentally. The origin of this behavior is analyzed and discussed based on our theoretical results. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.02.025
  • 2010 • 29 Ab initio study of the anomalous volume-composition dependence in Fe-Al alloys
    Friák, M. and Neugebauer, J.
    Intermetallics 18 1316-1321 (2010)
    The experimentally observed anomalous compositional dependence of the lattice constant of Fe-Al crystals has been theoretically investigated employing density functional theory (DFT) within the generalized gradient approximation (GGA). The formation energies, equilibrium volumes and magnetic states have been determined for a dense set of different aluminium concentrations and a large variety of atomic configurations. The spin-polarized calculations for Fe-rich compounds reproduce very well the anomalous lattice-constant behavior in contrast to both the nonmagnetic and fixed-spin-moment calculations that result in nearly linear trends without any anomaly. We thus identify the change in magnetism of iron atoms as caused by an increasing number of Al atoms in the first coordination spheres to be the decisive driving force of the anomalous behavior. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.03.014
  • 2010 • 28 Design and characterization of novel wear resistant multilayer CVD coatings with improved adhesion between Al2O3 and Ti(C,N)
    Garcia, J. and Pitonak, R. and Weißenbacher, R. and Köpf, A. and Soldera, F. and Suarez, S. and Miguel, F. and Pinto, H. and Kostka, A. and Mücklich, F.
    Advanced Engineering Materials 12 929-934 (2010)
    Multilayer CVD coatings for high speed cutting applications were designed to achieve high wear and heat resistance during machining of steel alloys. In this work the microstructure and cutting performance of these novel multilayer CVD coatings are investigated and compared with standard CVD multilayer coatings. 3D-FIB tomography is used to characterize the microstructure of the layers, especially the transition between the Ti(C,N) and the Al 2O3 layer. The 3D reconstruction of the surface of the Ti(C,N) layer shows the formation of protruded Ti(C,N) grains with a very particular architecture, which penetrate into the Al2O3 top-layer, providing a mechanical anchoring between both layers. Cemented carbides coated with the novel CVD multilayer present reduced crater and flank wear as well as improved adherence between the Al2O3 top-layer and the Ti(C,N) layer leading to a dramatic improvement of cutting performance. The microstructure and cutting performance of multilayer CVD coatings with a novel transition between the Ti(C,N) and the Al 2O3 layer are investigated. 3D-FIB tomography shows the formation of protruded Ti(C,N) grains with a particular architecture, which penetrate into the Al2O3 top-layer, providing a mechanical anchoring between both layers. Cutting tools coated with the novel CVD multilayer show dramatic improvement of cutting performance, due to reduced crater and flank wear and improved adherence between the Ti(C,N) and the Al 2O3 top-layer. © 2010 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201000130
  • 2010 • 27 Direct recycling of 1050 aluminum alloy scrap material mixed with 6060 aluminum alloy chips by hot extrusion
    Güley, V. and Ben Khalifa, N. and Tekkaya, A.E.
    International Journal of Material Forming 3 853-856 (2010)
    In this paper, a method for the direct recycling of aluminum scrap by hot extrusion is investigated. 1050 aluminum alloy material in the form of pins remained as scrap after a lateral extrusion process and was mixed with 6060 aluminum alloy chips resulting from a turning operation. Contrary to the conventional method of re-melting aluminum scrap to produce secondary aluminum, this aluminum scrap+chip mixture was cold compacted into billets and hot extruded at 500°C to full rectangular profiles. The extruded profiles were examined by tensile tests and microstructural investigations and compared to the conventionally extruded profiles from as-cast material. It was shown that not only the aluminum chips but also the aluminum scrap material can be recycled directly by hot extrusion, which requires only ~10% of the energy required for recycling by re-melting. Also, the profiles extruded from billets containing 1050 aluminum alloy scrap with and without cooling lubricant were compared and a deteriorating effect was determined. © 2010 Springer-Verlag France.
    view abstractdoi: 10.1007/s12289-010-0904-z
  • 2010 • 26 Simulation of the quench sensitivity of the aluminum alloy 6082
    Güzel, A. and Jäger, A. and Ben Khalifa, N. and Tekkaya, A.E.
    Key Engineering Materials 424 51-56 (2010)
    A method for the numerical estimation of the final hardness distribution of heat treated aluminum alloys was developed and implemented into a commercial finite element (FE) tool. Jominy end-quench tests were carried out in order to determine the quench sensitivity of the aluminum alloy EN AW-6082. The hardness distribution of the alloy after end-quenching was related to the corresponding cooling rates. The derived relation was tested for an industrial application by investigating the local heat treatment of a prototype crash absorbing structure. Numerical estimations were validated with experimental measurements. Effectiveness of the derived method and possible improvements were discussed. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2010 • 25 Investigation of the fatigue behavior of Al thin films with different microstructure
    Heinz, W. and Pippan, R. and Dehm, G.
    Materials Science and Engineering A 527 7757-7763 (2010)
    Cyclic compressive and tensile stresses occur in metallic films and interconnects applied in sensors and microelectronic devices when exposed to temperature changes. The stresses are induced by differences in the thermal expansion coefficients of the adjacent materials. Repeated cycling leads to damage evolution and, eventually, to failure. In this study we report on a successful strategy how to avoid thermal stress induced fatigue damage. We analysed the deformation structures of 0.2-2μm thick Al films subjected to thermal cycling between 100°C and 450°C up to 10,000 times. The investigations reveal that a reduction in film thickness or controlling the Al texture and the Al/substrate interface structure can be used to prevent thermo-mechanical fatigue damage. The findings are explained by orientation dependent plasticity and differences in dislocation mechanisms for different interface structures, and less accumulated plastic strain for thinner films. The approach is expected to apply in general for metallic films on substrates. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.08.046
  • 2010 • 24 Low cycle lifetime assessment of Al2024 alloy
    Khan, S. and Vyshnevskyy, A. and Mosler, J.
    International Journal of Fatigue 32 1270-1277 (2010)
    The 2024-T351 aluminum alloy is extensively used for fabricating aircraft parts. This alloy shows a relatively low ductility at room temperature and is generally heat treated in various conditions to suit particular applications. The present study experimentally and numerically analyzes the damage mechanism of an Al2024-T351 plate (short transverse direction) subjected to multi-axial stress states. The purpose of this work is to predict the cyclic lifetime of the considered alloy, based on the local approach of damage evolution using continuum damage modeling (CDM). The experimental program involves different kinds of specimens and loading conditions. Monotonic and cyclic tests have been conducted in order to measure the mechanical response and also to perform micromechanical characterization of damage and fracture processes. The cyclic plasticity behavior has been characterized by means of smooth cylindrical specimens. For analyzing the evolution of plastic deformation and damage under multi-axial stress conditions, cyclic loading tests in the low cycle regime have been conducted on different round notched bars. The predictions of the CDM were compared to the experimentally observed mechanical response and to the micromechanical characterization of damage. Emphasis was placed on the prediction of the number of cycles to failure. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2010.01.014
  • 2010 • 23 A novel isotropic quasi-brittle damage model applied to LCF analyses of Al2024
    Kintzel, O. and Khan, S. and Mosler, J.
    International Journal of Fatigue 32 1948-1959 (2010)
    The current paper deals with the assessment and the numerical simulation of low cycle fatigue of an aluminum 2024 alloy. According to experimental observations, the material response of Al2024 is highly direction-dependent showing a material behavior between ductile and brittle. In particular, in its corresponding (small transversal) S-direction, the material behavior can be characterized as quasi-brittle. For the modeling of such a mechanical response, a novel, fully coupled isotropic ductile-brittle continuum damage mechanics model is proposed. Since the resulting model shows a large number of material parameters, an efficient, hybrid parameter identification strategy is discussed. Within this strategy, as many parameters as possible have been determined a priori by exploiting analogies to established theories (like Paris' law), while the remaining free unknowns are computed by solving an optimization problem. Comparisons between the experimentally observed and the numerically simulated lifetimes reveal the prediction capability of the proposed model. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2010.07.001
  • 2010 • 22 Orientation dependence of local lattice rotations at precipitates: Example of κ-Fe3AlC carbides in a Fe3Al-based alloy
    Kobayashi, S. and Zambaldi, C. and Raabe, D.
    Acta Materialia 58 6672-6684 (2010)
    Local lattice rotations and in-grain orientation gradients at κ precipitates in matrix grains with orientations near the 45° rotated cube {0 0 1}〈1 1 0〉 (RC) and the γ-fiber components {1 1 1}〈1 1 2〉 were investigated in a Fe3Al alloy warm-rolled to reductions of between 10% and 60%. Near-RC grains showed larger local lattice rotations at precipitates than γ-fiber grains. In RC-oriented grains the local lattice rotations about the transverse direction (TD) were dominant at low reductions, but rotations about the rolling direction (RD) also occurred at higher strains. In the γ-fiber grains the axes of the in-grain lattice rotations were scattered between TD and RD. The rotations around the particles and their orientation dependence were analyzed using 3-D crystal plasticity finite-element simulations of a spherical inclusion in a plane strain deformed matrix of different orientations, namely RC, {1 1 1}〈1 1 2〉 and {1 1 1}〈0 1 1〉. © 2010 AWE and Crown Copyright. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.08.030
  • 2010 • 21 L21-ordered Fe-Al-Ti alloys
    Krein, R. and Friak, M. and Neugebauer, J. and Palm, M. and Heilmaier, M.
    Intermetallics 18 1360-1364 (2010)
    Fe-Al-Ti alloys with the ordered L21-structure (Heusler phase) belong to the few Fe-Al-based alloys which show comparably high-strength at high temperatures, e.g. at 800 °C. However, like many other high-temperature materials based on intermetallics they show limited ductility even at high temperatures. In order to further explore the possibilities in increasing their strength and ductility, alloys with four different microstructures, i.e. single-phase L21, L21 with incoherent precipitates of TiB2 or Laves phase, and coherent L21 + A2, were produced. Also, the influence of alloying with Cr and B has been investigated. The Young's modulus of Fe-25Al-20Ti-4Cr (at.%) in dependence of temperature up to 900 °C has been determined and results of the compressive flow stress, creep strength and brittle-toductile transition temperatures (BDTT) are summarised and compared to those of binary Fe3Al (D03), Fe-Al-Ti-based alloys, and some commercial alloys. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2009.12.036
  • 2010 • 20 Ion-enhanced oxidation of aluminum as a fundamental surface process during target poisoning in reactive magnetron sputtering
    Kuschel, T. and von Keudell, A.
    Journal of Applied Physics 107 103302 (2010)
    Plasma deposition of aluminum oxide by reactive magnetron sputtering (RMS) using an aluminum target and argon and oxygen as working gases is an important technological process. The undesired oxidation of the target itself, however, causes the so-called target poisoning, which leads to strong hysteresis effects during RMS operation. The oxidation occurs by chemisorption of oxygen atoms and molecules with a simultaneous ion bombardment being present. This heterogenous surface reaction is studied in a quantified particle beam experiment employing beams of oxygen molecules and argon ions impinging onto an aluminum-coated quartz microbalance. The oxidation and/or sputtering rates are measured with this microbalance and the resulting oxide layers are analyzed by x-ray photoelectron spectroscopy. The sticking coefficient of oxygen molecules is determined to 0.015 in the zero coverage limit. The sputtering yields of pure aluminum by argon ions are determined to 0.4, 0.62, and 0.8 at 200, 300, and 400 eV. The variation in the effective sticking coefficient and sputtering yield during the combined impact of argon ions and oxygen molecules is modeled with a set of rate equations. A good agreement is achieved if one postulates an ion-induced surface activation process, which facilitates oxygen chemisorption. This process may be identified with knock-on implantation of surface-bonded oxygen, with an electric-field-driven in-diffusion of oxygen or with an ion-enhanced surface activation process. Based on these fundamental processes, a robust set of balance equations is proposed to describe target poisoning effects in RMS. (C) 2010 American Institute of Physics. [doi:10.1063/1.3415531]
    view abstractdoi: 10.1063/1.3415531
  • 2010 • 19 Synthesis and brittle-to-ductile transition of the ω-Al0.7Cu0.2Fe0.1 tetragonal phase
    Laplanche, G. and Joulain, A. and Bonneville, J. and Gauthier-Brunet, V. and Dubois, S.
    Materials Science and Engineering A 527 4515-4518 (2010)
    Synthesis of ω-Al-Cu-Fe single phase material is reported. Microhardness tests performed over the temperature range 293-898K indicate a brittle-to-ductile transition between 673K and 823K. Fracture toughness was determined from indentation cracks at 293K. Both the hardness and fracture toughness are comparable to that of the icosahedral Al0.635Cu0.240Fe0.125 material. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.02.049
  • 2010 • 18 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 • 17 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 • 16 Effect of strain hardening on texture development in cold rolled Al-Mg alloy
    Liu, W.C. and Man, C.-S. and Raabe, D.
    Materials Science and Engineering A 527 1249-1254 (2010)
    The hot band of a continuous cast Al-Mg alloy possesses a typical deformed structure and a strong β fiber rolling texture. The hot band was heat-treated at 260 °C for 3 h to generate different degrees of strain hardening. The hot band and its counterpart after recovery treatment were cold rolled to different reductions along the original transverse direction. The effect of strain hardening on texture evolution was investigated by X-ray diffraction. The results show that a high degree of strain hardening reduces the formation rate of the β fiber rolling texture. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.msea.2009.09.059
  • 2010 • 15 Nanofilm metal layers as vacuum quality sensors
    Mader, S. and Haas, T. and Kunze, U. and Doll, T.
    Procedia Engineering 5 1144-1147 (2010)
    A monitoring device for vacuum quality is realized by lowest cost single use oxygen sensors for vacuum insulation panels. They use the pressure dependence of oxide layer growth thickness on electrically measured metal nanofUms. These films were manufactured by e-beam evaporation , characterized in terms of resistance change with subsequent modeling of underlying mechanisms.
    view abstractdoi: 10.1016/j.proeng.2010.09.313
  • 2010 • 14 Thin and dense ceramic coatings by plasma spraying at very low pressure
    Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 19 495-501 (2010)
    The very low pressure plasma spray (VLPPS) process operates at a pressure range of approximately 100 Pa. At this pressure, the plasma jet interaction with the surrounding atmosphere is very weak. Thus, the plasma velocity is almost constant over a large distance from the nozzle exit. Furthermore, at these low pressures the collision frequency is distinctly reduced and the mean free path is strongly increased. As a consequence, at low pressure the specific enthalpy of the plasma is substantially higher, but at lower density. These particular plasma characteristics offer enhanced possibilities to spray thin and dense ceramics compared to conventional processes which operate in the pressure range between 5 and 20 kPa. This paper presents some examples of gas-tight and electrically insulating coatings with low thicknesses <50 μm for solid oxide fuel cell applications. Furthermore, plasma spraying of oxygen conducting membrane materials such as perovskites is discussed. © 2009 ASM International.
    view abstractdoi: 10.1007/s11666-009-9416-0
  • 2010 • 13 An in situ tensile tester for studying electrochemical repassivation behavior: Fabrication and challenges
    Neelakantan, L. and Schönberger, B. and Eggeler, G. and Hassel, A.W.
    Review of Scientific Instruments 81 (2010)
    An in situ tensile rig is proposed, which allows performing electrochemical (repassivation) experiments during dynamic mechanical testing of wires. Utilizing the basic components of a conventional tensile tester, a custom-made minitensile rig was designed and fabricated. The maximal force that can be measured by the force sensor is 80 N, with a sensitivity of 0.5 mV/V. The maximum travel range of the crosshead induced by the motor is 10 mm with a minimum step size of 0.5 nm. The functionality of the tensile test rig was validated by investigating Cu and shape memory NiTi wires. Wires of lengths between 40 and 50 mm with varying gauge lengths can be tested. An interface between wire and electrochemical setup (noncontact) with a smart arrangement of electrodes facilitated the electrochemical measurements during tensile loading. Preliminary results on the repassivation behavior of Al wire are reported. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3292685
  • 2010 • 12 Salen-ligands based on a planar-chiral hydroxyferrocene moiety: Synthesis, coordination chemistry and use in asymmetric silylcyanation
    Niemeyer, J. and Cloppenburg, J. and Fröhlich, R. and Kehr, G. and Erker, G.
    Journal of Organometallic Chemistry 695 1801-1812 (2010)
    Condensation of the O-protected hydroxyferrocene carbaldehyde (Sp)-1 with suitable diamines, followed by liberation of the hydroxyferrocene moiety leads to a new type of ferrocene-based salen ligands (3). While the use of ethylenediamine in the condensation reaction yields the planar-chiral ethylene-bridged ligand [(Sp,Sp)-3a], reaction with the enantiomers of trans-1,2-cyclohexylendiamine gives rise to the corresponding diastereomeric cyclohexylene-bridged systems [(S,S,Sp,Sp)-3b and (R,R,Sp,Sp)-3c], which feature a combination of a planar-chiral ferrocene unit with a centrochiral diamine backbone. Starting with the ferrocene-aldehyde derivative (Rp)-1, the enantiomeric ligand series (3d/e/f) is accessible via the same synthetic route. The (Sp)-series of these newly developed N2O2-type ligands was used for the construction of the corresponding mononuclear bis(isopropoxy)titanium (4a/b/c), methylaluminum (5a/b/c) and chloroaluminum-complexes (6a/b/c), which were isolated in good yields and identified by X-ray diffraction in several cases. The aluminum complexes (5/6) were successfully used in the Lewis-acid catalyzed addition of trimethylsilylcyanide to benzaldehyde, yielding the corresponding cyanohydrins in 45-62% enantiomeric excess. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jorganchem.2010.04.008
  • 2010 • 11 Reduced wear and adhesion forces by laser dispersing of ceramics
    Nölke, C. and Claußen, S. and Dudziak, S. and Haferkamp, H. and Barcikowski, S.
    Physics Procedia 5 431-437 (2010)
    Laser dispersing offers a great potential to fabricate layers or tracks with tailored properties threat duce abrasive or adhesive wear at the surface of highly stressed components. Different ceramic powder materials like aluminum nitride, aluminum oxide and titanium carbide have been embedded in the surface of tool steels using laser dispersing. The created layers were investigated regarding their elemental composition, dimension, particle distribution and hardness curve.
    view abstractdoi: 10.1016/j.phpro.2010.08.165
  • 2010 • 10 An in vitro crystallization setup to assess the efficiency of different phosphate binders in nephrology: Quantitative analytical considerations
    Peitsch, T. and Matthes, M. and Brandenburg, V. and Epple, M.
    Analytical Methods 2 901-911 (2010)
    An increased phosphate level in the blood (hyperphosphatemia) is a severe problem for dialysis patients. Different phosphate binders which are used to prevent hyperphosphatemia were studied in a custom-made in vitro crystallization apparatus which simulated the stomach (pH 2) and the gut (pH 7). The effective phosphate binding capacity was measured and the resulting products were identified. This apparatus permits the quantitative analysis of the phosphate binding effect under the given in vitro conditions. In particular, phosphate binders on the basis of calcium acetate (Calciumacetat Nefro®), calcium carbonate (Calciumcarbonat® Fresenius), aluminium hydroxide (Antiphosphat®), lanthanum carbonate (Fosrenol®), and poly(allylamine hydrochloride) (Renagel®) were studied, and also compared with pure calcium acetate, calcium carbonate, and aluminium sulfate. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/b9ay00325h
  • 2010 • 9 Influence of the ball milling conditions on the preparation of rare earth aluminum hydrides
    Pommerin, A. and Felderhoff, M. and Schüth, F. and Weidenthaler, C.
    Scripta Materialia 63 1128-1131 (2010)
    The ball milling conditions in the preparation of rare earth aluminum hydrides from NaAlH4 and rare earth chlorides have a significant influence on product formation. Defined milling times and appropriate rotational speeds are required to obtain the desired products. It has been shown that starting directly from Na3AlH6 does not lead to the formation of REAlH6. Starting from rare earth iodides instead of chlorides allows dissolution of the alkali metal iodide formed and, therewith, the preparation of salt-free rare earth aluminum hydrides. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2010.08.020
  • 2010 • 8 Direct synthesis of pure complex aluminium hydrides by cryomilling
    Pommerin, A. and Weidenthaler, C. and Schüth, F. and Felderhoff, M.
    Scripta Materialia 62 576-578 (2010)
    Simple mechanochemical procedures can be used for the solid-state preparation of stable complex aluminium hydrides as hydrogen storage materials. For the synthesis of unstable complex hydrides, cryomilling at temperatures at which product decomposition does not take place under milling conditions appears to be a viable method. To probe the potential of cryomilling for the synthesis of complex aluminium hydrides, the reactions of different alkaline hydrides with AlH3 were tested under these conditions. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2009.12.041
  • 2010 • 7 In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals
    Rajagopalan, J. and Rentenberger, C. and Peter Karnthaler, H. and Dehm, G. and Saif, M.T.A.
    Acta Materialia 58 4772-4782 (2010)
    In situ transmission electron microscopy straining experiments with concurrent macroscopic stress-strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size dm = 70 nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (dm = 140 nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (dm = 120 nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.05.013
  • 2010 • 6 An experimental and numerical lifetime assessment of Al 2024 sheet
    Saleem Khan, S. and Hellmann, D. and Kintzel, O. and Mosler, J.
    Procedia Engineering 2 1141-1144 (2010)
    This work is concerned with the simulation of ductile damage evolution and the respective final material failure resulting from load reversals (LCF). For that purpose, a continuum damage mechanics (CDM) model proposed by Lemaitre is modified and utilized. This model has been validated by means of experiments of Al2024 alloy. These experiments involve specimens with different testing conditions. First, monotonic tensile tests have been considered. Subsequently, the cyclic yielding behavior has been characterized performing cyclic plasticity and damage tests on flat smooth specimen. The predictions of the model are compared to the experimentally observed results. Within the present work, special emphasis is placed on the experimental setup for fatigue testing of flat specimens as well as on the predictions of the number of cycles to final failure and the crack initiation loci. © 2010 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2010.03.123
  • 2010 • 5 Structural characterization of a completely alkyl-substituted Al-Sb Lewis acid-base adduct
    Schulz, S. and Kuczkowski, A. and Nieger, M. and Saxell, H.
    Journal of Organometallic Chemistry 695 2281-2283 (2010)
    The Lewis acid-base adduct t-Bu3Al-SbMe3 (1), which was synthesized by reaction of equimolar amounts of t-Bu3Al and trimethylstibine SbMe3, was characterized by multinuclear NMR ( 1H, 13C) spectroscopy, elemental analyses as well as by single crystal X-ray diffraction. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.jorganchem.2010.06.022
  • 2010 • 4 Monotonic and cyclic deformation behaviour of the SiC particle-reinforced aluminium matrix composite AMC225xe
    Smaga, M. and Walther, F. and Eifler, D.
    Advanced Engineering Materials 12 262-268 (2010)
    The monotonic and cyclic deformation behaviour of the aluminium matrix composite AMC225xe - i.e., the aerospace grade aluminium alloy AA 2124 reinforced with 25vol.-% ultrafine SiC particles - is characterised in detail on the basis of mechanical stress-strain hysteresis curves as well as temperature and electrical resistance measurements. A pronounced difference in plastic strain response is observed between tension and compression under monotonic and cyclic loading. ln fully reversed stress-controlled constant amplitude tests, negative plastic mean strains developed. The cyclic deformation behaviour ofAMC225xe is characterised by pronounced initial cyclic hardening. The endurance limit is reliably estimated in continuous load increase tests. ln particular, electrical resistance data are used as input parameters for fatigue life calculations analogous to the Basquin equation. Microstructural details are investigated by light and scanning electron microscopy. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.200900345
  • 2010 • 3 An investigation on low cycle lifetime of AL2024 alloy
    Vyshnevskyy, A. and Khan, S. and Mosler, J.
    Key Engineering Materials 417-418 289-292 (2010)
    One of the important considerations in the design of components is the estimation of cyclic lifetime and analysis of the critical regions of a construction. The local approach of lifetime estimation using continuum damage mechanics (CDM) has shown a great potential in predicting material failure not only for monotonic, but also for fully reversed loadings. In this paper, the CDM model of Desmorat-Lemaitre [1] was investigated regarding the prediction of cyclic lifetime. A series of experiments on tension specimens with different geometries were performed. The latter were used for the determination of model parameters as well as for the validation of the predictive capability of the model. © (2010) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2010 • 2 Atomic mobilities and diffusivities in the fcc, L12 and B2 phases of the Ni-Al system
    Zhang, L. and Du, Y. and Chen, Q. and Steinbach, I. and Huang, B.
    International Journal of Materials Research 101 1461-1475 (2010)
    A phenomenological model was utilized to describe diffusivities in the γ (fcc)/γ′ (L12) and A2/B2 phases of the Ni-Al system. An effective strategy, which takes the homogeneity range and defect concentration into account, was developed in the present work to optimize the atomic mobilities of γ′ phase. Such a strategy results in a dramatic decrease in the number of atomic mobility parameters to be evaluated for the L12 phase. The measured composition-and temperature-dependent diffusivities in the Ni-Al system have been well replicated by the present mobility descriptions. For the L12 phase, comprehensive comparisons show that with fewer model parameters the presently obtained mobilities yield a better fit to experimental diffusivities, compared with previous assessments. The mobility descriptions are further validated by comparing calculated and measured concentration profiles for various diffusion couples. The time-dependent Al composition profile for the annealed vapor Al/γ couple is accurately described for the first time. © Carl Hanser Verlag GmbH & Co. KG ISSN 1862-5282.
    view abstractdoi: 10.3139/146.110428
  • 2010 • 1 Diffusivities of an Al-Fe-Ni melt and their effects on the microstructure during solidification
    Zhang, L. and Du, Y. and Steinbach, I. and Chen, Q. and Huang, B.
    Acta Materialia 58 3664-3675 (2010)
    A systematical investigation of the diffusivities in an Al-Fe-Ni melt was presented. Based on the experimental and theoretical data about diffusivities, the temperature- and composition-dependent atomic mobilities were evaluated for the elements in Al-Ni, Al-Fe, Fe-Ni and Al-Fe-Ni melts via an effective approach. Most of the reported diffusivities can be reproduced well by the obtained atomic mobilities. In particular, for the first time the ternary diffusivity of the liquid in a ternary system is described in conjunction with the established atomic mobilities. The effect of the atomic mobilities in a liquid on microstructure and microsegregation during solidification was demonstrated with one Al-Ni binary alloy. The simulation results indicate that accurate databases of mobilities in the liquid phase are much needed for the quantitative simulation of microstructural evolution during solidification by using various approaches, including DICTRA and the phase-field method. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.03.002