Prof. Dr.-Ing. Gunther Eggeler

Institute for Materials
Ruhr-Universität Bochum

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  • Degradation behavior of the MgO/HA surface ceramic nano-composites in the simulated body fluid and its use as a potential bone implant
    Khalili, V. and Frenzel, J. and Eggeler, G.
    Materials Chemistry and Physics 258 (2021)
    In this work, we studied the effects of hot isostatic pressing and surface anodizing on the behavior of an in-situ surface modified magnesium matrix nano-composite with different wt.% of hydroxyapatite by stir-centrifugal casting. The hot isostatic pressing and anodizing were conducted to reduce the defects and to replace the surface of Mg/HA with a ceramic matrix nano-composite layer of MgO/HA, respectively. The composition of the conversion layer of anodizing was evaluated using energy dispersive spectroscopy and X-ray diffraction. The electrochemical tests were conducted in the simulated body fluid. The results show that the dominant deposition is vertical Mg(OH)2 nano-rods on the hot isostatic pressed-anodized surface during immersion in the simulated body fluid. According to the electrochemical results, a homogeneous distribution of 1.8 wt% nano-hydroxyapatite in the magnesium oxide matrix with a well-arranged nanostructure on the surface, after hot isostatic pressing and anodizing, reduces the H2 release and corrosion rate. Also, the mentioned specimen demonstrates the lowest thermodynamic tendency for corrosion (−1.345 V) and the corrosion rate of 3.8388 mm × year−1 with the highest protection efficiency of 42.26% compared to the as-cast pure magnesium. Therefore, it can be considered as a promising material in designing biomedical bone implants. © 2020 Elsevier B.V.
    view abstract10.1016/j.matchemphys.2020.123965
  • Dislocation networks in gamma/gamma’-microstructures formed during selective laser melting of a Ni-base superalloy
    Heep, L. and Schwalbe, C. and Heinze, C. and Dlouhy, A. and Rae, C.M.F. and Eggeler, G.
    Scripta Materialia 190 (2021)
    A dislocation network which formed during selective laser melting (SLM) of a Ni-base superalloy was analyzed using scanning transmission electron microscopy (STEM). This network traverses an ordered Gamma'-phase domain, in between two adjacent Gamma-solid solution regions. The Gamma’-phase region has formed when two Gamma’-phase particles have started to coalesce, trapping the dislocation network in this ordered region so that it formed two dislocation families with pairs of anti-phase boundary (APB) coupled super partial dislocations. The network features are presented and unusual features (twist character and low APB energies), not previously reported, are discussed. © 2020
    view abstract10.1016/j.scriptamat.2020.08.019
  • Effect of interface dislocations on mass flow during high temperature and low stress creep of single crystal Ni-base superalloys
    He, J. and Cao, L. and Makineni, S.K. and Gault, B. and Eggeler, G.
    Scripta Materialia 191 (2021)
    In this work, the nanometer-scale mass flow coupled to dislocation processes near the γ/γ′-interface during high temperature and low stress creep of a model Ni-base single crystal superalloy is investigated. In the early creep stages, the dislocation networks in the γ-phase at γ/γ′-interfaces attract γ-stabilizing elements like Cr, Co and in particular Re, resulting in compositional gradients close to the interface. At larger strains, where dislocations frequently cut into the γ′-phase, this local interfacial enrichment in these elements is no longer observed. The cutting dislocations take part of the segregated atoms away, whilst the remaining atoms are released and diffuse back into the γ-channels. © 2020
    view abstract10.1016/j.scriptamat.2020.09.016
  • On the size effect of additives in amorphous shape memory polymers
    Zirdehi, E.M. and Dumlu, H. and Eggeler, G. and Varnik, F.
    Materials 14 (2021)
    Small additive molecules often enhance structural relaxation in polymers. We explore this effect in a thermoplastic shape memory polymer via molecular dynamics simulations. The additiveto-monomer size ratio is shown to play a key role here. While the effect of additive-concentration on the rate of shape recovery is found to be monotonic in the investigated range, a non-monotonic dependence on the size-ratio emerges at temperatures close to the glass transition. This work thus identifies the additives’ size to be a qualitatively novel parameter for controlling the recovery process in polymer-based shape memory materials. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14020327
  • Surface metal matrix nano-composite of magnesium/hydroxyapatite produced by stir-centrifugal casting
    Khalili, V. and Moslemi, S. and Ruttert, B. and Frenzel, J. and Theisen, W. and Eggeler, G.
    Surface and Coatings Technology 406 (2021)
    The present study aims to investigate a liquid state method of stir and centrifugal casting as an in-situ and cost-attractive processing technology for the production of magnesium/hydroxyapatite surface metal matrix nano-composites (Mg/HA surface metal matrix nano-composite). The main contribution of this study is to design the best condition for achieving a uniform Mg/HA surface nano-composite as a potential bone implant. It was shown how casting parameters and the distribution of hydroxyapatite affect mechanical properties of nano-composites measured using nano-indentation, nano-scratch, and compression tests. Response surface method in Design Expert software was used to predict the best model and the optimum condition of casting based on the experimentally measured data. The surface metal matrix nano-composites, consisting of a magnesium matrix with different amounts of nano-sized hydroxyapatite and silicon-doped hydroxyapatite (0.75-3 wt%) particles, were prepared. Hot isostatic pressing was used to homogenize the nano-composites in terms of particle distribution and to reduce porosity. It was shown that the weight percent of hydroxyapatite reinforcement is the parameter which is best suited to tailor targeted strength values. The target values of maximum compression strength (187 MPa) and elastic modulus (33 GPa) were achieved with a combination of the following parameters: 1.83 wt% hydroxyapatite, 800 rpm mold rotational speed, and a propeller rotational time of 6.3 min. A specimen prepared under these conditions had a homogeneous distribution of nano-hydroxyapatite in magnesium metal matrix after hot isostatic pressing at 450 °C and 100 MPa for a holding time of 120 min. It indicated the best mechanical resistance in terms of hardness and material loss during the nano-scratch testing. Moreover, the XRD results show that there is no considerable chemical reaction between the reinforcement particles of n-HA and Mg metallic matrix during casting at 700 °C and thermo-mechanical treatment of HIP at 450 °C. © 2020
    view abstract10.1016/j.surfcoat.2020.126654
  • Analysis of strengthening due to grain boundaries and annealing twin boundaries in the CrCoNi medium-entropy alloy
    Schneider, M. and George, E.P. and Manescau, T.J. and Záležák, T. and Hunfeld, J. and Dlouhý, A. and Eggeler, G. and Laplanche, G.
    International Journal of Plasticity 124 (2020)
    CrCoNi exhibits the best combination of strength and ductility among all the equiatomic single-phase FCC subsets of the CrMnFeCoNi high-entropy alloy. Here, its yield strength was determined in compression as a function of grain size and temperature. Yield strength was also plotted as a function of "crystallite" size, which takes into account both annealing twin boundaries and grain boundaries. The resulting Hall-Petch slopes were straight lines but with different slopes that depend on the number of twin boundaries per grain. Scanning transmission electron microscopy of deformed specimens revealed the formation of dislocation pile-ups at grain and annealing twin boundaries indicating that the latter also act as obstacles to slip and contribute to strength. Using a simple pile-up model, the strengths of the grain and twin boundaries were estimated to lie in the range 900-1250 »MPa. Assuming that they have the same strength, in the case of twin boundaries this strength corresponds roughly to the stress required to constrict Shockley partials, which suggests that dissociated dislocations have to become compact before they can cross the annealing twin boundaries. © 2019 The Authors.
    view abstract10.1016/j.ijplas.2019.08.009
  • Bulk and surface low temperature phase transitions in the mg-alloy ez33a
    Straumal, A. and Mazilkin, I. and Tzoy, K. and Straumal, B. and Bryła, K. and Baranchikov, A. and Eggeler, G.
    Metals 10 (2020)
    Low-temperature phase transitions in the EZ33A Mg-cast alloy have been investigated. Based on the structure assessment of the alloy after annealing at 150◦ C (1826 h) and at 200◦ C (2371 h) a grain boundary wetting transition by a second solid phase was documented. Within a 50◦ C temperature range, substantial differences in the α(Mg) grain boundary fraction wetted by the (Mg,Zn)12 RE intermetallic were observed. In contrast to what was reported in the literature, two different types of precipitates were found within α(Mg) grains. With increasing annealing temperatures, both types of precipitates dissolved. © 2020 by the authors.
    view abstract10.3390/met10091127
  • Chemical complexity, microstructure and martensitic transformation in high entropy shape memory alloys
    Piorunek, D. and Frenzel, J. and Jöns, N. and Somsen, C. and Eggeler, G.
    Intermetallics 122 (2020)
    High entropy shape memory alloys (HESMAs) represent a relatively young class of functional materials. They show a reversible martensitic phase transformation which allows to exploit shape memory effects at relatively high temperatures. HESMAs represent ordered complex solid-solutions. Their high temperature phase is of B2 type, and various elements (e.g. Ni, Cu, Ti, Zr, Hf) occupy sites in specific sub-lattices. In the present work, we study the processing and the functional properties of HESMAs. We study effects of chemical complexity on solidification microstructures and martensitic transformations. Binary, ternary, quaternary, quinary and senary model alloys were investigated using advanced microstructural and thermal characterization methods. The results show that element partitioning during solidification results in a redistribution of individual alloy elements in dendritic/interdendritic regions. Surprisingly, the atomic ratios of the two groups of elements which occupy the Ni- (first group: Ni, Cu and Pd) and Ti-sub-lattice (second group: Ti, Zr, Hf) are maintained. This allows the material to form martensite throughout its heterogeneous microstructure. The effect of chemical complexity/composition on martensite start temperatures, MS, is discussed on the basis of valence electron concentrations, cV. Some of the alloys fall into MS(cV)-regimes which are uncommon for classical Ni-Ti-based shape memory alloys. In the present work, a new HESMA of type NiCuPdTiZrHf was identified which has the potential to provide maximum shape memory strains close to 15%. © 2020
    view abstract10.1016/j.intermet.2020.106792
  • Comparison of Spectra of Grain Boundaries Spontaneously Formed in Cu-Ag and Cu-In Systems
    Straumal, A.B. and Tsoi, K.V. and Mazilkin, I.A. and Rodin, A.O. and Eggeler, G.
    JETP Letters 111 (2020)
    Spectra of grain boundaries existing in the polycrystalline copper-silver system (positive enthalpy of mixing) have been studied in comparison with those of the copper-indium system (negative enthalpy of mixing). Spectra of grain boundaries are formed spontaneously upon an increase in the temperature, occurrence of eutectic and peritectic reactions, and subsequent relaxation of structures in the two-phase solid/liquid region of phase diagrams. It has been shown that the rate of grain growth and the relation between different grain boundary types in the total spectrum depend on the enthalpy of mixing. © 2020, Pleiades Publishing, Inc.
    view abstract10.1134/S0021364020080111
  • Corrigendum to ‘On the Re segregation at the low angle grain boundary in a single crystal Ni-base superalloy’ Scripta Materialia Volume 185, August 2020, Pages 88-93 (Scripta Materialia (2020) 185 (88–93), (S1359646220302475), (10.1016/j.scriptamat.2020.03.063))
    He, J. and Scholz, F. and Horst, O.M. and Thome, P. and Frenzel, J. and Eggeler, G. and Gault, B.
    Scripta Materialia 187 (2020)
    The authors regret that the acknowledgement for the funding in the initially version of the article was incomplete and should read “All authors acknowledge the financial support from DFG SFB TR 103 through project A1, A2, A4, B7”. The authors would like to apologise for any inconvenience caused. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2020.06.039
  • Effect of aspect ratio on the deformation behavior of dislocation-free Ni3 Al nanocubes
    Li, P. and Wang, X. and Zhou, Y. and Pfetzing-Micklich, J. and Somsen, C. and Eggeler, G.
    Nanomaterials 10 (2020)
    This study concentrates on several factors which govern the nanoscale plasticity of in situ compressed dislocation-free Ni3 Al nanocubes: cube size, aspect ratio and the presence of grooves. The yield strength of dislocation-free Ni3 Al nanocubes exhibits an apparent size dependence. The size dependence is strong when cubes are smaller than 300 nm. Compared with the strength of bulk Ni3 Al single crystals, the strength of nanocubes is two orders of magnitude higher, which clearly demonstrates that there is a size effect. Nanocube plasticity strongly depends on the alignment and the shape of the cubes. Deformed aligned nanocubes either display only a few localized deformation events (slip lines) or were homogenously compressed into flats due to multiple slip dislocation-mediated plasticity. For an aligned cube, crack initiation at the intersection of a slip line with a groove in the cube surface was observed. In case of a double cube, crack initiation occurs at surface irregularities, while subsequent crack propagation occurs along one or more slip planes. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10112230
  • Effect of off-stoichiometric compositions on microstructures and phase transformation behavior in Ni-Cu-Pd-Ti-Zr-Hf high entropy shape memory alloys
    Piorunek, D. and Oluwabi, O. and Frenzel, J. and Kostka, A. and Maier, H.J. and Somsen, C. and Eggeler, G.
    Journal of Alloys and Compounds (2020)
    High entropy shape memory alloys (HE-SMAs) show reversible martensitic phase transformations at elevated temperatures. HE-SMAs were derived from binary NiTi, to which the elements Cu, Pd, Zr and Hf are added. They represent ordered complex solid solutions. Their high temperature phase is of B2 type, where the added elements occupy sites in the Ni-(Cu, Pd) and Ti-sub-lattices (Zr, Hf). In the present study, advanced microstructural and thermal characterization methods were used to study the effects of the additional alloy elements on microstructures and phase transformations. The ratios of Ni-equivalent (Ni, Cu, Pd) and Ti-equivalent (Ti, Zr, Hf) elements in HE-SMAs were varied to establish systems that correspond to stoichiometric, under- and over-stoichiometric binary alloys. It is shown that basic microstructural features of cast and heat-treated HE-SMAs are inherited from the nine binary X–Y subsystems (X: Ni, Cu, Pd; Y: Ti, Zr, Hf). The phase transition temperatures that characterize the martensitic forward and reverse transformations depend on the concentrations of all alloy elements. The data obtained demonstrate how martensite start temperatures are affected by deviations from the composition of an ideal stoichiometric B2 phase. The findings are discussed in the light of previous work on the concentration dependence of SMA transformation temperatures, and directions for the development of new shape memory alloy compositions are proposed. © 2020 The Authors
    view abstract10.1016/j.jallcom.2020.157467
  • Experimental and Theoretical Investigation on Phase Formation and Mechanical Properties in Cr-Co-Ni Alloys Processed Using a Novel Thin-Film Quenching Technique
    Naujoks, D. and Schneider, M. and Salomon, S. and Pfetzing-Micklich, J. and Subramanyam, A.P.A. and Hammerschmidt, T. and Drautz, R. and Frenzel, J. and Kostka, A. and Eggeler, G. and Laplanche, G. and Ludwig, Al.
    ACS Combinatorial Science 22 (2020)
    The Cr-Co-Ni system was studied by combining experimental and computational methods to investigate phase stability and mechanical properties. Thin-film materials libraries were prepared and quenched from high temperatures up to 700 °C using a novel quenching technique. It could be shown that a wide A1 solid solution region exists in the Cr-Co-Ni system. To validate the results obtained using thin-film materials libraries, bulk samples of selected compositions were prepared by arc melting, and the experimental data were additionally compared to results from DFT calculations. The computational results are in good agreement with the measured lattice parameters and elastic moduli. The lattice parameters increase with the addition of Co and Cr, with a more pronounced effect for the latter. The addition of ∼20 atom % Cr results in a similar hardening effect to that of the addition of ∼40 atom % Co. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acscombsci.9b00170
  • Exploring the fundamentals of Ni-based superalloy single crystal (SX) alloy design: Chemical composition vs. microstructure
    Horst, O.M. and Adler, D. and Git, P. and Wang, H. and Streitberger, J. and Holtkamp, M. and Jöns, N. and Singer, R.F. and Körner, C. and Eggeler, G.
    Materials and Design 195 (2020)
    The present work contributes to a better understanding of the basic assumptions and principles behind the design of Ni-based single crystal superalloys (SXs). For this purpose, we cast and heat-treat four Ni-based single crystal superalloys (SXs) and compare their creep performances: ERBO/1 (with Re) and three ERBO/15 variants (no Re but increased levels of Ti, Mo and W). We show that Re can be replaced by other elements without losing creep strength. To come to this conclusion one has to consider both, alloy composition and microstructure. We analyze the mechanical, microstructural and chemical results (creep rates, γʼ-volume fractions, average γʼ-particle sizes, average γ-channel widths and the chemistry of γ- and γʼ-phases) obtained for ERBO/15 and its two leaner variants (less Mo: ERBO/15-Mo and less W: ERBO/15-W). ERBO/15-Mo and ERBO/15-W show higher creep rates than ERBO/15, because they exhibit lower Mo and W concentrations in the γ-channels. This results in higher diffusion rates, accelerated rafting and faster dislocation climb at γ/γʼ-interfaces. © 2020 The Authors
    view abstract10.1016/j.matdes.2020.108976
  • How nanoscale dislocation reactions govern low-temperature and high-stress creep of ni-base single crystal superalloys
    Bürger, D. and Dlouhý, A. and Yoshimi, K. and Eggeler, G.
    Crystals 10 (2020)
    The present work investigates γ-channel dislocation reactions, which govern low-temperature (T = 750◦C) and high-stress (resolved shear stress: 300 MPa) creep of Ni-base single crystal superalloys (SX). It is well known that two dislocation families with different b-vectors are required to form planar faults, which can shear the ordered γ’-phase. However, so far, no direct mechanical and microstructural evidence has been presented which clearly proves the importance of these reactions. In the mechanical part of the present work, we perform shear creep tests and we compare the deformation behavior of two macroscopic crystallographic shear systems [011](111) and [112](111). These two shear systems share the same glide plane but differ in loading direction. The [112](111) shear system, where the two dislocation families required to form a planar fault ribbon experience the same resolved shear stresses, deforms significantly faster than the [011](111) shear system, where only one of the two required dislocation families is strongly promoted. Diffraction contrast transmission electron microscopy (TEM) analysis identifies the dislocation reactions, which rationalize this macroscopic behavior. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/cryst10020134
  • Interdiffusion in Cr–Fe–Co–Ni medium-entropy alloys
    Durand, A. and Peng, L. and Laplanche, G. and Morris, J.R. and George, E.P. and Eggeler, G.
    Intermetallics 122 (2020)
    Diffusion in multi-component alloys is attracting renewed attention because of the worldwide interest in high- and medium-entropy alloys (HEAs/MEAs). In the present work, we used diffusion multiples made from MEAs of the quaternary Cr–Fe–Co–Ni system arranged as six distinct pseudo-binary diffusion couples (Cr29Fe13Co29Ni29–Cr29Fe29Co29Ni13, Cr29Fe29Co13Ni29–Cr29Fe29Co29Ni13, and so on, where the interdiffusing elements are italicized for clarity). In the two halves of each couple, the starting concentrations of the interdiffusing elements (Fe,Ni and Co,Ni in the above examples) were different while those of the background elements (Cr,Co and Cr,Fe in the above examples) were the same. The diffusion multiples were annealed at temperatures between 1153 and 1355 K at times from 100 to 900 h, after which the concentrations of the different elements were measured as a function of distance across each couple. Interdiffusion coefficients were derived from such concentration profiles using the standard Sauer-Freise method and compared with literature data as well as with published tracer diffusion coefficients. Although the background elements were homogeneously distributed initially, some of them developed distinct sine-wave shaped concentration gradients near the interfaces after annealing, implying that uphill diffusion of these elements had occurred. We show using a kinetic model for substitutional diffusion via vacancy hopping that such uphill diffusion can occur even in the absence of thermodynamic interactions, i.e. in ideal solid solutions in which the thermodynamic factor Φ of each element is equal to one (Φi=1+∂lnfi/∂lnci where fi and ci are the activity coefficient and mole fraction of element i, respectively). The model accounts for all elemental fluxes and also rationalizes the diffusion profiles of the major interdiffusing elements. © 2020
    view abstract10.1016/j.intermet.2020.106789
  • On the Influence of Alloy Composition on Creep Behavior of Ni-Based Single-Crystal Superalloys (SXs)
    Horst, O.M. and Ibrahimkhel, S. and Streitberger, J. and Wochmjakow, N. and Git, P. and Scholz, F. and Thome, P. and Singer, R.F. and Körner, C. and Frenzel, J. and  Eggeler, G.
    Minerals, Metals and Materials Series (2020)
    In the present work, three Ni-based single-crystal superalloys (SXs) were investigated, a Re-containing alloy ERBO/1 (CMSX-4 type) and two Re-free SXs referred to as ERBO/15 and ERBO/15-W, which differ in W content. The microstructural evolution of the three alloys during heat treatment and their creep behavior is investigated. When one applies one heat treatment to all three alloys, one obtains different γ/γ′-microstructures. Subjecting these three alloys to creep in the high-temperature low-stress creep regime, ERBO/15 outperforms ERBO/1. In order to separate the effects of alloy chemistry and microstructure, the kinetics of the microstructural evolution of the three alloys was measured. The results were used to establish similar microstructures in all three alloys. Comparing ERBO/15 with ERBO/15-W, it was found that in ERBO/15-W particles grow faster during the first precipitation heat treatment and that ERBO/15-W creeps significantly faster. At constant microstructures, ERBO/15 and ERBO/1 show similar creep behavior. In the high-temperature and low-stress creep regime, ERBO/15 shows lower minimum creep rates but ERBO/1 features a slower increase of creep rate in the tertiary creep regime. It was also found that in the high-temperature low-stress creep regime, ERBO/1 shows a double minimum creep behavior when particles are small. © 2020, The Minerals, Metals & Materials Society.
    view abstract10.1007/978-3-030-51834-9_6
  • On the influence of crystallography on creep of circular notched single crystal superalloy specimens
    Cao, L. and Thome, P. and Agudo Jácome, L. and Somsen, C. and Cailletaud, G. and Eggeler, G.
    Materials Science and Engineering A 782 (2020)
    The present work contributes to a better understanding of the effect of stress multiaxiality on the creep behavior of single crystal Ni-base superalloys. For this purpose we studied the creep deformation and rupture behavior of double notched miniature creep tensile specimens loaded in three crystallographic directions [100], [110] and [111] (creep conditions: 950 °C and 400 MPa net section stress). Crystal plasticity finite element method (CPFEM) was used to analyze the creep stress and strain distributions during creep. Double notched specimens have the advantage that when one notch fails, the other is still intact and allows to study a material state which is close to rupture. No notch root cracking was observed, while microstructural damage (pores and micro cracks) were frequently observed in the center of the notch root region. This is in agreement with the FEM results (high axial stress and high hydrostatic stress in the center of the notched specimen). Twinning was observed in the notch regions of [110] and [111] specimens, and <112> {111} twins were detected and analyzed using orientation imaging scanning electron microscopy. The present work shows that high lattice rotations can be detected in SXs after creep fracture, but they are associated with the high strains accumulated in the final rupture event. © 2020 The Authors
    view abstract10.1016/j.msea.2020.139255
  • On the rhenium segregation at the low angle grain boundary in a single crystal Ni-base superalloy
    He, J. and Scholz, F. and Horst, O.M. and Thome, P. and Frenzel, J. and Eggeler, G. and Gault, B.
    Scripta Materialia 185 (2020)
    Industrial scale single crystal (SX) Ni-base superalloys contain numerous low angle grain boundaries inherited from the solidification process. Here, we demonstrate that low angle grain boundaries in a fully heat-treated SX model Ni-base superalloy are strongly segregated with up to 12 at% Re. Some Re-rich dislocations forming this grain boundary are found located inside γ, others close to a γ/γ′ interface. Although these segregated Re atoms lose their solid-solution strengthening effect, they may enhance the creep resistance by pinning the low angle grain boundaries and slowing down dislocation reactions. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2020.03.063
  • On the stress and temperature dependence of low temperature and high stress shear creep in Ni-base single crystal superalloys
    Bürger, D. and Dlouhý, A. and Yoshimi, K. and Eggeler, G.
    Materials Science and Engineering A 795 (2020)
    In the present work, we investigate the stress and temperature dependence of low-temperature (750 ± 20 °C) and high-stress (300 ± 20 MPa) shear creep of a Ni-base single crystal superalloy. From continuous isothermal experiments and stress and temperature change tests the stress exponent n and the apparent activation energy Qapp of the phenomenological Sherby-Dorn equation were determined for the two macroscopic crystallographic shear systems (MCSS) [011¯](111) and [112¯](111). The activation parameters of creep, the stress exponents and the apparent activation energies were identified as 16 and 620 kJ/mol (MCSS: [011¯](111)) and 14 and 460 kJ/mol (MCSS: [112¯](111)). We show that during shear creep testing these phenomenological parameters do not change between the early (0.5–1% strain) and later stages of creep (4.5–5% strain), in contrast to what was observed for uniaxial tensile testing. The results are discussed in the light of what is known about stress and temperature dependencies of deformation rates in the creep literature and in view of the recent work by Bürger et al., 2020, who combined shear creep testing with analytical transmission electron microscopy to identify the elementary deformation mechanism, which governs low temperature and high stress creep. © 2020 The Authors
    view abstract10.1016/j.msea.2020.139961
  • Pattern-forming nanoprecipitates in NiTi-related high entropy shape memory alloys
    Hinte, C. and Barienti, K. and Steinbrücker, J. and Gerstein, G. and Swider, M.A. and Herbst, S. and Eggeler, G. and Maier, H.J.
    Scripta Materialia 186 (2020)
    The microstructure and the fracture behavior of TiZrHfCoNiCu high entropy shape memory alloys with two different compositions were investigated. An unusual microstructure featuring pattern-forming nanoprecipitates was observed in dendritic and the interdendritic regions of both alloys. The unique higher-level order of these precipitates does not follow concentration gradients but is influenced by homogeneity and mechanical stress. The results also demonstrate that high entropy alloys are not necessarily homogeneous single-phase solid solutions. Moreover, it appears that solid solution strengthening as the primary mechanism also has to be questioned. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2020.05.007
  • Processing of a single-crystalline CrCoNi medium-entropy alloy and evolution of its thermal expansion and elastic stiffness coefficients with temperature
    Laplanche, G. and Schneider, M. and Scholz, F. and Frenzel, J. and Eggeler, G. and Schreuer, J.
    Scripta Materialia 177 (2020)
    The equiatomic CrCoNi alloy is regarded as a model single-phase face-centered cubic medium-entropy alloy. A CrCoNi single crystal was grown by a Bridgman technique using a Ni-base superalloy seed. The elastic stiffnesses and thermal expansion coefficient were determined between 100 K and 673 K employing resonant ultrasound spectroscopy and dilatometry, respectively. All data were found to be in excellent agreement with those reported for polycrystalline CrCoNi. A comparison of the normalized Cauchy pressure of CrCoNi with those of other alloys indicates that interatomic bonds become more directional with increasing Cr-concentration while Co and Ni promote a metallic character. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2019.09.020
  • Publisher Correction: Unveiling the Re effect in Ni-based single crystal superalloys (Nature Communications, (2020), 11, 1, (389), 10.1038/s41467-019-14062-9)
    Wu, X. and Makineni, S.K. and Liebscher, C.H. and Dehm, G. and Mianroodi, J.R. and Shanthraj, P. and Svendsen, B. and Bürger, D. and Eggeler, G. and Raabe, D. and Gault, B.
    Nature Communications 11 (2020)
    The original version of this Article contained an error in Fig. 2h, in which the left-side label was given incorrectly as ‘(b3):SISF’, rather than the correct ‘(i):SISF’, and the right-side label was given incorrectly as ‘(c3):SESF’, rather than the correct ‘(j):SESF’. The original version of this Article also contained an error in Fig. 8, in which all three x-axis tick marks were incorrectly labelled ‘200, 250, 300’. The correct x-axis tick marks read ’20, 25, 30’. These errors have now been corrected in both the PDF and the HTML versions of the Article © 2020, The Author(s).
    view abstract10.1038/s41467-020-14820-0
  • Revealing the two-step nucleation and growth mechanism of vanadium carbonitrides in microalloyed steels
    Wang, H. and Li, Y. and Detemple, E. and Eggeler, G.
    Scripta Materialia 187 (2020)
    Combining high-resolution transmission electron microscopy (HR-TEM) and 3-dimensional atom probe tomography (3D-APT), the early stages of nucleation and growth of vanadium carbonitrides (VCN) were revealed. VCN nucleation starts with locally distorted body-centered cubic (bcc) lattices due to a substitution of Fe atoms by V atoms, which results in the formation of an intermediate coherent crystal structure within the ferrite matrix. Misfit strain self-accommodation leads to twining within the VCN particles. As the particles grow, the precipitates gradually lose coherency and grow into discs or plates. Simultaneously, the intermediate crystal structure of the nucleus transforms into the equilibrium VCN-structure. © 2020
    view abstract10.1016/j.scriptamat.2020.06.041
  • Unveiling the Re effect in Ni-based single crystal superalloys
    Wu, X. and Makineni, S.K. and Liebscher, C.H. and Dehm, G. and Rezaei Mianroodi, J. and Shanthraj, P. and Svendsen, B. and Bürger, D. and Eggeler, G. and Raabe, D. and Gault, B.
    Nature Communications 11 (2020)
    Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called “Re effect” have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO2 emissions in air-traffic. © 2020, The Author(s).
    view abstract10.1038/s41467-019-14062-9
  • A Kinetic Study on the Evolution of Martensitic Transformation Behavior and Microstructures in Ti–Ta High-Temperature Shape-Memory Alloys During Aging
    Paulsen, A. and Frenzel, J. and Langenkämper, D. and Rynko, R. and Kadletz, P. and Grossmann, L. and Schmahl, W.W. and Somsen, C. and Eggeler, G.
    Shape Memory and Superelasticity 5 (2019)
    Ti–Ta alloys represent candidate materials for high-temperature shape-memory alloys (HTSMAs). They outperform several other types of HTSMAs in terms of cost, ductility, and cold workability. However, Ti–Ta alloys are characterized by a relatively fast microstructural degradation during exposure to elevated temperatures, which gives rise to functional fatigue. In the present study, we investigate how isothermal aging affects the martensitic transformation behavior and microstructures in Ti70Ta30 HTSMAs. Ti–Ta sheets with fully recrystallized grain structures were obtained from a processing route involving arc melting, heat treatments, and rolling. The final Ti–Ta sheets were subjected to an extensive aging heat treatment program. Differential scanning calorimetry and various microstructural characterization techniques such as scanning electron microscopy, transmission electron microscopy, conventional X-ray, and synchrotron diffraction were used for the characterization of resulting material states. We identify different types of microstructural evolution processes and their effects on the martensitic and reverse transformation. Based on these results, an isothermal time temperature transformation (TTT) diagram for Ti70Ta30 was established. This TTT plot rationalizes the dominating microstructural evolution processes and related kinetics. In the present work, we also discuss possible options to slow down microstructural and functional degradation in Ti–Ta HTSMAs. © 2018, ASM International.
    view abstract10.1007/s40830-018-00200-7
  • Benchmark dataset of the effect of grain size on strength in the single-phase FCC CrCoNi medium entropy alloy
    Schneider, M. and George, E.P. and Manescau, T.J. and Záležák, T. and Hunfeld, J. and Dlouhý, A. and Eggeler, G. and Laplanche, G.
    Data in Brief 27 (2019)
    This data article presents the microstructural data as well as the mechanical properties of the CrCoNi medium-entropy alloy (MEA). The data presented in this article are related to the research article entitled “Analysis of strengthening due to grain boundaries and annealing twin boundaries in the CrCoNi medium-entropy alloy”, see Ref. Schneider et al., 2019. This article can be referred to for the analysis and interpretation of the data, as well as their comparison to other datasets in literature. Microstructural data available in the present paper are backscattered electron micrographs for sixteen different grain sizes. Also available are pdf reports of grain size analysis (annealing twin boundaries were neglected) and crystallite sizes (including annealing twin boundaries) as well as data describing the number of annealing twin boundaries per grain (n), corresponding Taylor factors (M) and average annealing twin thicknesses (t). Additionally, raw data of stress-strain curves at five different temperatures [77 K, 293 K, 473 K, 673 K and 873 K] are given for all sixteen grain sizes, which may be used for further research, e.g. data mining, machine learning and other analytical methods. Mechanical data such as yield stresses (σ0.2%), Hall-Petch parameters (σ0 and ky) and critical boundary strengths (τc) are provided along with a 1D discrete dislocation dynamics (1-D DDD) simulation results concerning the different boundary strengths. © 2019 The Author(s)
    view abstract10.1016/j.dib.2019.104592
  • Creep properties of single crystal Ni-base superalloys (SX): A comparison between conventionally cast and additive manufactured CMSX-4 materials
    Bürger, D. and Parsa, A.B. and Ramsperger, M. and Körner, C. and Eggeler, G.
    Materials Science and Engineering A 762 (2019)
    The present work compares the microstructures and the creep properties of two types of single crystal Ni-base superalloy CMSX-4 materials (SXs). One was produced by conventional directional solidification Bridgman processing. The other was manufactured by selective electron beam melting (SEBM). The microstructures of the two types of materials are compared with emphasis placed on the large (dendritic/interdendritic regions) and small scale (γ-matrix/γ′-precipitates) microstructural heterogeneities, which characterize SX microstructures and their evolution during processing, heat treatment and creep. It is shown that heat treated SEBM materials have creep properties, which match or even outperform those of conventionally processed SX materials. Creep properties were assessed using a miniature creep test technique where [001] miniature tensile creep specimens were tested in the high temperature/low stress (1050 °C, 160 MPa) and in the low temperature/high stress (850 °C, 600 MPa) creep regimes. The creep behavior is interpreted based on microstructural results, which were obtained using analytical scanning and transmission electron microscopy (SEM and TEM). © 2019 The Authors
    view abstract10.1016/j.msea.2019.138098
  • Discovery of ω -free high-temperature Ti-Ta- X shape memory alloys from first-principles calculations
    Ferrari, A. and Paulsen, A. and Langenkämper, D. and Piorunek, D. and Somsen, C. and Frenzel, J. and Rogal, J. and Eggeler, G. and Drautz, R.
    Physical Review Materials 3 (2019)
    The rapid degradation of the functional properties of many Ti-based alloys is due to the precipitation of the ω phase. In the conventional high-temperature shape memory alloy Ti-Ta, the formation of this phase compromises completely the shape memory effect, and high (>100°C) transformation temperatures cannot be maintained during cycling. A solution to this problem is the addition of other elements to form Ti-Ta-X alloys, which often modifies the transformation temperatures; due to the largely unexplored space of possible compositions, very few elements are known to stabilize the shape memory effect without decreasing the transformation temperatures below 100°C. In this study, we use transparent descriptors derived from first-principles calculations to search for new ternary Ti-Ta-X alloys that combine stability and high temperatures. We suggest four alloys with these properties, namely Ti-Ta-Sb, Ti-Ta-Bi, Ti-Ta-In, and Ti-Ta-Sc. Our predictions for the most promising of these alloys, Ti-Ta-Sc, are subsequently fully validated by experimental investigations, the alloy Ti-Ta-Sc showing no traces of ω phase after cycling. Our computational strategy is transferable to other materials and may contribute to suppress ω phase formation in a large class of alloys. ©2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.103605
  • Effect of Nb on improving the impact toughness of Mo-containing low-alloyed steels
    Wang, H.C. and Somsen, C. and Li, Y.J. and Fries, S.G. and Detemple, E. and Eggeler, G.
    Journal of Materials Science 54 (2019)
    The microalloying of low-alloyed steels with Nb can improve the strength-to-toughness balance. Such an effect of Nb is usually ascribed to the refinement of the grain structure occurring in the austenite regime during hot forming. In the present work, we report that Nb enhances the impact toughness of a low-alloyed Cr–Mo steel by a mechanism which has not been appreciated so far. The lower impact toughness in the Nb-free Cr–Mo steel is due to segregation of Mo to boundaries, which facilitates the formation of fine Mo-rich ξ-phase carbides lining up along the boundaries. This further promotes the nucleation and propagation of microcracks. The addition of Nb leads to the formation of Mo-enriched NbC particles. The interfaces between these particles and the matrix supply new preferential sites for precipitation of Mo-rich ξ-phase carbides upon subsequent tempering. In this way, Nb additions result in a decrease of Mo segregation to boundaries, significantly reducing the precipitation of ξ-phase carbides on grain boundaries, thus leading to improved impact toughness. In addition to the classical microstructural explanation (grain size effect), this chemical role of Nb sheds new light on the design strategies of advanced low-alloyed steels with optimized strength-to-toughness ratios. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s10853-019-03374-2
  • Impact of Heating–Cooling Rates on the Functional Properties of Ti–20Ta–5Al High-Temperature Shape Memory Alloys
    Krooß, P. and Lauhoff, C. and Langenkämper, D. and Paulsen, A. and Reul, A. and Degener, S. and Aminforoughi, B. and Frenzel, J. and Somsen, C. and Schmahl, W.W. and Eggeler, G. and Maier, H.J. and Niendorf, T.
    Shape Memory and Superelasticity 5 (2019)
    Due to their ability to provide a shape memory effect at elevated temperatures, high-temperature shape memory alloys (HT-SMAs) came into focus of academia and industry in the last decades. Ternary and quaternary Ni–Ti-based HT-SMAs have been in focus of a large number of studies so far. Ti–Ta HT-SMAs feature attractive shape memory properties along with significantly higher ductility and lower costs for alloying elements compared to conventional Ni–Ti-based HT-SMAs, which qualifies them as promising candidate alloys for high-temperature applications. Unfortunately, precipitation of undesired phases, e.g., the ω-phase, leads to significant functional degradation upon cyclic loading in binary Ti–Ta. Therefore, additions of ternary elements, such as Al, which suppress the ω-phase formation, are important. In the present study, the influence of different heating–cooling rates on the cyclic functional properties of a Ti–20Ta–5Al HT-SMA is investigated. Transmission electron microscopy as well as in situ synchrotron analysis revealed unexpected degradation mechanisms in the novel alloy studied. Elementary microstructural mechanisms leading to a degradation of the functional properties were identified, and the ramifications with respect to application of Ti–Ta–Al HT-SMAs are discussed. © 2019, ASM International.
    view abstract10.1007/s40830-019-00207-8
  • Microstructure – Property correlations for additively manufactured NiTi based shape memory alloys
    Kumar S, S. and Marandi, L. and Balla, V.K. and Bysakh, S. and Piorunek, D. and Eggeler, G. and Das, M. and Sen, I.
    Materialia 8 (2019)
    The research focuses on systematically studying the effect of processing parameters in controlling the microstructure and mechanical behavior including pseudoelasticity of a NiTi alloy. The alloy is prepared by laser engineered net shaping (LENS) based additive manufacturing technique. Laser energy densities of the manufacturing method are modified by altering laser scan speed and power. Corresponding effects on microstructure and phase evolution of NiTi alloy are evaluated thoroughly. Subsequently, mechanical properties are assessed implementing macro-, micro- and nano-indentations with load levels varying over five orders of magnitude. Layered microstructures, as a signature of the manufacturing process, are evident from the front and side planes of the products. Interestingly, existence of trace volume fraction of precipitates and martensite phase are noted in the alloy manufactured with highest laser energy density. These influence hardness, elastic modulus and indentation size effect of the alloy. Most importantly, pseudoelastic recovery of NiTi gets adversely affected. Spherical-nanoindentation is performed to precisely assess pseudoelasticity of NiTi alloys. Indentation-stress–strain graphs are generated from the spherical-nanoindentation. This is a unique and state-of-the-art way to assess the localized mechanical performance of materials. Considering that additive manufacturing is associated with microstructural inhomogeneity thereby leading to property variation along different regions, investigation is corroborated with bulk-scale tensile testing of the alloys, revealing similar results. Subsequent cyclic tensile tests complemented with digital image correlation are performed to appreciate their strain-recoverabilities. This study provides a scope to optimize the parameters of LENS to manufacture NiTi with the best combination of microstructure, phase stability and pseudoelasticity. © 2019 Elsevier Ltd
    view abstract10.1016/j.mtla.2019.100456
  • Ni-base superalloy single crystal (SX) mosaicity characterized by the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method
    Thome, P. and Medghalchi, S. and Frenzel, J. and Schreuer, J. and Eggeler, G.
    Ultramicroscopy 206 (2019)
    In the present work we present the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method, a new correlative orientation imaging method for scanning electron microscopy (OIM/SEM). The RVB-EBSD method was developed to study crystal mosaicity in as-cast Ni-base superalloy single crystals (SX). The technique allows to quantify small crystallographic deviation angles between individual dendrites and to interpret associated accommodation processes in terms of geometrically necessary dislocations (GNDs). The RVB-EBSD method was inspired by previous seminal approaches which use cross correlation EBSD procedures. It applies Gaussian band pass filtering to improve the quality of more than 500 000 experimental patterns. A rotation vector approximation and a correction procedure, which relies on a base line function, are used. The method moreover features a novel way of intuitive color coding which allows to easily appreciate essential features of crystal mosaicity. The present work describes the key elements of the method and shows examples which demonstrate its potential. © 2019 Elsevier B.V.
    view abstract10.1016/j.ultramic.2019.112817
  • On crystal mosaicity in single crystal Ni-based superalloys
    Hallensleben, P. and Scholz, F. and Thome, P. and Schaar, H. and Steinbach, I. and Eggeler, G. and Frenzel, J.
    Crystals 9 (2019)
    In the present work, we investigate the evolution of mosaicity during seeded Bridgman processing of technical Ni-based single crystal superalloys (SXs). For this purpose, we combine solidification experiments performed at different withdrawal rates between 45 and 720 mm/h with advanced optical microscopy and quantitative image analysis. The results obtained in the present work suggest that crystal mosaicity represents an inherent feature of SXs, which is related to elementary stochastic processes which govern dendritic solidification. In SXs, mosaicity is related to two factors: inherited mosaicity of the seed crystal and dendrite deformation. Individual SXs have unique mosaicity fingerprints. Most crystals differ in this respect, even when they were produced using identical processing conditions. Small differences in the orientation spread of the seed crystals and small stochastic orientation deviations continuously accumulate during dendritic solidification. Direct evidence for dendrite bending in a seeded Bridgman growth process is provided. It was observed that continuous or sudden bending affects the growth directions of dendrites. We provide evidence which shows that some dendrites continuously bend by 1.7° over a solidification distance of 25 mm. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/cryst9030149
  • 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 (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 abstract10.1016/j.msea.2019.05.087
  • On the evolution of dislocation cell structures in two Al-alloys (Al-5Mg and Al-11Zn) during reciprocal sliding wear at high homologous temperatures
    Parsa, A.B. and Walter, M. and Theisen, W. and Bürger, D. and Eggeler, G.
    Wear (2019)
    The formation of dislocation substructures in up to 10 µm deep subsurface regions of two aluminium alloys, Al-5Mg and Al-11Zn, was investigated under conditions of high homologous temperature reciprocal sliding wear (HT-RSW). Under creep conditions, Al-5Mg shows a solid solution type of inverse primary creep. In contrast, Al-11Zn creeps obstacle controlled and exhibits normal primary creep. These two materials were subjected to reciprocal sliding wear at 200 and 300 °C for 100 and 1000 cycles. Flat polished disks were exposed to the 1 mm reciprocal movements of a spherical aluminium oxide counterbody under normal forces of 5 and 10 N at an oscillation frequency of 1 Hz. Using focused ion beam (FIB) micromachining thin electron transparent foils were prepared from the surface regions of the as received and worn material states. Transmission electron microscopy (TEM) was used to study the evolution of nano and micro grain sizes in the surface regions. Despite the different creep behavior, the two materials behave similar under conditions of reciprocal sliding wear. The results obtained in the present work show that subgrain sizes decrease with increasing numbers of wear cycles and increasing normal forces. Subgrain sizes also increase with increasing temperature. At 300 °C, dynamic recrystallization was observed in both Al-alloys. The results of the present work are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. © 2018 Elsevier B.V.
    view abstract10.1016/j.wear.2018.10.018
  • On the Oxidation Behavior and Its Influence on the Martensitic Transformation of Ti–Ta High-Temperature Shape Memory Alloys
    Langenkämper, D. and Paulsen, A. and Somsen, C. and Frenzel, J. and Eggeler, G.
    Shape Memory and Superelasticity 5 (2019)
    In the present work, the influence of oxidation on the martensitic transformation in Ti–Ta high-temperature shape memory alloys is investigated. Thermogravimetric analysis in combination with microstructural investigations by scanning electron microscopy and transmission electron microscopy were performed after oxidation at 850 °C and at temperatures in the application regime of 450 °C and 330 °C for 100 h, respectively. At 850 °C, internal oxidation results in the formation of a mixed layered scale of TiO2 and β-Ta2O5, associated with decomposition into Ta-rich bcc β-phase and Ti-rich hexagonal α-phase in the alloy. This leads to a suppression of the martensitic phase transformation. In addition, energy dispersive X-ray analysis suggests an oxygen stabilization of the α-phase. At 450 °C, a slow decomposition into Ta-rich β-phase and Ti-rich α-phase is observed. After oxidation at 330 °C, the austenitic matrix shows strong precipitation of the ω-phase that suppresses the martensitic transformation on cooling. © 2019, ASM International.
    view abstract10.1007/s40830-018-00206-1
  • On the rejuvenation of crept Ni-Base single crystal superalloys (SX)by hot isostatic pressing (HIP)
    Horst, O.M. and Ruttert, B. and Bürger, D. and Heep, L. and Wang, H. and Dlouhý, A. and Theisen, W. and Eggeler, G.
    Materials Science and Engineering A 758 (2019)
    In the present work, we study the effect of HIP rejuvenation treatments on the creep behavior and residual life of a pre-crept single crystal Ni-base superalloy of type CMSX-4. The present work combines miniature creep experiments of precisely oriented [001]tensile creep specimens with HIP treatments and quantitative analysis of scanning and transmission electron micrographs. A HIP-rejuvenation treatment after 5.0% creep pre-strain significantly improves creep strength. However, the microstructural damage induced by the creep pre-deformation could not be fully removed. In a series of sequential creep/HIP/creep-experiments, increasing levels of pre-deformation result in increasing levels of creep rates even after identical HIP-rejuvenation treatments. The memory effect, which causes this phenomenon, is related to an accumulation of permanent microstructural damage, which is not associated with rafting or cavitation. The mechanical results obtained in the present work are interpreted based on microstructural results on the γ/γʼ-microstructure (γ-channel widths and γʼ-size), on the pore population (number density of pores, pore size distributions and pore area fractions)and dislocation substructures which have formed during creep. The results are discussed in the light of previous results reported in the literature. © 2019 The Authors
    view abstract10.1016/j.msea.2019.04.078
  • Repair of Ni-based single-crystal superalloys using vacuum plasma spray
    Kalfhaus, T. and Schneider, M. and Ruttert, B. and Sebold, D. and Hammerschmidt, T. and Frenzel, J. and Drautz, R. and Theisen, W. and Eggeler, G. and Guillon, O. and Vassen, R.
    Materials and Design 168 (2019)
    Turbine blades in aviation engines and land based gas-turbines are exposed to extreme environments. They suffer damage accumulation associated with creep, oxidation and fatigue loading. Therefore, advanced repair methods are of special interest for the gas-turbine industry. In this study, CMSX-4 powder is sprayed by Vacuum Plasma Spray (VPS) on single-crystalline substrates with similar compositions. The influence of the substrate temperature is investigated altering the temperature of the heating stage between 850 °C to 1000 °C. Different spray parameters were explored to identify their influence on the microstructure. Hot isostatic pressing (HIP) featuring fast quenching rates was used to minimize porosity and to allow for well-defined heat-treatments of the coatings. The microstructure was analysed by orientation imaging scanning electron microscopy (SEM), using electron backscatter diffraction (EBSD). The effects of different processing parameters were analysed regarding their influence on porosity and grain size. The results show that optimized HIP heat-treatments can lead to dense coatings with optimum γ/γ′ microstructure. The interface between the coating and the substrate is oxide free and shows good mechanical integrity. The formation of fine crystalline regions as a result of fast cooling was observed at the single-crystal surface, which resulted in grain growth during heat-treatment in orientations determined by the crystallography of the substrate. © 2019
    view abstract10.1016/j.matdes.2019.107656
  • Stress-induced formation of TCP phases during high temperature low cycle fatigue loading of the single-crystal Ni-base superalloy ERBO/1
    Meid, C. and Eggeler, M. and Watermeyer, P. and Kostka, A. and Hammerschmidt, T. and Drautz, R. and Eggeler, G. and Bartsch, M.
    Acta Materialia 168 (2019)
    The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950 °C (load-ratio: 0.6, tensile stress range: 420–740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 1000 - 3000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only μ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect, the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary μ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.02.022
  • Tension/Compression asymmetry of a creep deformed single crystal Co-base superalloy
    Lenz, M. and Eggeler, Y.M. and Müller, J. and Zenk, C.H. and Volz, N. and Wollgramm, P. and Eggeler, G. and Neumeier, S. and Göken, M. and Spiecker, E.
    Acta Materialia 166 (2019)
    The creep behavior of a multinary single crystal Co-base superalloy has been compared for uniaxial tension and compression of 400 MPa applied along [001] at 850 °C. Creep under tensile stress proceeds two times faster than creep under compression. A detailed TEM study shows that already after ∼0.3 % creep strain planar faults are formed in both samples. While extended a/2<112> ribbons with SISF loops embedded in APBs are observed in tension, extrinsic SFs are revealed in compression. At ∼5 % creep strain SISFs confined to the γ′ phase dominate in tension, whereas extrinsic SFs and microtwins extending across both phases are the prevalent planar faults in compression. In addition, dense networks of regular a/2<101> matrix dislocations develop at the γ/γ′ interfaces in both loading scenarios. In tensile creep and early compressive creep the direct contribution of planar faults to plastic deformation is minor and does not exceed 10 % of the measured plastic strain. In contrast, thickening of microtwins appears to become an efficient deformation channel in the later stages of compressive creep. A pronounced asymmetry regarding the rafting kinetics is observed resulting in a P-type rafted and topologically inverted microstructure after ∼5 % creep in tension while hardly any rafting has occurred under compression. The pronounced rafting and related recovery processes are likely responsible for the inferior creep behavior in tension. Finally, two novel diffusion-assisted degradation mechanisms related to microtwins are shown to be active: an expansion of the γ phase into γ′ precipitates along microtwins and the formation of γ phase nuclei at planar fault intersections inside γ′. Both phenomena are hypothesized to be triggered by segregation of γ formers like Co and Cr to planar faults. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.12.053
  • A phenomenological creep model for nickel-base single crystal superalloys at intermediate temperatures
    Gao, S. and Wollgramm, P. and Eggeler, G. and Ma, A. and Schreuer, J. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 26 (2018)
    For the purpose of good reproduction and prediction of creep deformation of nickel-base single crystal superalloys at intermediate temperatures, a phenomenological creep model is developed, which accounts for the typical γ/γ′ microstructure and the individual thermally activated elementary deformation processes in different phases. The internal stresses from γ/γ′ lattice mismatch and deformation heterogeneity are introduced through an efficient method. The strain hardening, the Orowan stress, the softening effect due to dislocation climb along γ/γ′ interfaces and the formation of dislocation ribbons, and the Kear-Wilsdorf-lock effect as key factors in the main flow rules are formulated properly. By taking the cube slip in slip systems and twinning mechanisms into account, the creep behavior for [110] and [111] loading directions are well captured. Without specific interaction and evolution of dislocations, the simulations of this model achieve a good agreement with experimental creep results and reproduce temperature, stress and crystallographic orientation dependences. It can also be used as the constitutive relation at material points in finite element calculations with complex boundary conditions in various components of superalloys to predict creep behavior and local stress distributions. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-651X/aabdbe
  • A TEM Investigation of Columnar-Structured Thermal Barrier Coatings Deposited by Plasma Spray-Physical Vapor Deposition (PS-PVD)
    Rezanka, S. and Somsen, C. and Eggeler, G. and Mauer, G. and Vaßen, R. and Guillon, O.
    Plasma Chemistry and Plasma Processing 38 (2018)
    The plasma spray-physical vapor deposition technique (PS-PVD) is used to deposit various types of ceramic coatings. Due to the low operating pressure and high enthalpy transfer to the feedstock, deposition from the vapor phase is very effective. The particular process conditions allow for the deposition of columnar microstructures when applying thermal barrier coatings (TBCs). These coatings show a high strain tolerance similar to those obtained by electron beam-physical vapor deposition (EB-PVD). But compared to EB-PVD, PS-PVD allows significantly reducing process time and costs. The application-related properties of PS-PVD TBCs have been investigated in earlier work, where the high potential of the process was described and where the good resistance to thermo-mechanical loading conditions was reported. But until now, the elementary mechanisms which govern the material deposition have not been fully understood and it is not clear, how the columnar structure is built up. Shadowing effects and diffusion processes are assumed to contribute to the formation of columnar microstructures in classical PVD processing routes. For such structures, crystallographic textures are characteristic. For PS-PVD, however, no crystallographic textures could initially be found using X-ray diffraction. In this work a more detailed TEM investigations and further XRD measurements of the columnar PS-PVD microstructure were performed. The smallest build units of the columnar TBC structure are referred to as sub-columns. The observed semi-single crystal structure of individual sub-columns was analyzed by means of diffraction experiments. The absence of texture in PS-PVD coatings is confirmed and elementary nucleation and growth mechanisms are discussed. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s11090-018-9898-y
  • Carbide types in an advanced microalloyed bainitic/ferritic Cr–Mo Steel – TEM observations and thermodynamic calculations [Karbide in einem mikrolegierten bainitisch-ferritischen Cr–Mo-Stahl – TEM Charakterisierung und thermodynamische Berechnungen]
    Wang, H. and Somsen, C. and Eggeler, G. and Detemple, E.
    Materialwissenschaft und Werkstofftechnik 49 (2018)
    The strength and toughness of low alloyed ferritic/bainitic steels depend on their microstructure, which evolves during thermo-mechanical treatments along the processing chain. Chromium-molybdenum steel microstructures are complex. Therefore, only a limited number of attempts have been made to fully characterize carbide populations in such steels. In the present work, analytical transmission electron microscopy is employed to study the microstructure of a low alloyed chromium-molybdenum steel, which features ferritic (F, mainly α-iron and niobium-carbides) and bainitic (B, α-phase, dislocation, grain/subgrain boundaries, various MxCy carbides) regions. The crystal structure and chemical nature of more than 200 carbides are determined and their distributions in the two microstructural regions are analyzed. The present work shows how particles can be identified in an effective manner and how the microstructural findings can be interpreted on the basis of thermodynamic calculations. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201700186
  • High-performance elastocaloric materials for the engineering of bulk- and micro-cooling devices
    Frenzel, J. and Eggeler, G. and Quandt, E. and Seelecke, S. and Kohl, M.
    MRS Bulletin 43 (2018)
    Pseudoelastic NiTi-based shape-memory alloys (SMAs) have recently received attention as candidate materials for solid-state refrigeration. The elastocaloric effect in SMAs exploits stress-induced martensitic transformation, which is associated with large latent heat. Most importantly, cyclic mechanical loading/unloading provides large adiabatic temperature drops exceeding 25 K at high process efficiencies. This article summarizes the underlying principles, important material parameters and process requirements, and reviews recent progress in the development of pseudoelastic SMAs with large coefficients of performance, as well as very good functional fatigue resistance. The application potential of SMA film and bulk materials is demonstrated for the case of cyclic tensile loading/unloading in prototypes ranging from miniature-scale devices to large-scale cooling units. Copyright © Materials Research Society 2018.
    view abstract10.1557/mrs.2018.67
  • How evolving multiaxial stress states affect the kinetics of rafting during creep of single crystal Ni-base superalloys
    Cao, L. and Wollgramm, P. and Bürger, D. and Kostka, A. and Cailletaud, G. and Eggeler, G.
    Acta Materialia 158 (2018)
    Miniature tensile creep specimens are used to investigate the effect of mild circular notches on microstructural evolution during [001] tensile creep of a Ni-base single crystal superalloy. Creep deformed material states from a uniaxial (950 °C, uniaxial stress: 300 MPa) and a circular notched creep specimen (950 °C, net section stress in notch root: 300 MPa) are compared. For both types of tests, creep experiments were interrupted after 81, 169 and 306 h. Quantitative scanning electron microscopy (SEM) is used to assess the evolution of the γ/γ′-microstructure from rafting to topological inversion. Scanning transmission electron microscopy (STEM) was applied to study the evolution of dislocation densities during creep. As a striking new result it is shown that in circular notched specimen, the microstructural evolution is well coupled to the kinetics of the stress redistribution during creep. Rafting, the directional coarsening of the γ′-phase, and the increase of γ-channel dislocation density, start in the notch root before the center of the specimen is affected. When stresses in the circular notched specimens are fully redistributed, the microstructural differences between the notch root and the center of the circular notched specimen disappear. The comparison of the mechanical data and the microstructural findings in uniaxial and circular notched specimens contribute to a better understanding of the role of mild notches, of stress multiaxiality and of strain accumulation in the microstructure evolution of single crystal Ni-base superalloys during creep. The results obtained in the present work are discussed in the light of previous work published in the literature. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.07.061
  • Interface dominated cooperative nanoprecipitation in interstitial alloys
    Wang, H. and Zhang, X. and Yan, D. and Somsen, C. and Eggeler, G.
    Nature Communications 9 (2018)
    Steels belong to one of the best established materials, however, the mechanisms of various phase transformations down to the nano length scale are still not fully clear. In this work, high-resolution transmission electron microscopy is combined with atomistic simulations to study the nanoscale carbide precipitation in a Fe–Cr–C alloy. We identify a cooperative growth mechanism that connects host lattice reconstruction and interstitial segregation at the growing interface front, which leads to a preferential growth of cementite (Fe3C) nanoprecipitates along a particular direction. This insight significantly improves our understanding of the mechanisms of nanoscale precipitation in interstitial alloys, and paves the way for engineering nanostructures to enhance the mechanical performance of alloys. © 2018, The Author(s).
    view abstract10.1038/s41467-018-06474-w
  • Martensite aging in 〈0 0 1〉 oriented Co49Ni21Ga30 single crystals in tension
    Lauhoff, C. and Krooß, P. and Langenkämper, D. and Somsen, C. and Eggeler, G. and Kireeva, I. and Chumlyakov, Y.I. and Niendorf, T.
    Functional Materials Letters 11 (2018)
    Co-Ni-Ga high-temperature shape memory alloys (HT-SMAs) are well-known candidate materials for damping applications at elevated temperatures. Recent studies showed that upon heat treatment in stress-induced martensite under compressive loads transformation temperatures can be increased significantly, qualifying Co-Ni-Ga for HT-actuation. The increase in transformation temperatures is related to a change in chemical order recently validated via neutron diffraction experiments. Since SMAs show distinct tension-compression asymmetry in terms of theoretical transformation strains and bearable stresses, understanding the impact of martensite aging in tension is crucial for future applications. The current results indicate that martensite aging in tension provides for a further improvement in functional properties. © 2018 The Author(s).
    view abstract10.1142/S1793604718500248
  • Microstructure and Mechanical Properties of CMSX-4 Single Crystals Prepared by Additive Manufacturing
    Körner, C. and Ramsperger, M. and Meid, C. and Bürger, D. and Wollgramm, P. and Bartsch, M. and Eggeler, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science (2018)
    Currently, additive manufacturing (AM) experiences significant attention in nearly all industrial sectors. AM is already well established in fields such as medicine or spare part production. Nevertheless, processing of high-performance nickel-based superalloys and especially single crystalline alloys such as CMSX-4® is challenging due to the difficulty of intense crack formation. Selective electron beam melting (SEBM) takes place at high process temperatures (~ 1000 °C) and under vacuum conditions. Current work has demonstrated processing of CMSX-4® without crack formation. In addition, by using appropriate AM scan strategies, even single crystals (SX SEBM CMSX-4®) develop directly from the powder bed. In this contribution, we investigate the mechanical properties of SX SEBM CMSX-4® prepared by SEBM in the as-built condition and after heat treatment. The focus is on hardness, strength, low cycle fatigue, and creep properties. These properties are compared with conventional cast and heat-treated material. © 2018 The Author(s)
    view abstract10.1007/s11661-018-4762-5
  • Multiscale Characterization of Microstructure in Near-Surface Regions of a 16MnCr5 Gear Wheel After Cyclic Loading
    Medghalchi, S. and Jamebozorgi, V. and Bala Krishnan, A. and Vincent, S. and Salomon, S. and Basir Parsa, A. and Pfetzing, J. and Kostka, A. and Li, Y. and Eggeler, G. and Li, T.
    JOM (2018)
    The dependence of the microstructure on the degree of deformation in near-surface regions of a 16MnCr5 gear wheel after 2.1 × 106 loading cycles has been investigated by x-ray diffraction analysis, transmission electron microscopy, and atom probe tomography. Retained austenite and large martensite plates, along with elongated lamella-like cementite, were present in a less deformed region. Comparatively, the heavily deformed region consisted of a nanocrystalline structure with carbon segregation up to 2 at.% at grain boundaries. Spheroid-shaped cementite, formed at the grain boundaries and triple junctions of the nanosized grains, was enriched with Cr and Mn but depleted with Si. Such partitioning of Cr, Mn, and Si was not observed in the elongated cementite formed in the less deformed zone. This implies that rolling contact loading induced severe plastic deformation as well as a pronounced annealing effect in the active contact region of the toothed gear during cyclic loading. © 2018 The Minerals, Metals & Materials Society
    view abstract10.1007/s11837-018-2931-z
  • NiTi-Based Elastocaloric Cooling on the Macroscale: From Basic Concepts to Realization
    Kirsch, S.-M. and Welsch, F. and Michaelis, N. and Schmidt, M. and Wieczorek, A. and Frenzel, J. and Eggeler, G. and Schütze, A. and Seelecke, S.
    Energy Technology 6 (2018)
    Solid-state cooling is an environmentally friendly, no global warming potential alternative to vapor compression-based systems. Elastocaloric cooling based on NiTi shape memory alloys exhibits excellent cooling capabilities. Due to the high specific latent heats activated by mechanical loading/unloading, large temperature changes can be generated in the material. The small required work input enables a high coefficient of performance. An overview of elastocaloric cooling from basic principles, such as elastocaloric cooling cycles, material characterization, modeling, and optimization, to the design of elastocaloric cooling devices is presented. Current work performed within the DFG (Deutsche Forschungsgemeinschaft) Priority Program SPP 1599 “Ferroic Cooling”, which is focused on the development and realization of a continuously operating elastocaloric cooling device, is highlighted. The cooling device operates in a rotatory mode with wires under tensile loading. The design allows maximization of cooling power by suitable wire diameter scaling as well as efficiency optimization by implementing a novel drive concept. Finally, computer-aided design (CAD) models of the discussed solid-state air cooling device are presented. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ente.201800152
  • On Shear Testing of Single Crystal Ni-Base Superalloys
    Eggeler, G. and Wieczorek, N. and Fox, F. and Berglund, S. and Bürger, D. and Dlouhy, A. and Wollgramm, P. and Neuking, K. and Schreuer, J. and Agudo Jácome, L. and Gao, S. and Hartmaier, A. and Laplanche, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science (2018)
    Shear testing can contribute to a better understanding of the plastic deformation of Ni-base superalloy single crystals. In the present study, shear testing is discussed with special emphasis placed on its strengths and weaknesses. Key mechanical and microstructural results which were obtained for the high-temperature (T ≈ 1000 °C) and low-stress (τ ≈ 200 MPa) creep regime are briefly reviewed. New 3D stereo STEM images of dislocation substructures which form during shear creep deformation in this regime are presented. It is then shown which new aspects need to be considered when performing double shear creep testing at lower temperatures (T < 800 °C) and higher stresses (τ > 600 MPa). In this creep regime, the macroscopic crystallographic [11−2](111) shear system deforms significantly faster than the [01−1](111) system. This represents direct mechanical evidence for a new planar fault nucleation scenario, which was recently suggested (Wu et al. in Acta Mater 144:642–655, 2018). The double shear creep specimen geometry inspired a micro-mechanical in-situ shear test specimen. Moreover, the in-situ SEM shear specimen can be FIB micro-machined from prior dendritic and interdendritic regions. Dendritic regions, which have a lower γ′ volume fraction, show a lower critical resolved shear stress. © 2018 The Author(s)
    view abstract10.1007/s11661-018-4726-9
  • 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 (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 abstract10.1016/j.msea.2018.05.012
  • On the diffusive phase transformation mechanism assisted by extended dislocations during creep of a single crystal CoNi-based superalloy
    Makineni, S.K. and Kumar, A. and Lenz, M. and Kontis, P. and Meiners, T. and Zenk, C. and Zaefferer, S. and Eggeler, G. and Neumeier, S. and Spiecker, E. and Raabe, D. and Gault, B.
    Acta Materialia 155 (2018)
    We propose here a deformation-induced diffusive phase transformation mechanism occurring during shearing of γ′ ordered phase in a γ/γ′ single crystalline CoNi-based superalloy. Shearing involved the creation and motion of a high density of planar imperfections. Through correlative electron microscopy and atom probe tomography, we captured a superlattice intrinsic stacking fault (SISF) and its associated moving leading partial dislocation (LPD). The structure and composition of these imperfections reveal characteristic chemical – structural contrast. The SISF locally exhibits a D019 ordered structure coherently embedded in the L12 γ′ and enriched in W and Co. Interestingly, the LPD is enriched with Cr and Co, while the adjoining planes ahead of the LPD are enriched with Al. Quantitative analysis of the three-dimensional compositional field in the vicinity of imperfections sheds light onto a new in-plane diffusion mechanism as the LPD moves on specific {111} planes upon application of stress at high temperature. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.05.074
  • On the Electropolishing Mechanism of Nickel Titanium in Methanolic Sulfuric acid − An Electrochemical Impedance Study
    Fushimi, K. and Neelakantan, L. and Eggeler, G. and Hassel, A.W.
    Physica Status Solidi (A) Applications and Materials Science 215 (2018)
    Electropolishing of NiTi shape memory alloys is possible in methanolic 3 m H2SO4. The electro-dissolution behavior of NiTi in methanolic 3 m H2SO4 is ascertained in terms of Nyquist plots using electrochemical impedance spectroscopy (EIS) under limiting current flow (mass transfer control) condition. The electro-dissolution behavior is studied under convective conditions using a rotating disc electrode. The influence of changes in rotation rate, applied potential, and temperature are determined. This study demonstrates that electro-dissolution under mass transfer condition follows a compact salt-film mechanism. In order to quantitatively characterize the salt film formed during electropolishing, EIS is performed under stationary conditions. The increase in applied voltage causes an increase in polarization resistance and decrease in capacitance of the interface film. © 2018 The Authors. Published by Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssa.201800011
  • On the influence of crystallography and dendritic microstructure on micro shear behavior of single crystal Ni-based superalloys
    Laplanche, G. and Wieczorek, N. and Fox, F. and Berglund, S. and Pfetzing-Micklich, J. and Kishida, K. and Inui, H. and Eggeler, G.
    Acta Materialia 160 (2018)
    An in-situ SEM micromechanical test technique is used to investigate the response of a Ni-based single crystal superalloy to double shear loading. The present work shows that micro double shear testing can detect mechanical differences between interdendritic and dendritic regions with γ′-volume fractions of 77% and 72%, respectively, i.e., the interdendritic regions exhibit a larger flow stress than the dendritic regions. These micromechanical differences are apparent when micro double shear specimens are oriented for single-slip while they appear to be overshadowed by dislocation interactions, when multiple-slip is promoted. Sudden deformation events are observed to occur concomitantly with the formation of shear steps (localized plastic deformation) at the surface of the shear zones during single-slip. The micro double shear specimens oriented for single-slip show very low work-hardening. In contrast, much higher stresses are required to plastically deform micro double shear specimens oriented for multiple-slip which exhibit stronger work-hardening. No sudden deformation events could be detected for multiple-slip which results in a more homogeneous deformation of the shear zones (absence of localized plastic deformation). © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.08.052
  • On the nucleation of planar faults during low temperature and high stress creep of single crystal Ni-base superalloys
    Wu, X. and Dlouhy, A. and Eggeler, Y.M. and Spiecker, E. and Kostka, A. and Somsen, C. and Eggeler, G.
    Acta Materialia 144 (2018)
    The present work studies the nucleation of planar faults in the early stages of low temperature (750 °C) and high stress (800 MPa) creep of a Ni-base single crystal superalloy (SX). Two families of 60° dislocations with different Burgers vectors were detected in the transmission electron microscope (TEM). These can react and form a planar fault in the γ′ phase. A 2D discrete dislocation model helps to rationalize a sequence of events which lead to the nucleation of a planar fault. First, one 60° channel dislocation approaches another 60° interface dislocation with a different Burgers vector. At a distance of 5 nm, it splits up into two Shockley partials. The interface dislocation is pushed into the γ′-phase where it creates a small antiphase boundary. It can only move on when the leading Shockley partial joins it and creates an overall 1/3<112> superdislocation. This process is fast and therefore is difficult to observe. The results obtained in the present work contribute to a better understanding of the processes which govern the early stages of low temperature and high stress primary creep of SX. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.09.063
  • Rejuvenation of Single-Crystal Ni-Base Superalloy Turbine Blades: Unlimited Service Life?
    Ruttert, B. and Horst, O. and Lopez-galilea, I. and Langenkämper, D. and Kostka, A. and Somsen, C. and Goerler, J.V. and Ali, M.A. and Shchyglo, O. and Steinbach, I. and Eggeler, G. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science (2018)
    Rejuvenation of the initially hot isostatic pressing (HIP) heat-treated single-crystal Ni-base superalloy (SX) ERBO/1 was examined experimentally and via phase field simulation to establish rejuvenation treatments as a cost-effective alternative for another interval of service life. Creep was performed at 950 °C and 350 MPa, and the specimens were crept to 0.6 pct (creep rate minimum) or 2 pct strain, respectively. A slight coarsening of the γ/γ′ microstructure was observed experimentally and via simulation at 0.6 pct and rafting at 2 pct strain. The damaged microstructures were rejuvenated in a novel hot isostatic press that provides fast quenching rates before the same specimens were recrept under the same initial creep conditions. High-resolution microscopy proves that the rejuvenation re-establishes the original γ/γ′ microstructure in the dendrite core of the precrept specimens (0.6 and 2 pct). However, the interdendritic areas of the 2 pct precrept and rejuvenated specimen still contain elongated γ′ particles enwrapped by interfacial dislocation networks that survived the applied rejuvenation. The subsequent experimental and simulated creep tests after rejuvenation demonstrated that the creep behavior is only reproducible by the proposed rejuvenation for specimens that had crept until the end of the primary creep regime. © 2018 The Minerals, Metals & Materials Society and ASM International
    view abstract10.1007/s11661-018-4745-6
  • Temperature-induced transformations and martensitic reorientation processes in ultra-fine-grained Ni rich pseudoelastic NiTi wires studied by electrical resistance
    Pelegrina, J.L. and Olbricht, J. and Yawny, A. and Eggeler, G.
    Journal of Alloys and Compounds 735 (2018)
    Temperature-induced, stress-induced martensitic phase transitions and martensite reorientation process in Ni rich (50.9 at.%) NiTi pseudoelastic NiTi wires with ultra-fine grained (UFG) microstructure were studied by electrical resistance measurements. Measurements of the electrical resistance as a function of temperature at different constant mechanical loads accompanied by complementary experiments with variable loads at constant temperature were performed. Results show that the transformation mechanisms in UFG microstructures exhibit a higher level of complexity when compared with those characterizing the behavior of other microstructures (e.g., recrystallized or larger grains size). It was found that a threshold stress level below 150 MPa delimits the transition from a homogeneous (low stress) to localized but reversible Lüders type transformation (high stress) when the transformations are induced under constant applied stress and that reorientation processes require stresses of 100 MPa in the present UFG wires. Even though the strain evolutions do not always show two distinct yielding events during cooling or heating, electrical resistance measurements proved that a two-step transformation involving R-phase and B19′ martensite was always present in the extended range of temperatures and stresses investigated here. © 2017 Elsevier B.V.
    view abstract10.1016/j.jallcom.2017.12.009
  • Testing of Ni-base superalloy single crystals with circular notched miniature tensile creep (CNMTC) specimens
    Cao, L. and Bürger, D. and Wollgramm, P. and Neuking, K. and Eggeler, G.
    Materials Science and Engineering A 712 (2018)
    The present work introduces a novel circular notched miniature tensile creep (CNMTC) specimen which is used to study the influence of notches on creep and of multiaxial stress states on microstructural evolution in Ni-based single crystal (SX) superalloys. It is briefly discussed how mild circular notches affect the stress state in the notch root of a tensile bar during elastic loading. Then the stress redistribution under creep conditions is calculated using the finite element method (FEM), assuming isotropic material behavior. Two series of interrupted creep experiments with the Ni-based single crystal superalloy ERBO1 (CMSX-4 type) were then performed at 950 °C, with flat uniaxial miniature tensile creep (FUMTC) and CNMTC <100> specimens, respectively. The evolution of cavities and microcracks in both types of specimens was carefully analyzed after 81, 169, and 306 h. In the uniaxial experiments, a growth of cast pores and the formation of new creep cavities were observed. These degradation processes were much less pronounced in the circular notched specimens. The results of the present work are discussed in the light of previous findings on the influence of multiaxial stress states on creep in single crystal superalloys. © 2017 Elsevier B.V.
    view abstract10.1016/j.msea.2017.11.102
  • The influence of water and solvent uptake on functional properties of shape-memory polymers
    Ghobadi, E. and Marquardt, A. and Zirdehi, E.M. and Neuking, K. and Varnik, F. and Eggeler, G. and Steeb, H.
    International Journal of Polymer Science 2018 (2018)
    In this contribution, diffusion of water, acetone, and ethanol into a polymer matrix has been studied experimentally and numerically by finite element approaches. Moreover, the present study reports an assessment of different thermomechanical conditions of the shape-memory (SM) performance, for example, stress-or strain-holding times in stress-or strain-controlled thermomechanical cycles and the effect of maximum strain. According to the results presented here, the uptake of acetone in Estane is much higher than ethanol and follows classical Fickian diffusion. Further, a series of thermomechanical measurements conducted on dry and physically (hydrolytically) aged polyether urethanes revealed that incorporation of water seems to have an appreciable impact on the shape recovery ratios which can be attributed to the additional physical crosslinks. However, no obvious difference in shape fixation of dry and physically (hydrolytically) aged samples could be recognized. Furthermore, by decreasing the strain-holding time, shape recovery improves significantly. Moreover, the shape fixity is found to be independent of holding time. The shape recovery ratio decreased dramatically with an increase in the stress-holding time. Copyright © 2018 Ehsan Ghobadi et al.
    view abstract10.1155/2018/7819353
  • Ultrahigh-temperature tensile creep of TiC-reinforced Mo-Si-B-based alloy
    Kamata, S.Y. and Kanekon, D. and Lu, Y. and Sekido, N. and Maruyama, K. and Eggeler, G. and Yoshimi, K.
    Scientific Reports 8 (2018)
    In this study, the ultrahigh-temperature tensile creep behaviour of a TiC-reinforced Mo-Si-B-based alloy was investigated in the temperature range of 1400-1600 °C at constant true stress. The tests were performed in a stress range of 100-300 MPa for 400 h under vacuum, and creep rupture data were rationalized with Larson-Miller and Monkman-Grant plots. Interestingly, the MoSiBTiC alloy displayed excellent creep strength with relatively reasonable creep parameters in the ultrahigh-temperature range: a rupture time of ~400 h at 1400 °C under 137 MPa with a stress exponent (n) of 3 and an apparent activation energy of creep (Q app ) of 550 kJ/mol. The increasing rupture strains with decreasing stresses (up to 70%) and moderate strain-rate oscillations in the creep curves suggest that two mechanisms contribute to the creep: phase boundary sliding between the hard T2 and (Ti,Mo)C phases and the Moss phase, and dynamic recovery and recrystallization in Moss, observed with orientation imaging scanning electron microscopy. The results presented here represent the first full analysis of creep for the MoSiBTiC alloy in an ultrahigh-temperature range. They indicate that the high-temperature mechanical properties of this material under vacuum are promising. © 2018 The Author(s).
    view abstract10.1038/s41598-018-28379-w
  • Unusual composition dependence of transformation temperatures in Ti-Ta-X shape memory alloys
    Ferrari, A. and Paulsen, A. and Frenzel, J. and Rogal, J. and Eggeler, G. and Drautz, R.
    Physical Review Materials 2 (2018)
    Ti-Ta-X (X = Al, Sn, Zr) compounds are emerging candidates as high-temperature shape memory alloys (HTSMAs). The stability of the one-way shape memory effect (1WE), the exploitable pseudoelastic (PE) strain intervals, as well as the transformation temperature in these alloys depend strongly on composition, resulting in a trade-off between a stable shape memory effect and a high transformation temperature. In this work, experimental measurements and first-principles calculations are combined to rationalize the effect of alloying a third component to Ti-Ta-based HTSMAs. Most notably, an increase in the transformation temperature with increasing Al content is detected experimentally in Ti-Ta-Al for low Ta concentrations, in contrast to the generally observed dependence of the transformation temperature on composition in Ti-Ta-X. This inversion of trend is confirmed by the ab initio calculations. Furthermore, a simple analytical model based on the ab initio data is derived. The model can not only explain the unusual composition dependence of the transformation temperature in Ti-Ta-Al but also provide a fast and elegant tool for a qualitative evaluation of other ternary systems. This is exemplified by predicting the trend of the transformation temperature of Ti-Ta-Sn and Ti-Ta-Zr alloys, yielding a remarkable agreement with available experimental data. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.073609
  • Bioactivity and electrochemical behavior of hydroxyapatite-silicon-multi walled carbon nano-tubes composite coatings synthesized by EPD on NiTi alloys in simulated body fluid
    Khalili, V. and Khalil-Allafi, J. and Frenzel, J. and Eggeler, G.
    Materials Science and Engineering C 71 (2017)
    In order to improve the surface bioactivity of NiTi bone implant and corrosion resistance, hydroxyapatite coating with addition of 20 wt% silicon, 1 wt% multi walled carbon nano-tubes and both of them were deposited on a NiTi substrate using a cathodic electrophoretic method. The apatite formation ability was estimated using immersion test in the simulated body fluid for 10 days. The SEM images of the surface of coatings after immersion in simulated body fluid show that the presence of silicon in the hydroxyapatite coatings accelerates in vitro growth of apatite layer on the coatings. The Open-circuit potential and electrochemical impedance spectroscopy were measured to evaluate the electrochemical behavior of the coatings in the simulated body fluid at 37 °C. The results indicate that the compact structure of hydroxyapatite-20 wt% silicon and hydroxyapatite-20 wt% silicon-1 wt% multi walled carbon nano-tubes coatings could efficiently increase the corrosion resistance of NiTi substrate. © 2016
    view abstract10.1016/j.msec.2016.10.036
  • Effect of temperature and texture on the reorientation of martensite variants in NiTi shape memory alloys
    Laplanche, G. and Birk, T. and Schneider, S. and Frenzel, J. and Eggeler, G.
    Acta Materialia 127 (2017)
    Martensitic Ni50Ti50 wires and sheets with different textures were tensile tested in the temperature range between −100 °C and 60 °C. The effect of texture and temperature on reorientation of martensite variants was investigated. After deformation, all material states were heated into the austenite regime to study their shape memory behavior. During room temperature tensile testing, in-situ digital image correlation revealed that the reorientation of martensite variants is associated with the nucleation and propagation of a macroscopic Lüders band. A comparison between the mechanical data obtained for wire and sheet specimens revealed a strong effect of texture. The plateau stresses of sheets were found to be 25–33% larger and their recoverable strains were 30% lower than for wires. However, the product of plateau stress and recoverable strain, which represents the external work per unit volume required for martensite variants reorientation does not depend on texture. The tensile tests performed at different temperatures revealed that in the temperature range considered the recoverable strain does not depend significantly on temperature. In contrast, the plateau stress as well as the external work required to reorient martensite decrease with increasing testing temperature. We use a thermodynamic approach involving the elastic strain energy associated with the growth of reoriented martensite variants to rationalize these temperature dependencies. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.01.023
  • Identification of a ternary μ-phase in the Co-Ti-W system – An advanced correlative thin-film and bulk combinatorial materials investigation
    Naujoks, D. and Eggeler, Y.M. and Hallensleben, P. and Frenzel, J. and Fries, S.G. and Palumbo, M. and Koßmann, J. and Hammerschmidt, T. and Pfetzing-Micklich, J. and Eggeler, G. and Spiecker, E. and Drautz, R. and Ludwig, Al.
    Acta Materialia 138 (2017)
    The formation of a ternary μ-phase is documented for the system Co-Ti-W. The relevant compositional stability range is identified by high-throughput energy dispersive X-ray spectroscopy, electrical resistance and X-ray diffraction maps from a thin-film materials library (1 μm thickness). Bulk samples of the identified compositions were fabricated to allow for correlative film and bulk studies. Using analytical scanning and transmission electron microscopy, we demonstrate that in both, thin film and bulk samples, the D85 phase (μ-phase) coexists with the C36-phase and the A2-phase at comparable average chemical compositions. Young's moduli and hardness values of the μ-phase and the C36-phase were determined by nanoindentation. The trends of experimentally obtained elastic moduli are consistent with density functional theory (DFT) calculations. DFT analysis also supports the experimental findings, that the μ-phase can solve up to 18 at.% Ti. Based on the experimental and DFT results it is shown that CALPHAD modeling can be modified to account for the new findings. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.07.037
  • Introduction
    Göken, M. and Eggeler, G. and Zhu, Y. and Lu, L.
    Journal of Materials Research 32 (2017)
    view abstract10.1557/jmr.2017.469
  • Microstructural evolution and functional fatigue of a Ti–25Ta high-temperature shape memory alloy
    Maier, H.J. and Karsten, E. and Paulsen, A. and Langenkämper, D. and Decker, P. and Frenzel, J. and Somsen, C. and Ludwig, Al. and Eggeler, G. and Niendorf, T.
    Journal of Materials Research (2017)
    Titanium–tantalum based alloys can demonstrate a martensitic transformation well above 100 °C, which makes them attractive for shape memory applications at elevated temperatures. In addition, they provide for good workability and contain only reasonably priced constituents. The current study presents results from functional fatigue experiments on a binary Ti–25Ta high-temperature shape memory alloy. This material shows a martensitic transformation at about 350 °C along with a transformation strain of 2 pct at a bias stress of 100 MPa. The success of most of the envisaged applications will, however, hinge on the microstructural stability under thermomechanical loading. Thus, light and electron optical microscopy as well X-ray diffraction were used to uncover the mechanisms that dominate functional degradation in different temperature regimes. It is demonstrated the maximum test temperature is the key parameter that governs functional degradation in the thermomechanical fatigue tests. Specifically, ω-phase formation and local decomposition in Ti-rich and Ta-rich areas dominate when T max does not exceed ≈430 °C. As T max is increased, the detrimental phases start to dissolve and functional fatigue can be suppressed. However, when T max reaches ≈620 °C, structural fatigue sets in, and fatigue life is again deteriorated by oxygen-induced crack formation. Copyright © Materials Research Society 2017
    view abstract10.1557/jmr.2017.319
  • Molecular dynamics simulations of entangled polymers: The effect of small molecules on the glass transition temperature
    Mahmoudinezhad, E. and Marquardt, A. and Eggeler, G. and Varnik, F.
    Procedia Computer Science 108 (2017)
    Effect of small molecules, as they penetrate into a polymer system, is investigated via molecular dynamics simulations. It is found that small spherical particles reduce the glass transition temperature and thus introduce a softening of the material. Results are compared to experimental findings for the effect of different types of small molecules such as water, acetone and ethanol on the glass transition temperature of a polyurethane-based shape memory polymer. Despite the simplicity of the simulated model, MD results are found to be in good qualitative agreement with experimental data. © 2017 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procs.2017.05.152
  • On the competition between the stress-induced formation of martensite and dislocation plasticity during crack propagation in pseudoelastic NiTi shape memory alloys
    Ungár, T. and Frenzel, J. and Gollerthan, S. and Ribárik, G. and Balogh, L. and Eggeler, G.
    Journal of Materials Research (2017)
    The present work addresses the competition between dislocation plasticity and stress-induced martensitic transformations in crack affected regions of a pseudoelastic NiTi miniature compact tension specimen. For this purpose X-ray line profile analysis was performed after fracture to identify dislocation densities and remnant martensite volume fractions in regions along the crack path. Special emphasis was placed on characterizing sub fracture surface zones to obtain depth profiles. The stress affected zone in front of the crack-tip is interpreted in terms of a true plastic zone associated with dislocation plasticity and a pseudoelastic zone where stress-induced martensite can form. On unloading, most of the stress-induced martensite transforms back to austenite but a fraction of it is stabilized by dislocations in both, the irreversible martensite and the surrounding austenite phase. The largest volume fraction of the irreversible or remnant martensite along with the highest density of dislocations in this phase was found close to the primary crack-tip. With increasing distance from the primary crack-tip both, the dislocation density and the volume fraction of irreversible martensite decrease to lower values. Copyright © Materials Research Society 2017
    view abstract10.1557/jmr.2017.267
  • On the evolution of cast microstructures during processing of single crystal Ni-base superalloys using a Bridgman seed technique
    Hallensleben, P. and Schaar, H. and Thome, P. and Jöns, N. and Jafarizadeh, A. and Steinbach, I. and Eggeler, G. and Frenzel, J.
    Materials and Design 128 (2017)
    The present work takes a new look at a modified Bridgman process (Bridgman seed technique, BST) for the production of laboratory Ni-base single crystal (SX) superalloy cylinders of 12/120 mm diameter/length. This type of specimen is needed to perform inexpensive parametric studies for the development of new SX and for understanding the evolution of microstructures during SX casting. During melting, the seed partially melts back. The elementary segregation processes cause a so far unknown type of constitutional heating/cooling. Competitive growth eventually establishes a constant average dendrite spacing. In the present work it is documented how this dendrite spacing varies in one cylindrical ingot, and how it scatters when a series of SX ingots is produced. This type of information is scarce. The calculated temperature gradient across the solid/liquid interface (calculated by FEM) is in excellent agreement with predictions from the Kurz-Fisher equation which yields a dendrite spacing based on the experimental withdrawal rate and the microstructurally determined average dendrite spacing. The presence of small angle grain boundaries on cross sections which were taken perpendicular to the solidification direction can be rationalized on the basis of small deviations from the ideal growth directions of individual primary dendrites. © 2017 Elsevier Ltd
    view abstract10.1016/j.matdes.2017.05.001
  • Optimizing Ni–Ti-based shape memory alloys for ferroic cooling
    Wieczorek, A. and Frenzel, J. and Schmidt, M. and Maaß, B. and Seelecke, S. and Schütze, A. and Eggeler, G.
    Functional Materials Letters 10 (2017)
    Due to their large latent heats, pseudoelastic Ni–Ti-based shape memory alloys (SMAs) are attractive candidate materials for ferroic cooling, where elementary solid-state processes like martensitic transformations yield the required heat effects. The present work aims for a chemical and microstructural optimization of Ni–Ti for ferroic cooling. A large number of Ni–Ti-based alloy compositions were evaluated in terms of phase transformation temperatures, latent heats, mechanical hysteresis widths and functional stability. The aim was to identify material states with superior properties for ferroic cooling. Different material states were prepared by arc melting, various heat treatments and thermo-mechanical processing. The cooling performance of selected materials was assessed by differential scanning calorimetry, uniaxial tensile loading/unloading, and by using a specially designed ferroic cooling demonstrator setup. A Ni(Formula presented.)Ti(Formula presented.)Cu5V(Formula presented.) SMA was identified as a potential candidate material for ferroic cooling. This material combines extremely stable pseudoelasticity at room temperature and a very low hysteresis width. The ferroic cooling efficiency of this material is four times higher than in the case of binary Ni–Ti. © 2017 World Scientific Publishing Company
    view abstract10.1142/S179360471740001X
  • Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi
    Laplanche, G. and Kostka, A. and Reinhart, C. and Hunfeld, J. and Eggeler, G. and George, E.P.
    Acta Materialia 128 (2017)
    The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77 K) and room temperature (293 K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2&lt;110&gt; dislocations dissociated into 1/6&lt;112&gt; Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22 ± 4 mJ m−2, which is ∼25% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16 μm grain size is 260 ± 30 MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.02.036
  • Rejuvenation of creep resistance of a Ni-base single-crystal superalloy by hot isostatic pressing
    Ruttert, B. and Bürger, D. and Roncery, L.M. and Parsa, A.B. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Materials and Design 134 (2017)
    Ni-base single-crystal turbine blades are exposed to a combination of high temperatures and high stresses during their service life in high-pressure turbines of aero engines or stationary gas turbines. This unavoidably leads to various internal microstructural changes such as rafting and the formation of cavities. This study introduces a creep-rejuvenation-creep test cycle using one miniature Ni-base single-crystal creep specimen. A novel hot isostatic press providing high quenching rates was applied to rejuvenate the damaged microstructure of the specimen after the first high-temperature creep degradation before the same specimen was repeatedly creep-tested under the same initial creep conditions. After rejuvenating, microstructural results obtained from high-resolution microscopy prove that the creep cavities were closed, dislocation densities were re-set, and the original but now slightly finer γ/γ′-microstructure was restored without any recrystallization. The subsequent creep test of the rejuvenated specimen demonstrated that the proposed rejuvenation procedure in this work is a suitable method to reproduce the initial creep behavior and to thus prolong the lifetime of an already crept Ni-base single-crystal specimen. © 2017 Elsevier Ltd
    view abstract10.1016/j.matdes.2017.08.059
  • Transmission electron microscopy study of the microstructural evolution during higherature and low-stress (011) [11] shear creep deformation of the superalloy single crystal LEK 94
    Agudo Jácome, L. and Göbenli, G. and Eggeler, G.
    Journal of Materials Research 32 (2017)
    The present work describes the shear creep behavior of the superalloy LEK 94 at temperatures between 980 and 1050 °C and shear stresses between 50 and 140 MPa for loading on the macroscopic crystallographic shear system (MCSS) (011) . The strain rate versus strain curves show short primary and extended secondary creep regimes. We find an apparent activation energy for creep of Q app = 466 kJ/mol and a Norton-law stress exponent of n = 6. With scanning transmission electron microscopy, we characterize three material states that differ in temperature, applied stress, and accumulated strain/time. Rafting develops perpendicular to the maximum principal stress direction, γ channels fill with dislocations, superdislocations cut γ′ particles, and dislocation networks form at γ/γ′ interfaces. Our findings are in agreement with previous results for higherature and low-stress [001] and [110] tensile creep testing, and for shear creep testing of the superalloys CMSX-4 and CMSX-6 on the MCSSs (111) and (001)[100]. The parameters that characterize the evolving γ/γ′ microstructure and the evolving dislocation substructures depend on creep temperature, stress, strain, and time. © 2017 Materials Research Society.
    view abstract10.1557/jmr.2017.336
  • Assessment of strain hardening in copper single crystals using in situ SEM microshear experiments
    Wieczorek, N. and Laplanche, G. and Heyer, J.-K. and Parsa, A.B. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 113 (2016)
    The effect of a pre-strain on the plasticity of copper single crystals subjected to in situ microshear deformation in a scanning electron microscope (SEM) is investigated. Pre-strains of 6.5 and 20% are imposed using [1 0 0] tensile testing. During tensile pre-deformation, several slip systems are activated and irregularly spaced slip bands form. A trace analysis revealed the presence of several slip bands on the tensile specimen near the grips while one family of slip bands parallel to the (1 1 1) crystallographic plane were detected in the middle of the tensile specimen. From the middle of the pre-deformed tensile specimens double microshear samples were prepared using focused ion beam (FIB) machining such that the [0 -1 -1] (1 -1 1) slip system could be directly activated. The results show how microshear behavior reacts to different levels of tensile pre-deformation. Sudden deformation events (SDEs) are observed during microshear testing. The critical stress associated with the first SDE is shown to increase with increasing pre-deformation as a result of an increasing number of slip bands introduced during pre-deformation per shear zone. The results allow also to obtain information on the interaction between dislocations activated during microshearing ([0 -1 -1] (1 -1 1)) and those which were introduced during tensile pre-deformation ([1 0 -1] (1 1 1) and [1 -1 0] (1 1 1)). When these slip systems interact glissile junctions and Lomer-Cottrell locks are likely to form. In the light of this analysis, we rationalize the occurrence of sudden deformation events based on piled up dislocation assemblies which overcome Lomer-Cottrell lock barriers. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.04.055
  • Characterization of mechanical properties of hydroxyapatite-silicon-multi walled carbon nano tubes composite coatings synthesized by EPD on NiTi alloys for biomedical application
    Khalili, V. and Khalil-Allafi, J. and Sengstock, C. and Motemani, Y. and Paulsen, A. and Frenzel, J. and Eggeler, G. and Köller, M.
    Journal of the Mechanical Behavior of Biomedical Materials 59 (2016)
    Release of Ni1+ ions from NiTi alloy into tissue environment, biological response on the surface of NiTi and the allergic reaction of atopic people towards Ni are challengeable issues for biomedical application. In this study, composite coatings of hydroxyapatite-silicon multi walled carbon nano-tubes with 20 wt% Silicon and 1 wt% multi walled carbon nano-tubes of HA were deposited on a NiTi substrate using electrophoretic methods. The SEM images of coated samples exhibit a continuous and compact morphology for hydroxyapatite-silicon and hydroxyapatite-silicon-multi walled carbon nano-tubes coatings. Nano-indentation analysis on different locations of coatings represents the highest elastic modulus (45.8 GPa) for HA-Si-MWCNTs which is between the elastic modulus of NiTi substrate (66.5 GPa) and bone tissue (≈30 GPa). This results in decrease of stress gradient on coating-substrate-bone interfaces during performance. The results of nano-scratch analysis show the highest critical distance of delamination (2.5 mm) and normal load before failure (837 mN) as well as highest critical contact pressure for hydroxyapatite-silicon-multi walled carbon nano-tubes coating. The cell culture results show that human mesenchymal stem cells are able to adhere and proliferate on the pure hydroxyapatite and composite coatings. The presence of both silicon and multi walled carbon nano-tubes (CS3) in the hydroxyapatite coating induce more adherence of viable human mesenchymal stem cells in contrast to the HA coated samples with only silicon (CS2). These results make hydroxyapatite-silicon-multi walled carbon nano-tubes a promising composite coating for future bone implant application. © 2016 Elsevier Ltd.
    view abstract10.1016/j.jmbbm.2016.02.007
  • Decomposition of the single-phase high-entropy alloy CrMnFeCoNi after prolonged anneals at intermediate temperatures
    Otto, F. and Dlouhý, A. and Pradeep, K.G. and Kuběnová, M. and Raabe, D. and Eggeler, G. and George, E.P.
    Acta Materialia 112 (2016)
    Among the vast number of multi-principal-element alloys that are referred to as high-entropy alloys (HEAs) in the literature, only a limited number solidify as single-phase solid solutions. The equiatomic HEA, CrMnFeCoNi, is a face-centered cubic (FCC) prototype of this class and has attracted much attention recently because of its interesting mechanical properties. Here we evaluate its phase stability after very long anneals of 500 days at 500-900 °C during which it is reasonable to expect thermodynamic equilibrium to have been established. Microstructural analyses were performed using complementary analysis techniques including scanning and transmission electron microscopy (SEM/TEM/STEM), energy dispersive X-ray (EDX) spectroscopy, selected area electron diffraction (SAD), and atom probe tomography (APT). We show that the alloy is a single-phase solid solution after homogenization for 2 days at 1200 °C and remains in this state after a subsequent anneal at 900 °C for 500 days. However, it is unstable and forms second-phase precipitates at 700 and 500 °C. A Cr-rich σ phase forms at 700 °C, whereas three different phases (L10-NiMn, B2-FeCo and a Cr-rich body-centered cubic, BCC, phase) precipitate at 500 °C. These precipitates are located mostly at grain boundaries, but also form at intragranular inclusions/pores, indicative of heterogeneous nucleation. Since there is limited entropic stabilization of the solid solution state even in the extensively investigated CrMnFeCoNi alloy, the stability of other HEAs currently thought to be solid solutions should be carefully evaluated, especially if they are being considered for applications in vulnerable temperature ranges. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2016.04.005
  • Diffusion of small molecules in a shape memory polymer
    Marquardt, A. and Mogharebi, S. and Neuking, K. and Varnik, F. and Eggeler, G.
    Journal of Materials Science 51 (2016)
    The present work studies the diffusion of small molecules (acetone, ethanol, and water) in a shape memory polymer (SMP) of type Estane ETE 75DT3 (SMP-E), which represents a thermoplastic polyurethane. The work aims at providing background information on the chemical reaction between SMPs and small molecules which can limit the service life of SMP actuators operating in harsh chemical environments. Weight gain studies after immersion of plate specimens in liquid acetone, ethanol, and water yield data which can be assessed on the basis of analytical and numerical solutions of Fick’s second law. The diffusion coefficients which are obtained for 21, 30, and 40 °C in the present study scale as Dacetone &gt; Dethanol &gt; Dwater. The diffusion coefficients show Arrhenius types of temperature dependencies with apparent activation energies of 33 (acetone), 59 (ethanol), and 58 (water) kJ mol−1. The diffusion coefficients and the apparent activation energies obtained in the present work are in reasonable agreement with data which were reported for the reaction of the three small molecules with similar polymers in the literature. It is not straightforward to correlate differences in molecular mobility with individual physical properties. The Hansen solubility parameter (originally derived to explain solubility not mobility) qualitatively rationalizes the observed differences. © 2016, Springer Science+Business Media New York.
    view abstract10.1007/s10853-016-0213-0
  • Double minimum creep of single crystal Ni-base superalloys
    Wu, X. and Wollgramm, P. and Somsen, C. and Dlouhy, A. and Kostka, A. and Eggeler, G.
    Acta Materialia 112 (2016)
    Low temperature (750°C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the (001) direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2016.04.012
  • Experimental methods for investigation of shape memory based elastocaloric cooling processes and model validation
    Schmidt, M. and Ullrich, J. and Wieczorek, A. and Frenzel, J. and Eggeler, G. and Schütze, A. and Seelecke, S.
    Journal of Visualized Experiments 2016 (2016)
    Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor compression based cooling process. Nickel-Titanium (Ni-Ti) based alloy systems, especially, show large elastocaloric effects. Furthermore, exhibit large latent heats which is a necessary material property for the development of an efficient solid-state based cooling process. A scientific test rig has been designed to investigate these processes and the elastocaloric effects in SMAs. The realized test rig enables independent control of an SMA's mechanical loading and unloading cycles, as well as conductive heat transfer between SMA cooling elements and a heat source/sink. The test rig is equipped with a comprehensive monitoring system capable of synchronized measurements of mechanical and thermal parameters. In addition to determining the process-dependent mechanical work, the system also enables measurement of thermal caloric aspects of the elastocaloric cooling effect through use of a high-performance infrared camera. This combination is of particular interest, because it allows illustrations of localization and rate effects - both important for efficient heat transfer from the medium to be cooled. The work presented describes an experimental method to identify elastocaloric material properties in different materials and sample geometries. Furthermore, the test rig is used to investigate different cooling process variations. The introduced analysis methods enable a differentiated consideration of material, process and related boundary condition influences on the process efficiency. The comparison of the experimental data with the simulation results (of a thermomechanically coupled finite element model) allows for better understanding of the underlying physics of the elastocaloric effect. In addition, the experimental results, as well as the findings based on the simulation results, are used to improve the material properties. © 2016 Journal of Visualized Experiments.
    view abstract10.3791/53626
  • Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy
    Laplanche, G. and Kostka, A. and Horst, O.M. and Eggeler, G. and George, E.P.
    Acta Materialia 118 (2016)
    At low homologous temperatures (down to cryogenic temperatures), the CrMnFeCoNi high-entropy alloy possesses good combination of strength, work hardening rate (WHR), ductility, and fracture toughness. To improve understanding of the deformation mechanisms responsible for its mechanical properties, tensile tests were performed at liquid nitrogen and room temperature (77 K and 293 K) and interrupted at different strains to quantify the evolution of microstructure by transmission electron microscopy. Dislocation densities, and twin widths, their spacings, and volume fractions were determined. Nanotwins were first observed after true strains of ∼7.4% at 77 K and ∼25% at 293 K; at lower strains, deformation occurs by dislocation plasticity. The tensile stress at which twinning occurs is 720 ± 30 MPa, roughly independent of temperature, from which we deduce a critical resolved shear stress for twinning of 235 ± 10 MPa. In the regime where deformation occurs by dislocation plasticity, the shear modulus normalized WHR decreases with increasing strain at both 77 K and 293 K. Beyond ∼7.4% true strain, the WHR at 77 K remains constant at a high value of G/30 because twinning is activated, which progressively introduces new interfaces in the microstructure. In contrast, the WHR at room temperature continues to decrease with increasing strain because twinning is not activated until much later (close to fracture). Thus, the enhanced strength-ductility combination at 77 K compared to 293 K is primarily due to twinning starting earlier in the deformation process and providing additional work hardening. Consistent with this, when tensile specimens were pre-strained at 77 K to introduce nanotwins, and subsequently tested at 293 K, flow stress and ductility both increased compared to specimens that were not pre-strained. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.07.038
  • Nanostructured Ti-Ta thin films synthesized by combinatorial glancing angle sputter deposition
    Motemani, Y. and Khare, C. and Savan, A. and Hans, M. and Paulsen, A. and Frenzel, J. and Somsen, C. and Mücklich, F. and Eggeler, G. and Ludwig, Al.
    Nanotechnology 27 (2016)
    Ti-Ta alloys are attractive materials for applications in actuators as well as biomedical implants. When fabricated as thin films, these alloys can potentially be employed as microactuators, components for micro-implantable devices and coatings on surgical implants. In this study, Ti100-xTa x (x = 21, 30) nanocolumnar thin films are fabricated by glancing angle deposition (GLAD) at room temperature using Ti73Ta27 and Ta sputter targets. Crystal structure, morphology and microstructure of the nanostructured thin films are systematically investigated by XRD, SEM and TEM, respectively. Nanocolumns of ∼150-160 nm in width are oriented perpendicular to the substrate for both Ti79Ta21 and Ti70Ta30 compositions. The disordered α″ martensite phase with orthorhombic structure is formed in room temperature as-deposited thin films. The columns are found to be elongated small single crystals which are aligned perpendicular to the and planes of α″ martensite, indicating that the films' growth orientation is mainly dominated by these crystallographic planes. Laser pre-patterned substrates are utilized to obtain periodic nanocolumnar arrays. The differences in seed pattern, and inter-seed distances lead to growth of multi-level porous nanostructures. Using a unique sputter deposition geometry consisting of Ti73Ta27 and Ta sputter sources, a nanocolumnar Ti-Ta materials library was fabricated on a static substrate by a co-deposition process (combinatorial-GLAD approach). In this library, a composition spread developed between Ti72.8Ta27.2 and Ti64.4Ta35.6, as confirmed by high-throughput EDX analysis. The morphology over the materials library varies from well-isolated nanocolumns to fan-like nanocolumnar structures. The influence of two sputter sources is investigated by studying the resulting column angle on the materials library. The presented nanostructuring methods including the use of the GLAD technique along with pre-patterning and a combinatorial materials library fabrication strategy offer a promising technological approach for investigating Ti-Ta thin films for a range of applications. The proposed approaches can be similarly implemented for other materials systems which can benefit from the formation of a nanocolumnar morphology. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/27/49/495604
  • On Local Phase Equilibria and the Appearance of Nanoparticles in the Microstructure of Single-Crystal Ni-Base Superalloys
    Yardley, V. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Parsa, A.B. and Kostka, A. and Somsen, C. and Dlouhy, A. and Neuking, K. and George, E.P. and Eggeler, G.
    Advanced Engineering Materials 18 (2016)
    High-resolution characterization techniques are combined with thermodynamic calculations (CALPHAD) to rationalize microstructural features of single crystal Ni-base superalloys. Considering the chemical compositions of dendritic and interdendritic regions one can explain differences in γ′-volume fractions. Using thermodynamic calculations one can explain, why γ-nanoparticles are observed in the central regions of large cuboidal γ′-particles and why tertiary γ′-nanoparticles form in the γ-channels. The chemical compositions of the γ-channels and of the newly formed γ-particles differ because of the Gibbs–Thomson pressure which acts on the small particles. With increasing size of secondary γ′-particles, their shape changes from spherical to cuboidal. Some general thermodynamic aspects including the temperature dependencies of the Gibbs free energy G, the enthalpy H, and the entropy S and site occupancies in the ordered L12 (γ′) phase are considered. The importance of cooling rate after homogenization is discussed. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.201600237
  • On the Effect of Hot Isostatic Pressing on the Creep Life of a Single Crystal Superalloys
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Bürger, D. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Advanced Engineering Materials 18 (2016)
    The creep behavior of a single-crystal Ni-base superalloy in two microstructural states is compared. One is obtained by casting followed by a conventional heat treatment. The other results from the same nominal heat treatment integrated into a hot isostatic pressing process. The microstructure after HIP differed from that in the conventional route in two respects. First, the γ′ particles are smaller and the γ channels are narrower. Second, after HIP, the number density of pores is lower and the pore sizes are smaller. The HIP microstructure improves creep in two respects: the finer γ/γ′-microstructure results in lower minimum creep rates. Moreover, the shrinkage of cast porosity during HIP delays the nucleation and growth of micro cracks and results in higher rupture strains in the low-temperature high stress regime. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.201600071
  • Oxidation Behavior of the CrMnFeCoNi High-Entropy Alloy
    Laplanche, G. and Volkert, U.F. and Eggeler, G. and George, E.P.
    Oxidation of Metals 85 (2016)
    Oxidation of the Cr20Mn20Fe20Co20Ni20 (at%) high-entropy alloy (HEA) was investigated at 500–900 °C in laboratory air. At 600 °C the oxide was mainly Mn2O3 with a thin inner Cr2O3 layer; at 700 and 800 °C it was mainly Mn2O3 with some Cr enrichment; at 900 °C it was Mn3O4. The oxidation rate was initially linear but became parabolic at longer times with an activation energy of 130 kJ/mol, comparable to that of Mn diffusion in Mn oxides but much lower than that for sluggish diffusion of Mn in the HEA. The diffusion of Mn through the oxide is considered to be the rate-limiting process. © 2016, Springer Science+Business Media New York.
    view abstract10.1007/s11085-016-9616-1
  • Preparing hydroxyapatite-silicon composite suspensions with homogeneous distribution of multi-walled carbon nano-tubes for electrophoretic coating of NiTi bone implant and their effect on the surface morphology
    Khalili, V. and Khalil-Allafi, J. and Xia, W. and Parsa, A.B. and Frenzel, J. and Somsen, C. and Eggeler, G.
    Applied Surface Science 366 (2016)
    Preparing a stable suspension is a main step towards the electrophoretically depositing of homogeneous and dense composite coatings on NiTi for its biomedical application. In the present study, different composite suspensions of hydroxyapatite, silicon and multi-walled carbon nano-tubes were prepared using n-butanol and triethanolamine as media and dispersing agent, respectively. Multi-walled carbon nanotubes were first functionalized in the nitric acid vapor for 15 h at 175 °C, and then mixed into suspensions. Thermal desorption spectroscopy profiles indicate the formation of functional groups on multi-walled carbon nano-tubes. An excellent suspension stability can be achieved for different amounts of triethanolamine. The amount of triethanolamine can be increased by adding a second component to a stable hydroxyapatite suspension due to an electrostatic interaction between components in suspension. The stability of composite suspension is less than that of the hydroxyapatite suspension, due to density differences, which under the gravitational force promote the demixing. The scanning electron microscopy images of the coatings surface show that more dense coatings are developed on NiTi substrate using electrophoretic deposition and sintering at 850 °C in the simultaneous presence of silicon and multi-walled carbon nanotubes in the hydroxyapatite coatings. The atomic force microscopy results of the coatings surface represent that composite coatings of hydroxyapatite-20 wt.% silicon and hydroxyapatite-20 wt.% silicon-1 wt.% multi-walled carbon nano-tubes with low zeta potential have rougher surfaces. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apsusc.2016.01.053
  • The effect of stress, temperature and loading direction on the creep behaviour of Ni-base single crystal superalloy miniature tensile specimens
    Wollgramm, P. and Bürger, D. and Parsa, A.B. and Neuking, K. and Eggeler, G.
    Materials at High Temperatures 33 (2016)
    In the present work, we use a miniature test procedure to investigate the tensile creep behaviour of the single crystal superalloy ERBO1. We test precisely oriented [0 0 1], [1 1 0] and [1 1 1] creep specimens and determine the stress and the temperature dependence of characteristic creep rates in limited stress and temperature regimes, where the stress and temperature dependence of characteristic creep rates can be well described by power law and Arrhenius type of relations, with stress exponents n and apparent activation energies Qapp. n-values increase with stress and decrease with temperature. Qapp-values, on the other hand, increase with increasing temperature and decrease with increasing stress. Creep curve shapes gradually evolve from the high temperature low stress to the low temperature high stress (LTHS) regime. This implies that there is a gradual change in elementary deformation and softening mechanisms, which is qualitatively confirmed using transmission electron microscopy. While at high temperatures different loading directions only have a moderate influence on creep, there is a very strong effect of loading direction at low temperatures. The [1 1 0] tests show the fastest deformation rates and the shortest rupture times. In the LTHS creep regime, we confirm the double minimum (DM) type of creep behaviour, which was previously reported but never explained. Further work is required to rationalise DM-creep. The implications of this type of creep behaviour on scatter and on extrapolation of creep data is discussed in the light of previous results published in the literature. © 2016 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/09603409.2016.1186414
  • The influence of Si as reactive bonding agent in the electrophoretic coatings of HA–Si–MWCNTs on NiTi alloys
    Khalili, V. and Khalil-Allafi, J. and Maleki-Ghaleh, H. and Paulsen, A. and Frenzel, J. and Eggeler, G.
    Journal of Materials Engineering and Performance 25 (2016)
    In this study, different composite coatings with 20 wt.% silicon and 1 wt.% multi-walled carbon nanotubes of hydroxyapatite were developed on NiTi substrate using a combination of electrophoretic deposition and reactive bonding during the sintering. Silicon was used as reactive bonding agent. During electrophoretic deposition, the constant voltage of 30 V was applied for 60 s. After deposition, samples were dried and then sintered at 850 °C for 1 h in a vacuum furnace. SEM, XRD and EDX were used to characterize the microstructure, phase and elemental identification of coatings, respectively. The SEM images of the coatings reveal a uniform and compact structure for HA–Si and HA–Si–MWCNTs. The presence of silicon as a reactive bonding agent as well as formation of new phases such as SiO2, CaSiO3 and Ca3SiO5 during the sintering process results in compact coatings and consumes produced phases from HA decomposition. Formation of the mentioned phases was confirmed using XRD analysis. The EDX elemental maps show a homogeneous distribution of silicon all over the composite coatings. Also, the bonding strength of HA–Si–MWCNTs coating is found to be 27.47 ± 1 MPa. © 2015, ASM International.
    view abstract10.1007/s11665-015-1824-3
  • Transmission electron microscopy of a CMSX-4 Ni-base superalloy produced by selective electron beam melting
    Parsa, A.B. and Ramsperger, M. and Kostka, A. and Somsen, C. and Körner, C. and Eggeler, G.
    Metals 6 (2016)
    In this work, the microstructures of superalloy specimens produced using selective electron beam melting additive manufacturing were characterized. The materials were produced using a CMSX-4 powder. Two selective electron beam melting processing strategies, which result in higher and lower effective cooling rates, are described. Orientation imaging microscopy, scanning transmission electron microscopy and conventional high resolution transmission electron microscopy are used to investigate the microstructures. Our results suggest that selective electron beam melting processing results in near equilibrium microstructures, as far as γ′ volume fractions, the formation of small amounts of TCP phases and the partitioning behavior of the alloy elements are concerned. As expected, higher cooling rates result in smaller dendrite spacings, which are two orders of magnitude smaller than observed during conventional single crystal casting. During processing, columnar grains grow in <100> directions, which are rotated with respect to each other. There are coarse γ/γ′ microstructures in high angle boundary regions. Dislocation networks form low angle boundaries. A striking feature of the as processed selective electron beam melting specimens is their high dislocation density. From a fundamental point of view, this opens new possibilities for the investigation of elementary dislocation processes which accompany solidification. © 2016 by the authors; licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met6110258
  • A quantitative metallographic assessment of the evolution of porosity during processing and creep in single crystal Ni-base super alloys
    Buck, H. and Wollgramm, P. and Parsa, A.B. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 46 (2015)
    The present work reviews previous research on the evolution of porosity. It presents new results from a detailed study on the evolution of porosity during casting, heat treatment and creep of a single crystal Ni-base superalloy subjected to uniaxial tensile creep at 1050 °C and 160 MPa in [001] and [110] directions. A quantitative metallographic study was performed on carefully polished metallographic cross sections, monitoring sampling fields of 4500 × 1000 μm2 using the back scatter contrast of an analytical scanning electron microscope; evolutions of pore sizes and pore form factors were analyzed and all important details which were previously revealed in a synchrotron study could be reproduced. In addition, it was observed that micro cracks form at larger cast pores. They interlink and thus initiate final rupture. The [110] tensile creep tests showed lower rupture strains than the [001] experiments. In agreement with earlier work, this can be rationalized on the basis of aligned porosity along primary dendrites. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201500379
  • 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 (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 abstract10.1002/adem.201400136
  • Atomic layer-by-layer construction of Pd on nanoporous gold via underpotential deposition and displacement reaction
    Yan, X. and Xiong, H. and Bai, Q. and Frenzel, J. and Si, C. and Chen, X. and Eggeler, G. and Zhang, Z.
    RSC Advances 5 (2015)
    Atomic layer-by-layer construction of Pd on nanoporous gold (NPG) has been investigated through the combination of underpotential deposition (UPD) with displacement reaction. It has been found that the UPD of Cu on NPG is sensitive to the applied potential and the deposition time. The optimum deposition potential and time were determined through potential- and time-sensitive stripping experiments. The NPG-Pd electrode shows a different voltammetric behavior in comparison to the bare NPG electrode, and the deposition potential was determined through the integrated charge control for the monolayer UPD of Cu on the NPG-Pd electrode. Five layers of Pd were constructed on NPG through the layer-by-layer deposition. In addition, the microstructure of the NPG-Pdx (x = 1, 2, 3, 4 and 5) films was probed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The microstructural observation demonstrates that the atomic layers of Pd form on the ligament surface of NPG through epitaxial growth, and have no effect on the nanoporous structure of NPG. In addition, the hydrogen storage properties of the NPG-Pdx electrodes have also been addressed. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c4ra17014h
  • Cyclic degradation of titanium-tantalum high-temperature shape memory alloys - The role of dislocation activity and chemical decomposition
    Niendorf, T. and Krooß, P. and Somsen, C. and Rynko, R. and Paulsen, A. and Batyrshina, E. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Functional Materials Letters 8 (2015)
    Titanium-tantalum shape memory alloys (SMAs) are promising candidates for actuator applications at elevated temperatures. They may even succeed in substituting ternary nickel-titanium high temperature SMAs, which are either extremely expensive or difficult to form. However, titanium-tantalum alloys show rapid functional and structural degradation under cyclic thermo-mechanical loading. The current work reveals that degradation is not only governed by the evolution of the ω-phase. Dislocation processes and chemical decomposition of the matrix at grain boundaries also play a major role. © 2015 The Author(s).
    view abstract10.1142/S1793604715500629
  • Damage evolution in pseudoelastic polycrystalline Co-Ni-Ga high-temperature shape memory alloys
    Vollmer, M. and Krooß, P. and Segel, C. and Weidner, A. and Paulsen, A. and Frenzel, J. and Schaper, M. and Eggeler, G. and Maier, H.J. and Niendorf, T.
    Journal of Alloys and Compounds 633 (2015)
    Due to its transformation behavior, Co-Ni-Ga represents a very promising high temperature shape memory alloy (HT SMA) for applications at elevated temperatures. Co-Ni-Ga single crystals show a fully reversible pseudoelastic shape change up to temperatures of 400 °C. Unfortunately, polycrystalline Co-Ni-Ga suffers from brittleness and early fracture mainly due to intergranular constraints. In the current study, different thermo-mechanical processing routes produced various microstructures which differ in grain size and texture. A bicrystalline bamboo-like grain structure results in the highest reversible transformation strains and excellent cyclic stability. Moreover, solution-annealed and hot-rolled conditions also showed cyclic stability. Using in situ high-resolution electron microscopy, the elementary processes, which govern the microstructural evolution during pseudoelastic cycling were investigated and the mechanisms that govern structural and functional degradation were identified. The observations documented in the present work suggest that the formation of the ductile γ-phase on and near grain boundaries as well as the activation of multiple martensite variants at grain boundaries are beneficial for improved cyclic performance of polycrystalline Co-Ni-Ga HT SMAs. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2015.01.282
  • Elastocaloric cooling with ni-Ti based alloys - material characterization and process variation
    Schmidt, M. and Ullrich, J. and Wieczorek, A. and Frenzel, J. and Schötze, A. and Eggeler, G. and Seelecke, S.
    ASME 2015 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2015 1 (2015)
    Solid state refrigeration processes, such as magnetocaloric and electrocaloric refrigeration, have recently shown to be a promising alternative to conventional compression refrigeration. A new solid state elastocaloric refrigeration process using the latent heats within Shape Memory Alloys (SMA) could also hold potential in this field. This work investigates the elastocaloric effects in Ni-Ti-based superelastic Shape Memory Alloy (SMA) systems for use in an elastocaloric cooling processes. Ni-Ti alloys exhibits large latent heats and a small mechanical hysteresis, which may potentially lead to the development of an efficient environmentally friendly solid-state cooling system, without the need for ozone-depleting refrigerants. A systematic investigation of the SMA is conducted using a novel custombuilt scientific testing platform specifically designed to measure cooling process related phenomena. This testing system is capable of performing tensile tests at high rates as well as measuring and controlling the solid-state heat transfer between SMA and heat source/heat sink. Tests are conducted following a cooling process related training cycle where the material has achieved stabilized behavior. First, a characterization of the elastocaloric material properties is performed followed by an investigation of the material under cooling process conditions. A comprehensive monitoring of the mechanical and thermal parameters enables the observation of temperature changes during mechanical cycling of the SMA at high strain rates. These observations can be used to study the rate dependent efficiency of the elastocaloric material. The measurement of the temperature of both the heat source/heat sink and the SMA itself, as well as the required mechanical work during a running cooling process, reveals the influence of the operating conditions on the elastocaloric effect of the material. Furthermore investigations of the process efficiency at different thermal boundary conditions (temperature of heat source/heat sink), indicates that the process is dependent on the boundary conditions which have to be controlled in order to optimize the efficiency. © Copyright 2015 by ASME.
    view abstract10.1115/SMASIS2015-8944
  • In vitro comparison of the sagittal split osteotomy with and without inferior border osteotomy
    Böckmann, R. and Schön, P. and Neuking, K. and Meyns, J. and Kessler, P. and Eggeler, G.
    Journal of Oral and Maxillofacial Surgery 73 (2015)
    Purpose By adding an osteotomy of the inferior border of the mandibular body to the classic sagittal split osteotomy, the authors expected to prevent unfavorable splits and damage to the inferior alveolar nerve. Materials and Methods Thirty-five human mandibles were used to perform 70 sagittal split osteotomies as an in vitro study. Conducted as a split-mouth model, each mandible was split at the midline. One side of the mandible was split using the traditional Obwegeser-Dal Pont technique, and the other side was split in the same manner with an additional osteotomy of the inferior mandible border. The torque used to split the mandible was measured, and the fracture line of the mandible was recorded. Results The average torque associated with the original technique was 1.38 Nm (standard deviation, 0.60 Nm), with a fracture line along the mandibular canal. The average torque required to split the hemimandible with the modified technique was 1.02 Nm (standard deviation, 0.50 Nm), a significant (P <.001) difference, with a fracture line parallel to the posterior ramus of the mandible. The fracture pattern depended significantly on the technique used (P <.001), but not on the applied torque force. Conclusion By adding an osteotomy of the inferior mandibular border to the sagittal split osteotomy, less torque was needed to split the mandible. The fracture line was more predictable, even when all the surgical manipulations were performed at a safe distance from the inferior alveolar nerve. © 2015 American Association of Oral and Maxillofacial Surgeons.
    view abstract10.1016/j.joms.2014.08.005
  • Influence of microstructure on macroscopic elastic properties and thermal expansion of nickel-base superalloys ERBO/1 and LEK94
    Demtröder, K. and Eggeler, G. and Schreuer, J.
    Materialwissenschaft und Werkstofftechnik 46 (2015)
    In the present work the thermal expansion and the elastic properties of second generation nickel-base superalloy single crystals ERBO/1 (CMSX-4 variation) and LEK94 have been studied between about 100 K and 1273 K using dilatometry and resonant ultrasound spectroscopy, respectively. Inhomogeneity related to the large scale microstructure of the samples can act as a potential source of scatter for the propagation of ultrasonic waves. This can be overcome by choosing samples of sufficient size so that they appear as homogeneous media at the scale of the elastic wave length. Our final results are in good agreement with data reported in literature for similar alloy systems. In particular, the elastic material properties are only weekly affected by moderate variations in chemical composition and microstructure. Taking into account literature data for other superalloys like CMSX-4, we derive general polynomial functions which describe the temperature dependence of the elastic moduli E<inf>〈100〉</inf>, E<inf>〈110〉</inf> and E<inf>〈111〉</inf> in nickel-base superalloys to within about ±3%. It was also observed that the alloys ERBO/1 and LEK94 show weak but significant anomalies in both thermal expansion and temperature coefficients of elastic constants above about 900 K. These anomalies are probably related to the gradual dissolution of the γ′-precipitates at higher temperatures. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201500406
  • Ledges and grooves at γ/γ′ interfaces of single crystal superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Kostka, A. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 90 (2015)
    In the present work we study the formation of grooves and ledges (typical size: <100 nm) at γ/γ′ interfaces of single crystal Ni-base superalloys. We highlight previous work which documents the presence of such interface irregularities and shows that their number and size increases during high temperature exposure and creep. We use diffraction contrast stereo transmission electron microscopy (TEM) to provide new evidence for the presence of ledges and grooves near dislocations at γ/γ′ interfaces after heat treatment and creep. We present a 2D model of the interfacial region which shows how dislocation stress fields alter local chemical potentials and drive diffusional fluxes which result in the formation of a groove. The results of the numerical study yield realistic groove sizes in relevant time scales. The results obtained in the present study suggest that the formation of grooves and ledges represents an elementary process which needs to be considered when rationalizing the kinetics of rafting, the directional coarsening of the γ′ phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.02.005
  • Martensite aging - Avenue to new high temperature shape memory alloys
    Niendorf, T. and Krooß, P. and Somsen, C. and Eggeler, G. and Chumlyakov, Y.I. and Maier, H.J.
    Acta Materialia 89 (2015)
    High-temperature shape memory alloys are attractive for efficient solid state actuation. A key criterion for shape memory alloys is the martensite start temperature. The current study introduces a concept for increasing this temperature of alloys initially not suited for high-temperature actuation. Aging of stress-induced martensite, referred to as SIM-aging in the current work, is able to increase the martensite start temperature by about 130 °C as demonstrated in the present study for a Co-Ni-Ga shape memory alloy. The increase of transformation temperatures can be explained based on the concept of symmetry-conforming short-range order. Following SIM-aging the Co-Ni-Ga alloy shows cyclic actuation stability at elevated temperatures. While martensite aging has always been viewed as detrimental in the past, it can actually be exploited to design new classes of high-temperature shape memory alloys with excellent properties. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.01.042
  • Microstructural evolution in a Ti-Ta hightemperature shape memory alloy during creep
    Rynko, R. and Marquardt, A. and Paulsen, A. and Frenzel, J. and Somsen, C. and Eggeler, G.
    International Journal of Materials Research 106 (2015)
    Alloys based on the titanium-tantalum system are considered for application as high-temperature shape memory alloys due to their martensite start temperatures, which can surpass 200 °C. In the present work we study the evolution of microstructure and the influence of creep on the phase transformation behavior of a Ti70Ta30 (at.%) high-temperature shape memory alloy. Creep tests were performed in a temperature range from 470 to 530 °C at stresses between 90 and 150 MPa. The activation energy for creep was found to be 307 kJ mol-1 and the stress exponent n was determined as 3.7. Scanning and transmission electron microscopy investigations were carried out to characterize the microstructure before and after creep. It was found that the microstructural evolution during creep suppresses subsequent martensitic phase transformations. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.111189
  • Microstructural evolution of a CoCrFeMnNi high-entropy alloy after swaging and annealing
    Laplanche, G. and Horst, O. and Otto, F. and Eggeler, G. and George, E.P.
    Journal of Alloys and Compounds 647 (2015)
    Abstract The processing parameters which govern the evolution of microstructure and texture during rotary swaging and subsequent heat treatments were studied in an equiatomic single-phase CoCrFeMnNi high-entropy alloy. After vacuum induction melting and casting, the diameter of the 40 mm cast ingot was reduced at room temperature to a final diameter of 16.5 mm by rotary swaging (diameter reduction of 60%/area reduction of 80%) and the alloy was then annealed at different temperatures for 1 h. The resulting microstructures were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction and correlated with results of microhardness measurements. It was found that the microhardness first increases slightly upon annealing below the recrystallization temperature but then drops steeply at higher annealing temperatures due to the onset of recrystallization. Special emphasis was placed on how the microstructure evolves with respect to the radial and longitudinal position in the rod. Finally, a combination of swaging and heat treatment parameters were identified that can produce CoCrFeMnNi high-entropy alloys with a homogeneous composition and grain size and almost no texture. © 2015 Published by Elsevier B.V.
    view abstract10.1016/j.jallcom.2015.05.129
  • Microstructure, Shape Memory Effect and Functional Stability of Ti67Ta33 Thin Films
    Motemani, Y. and Kadletz, P.M. and Maier, B. and Rynko, R. and Somsen, C. and Paulsen, A. and Frenzel, J. and Schmahl, W.W. and Eggeler, G. and Ludwig, Al.
    Advanced Engineering Materials 17 (2015)
    Ti-Ta based alloys are an interesting class of high-temperature shape memory materials. When fabricated as thin films, they can be used as high-temperature micro-actuators with operation temperatures exceeding 100 °C. In this study, microstructure, shape memory effect and thermal cycling stability of room-temperature sputter deposited Ti<inf>67</inf>Ta<inf>33</inf> thin films are investigated. A disordered α martensite (orthorhombic) phase is formed in the as-deposited Ti<inf>67</inf>Ta<inf>33</inf> films. The films show a columnar morphology with the columns being oriented perpendicular to the substrate surface. They are approximately 200 nm in width. XRD texture analysis reveals a martensite fiber texture with {120} and {102} fiber axes. The XRD results are confirmed by TEM analysis, which also shows columnar grains with long axes perpendicular to the {120} and {102} planes of α martensite. The shape memory effect is analyzed in the temperature range of -10 to 240 °C using the cantilever deflection method, with special emphasis placed on cyclic stability. Ti<inf>67</inf>Ta<inf>33</inf> thin films undergo a forward martensitic transformation at M<inf>s</inf> ≈ 165 °C, with a stress relaxation of approximately 33 MPa during the transformation. The actuation response of the film actuators degrades significantly during thermal cycling. TEM analysis shows that this degradation is related to the formation of nanoscale ω precipitates (5-13 nm) which form above the austenite finish temperature. These precipitates suppress the martensitic transformation, as they act as obstacles for the growth of martensite variants. Ti-Ta thin films can be used as high-temperature micro-actuators. In this study, microstructure, shape memory effect, and functional stability of room-temperature sputter deposited Ti<inf>67</inf>Ta<inf>33</inf> thin films are systematically investigated. The actuation response of the film actuators degrades significantly during thermal cycling. This degradation is related to the formation of nanoscale ω precipitates (5-13 nm) which form above the austenite finish temperature. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201400576
  • Modeling thermally induced martensitic transformations in nickel titanium shape memory alloys
    Jaeger, S. and Eggeler, G. and Kastner, O.
    Continuum Mechanics and Thermodynamics 27 (2015)
    During stress-free thermal analysis with differential scanning calorimetry (DSC), nickel titanium (NiTi) shape memory alloys show a thermal hysteresis which is affected by cooling/heating rates. Moreover, the Ni content of near equiatomic alloys governs the phase transition temperatures. This contribution aims at establishing a constitutive equation which can account for these effects, building on earlier work by Müller, Achenbach and Seelecke (MAS). To be specific, we discuss our new method with a focus on NiTi alloys. As in the original MAS model, our approach is rooted in a non-convex free energy representation and rate equations are utilized to incorporate history dependence during non-equilibrium processes. The relaxation times of these rate equations are determined by characteristic transformation probabilities which in turn are governed by the free energy landscape of our system. We show how the model can be parameterized to rationalize experimental DSC data observed for NiTi samples of variable composition and measured at variable cooling/heating rates. The good agreement between model predictions and experimental results suggests that thermal hystereses are not only related to interfacial strain energy effects but also affected by the transient character of the transformation process incorporating specific thermal relaxation times. Our analysis shows that we observe strong hysteretic effects when the cooling/heating rates exceed these characteristic relaxation rates. © 2014, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00161-014-0375-4
  • Nanoindentation studies of the mechanical properties of the μ phase in a creep deformed Re containing nickel-based superalloy
    Rehman, H.U. and Durst, K. and Neumeier, S. and Parsa, A.B. and Kostka, A. and Eggeler, G. and Göken, M.
    Materials Science and Engineering A 634 (2015)
    Addition of Re in nickel-based superalloys results in an increase of the creep life. However, Re is also known to segregate to the dendrite core and to promote the formation of topologically closed packed (TCP) phases. In the present work, the local segregation of Re was studied in the heat treated and creep deformed state of a nickel-based superalloy. Tensile creep deformation at 1050°C resulted in the formation of TCP phases in the dendritic regions. Characterization using TEM confirmed the presence of μ phase that grows on {111} planes. Measurements with a nanoindenting AFM show that the μ phase is harder and shows less work hardening than both the γ and the γ' phases. Furthermore, in the creep deformed state the hardness of the matrix phase is very similar in the dendrite core and in interdendritic areas, although Re is still enriched in the dendrite core. It is shown that Re is consumed in the dendrite core by the TCP phases. © 2015 Elsevier B.V.
    view abstract10.1016/j.msea.2015.03.045
  • On the effect of alloy composition on martensite start temperatures and latent heats in Ni-Ti-based shape memory alloys
    Frenzel, J. and Wieczorek, A. and Opahle, I. and Maaß, B. and Drautz, R. and Eggeler, G.
    Acta Materialia 90 (2015)
    In the present work we explain the concentration dependence of the martensite start temperature (MS) in Ni-Ti-based shape memory alloys (SMAs). We briefly review the present level of understanding and show that there is a need for further work. We then investigate the strong dependence of MS on alloy composition in binary Ni-Ti, ternary Ni-Ti-X (X = Cr, Cu, Hf, Pd, V, Zr) and quaternary Ni-Ti-Cu-Y (Y = Co, Pd) SMAs. For binary Ni-Ti, we combine differential scanning calorimetry experiments with insight gained through the application of the density functional theory (DFT) to show that heats of transformation ΔH decrease as Ni concentrations increase from 50.0 to 51.2 at.%. This causes a shift in the Gibbs free energy curves of austenite GA(T) and martensite GM(T), which in turn results in a lower MS temperature. Our DFT results suggest that the strong decrease of ΔH is caused by a stabilization of the B2 phase by structural relaxations around Ni antisite atoms, together with a gradual destabilization of B19′. The martensite start temperatures and the latent heats of transformation for binary, ternary and quaternary Ni-Ti-based SMAs are closely related. We observe smaller latent heats when the geometrical differences between the crystal structures of austenite and martensite decrease. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.02.029
  • On the identification of superdislocations in the γ′-phase of single-crystal Ni-base superalloys - An application of the LACBED method to complex microstructures
    Müller, J. and Eggeler, G. and Spiecker, E.
    Acta Materialia 87 (2015)
    Ni-base superalloys are used for turbine blades, which operate in the creep range at temperatures above 1000 °C. One of the objectives of modern materials science is to analyze the combination of elementary deformation and microstructural coarsening processes and to identify physically based micromechanical models which allow one to predict the mechanical behavior on the macroscale. High-temperature creep of single-crystal Ni-base superalloys is governed by dislocation plasticity in the well-known γ/γ′-microstructure. For a comprehensive description of plasticity, it is important to understand the nucleation, glide and climb of superdislocations in the γ′-phase. The rate-controlling dislocation processes have to be identified and therefore a reliable Burgers vector analysis of superdislocations is essential. Superdislocations exhibit complex dislocation cores, typically comprising superpartial dislocations and planar defects. Therefore, conventional Burgers vector analysis based on the invisibility criterion often fails, due to the presence of pronounced residual contrast. In the present work, large-angle convergent-beam electron diffraction (LACBED) is employed for Burgers vector determination of two characteristic superdislocations, of the standard <1 1 0> and the more complex <1 0 0> type. LACBED results are compared with results obtained using the conventional invisibility analysis. While both techniques work for the standard superdislocation, the conventional analysis fails to analyze the <1 0 0> superdislocation, which shows pronounced residual contrast even under conditions of g · b = 0 and g · b × u = 0. In contrast, the LACBED technique allows for an unambiguous determination of the Burgers vector, including its magnitude and absolute sense. In the present study, the use of LACBED to identify dislocations in the complex microstructure of an Ni-base superalloy is outlined and the better performance of LACBED as compared to the conventional gb-analysis is discussed. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.12.029
  • On the role of Re in the stress and temperature dependence of creep of Ni-base single crystal superalloys
    Wollgramm, P. and Buck, H. and Neuking, K. and Parsa, A.B. and Schuwalow, S. and Rogal, J. and Drautz, R. and Eggeler, G.
    Materials Science and Engineering A 628 (2015)
    In the present study we investigate the creep behavior of a Ni-base single crystal superalloy. We evaluate the stress and temperature dependence of the minimum creep rate, which shows a power law type of stress dependence (characterized by a stress exponent n) and an exponential type of temperature dependence (characterized by an apparent activation energy Qapp). Under conditions of high temperature (1323K) and low stress (160MPa) creep, n and Qapp are determined as 5.3 and 529kJ/mol, respectively. For lower temperatures (1123K) and higher stresses (600MPa) the stress exponent n is higher (8.5) while the apparent activation energy of creep is lower (382kJ/mol). We show that there is a general trend: stress exponents n increase with increasing stress and decreasing temperature, while higher apparent activation energies are observed for lower stresses and higher temperatures. We use density functional theory (DFT) to calculate the activation energy of diffusion for Re in a binary Ni-Re alloy with low Re-concentrations. The resulting energy is almost a factor 2 smaller than the apparent activation energy of creep. We explain why it is not straightforward to rationalize the temperature dependence of creep merely on the basis of the diffusion of one alloying element. We show that the evolution of the microstructure also must be considered. © 2015 Elsevier B.V.
    view abstract10.1016/j.msea.2015.01.010
  • On the widths of the hysteresis of mechanically and thermally induced martensitic transformations in Ni-Ti-based shape memory alloys
    Jaeger, S. and Maaß, B. and Frenzel, J. and Schmidt, M. and Ullrich, J. and Seelecke, S. and Schütze, A. and Kastner, O. and Eggeler, G.
    International Journal of Materials Research 106 (2015)
    It is well known that a good crystallographic compatibility between austenite and martensite in Ni-Ti-based shape memory alloys results in narrow thermal hystereses (e. g. Ball and James, Arch. Ration. Mech. Anal., 1987). The present work suggests that a good crystallographic fit is moreover associated with a small mechanical hysteresis width, observed during a forward and reverse stress-induced transformation. Furthermore, shape memory alloys with a good crystallographic fit show smaller transformation strains. The results obtained in the present study suggest that these correlations are generic and apply to binary Ni-Ti (with varying Ni contents) and quaternary Ni-Ti-Cu-X (X = Cr, Fe, V) alloys. For binary Ni-Ti, it was observed that Ni-rich compositions (good lattice fit) show a lower accummulation of irreversible strains during pseudoelastic cycling. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.111284
  • Processing of NiTi shape memory sheets - Microstructural heterogeneity and evolution of texture
    Laplanche, G. and Kazuch, A. and Eggeler, G.
    Journal of Alloys and Compounds 651 (2015)
    In the present paper we study the evolution of microstructure and texture during processing of Ni<inf>51</inf>Ti<inf>49</inf> shape memory sheets using electron backscatter diffraction. Hot rolling results in a heterogeneous microstructure which reflects a temperature gradient in the sheet. Equiaxed and randomly oriented grains are observed close to the surface of the hot rolled sheet while the sheet interior shows a strong texture containing two main texture components {111}<110> and {110}<110> with grains elongated along the rolling direction. In contrast, cold rolling in combination with a recrystallization heat treatment produces a more homogeneous microstructure in terms of grain morphology and grain size. It also promotes a random grain orientation along the rolling and transverse directions while the normal direction shows a strong γ-fiber {111}<uvw> texture. To get a better understanding of the elementary deformation mechanisms which control the texture evolution during rolling, textures assessed in the present study are compared with simulations reported in the literature. © 2015, Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2015.08.127
  • The effect of cast microstructure and crystallography on rafting, dislocation plasticity and creep anisotropy of single crystal Ni-base superalloys
    Nörtershäuser, P. and Frenzel, J. and Ludwig, Al. and Neuking, K. and Eggeler, G.
    Materials Science and Engineering A 626 (2015)
    In the present work we investigate three mechanical and microstructural aspects of high temperature and low stress creep of the single crystal superalloy LEK 94. First, we compare the tensile creep behavior of specimens loaded in precise [001] and [110] directions and show that tensile creep specimens with precise [110] directions show significantly lower minimum creep rates. However, small deviations from precise [110] orientations result in a significant increase of creep rate. Second, we use a novel SEM technique to measure dislocation densities. We show that after short periods of creep, dislocation densities in dendritic regions are always higher than in interdendritic regions. This finding is probably associated with wider γ-channels, higher concentrations of W and Re and higher misfit stresses in the γ-channels of dendrites. Finally, we show that internal stresses associated with solidification can drive complex rafting processes during high temperature exposure, which differ between dendrite cores and interdendritic regions. © 2014 Elsevier B.V.
    view abstract10.1016/j.msea.2014.12.030
  • The nucleation of Mo-rich Laves phase particles adjacent to M23C6 micrograin boundary carbides in 12% Cr tempered martensite ferritic steels
    Isik, M.I. and Kostka, A. and Yardley, V.A. and Pradeep, K.G. and Duarte, M.J. and Choi, P.P. and Raabe, D. and Eggeler, G.
    Acta Materialia 90 (2015)
    We study the nucleation of Mo-rich Laves phase particles during aging and creep of 12 wt.% Cr tempered martensite ferritic steels (TMFS). Recently, in Isik et al. (2014) we reported that Laves phase particles tend to form at micrograin boundaries of TMFSs after Mo and Si had segregated from the ferritic matrix to these internal interfaces. In the present work, we employ transmission electron microscopy (TEM) and atom probe tomography (APT) to study the formation of Laves phase particles. We investigate the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides. Our results suggest that this joint precipitation effect is due to the combined segregation of Mo and Si from the matrix to the micrograin boundaries and Si and P enrichment around the growing carbides.
    view abstract10.1016/j.actamat.2015.01.027
  • Three-dimensional Cu foam-supported single crystalline mesoporous Cu2O nanothorn arrays for ultra-highly sensitive and efficient nonenzymatic detection of glucose
    Dong, C. and Zhong, H. and Kou, T. and Frenzel, J. and Eggeler, G. and Zhang, Z.
    ACS Applied Materials and Interfaces 7 (2015)
    Highly sensitive and efficient biosensors play a crucial role in clinical, environmental, industrial, and agricultural applications, and tremendous efforts have been dedicated to advanced electrode materials with superior electrochemical activities and low cost. Here, we report a three-dimensional binder-free Cu foam-supported Cu<inf>2</inf>O nanothorn array electrode developed via facile electrochemistry. The nanothorns growing in situ along the specific direction of <011> have single crystalline features and a mesoporous surface. When being used as a potential biosensor for nonenzyme glucose detection, the hybrid electrode exhibits multistage linear detection ranges with ultrahigh sensitivities (maximum of 97.9 mA mM-1 cm-2) and an ultralow detection limit of 5 nM. Furthermore, the electrode presents outstanding selectivity and stability toward glucose detection. The distinguished performances endow this novel electrode with powerful reliability for analyzing human serum samples. These unprecedented sensing characteristics could be ascribed to the synergistic action of superior electrochemical catalytic activity of nanothorn arrays with dramatically enhanced surface area and intimate contact between the active material (Cu<inf>2</inf>O) and current collector (Cu foam), concurrently supplying good conductivity for electron/ion transport during glucose biosensing. Significantly, our findings could guide the fabrication of new metal oxide nanostructures with well-organized morphologies and unique properties as well as low materials cost. © 2015 American Chemical Society.
    view abstract10.1021/acsami.5b05738
  • Transformation activity in ultrafine grained pseudoelastic NiTi wires during small amplitude loading/unloading experiments
    Pelegrina, J.L. and Yawny, A. and Olbricht, J. and Eggeler, G.
    Journal of Alloys and Compounds 651 (2015)
    Martensitic phase transformations were studied in ultrafine grained Ni-rich pseudoelastic NiTi wires during cyclic deformation under small imposed strain amplitudes. Small strain variation tests were complemented by a thermographic analysis of the emerging temperature distributions in the specimens. The characteristics of the observed thermal profiles result from the specific phase transitions which take place at different stress levels. Homogeneous temperature changes, corresponding to a non-localized transformation activity along the specimen length, were observed throughout the whole range of applied stresses, starting from values as low as 100 MPa. This behavior is in line with previous literature reports for the stress-induced transformation from B2 austenite to R-phase as long as the critical stress for B19′ martensite formation is not reached. In the present study, similar type of transformation activity could also be demonstrated at higher strains/stresses, even after the stress induced transformation to B19′ was apparently completed. These findings suggest that transformation activity involving the B2 phase is present throughout the whole pseudoelastic stress-strain cycle; i.e., it is not restricted to the initial loading portion. Finally, non-localized transformation to or from B19′ was identified during small amplitude strain variations in the plateau-like coexistence ranges of the pseudoelastic cycle. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2015.08.159
  • Variational prediction of the mechanical behavior of shape memory alloys based on thermal experiments
    Junker, P. and Jaeger, S. and Kastner, O. and Eggeler, G. and Hackl, K.
    Journal of the Mechanics and Physics of Solids 80 (2015)
    In this work, we present simulations of shape memory alloys which serve as first examples demonstrating the predicting character of energy-based material models. We begin with a theoretical approach for the derivation of the caloric parts of the Helmholtz free energy. Afterwards, experimental results for DSC measurements are presented. Then, we recall a micromechanical model based on the principle of the minimum of the dissipation potential for the simulation of polycrystalline shape memory alloys. The previously determined caloric parts of the Helmholtz free energy close the set of model parameters without the need of parameter fitting. All quantities are derived directly from experiments. Finally, we compare finite element results for tension tests to experimental data and show that the model identified by thermal measurements can predict mechanically induced phase transformations and thus rationalize global material behavior without any further assumptions. © 2015 Elsevier Ltd.
    view abstract10.1016/j.jmps.2015.04.015
  • [001] Preferentially-oriented 2D tungsten disulfide nanosheets as anode materials for superior lithium storage
    Yang, W. and Wang, J. and Si, C. and Peng, Z. and Frenzel, J. and Eggeler, G. and Zhang, Z.
    Journal of Materials Chemistry A 3 (2015)
    Rechargeable lithium ion batteries (LIBs) have transformed portable electronics and will play a crucial role in transportation, such as electric vehicles. For higher energy storage in LIBs, two issues should be addressed, that is, the fundamental understanding of the chemistry taking place in LIBs and the discovery of new materials. Here we design and fabricate two-dimensional (2D) WS<inf>2</inf> nanosheets with preferential [001] orientation and perfect single crystalline structures. Being used as an anode for LIBs, the WS<inf>2</inf>-nanosheet electrode exhibits a high specific capacity, good cycling performance and excellent rate capability. Considering the controversy in the lithium storage mechanism of WS<inf>2</inf>, ex-situ X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) analyses clearly verify that the recharge product (3.0 V vs. Li+/Li) of the WS<inf>2</inf> electrode after fully discharging to 0.01 V (vs. Li+/Li) tends to reverse to WS<inf>2</inf>. More remarkably, the [001] preferentially-oriented 2D WS<inf>2</inf> nanosheets are also promising candidates for applications in photocatalysis, water splitting, and so forth. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c5ta04176g
  • Comparative studies on the accumulation of strain and recovery ratio of Veriflex®, a shape-memory polymer for a high strain (∈m = 210%): Atomic force microscopic experiments
    Chowdhury, A.M.S. and Schmidt, C. and Neuking, K. and Eggeler, G.
    High Performance Polymers 26 (2014)
    In the present study, the functional fatigue in the commercial SMP Veriflex®, which is associated with repeating up to maximum 40 programming/one-way effect (1WE) cycles, has been examined. The material is characterized by a glass transition temperature (Tg) of 67 C, above which it looses all its strength. An interesting comparative investigation on thermomechanical cycles, including programming, cooling, unloading, and heating to trigger the 1WE, were carried out for Veriflex at 62 C (T < Tg, below 5 C but near to Tg) and also at 72 C (T > Tg, above 5 C but near to Tg) for strains of 140% and for a recovery time of 10 min. Accumulation of strain was estimated during the thermomechanical treatments for using strains of 140% at 62 C (T < Tg) as well as at 72 C (T > Tg). Recovery ratios for strains of 140% at 62 C (T < Tg) as well as at 72 C (T > Tg) were also estimated. It turns out that programming, cooling, unloading, and heating to trigger the 1WE causes an increase in irreversible strain and is associated with a corresponding decrease in the intensity of the 1WE in particular during the first thermomechanical cycles. Confocal laser scanning microscopic) study shows a very little wavy surface structure evolved during cycling up to strains of 140% at 72 C (T > Tg). Infrared study features the chemical nature after cycling, processing, and programming of Veriflex. © The Author(s) 2013.
    view abstract10.1177/0954008313494907
  • Cyclic degradation mechanisms in aged FeNiCoAlTa shape memory single crystals
    Krooß, P. and Somsen, C. and Niendorf, T. and Schaper, M. and Karaman, I. and Chumlyakov, Y. and Eggeler, G. and Maier, H.J.
    Acta Materialia 79 (2014)
    This study focuses on the functional stability of [0 0 1]-oriented Fe 41Ni28Co17Al11.5Ta2.5 (at.%) single crystals. It is shown that functional degradation of aged FeNiCoAlTa, containing fine dispersed γ′-particles ∼5-8 nm in diameter is caused by the interaction of different martensite variants under cyclic loading in tension. Superelastic cycling experiments up to 4.5% total strain resulted in the accumulation of permanent strain mainly caused by the formation of retained martensite. In situ observations were conducted in order to evaluate the local strain evolution and martensite variant interactions on the meso- and microscale. Optical microscopy and transmission electron microscopy observations revealed various differently oriented martensite variants which were retained upon 100 superelastic cycles. In addition, fine martensitic structures remaining in the vicinity of the γ′ precipitates were found after mechanical cycling, which are shown to be important for cyclic degradation in Fe-based shape memory alloys. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.06.019
  • Direct microstructural evidence for the stress induced formation of martensite during nanonindentation of NiTi
    Pfetzing-Micklich, J. and Wieczorek, N. and Simon, T. and Maaß, B. and Eggeler, G.
    Materials Science and Engineering A 591 (2014)
    When a pseudoelastic NiTi alloy is loaded and subsequently unloaded, stress induced martensite forms and disappears. It is challenging to directly observe and characterize the local martensitic features in such alloys. In the present study we use a specific NiTi alloy where stress induced martensite is thermally stable during unloading to prove the formation of martensite during nanoindentation. TEM investigations provide direct microstructural evidence for the martensitic phase transformation during nanoindentation. Subsequent in-situ heating in TEM shows how the stress induced martensite transforms back to austenite. © 2013 Elsevier B.V.
    view abstract10.1016/j.msea.2013.10.035
  • Effect of nitrogen doping on the reducibility, activity and selectivity of carbon nanotube-supported iron catalysts applied in CO2 hydrogenation
    Chew, L.M. and Kangvansura, P. and Ruland, H. and Schulte, H.J. and Somsen, C. and Xia, W. and Eggeler, G. and Worayingyong, A. and Muhler, M.
    Applied Catalysis A: General 482 (2014)
    CO2 hydrogenation to short-chain hydrocarbons was investigated over iron catalysts supported on oxygen- and nitrogen-functionalized multi-walled carbon nanotubes (CNTs) and on silica, which were synthesized by the dry impregnation method using ammonium ferric citrate as precursor. The reduction of the calcined catalysts was examined in detail using temperature-programmed reduction in H2 and in situ X-ray absorption near-edge structure (XANES) analysis. The XANES results revealed that the mixture of hematite and magnetite was gradually transformed into wustite and metallic iron during heating in H2. Iron oxide nanoparticles supported on nitrogen-functionalized CNTs were easier to reduce compared to those on oxygen-functionalized CNTs indicating a promoting effect of the nitrogen functional groups. The interaction between iron oxide and silica was found to be much stronger inhibiting the reduction to metallic iron. As a result, the catalytic activity of iron nanoparticles supported on CNTs in CO2 hydrogenation at 360 °C, 25 bar and a H2:CO 2 ratio of 3 was almost twofold higher compared with iron supported on silica. CO2 was converted into C1-C5 hydrocarbons with CO and methane as major products over all catalysts. The Fe/NCNT catalyst achieved the highest olefin selectivity of 11% in the hydrocarbons range of C2-C5. In contrast, mostly paraffins were formed over the Fe/SiO2 catalyst. © 2014 Elsevier B.V.
    view abstract10.1016/j.apcata.2014.05.037
  • Effect of ternary element addition on the corrosion behaviour of NiTi shape memory alloys
    Kassab, E. and Neelakantan, L. and Frotscher, M. and Swaminathan, S. and Maaß, B. and Rohwerder, M. and Gomes, J. and Eggeler, G.
    Materials and Corrosion 65 (2014)
    The goal of this study is to compare the corrosion behaviour of selected ternary nickel titanium (NiTi)-based alloys (Ni45Ti 50Cu5, Ni47Ti50Fe3 and Ni39Ti50Pd11) with a binary, pseudoelastic Ni50.7Ti49.3 alloy. We examine the influence of the ternary elements on the corrosion behaviour using standard electrochemical techniques. All measurements were done in a physiological solution (0.9% NaCl) simulating a body temperature of 37 ± 1 °C. The influence of Cu and Pd addition on the surface oxide film was characterised by X-ray photoelectron spectroscopy (XPS). The results revealed that, the localised corrosion resistance of these ternary alloys is lower than the binary NiTi alloy. By comparing the different NiTi-based alloys, the following relation has been proposed for their localised corrosion resistances: NiTiCu < NiTiFe < NiTiPd < NiTi. Depth profiling by XPS showed that the surface oxide film on all the investigated NiTi-based alloys is mainly of TiO2, however, the NiTiPd and NiTiCu alloys showed metallic ternary element distributed within TiO2 layer. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/maco.201206587
  • Experimental investigation and numerical simulation of the mechanical and thermal behavior of a superelastic shape memory alloy beam during bending
    Ullrich, J. and Schmidt, M. and Schütze, A. and Wieczorek, A. and Frenzel, J. and Eggeler, G. and Seelecke, S.
    ASME 2014 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2014 2 (2014)
    Superelastic Shape Memory Alloys (SMA) are typically used in applications where the martensitic phase transformation is exploited for its reversible, large deformation such as medical applications (e.g. stents). In this work, we focus on the mechanical and thermal behavior of a Nickel-Titanium SMA strip in bending mode. One possible application of this mode is to provide a restoring force when used in joints of SMA wire actuator systems making the need for an antagonistic SMA actuator redundant. In these applications mentioned above, typically only the mechanical properties are of interest while the temperature is considered constant, even though the martensitic phase transformation in SMA is a thermomechanically coupled process. As a part of the DFG (German Research Association) Priority Programme SPP1599 "Ferroic Cooling" which aims at advancing the development of solid state cooling devices, we have an equally large interest for the thermal evolution of Nickel-Titanium SMA during deformation and its induced phase transformation. In this paper we investigate the thermal and the mechanical response of a SMA beam during bending experiments in which the deformation is induced by holding one end of a SMA strip fixed while the other end is subject to a prescribed deflection. Sensors and high speed thermal cameras are used to capture reaction forces, deformations and temperature changes. We compare these experimental results with numerical simulation results obtained from Finite Element simulations where a thermo-mechanically coupled SMA model is implemented into a finite deformation framework. © 2014 by ASME.
    view abstract10.1115/SMASIS20147619
  • Functional and structural fatigue of titanium tantalum high temperature shape memory alloys (HT SMAs)
    Niendorf, T. and Krooß, P. and Batyrsina, E. and Paulsen, A. and Motemani, Y. and Ludwig, Al. and Buenconsejo, P. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Materials Science and Engineering A 620 (2014)
    Due to their high work output and good mechanical properties, actuators made from shape memory alloys (SMAs) are used in numerous applications. Unfortunately, SMAs such as nickel-titanium (Ni-Ti) can only be employed at temperatures up to about 100°C. Lately, high-temperature shape memory alloys (HT SMAs) have been introduced to overcome this limitation. Ternary systems based on Ni-Ti have been intensively characterized and alloys are available that can operate at elevated temperatures. However, these alloys either contain substantial amounts of expensive noble elements like platinum and palladium, or the materials are brittle. The titanium-tantalum (Ti-Ta) system has been developed to overcome these issues. Binary Ti-Ta provides relatively high MS temperature combined with excellent workability, but it suffers from fast cyclic degradation. By alloying with third elements this drawback can be overcome: The ternary Ti-Ta-Al alloy shows overall promising properties as will be shown in the present work. In-situ thermo-mechanical cycling experiments were conducted and allowed for evaluation of the factors affecting the functional and structural fatigue of this alloy. Functional fatigue is dominated by ω-phase evolution, while structural fatigue is triggered by an interplay of ω-phase induced embrittlement and deformation constraints imposed by unsuitable texture. In addition, a concept for fatigue life extension proposed very recently for binary Ti-Ta, is demonstrated to be also applicable for the ternary Ti-Ta-Al. © 2014 Elsevier B.V.
    view abstract10.1016/j.msea.2014.10.038
  • Infrared transmission spectroscopy of charge carriers in self-assembled InAs quantum dots under surface electric fields
    Pal, S. and Valentin, S.R. and Kukharchyk, N. and Nong, H. and Parsa, A.B. and Eggeler, G. and Ludwig, Ar. and Jukam, N. and Wieck, A.D.
    Journal of Physics Condensed Matter 26 (2014)
    We present a study on the intersublevel spacings of electrons and holes in a single layer of InAs self-assembled quantum dots. We use Fourier transform infrared transmission spectroscopy via a density chopping scheme for direct experimental observation of the intersublevel spacings of electrons without any external magnetic field. Epitaxial, complementary-doped and semi-transparent electrostatic gates are grown within the ultra high vacuum conditions of molecular beam epitaxy to voltage-tune the device, while a two dimensional electron gas (2DEG) serves as a back contact. Spacings of the hole sublevels are indirectly calculated from the photoluminescence spectrum by using a simple model given by Warburton et al [1]. Additionally, we observe that the intersubband resonances of the 2DEG are enhanced due to the quantum dot layer on top of the device. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/26/50/505801
  • Ingot metallurgy and microstructural characterization of Ti-Ta alloys
    Zhang, J. and Rynko, R. and Frenzel, J. and Somsen, C. and Eggeler, G.
    International Journal of Materials Research 105 (2014)
    In the present work we perform a detailed investigation of ingot metallurgy processing routes of Ti-30Ta, a shape memory alloy with a good potential for applications at higher temperatures. There is currently considerable interest in high temperature shape memory alloys, both in industry (automotive and aerospace applications) and in academia. By means of scanning electron microscopy, we provide recommendations on the number of remelting cycles in the vacuum arc melting process, and on annealing temperatures/times in order to obtain chemical homogeneity. It is also shown that this is required to obtain well defined differential scanning calorimeter charts, which facilitates characterization and investigations of martensitic transformation in this alloy. Areas in need of further work are identified. Copyright © 2014 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.111010
  • Interface reactions of Ag@TiO2 nanocomposite films
    Zuo, J. and Rao, J. and Eggeler, G.
    Materials Chemistry and Physics 145 (2014)
    TiO2 films were sputtered on 100-nm-thick Ag layers at various O2 partial pressures to study forming processes at the interface. The interfacial reactions during the deposition process were investigated by means of transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and UV-vis reflection spectra. The sputtering process led to formation of Ag nanoparticles surrounded by Ag 2O and TiO2 in the TiO2 film matrix as well as on the surface. The presence of oxygen in the plasma resulted in enrichment of silver oxides on the surface and an intermixing of Ag in the TiO2 matrix. The film structures could be explained based on the interplay among the formation of silver oxide, the nucleation and growth of TiO2, as well as the mobility of silver and silver oxides within the growing TiO2 films. © 2014 Elsevier B.V.
    view abstract10.1016/j.matchemphys.2014.01.041
  • Large-scale synthesis and catalytic activity of nanoporous Cu-O system towards CO oxidation
    Kou, T. and Si, C. and Gao, Y. and Frenzel, J. and Wang, H. and Yan, X. and Bai, Q. and Eggeler, G. and Zhang, Z.
    RSC Advances 4 (2014)
    Nanoporous Cu-O system catalysts with different oxidation states of Cu have been fabricated through a combination of dealloying as-milled Al66.7Cu33.3 alloy powders and subsequent thermal annealing. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) have been used to characterize the microstructure and surface chemical states of Cu-O catalysts. The peculiar nanoporous structure can be retained in Cu-O catalysts after thermal treatment. Catalytic experiments reveal that all the Cu-O samples exhibit complete CO conversion below 170 °C. The optimal catalytic performance could be achieved through the combination of annealing in air with hydrogen treatment for the Cu-O catalyst, which shows a near complete conversion temperature (T90%) of 132 °C and an activation energy of 91.3 KJ mol-1. In addition, the present strategy (ball milling, dealloying and subsequent thermal treatment) could be scaled up to fabricate high-performance Cu-O catalysts towards CO oxidation. This journal is © The Royal Society of Chemistry 2014.
    view abstract10.1039/c4ra12227e
  • Mesostructural design and manufacturing of open-pore metal foams by investment casting
    Matz, A.M. and Mocker, B.S. and Müller, D.W. and Jost, N. and Eggeler, G.
    Advances in Materials Science and Engineering 2014 (2014)
    The present paper describes the manufacturing process of open-pore metal foams by investment casting and the mesostructural/morphological evolution resulting from a new technique of modifying the precursor. By this technique, the precursor is coated with a polymer layer whereby a thickening of the struts occurs. Relative densities in the range of 1.85≤ρrel≤25% of open-pore metal foams can be achieved with high accuracy. The samples investigated have pore densities of ρP=7 ppi, 10 ppi, and 13 ppi. The relevant processing parameters needed for a homogenous formation of the polymer layer are determined for two different coating materials and the resulting open-pore foam's mesostructure is characterized qualitatively and quantitatively. The alloy used for investment casting open-pore metal foamsis AlZn11. The microstructural evolution of these foams is evaluated as a function of the mesostructure. Differences in the microstructure are observed for foams with low and high relative densities and discussed in terms of cooling subsequent to investment casting. © 2014 Alexander Martin Matz et al.
    view abstract10.1155/2014/421729
  • Micromechanical investigations and modelling of a Copper-Antimony-Alloy under creep conditions
    Vöse, M. and Otto, F. and Fedelich, B. and Eggeler, G.
    Mechanics of Materials 69 (2014)
    In many practical applications, creep damage is the limiting factor of a component's lifetime. A micromechanical model of creep induced grain boundary damage is proposed, which allows for the simulation of creep damage in a polycrystal within the framework of finite element analysis. The model considers grain boundary cavitation and sliding according to a micromechanically motivated cohesive zone model while creep deformation of the grains is described following the slip system theory. The model can be applied to idealised polycrystalline structures, such as a Voronoi tessellation or, like demonstrated here, to real grain structures of miniature creep specimens. Creep tests with pure Cu single crystals and with a coarse-grained polycrystalline Cu-1 wt.% Sb alloy at 823 K have been performed and used to calibrate the polycrystal model. The grain structure of the polycrystalline Cu-Sb specimens has been revealed by the EBSD method. Extensive grain boundary sliding and cavitation has been observed in the crept specimens. Grain boundary sliding has been found to promote wedge-type damage at grain boundary triple junctions and to contribute significantly to the total creep strain. Furthermore, the assumed stress sensitivity of the models grain boundary cavity nucleation rate strongly influences the development of wedge-type damage. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.mechmat.2013.09.013
  • Microshear deformation of gold single crystals
    Heyer, J.-K. and Brinckmann, S. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 62 (2014)
    We perform microshear experiments on Au single crystals, directly imposing shear loading on the microscopic crystallographic h1-10i {111} slip system. We use a focused ion beam machined micro-double shear specimen which we load with a flat punch indenter inside a scanning electron microscope. Our method yields reproducible mechanical data (e.g. critical shear stresses of 63.5 ± 2.5 MPa). We study small-scale plasticity up to high strains (>50%) at constant slip geometry and document localized plastic deformation and sudden plastic deformation events. Strain bursts are observed, which can be related to the formation of new shear bands. Alternatively, they can result from sudden shear strain accumulation events in existing shear bands. Due to the stochastic nature of plastic deformation, the nature and the number of strain bursts can vary. We show and discuss how our in situ test technique captures these effects and how this affects the corresponding load-displacement curves. We discuss the advantages and inconveniences of our microshear test technique compared to other small-scale testing methods and relate our mechanical results to previous results reported for the micromechanical behavior of Au. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.10.002
  • Multi-component nanoporous platinum-ruthenium-copper-osmium-iridium alloy with enhanced electrocatalytic activity towards methanol oxidation and oxygen reduction
    Chen, X. and Si, C. and Gao, Y. and Frenzel, J. and Sun, J. and Eggeler, G. and Zhang, Z.
    Journal of Power Sources 273 (2014)
    Multi-component nanoporous platinum-ruthenium-copper-osmium-iridium (np-PtRuCuOsIr) electrocatalyst has been facilely fabricated by chemical dealloying of mechanically alloyed AlCuPtRuOsIr precursor. The np-PtRuCuOsIr catalyst exhibits a typical three-dimensional bi-continuous interpenetrating ligament/channel structure with a length scale of ∼2.5 nm. The np-PtRuCuOsIr catalyst reaches a higher level in the mass activity (857.5 mA mgPt-1) and specific activity (3.0 mA cm-2) towards methanol oxidation compared to the commercial PtC catalyst (229.5 mA mgPt-1 and 0.5 mA cm-2 respectively). Moreover, the CO stripping peak of np-PtRuCuOsIr is 0.54 V (vs. SCE), 130 mV negative shift in comparison with the commercial PtC (0.67 V vs. SCE). The half-wave potential of np-PtRuCuOsIr is 0.900 V vs. RHE, 36 mV positive compared with that of the commercial PtC (0.864 V vs. RHE). The np-PtRuCuOsIr catalyst also shows 1.8 and 3.8 times enhancement in the mass and specific activity towards oxygen reduction than the commercial PtC. Moreover, the np-PtRuCuOsIr alloy exhibits superior oxygen reduction activities even after 15 K cycles, indicating its excellent long-term stability. The present np-PtRuCuOsIr can act as a promising candidate for the electrocatalyst in direct methanol fuel cells (DMFCs). © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2014.09.076
  • On the electropolishing of NiTi braided stents - Challenges and solutions
    Kassab, E. and Marquardt, A. and Neelakantan, L. and Frotscher, M. and Schreiber, F. and Gries, T. and Jockenhoevel, S. and Gomes, J. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 45 (2014)
    Nickel Titanium (NiTi) alloys possess special mechanical properties and good biocompatibility hence used as base material for the production of vascular stents. Normally, vascular stents are machined from NiTi tubes, using laser cutting processes. Braiding is a promising alternative for the machining of certain NiTi stents. However, a surface finish treatment, such as electropolishing of the braided stents, is still required in order to achieve a medical-grade surface finish. The thermally-grown oxide resulting from the shape-setting heat treatment, following the braiding must be removed. Moreover, electropolishing is required to achieve optimum corrosion resistance. Therefore, the aim of this study is to find suitable parameters for the effective electropolishing of NiTi textile stents. Electropolishing of a device with such a complex geometry is challenging, hence a custom-designed electrolytic cell was constructed and used in this study. We examined the stent surfaces before and after electropolishing, using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Potentiodynamic tests were performed in NaCl 0.9% solution for as-received and electropolished samples. The results from the present study indicate an improvement in surface quality of the braided stents after electropolishing. Potentiodynamic tests revealed that electropolishing improves the corrosion resistance of the NiTi stents. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201400220
  • On the functional degradation of binary titanium-tantalum high-temperature shape memory alloys - A new concept for fatigue life extension
    Niendorf, T. and Krooß, P. and Batyrsina, E. and Paulsen, A. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Functional Materials Letters 7 (2014)
    High-temperature shape memory alloys are promising candidates for actuator applications at elevated temperatures. Ternary nickel-titanium-based alloys either contain noble metals which are very expensive, or suffer from poor workability. Titanium-tantalum shape memory alloys represent a promising alternative if one can avoid the cyclic degradation due to the formation of the omega phase. The current study investigates the functional fatigue behavior of Ti-Ta and introduces a new concept providing for pronounced fatigue life extension. © 2014 The Authors.
    view abstract10.1142/S1793604714500428
  • On the nature of γ′ phase cutting and its effect on high temperature and low stress creep anisotropy of Ni-base single crystal superalloys
    Agudo Jácome, L. and Nörtershäuser, P. and Somsen, C. and Dlouhý, A. and Eggeler, G.
    Acta Materialia 69 (2014)
    The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [0 0 1] and [1 1 0] directions at 1293 K and 160 MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [1 1 0] as compared to [0 0 1] tensile loading were identified. [1 1 0] tensile creep is associated with a higher number of γ′ phase cutting events, where two dislocations with equal Burgers vectors of type <1 1 0> jointly shear the γ′ phase. The resulting <2 2 0>-type superdislocation can move by glide. In contrast, during [0 0 1] tensile loading, two dislocations with different <1 1 0>-type Burgers vectors must combine for γ′ phase cutting. The resulting <2 0 0>-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the γ′ phase cutting events. The higher [1 1 0] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in γ′) and a higher superdislocation mobility (<2 2 0>glide) in the γ′ phase. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.01.021
  • On the nucleation of Laves phase particles during high-temperature exposure and creep of tempered martensite ferritic steels
    Isik, M.I. and Kostka, A. and Eggeler, G.
    Acta Materialia 81 (2014)
    This paper reports on the formation of an Mo-rich Laves phase during high-temperature exposure and creep of a tempered martensite ferritic steel with 12 wt.% Cr and 1 wt.% Mo. The material was exposed to 550 °C for time intervals between 864 and 81,984 h. For comparison, a few creep tests were carried out at 550 °C and 120 MPa (duration between 864 and 12,456 h). All tests were interrupted after specific time periods and microstructures were investigated using transmission electron microscopy and atom probe tomography. Laves phase formation occurs during both heat treatment and creep. Creep stress and strain have no significant effect on the early stages of Laves phase formation. In the present work we show that prior to Laves phase nucleation Si and Mo segregate to micrograin boundaries, where subsequently Laves phase particles appear next to M23C6carbides. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.08.008
  • On the physical nature of tribolayers and wear debris after sliding wear in a superalloy/steel tribosystem at 25 and 300°C
    Rynio, C. and Hattendorf, H. and Klöwer, J. and Eggeler, G.
    Wear 317 (2014)
    Dry sliding wear of metals is strongly affected by the formation of oxide particles and their incorporation into compacted oxide layers, so-called glaze layers. A high-temperature reciprocating pin-on-disc tribometer was used to study the tribological response of a Ni-based Alloy 80A pin on a cast iron disc at ambient temperature and at 300. °C. Alloy 80A is used for valves and specific cast irons are used for valve seat-inserts in automotive diesel engines, where wear limits the service life of the valve/seat-insert tribosystems. Measurements of the friction coefficient, the total linear wear and the electrical contact resistance were used to monitor the formation of oxide layers during the experiments. Electron dispersive X-ray (EDX) element mappings from the surface regions with wear scars provide clear evidence for the formation of glaze layers and material transfer between pin and disc. Focused ion beam (FIB) micromachining was used to cut out thin lamellae from specific surface regions of glaze layers and from metallic wear particles. These lamellae were investigated in a transmission electron microscope (TEM). It was shown that the glaze layers generated at 25 and 300. °C exhibit distinct differences, which led to a reduction in wear rate by a factor of five at the higher temperature. We also report on the mechanical mixing of oxide particles and metal matrix that results in a metal/oxide nanocomposite directly below the sliding surfaces. Such composite structures were also observed inside of metallic wear debris. © 2014 Elsevier B.V.
    view abstract10.1016/j.wear.2014.04.022
  • Orientation dependence of stress-induced martensite formation during nanoindentation in NiTi shape memory alloys
    Laplanche, G. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 68 (2014)
    In the present work we used nanoindentation with a spherical indenter tip to study the formation of stress-induced martensite in NiTi shape memory alloys. Prior to nanoindentation, orientation imaging was performed to select austenite grains with specific crystallographic orientations, including the principal crystallographic directions [0 0 1], [1 0 1] and [1 1 1]. We studied a material where stress-induced martensite is stable at room temperature and found surface patterns with four-, two- and threefold symmetries for the [0 0 1], [1 0 1] and [1 1 1] crystallographic indentation directions, respectively. Atomic force microscopy investigations of the topography showed that the surface patterns were associated with sink-ins. The crystallographic sink-in patterns disappeared during heating, which proved their martensitic origin. Our results provide clear experimental evidence which shows that the crystallographic anisotropy of nanoindentation is governed by the crystallographic anisotropy of the stress-induced formation of martensite.©2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.01.006
  • Sigma phase evolution in Co-Re-Cr-based alloys at 1100 C
    Depka, T. and Somsen, C. and Eggeler, G. and Mukherji, D. and Rösler, J.
    Intermetallics 48 (2014)
    Co-Re-based alloys have been introduced as a novel metallic system that possesses a wide exploitable composition range and high melting temperatures. The poor oxidation resistance of the binary system can be improved by alloying chromium. However, adding chromium also leads to the occurrence of the sigma phase of type Cr2Re3. In the present study, we investigate the evolution of the sigma phase during creep and aging at 1100 C for three selected alloys based on the ternary composition Co-17Re-23Cr (at.%). In all alloys, sigma phase populates the grain boundaries of the hexagonally close-packed (HCP) matrix phase in a blocky morphology. Additionally, a fine dispersion of lamellar sigma phase in the grain interiors has formed during the initial processing or forms during thermal exposure. This precipitation takes place by a cellular reaction that transforms a supersaturated HCP phase into alternating lamellae of a near-equilibrium HCP phase and the sigma phase. The process therefore has the character of a discontinuous precipitation. Using orientation imaging microscopy, we observe an orientation relationship between the lamellae, which describes the basal plane of the HCP phase to form a coherent interface with the base layer of atoms of the tetragonal sigma phase. After long-term thermal exposure to 1100 C, overaging of the lamellar structure results in spheroidization of the sigma lamellae and subsequent Ostwald ripening. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2013.11.003
  • Spectroelectrochemical and morphological studies of the ageing of silver nanoparticles embedded in ultra-thin perfluorinated sputter deposited films
    Ebbert, C. and Alissawi, N. and Somsen, C. and Eggeler, G. and Strunskus, T. and Faupel, F. and Grundmeier, G.
    Thin Solid Films 571 (2014)
    This paper focuses on the investigation of the ageing behaviour of silver nanoparticle containing polytetrafluoroethylene thin films during exposure to phosphate buffer solution (pH = 7.5). In order to investigate the effect of the electrical connection between the silver nanoparticles via a conductive substrate, two kinds of composite films were compared. One model where the nanoparticles are directly deposited on an inert conducting substrate and then covered by an ultra-thin polytetrafluoroethylene like film. In the second case a polytetrafluoroethylene/silver nanoparticle/polytetrafluoroethylene sandwich film was prepared on the same substrate to prevent electrical connection of the silver nanoparticles. Degradation was followed in-situ by means of the combination of ultraviolet-visible spectroscopy and electrochemical impedance spectroscopy. In the case of electrically connected nanoparticles electrochemical Ostwald ripening took place, while this process was not observed for the insulated nanoparticles. The electrochemical impedance spectroscopy studies allowed for the parallel study of the correlated loss of barrier properties. Transmission electron microscopy images of both composite films confirmed the results obtained by means of the in situ electrochemical ultraviolet-visible studies. © 2014 Elsevier B.V.
    view abstract10.1016/j.tsf.2014.10.054
  • Sudden stress-induced transformation events during nanoindentation of NiTi shape memory alloys
    Laplanche, G. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 78 (2014)
    This study investigates the stress-induced formation of martensite during nanoindentation of an austenitic NiTi shape memory alloy, where stress-induced martensite is stable at room temperature. An individual grain with a [1 1 1] surface normal was selected for spherical ex situ and in situ nanoindentation in a scanning electron microscope. The in situ load-displacement curves show several pop-ins which occur concomitantly with the formation of traces around the contact zone between the indenter tip and the sample. These traces exhibit a threefold symmetry around the remnant indent. A detailed study of the indentation-induced surface relief by atomic force microscopy before and after shape recovery allows to identify the formation of six twinned martensite plates. Post-mortem microstructural characterization shows that these twinned martensite plates are growing as the applied load is increasing. The activation of the experimentally observed twinned martensite plates is rationalized by analytical calculations of resolved shear stress and mechanical interaction energy density. Finally, the in situ nanoindentation results in combination with the post-mortem microstructural characterization show that the most likely deformation mechanism responsible for pop-in events corresponds to sudden increases of the thicknesses of twinned martensite plates. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.05.061
  • Temperature dependencies of the elastic moduli and thermal expansion coefficient of an equiatomic, single-phase CoCrFeMnNi high-entropy alloy
    Laplanche, G. and Gadaud, P. and Horst, O. and Otto, F. and Eggeler, G. and George, E.P.
    Journal of Alloys and Compounds 623 (2014)
    The equiatomic CoCrFeMnNi alloy is now regarded as a model face-centered cubic single-phase high-entropy alloy. Therefore, determination of its intrinsic properties such as the temperature dependencies of elastic moduli and thermal expansion coefficient are important to improve understanding of this new class of material. These temperature dependencies were measured over a large temperature range (200-1270 K) in this study. © 2014 Elsevier B.V.
    view abstract10.1016/j.jallcom.2014.11.061
  • The evolution of tribolayers during high temperature sliding wear
    Rynio, C. and Hattendorf, H. and Klöwer, J. and Eggeler, G.
    Wear 315 (2014)
    High temperature reciprocating sliding wear experiments of a Ni-based superalloy pin against a cast iron disc were performed at 600 and 800. °C (load: 20. N, frequency: 20. Hz, stroke: 1. mm). The evolution of tribolayers was investigated using scanning and transmission electron microscopy (SEM and TEM) and energy dispersive X-ray spectroscopy (EDX). Four distinct subsurface zones are identified and discussed in terms of plastic strain accumulation and microstructure evolution. The development of protective nanocrystalline oxide-layers (glaze-layers) on top of the wear surfaces leads to very low wear rates due to a suppression of the direct metal-metal contact between the pin and the disc. The nanohardness, microstructure and chemical composition of the glaze-layers are reported. © 2014 Elsevier B.V.
    view abstract10.1016/j.wear.2014.03.007
  • A critical assessment of experimental methods for determining the dynamic mechanical characteristics of shape memory polymers
    Kazakevičiute-Makovska, R. and Mogharebi, S. and Steeb, H. and Eggeler, G. and Neuking, K.
    Advanced Engineering Materials 15 (2013)
    Experimental study of temperature- and frequency-dependent properties of the commercially available shape memory polymer Tecoflex™ EG 72D (TFX) (Lubrizol, USA) using dynamic mechanical analysis (DMA) technique is presented. Temperature scan DMA tests have been carried out in three distinct deformation modes, uniaxial tension, three-point bending, and simple torsion, using two different testing rigs at different test parameters (frequency, strain amplitude, and heating rate) in temperatures ranging from below to above the glass transition temperature. The influence of different test parameters and some discrepancies in the temperature-dependent storage and loss moduli (and hence the loss factor) measured by DMA in different deformation modes are discussed. Shape memory polymer Tecoflex™ is experimentally investigated by running DMA tests in three distinct deformation modes, uniaxial tension, three-point bending, and simple torsion, using two test rigs, Eplexor 500N of Gabo Qualimeter® and Modular Compact Rheometer MCR-301 of Anton Paar. It is observed that the temperature-dependent storage and loss moduli measured in different deformation modes show certain discrepancies that cannot be easily reconciled. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200341
  • Bending rotation HCF testing of pseudoelastic Ni-Ti shape memory alloys
    Rahim, M. and Frenzel, J. and Frotscher, M. and Heuwer, B. and Hiebeler, J. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 44 (2013)
    In the present work, a computer-controlled test rig for simultaneous fatigue testing of several pseudoelastic NiTi wires through bending rotation is described. Bending rotation fatigue (BRF) testing represents a displacement-controlled experiment where a straight wire is bent into a semi-circle und forced to rotate around its axis. Thus, each point on the wire surface is subjected to alternating tension and compression. A test rig, which allows to control loading amplitudes, rotation frequencies and temperatures is described. We report preliminary results of an experimental program, which aims for a better understanding of fatigue lives, crack initiation, and crack growth in pseudoelastic NiTi wires. It was found that a good surface quality is of utmost importance to avoid early crack initiation. Wöhler curves of pseudoelastic NiTi wires typically show two different regimes depending on the maximum imposed surface strain during bending rotation fatigue testing. Larger strain amplitudes, which are associated with macroscopic formation of stress-induced martensite, result in relatively low fatigue lives (LCF regime). In contrast, cycle numbers exceeding 107were obtained for strain amplitudes where no large scale stress-induced formation of martensite occurred (HCF regime). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201300118
  • Characteristics of the stress-induced formation of R-phase in ultrafine-grained NiTi shape memory wire
    Olbricht, J. and Yawny, A. and Pelegrina, J.L. and Eggeler, G. and Yardley, V.A.
    Journal of Alloys and Compounds 579 (2013)
    The transformation between the cubic B2 and monoclinic B19' phases in ultrafine-grained pseudoelastic NiTi can occur as a two-step process involving the intermediate rhombohedral R-phase. Experimental work using differential scanning calorimetry, electrical resistance measurements and transmission electron microscopy has demonstrated the formation of this intermediate phase during thermal cycling and during mechanical loading. In the present paper, complementary mechanical and thermographic results are presented which allow to further assess the character of the stress-induced R-phase formation. The transformation from B2 to R-phase is demonstrated to occur homogeneously within the gauge length rather than via advancing Lüders-type transition regions as it is the case in the localized transformation from B2 or R-phase to B19'. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2013.06.056
  • Comparative studies on the accumulation of strain and recovery ratio of Veriflex®, a shape-memory polymer: Infrared and laser experiments
    Chowdhury, A.M.S. and Schmidt, C. and Neuking, K. and Eggeler, G.
    High Performance Polymers 25 (2013)
    A comparative investigation on thermomechanical cycles including programming, cooling, unloading and heating to trigger the one-way effect (1-WE) was carried out for Veriflex® between temperature (T) and glass transition temperature (Tg) at 62 C (T < Tg) and 72 C (T > Tg) for 140% strains and 10 min recovery time. Accumulation of strain and recovery ratios was estimated during the treatments. It turns out that the process causes an increase in irreversible strain and is associated with a corresponding decrease in intensity of the 1-WE in particular during the first thermomechanical cycle. The confocal laser scanning microscopic study shows a very little wavy surface structure evolved at 72 C. Infrared study features the chemical nature of Veriflex. © The Author(s) 2013.
    view abstract10.1177/0954008313487929
  • Cyclic deformation and lifetime of Alloy 617B during thermo-mechanical fatigue
    Maier, G. and Riedel, H. and Nieweg, B. and Somsen, C. and Eggeler, G. and Klöwer, J. and Mohrmann, R.
    Materials at High Temperatures 30 (2013)
    Different heats of the nickel-base Alloy 617B are tested under in-phase and out-of-phase thermo-mechanical fatigue (TMF) conditions at temperatures between 50 and 900 °C. During one of the TMF tests the growth of microcracks is observed using the replica technique. After the tests, some of the specimens are inspected by scanning electron microscopy in order to analyse the prevailing damage mechanisms compared with those observed in isothermal low-cycle fatigue (LCF) tests. In addition, a Chaboche-type model and a fracture-mechanics-based lifetime model are employed to describe the cyclic viscoplastic deformation and fatigue lifetime. The Chaboche model adjusted to isothermal data is found to reasonably predict the cyclic viscoplastic behaviour of thermo-mechanically loaded specimens. Lifetime data of TMF tests fall into a common scatter band with LCF tests at temperatures above 400 °C if the test results are analysed based on the introduced lifetime model.
    view abstract10.3184/096034013X13636905345685
  • Design and fabrication of a bending rotation fatigue test rig for in situ electrochemical analysis during fatigue testing of NiTi shape memory alloy wires
    Neelakantan, L. and Zglinski, J.K. and Frotscher, M. and Eggeler, G.
    Review of Scientific Instruments 84 (2013)
    The current investigation proposes a novel method for simultaneous assessment of the electrochemical and structural fatigue properties of nickel-titanium shape memory alloy (NiTi SMA) wires. The design and layout of an in situ electrochemical cell in a custom-made bending rotation fatigue (BRF) test rig is presented. This newly designed test rig allows performing a wide spectrum of experiments for studying the influence of fatigue on corrosion and vice versa. This can be achieved by performing ex situ andor in situ measurements. The versatility of the combined electrochemicalmechanical test rig is demonstrated by studying the electrochemical behavior of NiTi SMA wires in 0.9 NaCl electrolyte under load. The ex situ measurements allow addressing various issues, for example, the influence of pre-fatigue on the localized corrosion resistance, or the influence of hydrogen on fatigue life. Ex situ experiments showed that a pre-fatigued wire is more susceptible to localized corrosion. The synergetic effect can be concluded from the polarization studies and specifically from an in situ study of the open circuit potential (OCP) transients, which sensitively react to the elementary repassivation events related to the local failure of the oxide layer. It can also be used as an indicator for identifying the onset of the fatigue failure. © 2013 American Institute of Physics.
    view abstract10.1063/1.4793488
  • Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study
    Hafez Haghighat, S.M. and Eggeler, G. and Raabe, D.
    Acta Materialia 61 (2013)
    Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.03.003
  • Erratum: Pseudoelastic deformation and size effects during in situ transmission electron microscopy tensile testing of NiTi (Acta Materialia 60 (2012) (2770-2777))
    Manchiraju, S. and Kroeger, A. and Somsen, C. and Dlouhy, A. and Eggeler, G. and Sarosi, P.M. and Anderson, P.M. and Mills, M.J.
    Acta Materialia 61 (2013)
    view abstract10.1016/j.actamat.2013.02.020
  • High temperature wear testing of a Ni-based superalloy pin on a cast iron disc
    Rynio, C. and Hattendorf, H. and Klöwer, J. and Lüdecke, H.-G. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 44 (2013)
    A pin-on-disc wear test rig is described, which allows to extract reproducible mechanical and microstructural wear data from very small sample volumes at temperatures up to 900°C. The friction and wear behavior of Alloy 80A against a cast iron is evaluated at temperatures from ambient to 800°C. The wear rate of Alloy 80A decreased with increasing temperature. This was attributed to the development of protective tribolayers, which prevented a direct contact between the two sliding partners. Energy Dispersive X-Ray (EDX) mapping of surface wear products and Transmission Electron Microscopy (TEM) results for the evolution of subsurface microstructures are exemplarily presented for wear experiments performed for 10 min at 300°C (frequency: 20 Hz, load: 20 N, stroke: 1 mm). EDX investigations provide a good insight into material transfers and oxide layer generations during sliding wear. TEM-micrographs revealed cell structure formation and very small nanograins directly beneath the surface. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201300165
  • High-temperature and low-stress creep anisotropy of single-crystal superalloys
    Agudo Jácome, L. and Nörtershäuser, P. and Heyer, J.-K. and Lahni, A. and Frenzel, J. and Dlouhy, A. and Somsen, C. and Eggeler, G.
    Acta Materialia 61 (2013)
    The high-temperature and low-stress creep (1293 K, 160 MPa) of the single-crystal Ni-based superalloy LEK 94 is investigated, comparing the tensile creep behavior of miniature creep specimens in [0 0 1] and [1 1 0] directions. In the early stages of creep, the [0 0 1]-direction loading shows higher minimum creep rates, because a greater number of microscopic crystallographic slip systems are activated, the dislocation networks at γ/γ′ interfaces accommodate lattice misfit better, and γ channels are wider. After the creep rate minimum, creep rates increase more strongly as a function of strain for [1 1 0] tensile loading. This may be related to the nature of rafting during [1 1 0] tensile creep, which results in a more open topology of the γ channels. It may also be related to more frequent γ′ cutting events compared with [1 0 0] tensile creep. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.01.052
  • Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys
    Rahim, M. and Frenzel, J. and Frotscher, M. and Pfetzing-Micklich, J. and Steegmüller, R. and Wohlschlögel, M. and Mughrabi, H. and Eggeler, G.
    Acta Materialia 61 (2013)
    In the present work we show how different oxygen (O) and carbon (C) levels affect fatigue lives of pseudoelastic NiTi shape memory alloys. We compare three alloys, one with an ultrahigh purity and two which contain the maximum accepted levels of C and O. We use bending rotation fatigue (up to cycle numbers &gt;108) and scanning electron microscopy (for investigating microstructural details of crack initiation and growth) to study fatigue behavior. High cycle fatigue (HCF) life is governed by the number of cycles required for crack initiation. In the low cycle fatigue (LCF) regime, the high-purity alloy outperforms the materials with higher number densities of carbides and oxides. In the HCF regime, on the other hand, the high-purity and C-containing alloys show higher fatigue lives than the alloy with oxide particles. There is high experimental scatter in the HCF regime where fatigue cracks preferentially nucleate at particle/void assemblies (PVAs) which form during processing. Cyclic crack growth follows the Paris law and does not depend on impurity levels. The results presented in the present work contribute to a better understanding of structural fatigue of pseudoelastic NiTi shape memory alloys. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.02.054
  • Interaction effects and transport properties of Pt capped Co nanoparticles
    Ludwig, Ar. and Agudo, L. and Eggeler, G. and Ludwig, Al. and Wieck, A.D. and Petracic, O.
    Journal of Applied Physics 113 (2013)
    We studied the magnetic and transport properties of Co nanoparticles (NPs) being capped with varying amounts of Pt. Beside field and temperature dependent magnetization measurements, we performed δΜ measurements to study the magnetic interactions between the Co NPs. We observe a transition from demagnetizing towards magnetizing interactions between the particles for an increasing amount of Pt capping. Resistivity measurements show a crossover from giant magnetoresistance towards anisotropic magnetoresistance. © 2013 American Institute of Physics.
    view abstract10.1063/1.4789422
  • On the crystallographic anisotropy of nanoindentation in pseudoelastic NiTi
    Pfetzing-Micklich, J. and Somsen, C. and Dlouhy, A. and Begau, C. and Hartmaier, A. and Wagner, M.F.-X. and Eggeler, G.
    Acta Materialia 61 (2013)
    We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈0 0 1〉, 〈1 0 1〉 and 〈1 1 1〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈0 0 1〉 direction and lowest along the 〈1 1 1〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.09.081
  • On the cyclic material stability of shape memory polymer
    Mogharebi, S. and Kazakeviciute-Makovska, R. and Steeb, H. and Eggeler, G. and Neuking, K.
    Materialwissenschaft und Werkstofftechnik 44 (2013)
    This paper presents the experimental study of a Thermoplastic Polyurethane (TPU) based Shape Memory Polymer (SMP) produced from granulates of commercially available EstaneTM ETE75DT3 NAT022 (Oevel Westerlo, Belgium). This polymer is characterized by Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Thermal Analysis (DMTA). The experimental procedure was designed to study the functional properties (shape fixity and shape recovery) of this shape memory polymer in multiple programming/shape-recovery cycles at three different programming temperatures. The results are displayed in the temperature-stress- strain space with focus on parameters which characterize the functional fatigue and material stability of the tested polymer during consecutive cycles. These results give a better understanding of this material class that has a potential for actuator applications in engineering. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201300023
  • Shift of the blocking temperature of Co nanoparticles by Cr capping
    Ewerlin, M. and Petracic, O. and Demirbas, D. and Agudo, L. and Eggeler, G. and Brüssing, F. and Abrudan, R. and Zabel, H.
    Journal of Applied Physics 114 (2013)
    We have studied the effect of Cr capping on the magnetic properties of Co nanoparticles (NPs). The NPs have an average diameter of 2.2 nm. The blocking temperature TB of the bare Co particles is 13.2 K. By capping with a thin Cr layer up to a thickness of tCr = 0.52 nm, we first observe a decrease of TB up to tCr = 0.14 nm, followed by an increase of TB for larger thicknesses 0.14 nm ≤ tCr ≤ 0.52 nm. X-ray magnetic circular dichroism measurements at the resonant Co and Cr L3 edges confirm a magnetic polarization of Cr which is opposite to the magnetization of Co. The antiparallel alignment of Co and Cr spins at the Co/Cr interface can explain the decrease at low capping layer thickness. However, for larger Cr capping layer thicknesses, the Cr film bridges the Co NPs, mediating interparticle exchange coupling and enhancing dipolar coupling that leads to an increase of the blocking temperature. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4851677
  • Strain mapping of crack extension in pseudoelastic NiTi shape memory alloys during static loading
    Young, M.L. and Gollerthan, S. and Baruj, A. and Frenzel, J. and Schmahl, W.W. and Eggeler, G.
    Acta Materialia 61 (2013)
    Crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) compact tension (CT) specimens was examined during static loading. The material composition of 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using high-energy synchrotron X-ray diffraction. A miniature CT specimen was developed, which is small enough to allow in situ testing in a synchrotron beam line to identify phases, textures and lattice strains in front of a crack tip. Stress-induced martensite in pseudoelastic NiTi SMAs was mapped in front of the crack of a CT specimen during static loading using synchrotron radiation. The phase volume fraction and lattice microstrain results are discussed and compared with results from thermographic measurements. The Poisson effect is observed by comparing the lattice strains in the loading direction and transverse to the loading direction. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.06.024
  • The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy
    Otto, F. and Dlouhý, A. and Somsen, C. and Bei, H. and Eggeler, G. and George, E.P.
    Acta Materialia 61 (2013)
    An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4-160 μm. Quasi-static tensile tests at an engineering strain rate of 10-3 s-1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ∼2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (∼20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing ("dynamic Hall-Petch"), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.06.018
  • Advanced scanning transmission stereo electron microscopy of structural and functional engineering materials
    Agudo Jácome, L. and Eggeler, G. and Dlouhý, A.
    Ultramicroscopy 122 (2012)
    Stereo transmission electron microscopy (TEM) provides a 3D impression of the microstructure in a thin TEM foil. It allows to perform depth and TEM foil thickness measurements and to decide whether a microstructural feature lies inside of a thin foil or on its surface. It allows appreciating the true three-dimensional nature of dislocation configurations. In the present study we first review some basic elements of classical stereo TEM. We then show how the method can be extended by working in the scanning transmission electron microscope (STEM) mode of a modern analytical 200. kV TEM equipped with a field emission gun (FEG TEM) and a high angle annular dark field (HAADF) detector. We combine two micrographs of a stereo pair into one anaglyph. When viewed with special colored glasses the anaglyph provides a direct and realistic 3D impression of the microstructure. Three examples are provided which demonstrate the potential of this extended stereo TEM technique: a single crystal Ni-base superalloy, a 9% Chromium tempered martensite ferritic steel and a NiTi shape memory alloy. We consider the effect of camera length, show how foil thicknesses can be measured, and discuss the depth of focus and surface effects. © 2012 Elsevier B.V.
    view abstract10.1016/j.ultramic.2012.06.017
  • Atomic ordering effect in Ni 50Mn 37Sn 13 magnetocaloric ribbons
    Wu, D. and Xue, S. and Frenzel, J. and Eggeler, G. and Zhai, Q. and Zheng, H.
    Materials Science and Engineering A 534 (2012)
    High-performance Ni 50Mn 37Sn 13 magnetocaloric materials are produced using melt spinning technique in the present work and the atomic order dependence of phase transition behaviors and magnetic properties is established. The effective refrigeration capacity of the melt-spun ribbon annealed at 1273K for 15min reaches 95.27J/kg for a magnetic field change of 18kOe, demonstrating great potential for magnetic refrigeration applications near ambient temperature. © 2011 Elsevier B.V.
    view abstract10.1016/j.msea.2011.12.009
  • Caloric effects in ferroic materials: New concepts for cooling
    Fähler, S. and Rößler, U.K. and Kastner, O. and Eckert, J. and Eggeler, G. and Emmerich, H. and Entel, P. and Müller, S. and Quandt, E. and Albe, K.
    Advanced Engineering Materials 14 (2012)
    Refrigeration is one of the main sinks of the German and European electricity consumption and accordingly contributes to worldwide CO 2 emissions. High reduction potentials are envisaged if caloric effects in solid materials are used. The recent discovery of giant entropy changes associated with ferroelastic phase transformations promises higher efficiency. Ferroic transitions enhance the entropy change of magneto-, elasto-, baro-, and electro-caloric effects. Furthermore, because the refrigerant is in a solid state, this technology completely eliminates the need for halofluorocarbon refrigerants having a high global-warming potential. The smaller footprint for operation and the scalable mechanism open up further applications such as cooling of microsystems. While the principal feasibility of magnetocaloric refrigeration is already evident, it requires large magnetic fields (&gt;2a T) which hampers wide industrial and commercial application. It is expected that this obstacle can be overcome by materials with lower hysteresis and by using stress- or electric fields. In order to accelerate research on ferroic cooling, the Deutsche Forschungsgemeinschaft (DFG) decided to establish the priority program SPP 1599 in April 2011. In this article we will address the major challenges for introducing ferroic materials in practical cooling applications: energy efficiency, effect size, and fatigue behavior. Solid state cooling in this sense can be based on the following "ferroic-caloric" classes of materials: ferroelastic shape memory alloys, ferromagnetic shape memory alloys, and ferroelectric materials and their possible combinations in materials with "multicaloric" effects. The open questions require the interdisciplinary collaboration of material scientists, engineers, physicists, and mathematicians. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201100178
  • Dealloying strategy to fabricate ultrafine nanoporous gold-based alloys with high structural stability and tunable magnetic properties
    Zhang, Z. and Zhang, C. and Gao, Y. and Frenzel, J. and Sun, J. and Eggeler, G.
    CrystEngComm 14 (2012)
    In the present work, the dealloying of Al-Au-based precursors and formation of nanoporous Au-based alloys have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and energy dispersive X-ray (EDX) analysis. The results show that the addition of Ni and/or Co has no influence on phase constitution of rapidly solidified Al-Au-M (M = Ni, Co, or Ni/Co) alloys and a single-phase Al 2(Au,M) intermetallic compound can be identified in these ternary and quarternary precursor alloys. The Al-Au-based precursors can be fully dealloyed in an alkaline solution under free corrosion conditions, and the dealloying results in the formation of novel ultrafine nanoporous Au-based alloys (Au(Ni), Au(Co) and Au(Ni,Co)) with ligaments/channels of ∼5 nm. The ultrafine nanoporous Au-based alloys possess extraordinarily high structural stability against thermal annealing. Moreover, due to the intrinsic magnetism of Ni and Co, the addition of Ni and/or Co leads to the formation of novel magnetic nanoporous alloys. The dealloying mechanism of these Al-Au-based precursors has been discussed based upon surface diffusion of Au adatoms and interaction between Au and additional elements. The present findings provide a new dealloying route to fabricate ultrafine nanoporous Au-based alloys with high stability and magnetic properties through alloy design of precursors. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2ce26187a
  • Effect of Si addition on the oxidation resistance of Co-Re-Cr-alloys: Recent attainments in the development of novel alloys
    Gorr, B. and Burk, S. and Depka, T. and Somsen, C. and Abu-Samra, H. and Christ, H.-J. and Eggeler, G.
    International Journal of Materials Research 103 (2012)
    The influence of silicon on the oxidation behaviour of Co- Re- Cr-alloys has been studied at 1 000 °C and 1 100 °C. Consideration was given to the synergetic effects between chromium and silicon with respect to the development of a protective Cr 2O 3 layer. The Si addition to the Co- Re-alloys produces a significant decrease in the evaporation rate of Re oxides. Moreover, the beneficial influence in the transient oxidation period results in a rapid formation of Cr2O3 scale. While the addition of 1 and 2 at.% Si to the ternary Co-17Re-23Cr alloy was insufficient to form a continuous Cr2O3 scale, the addition of 3 at.% silicon caused a change in the oxidation mode resulting in the formation of a nearly continuous Cr 2O 3 scale. On the oxide/alloy interface of the alloy Co-17Re-30Cr-2Si, a continuous and dense Cr 2O 3 scale was observed, which remained stable after 100 h exposure protecting the metallic substrate. © 2012 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.110626
  • 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 (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 abstract10.1016/j.actamat.2011.09.022
  • Leaf-like dislocation substructures and the decrease of martensitic start temperatures: A new explanation for functional fatigue during thermally induced martensitic transformations in coarse-grained Ni-rich Ti-Ni shape memory alloys
    Zhang, J. and Somsen, C. and Simon, T. and Ding, X. and Hou, S. and Ren, S. and Ren, X. and Eggeler, G. and Otsuka, K. and Sun, J.
    Acta Materialia 60 (2012)
    During repeatedly imposed thermally induced martensitic transformations in Ti-Ni shape memory alloys, the martensite start temperature M s decreases. This has been rationalized on the basis of a scenario where an increasing dislocation density makes it more and more difficult for martensite to form. However, it is not clear why dislocations which form because they accommodate the growth of martensite during the first cooling cycle should act as obstacles during subsequent transformation cycles. In the present work we use diffraction contrast transmission electron microscopy to monitor the formation of unique leaf-like dislocation substructures which form as the martensite start temperature decreases during thermal cycling. We interpret our microstructural results on the basis of a microstructural scenario where dislocations play different roles with respect to the propagation of a big martensite needle in one transformation cycle and the nucleation and growth of new martensite needles in the following cycles. As a consequence, chestnut-leaf-like dislocation arrays spread out in different crystallographic directions. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.12.014
  • Nanoindentation of pseudoelastic NiTi containing Ni4Ti 3 precipitates
    Young, M.L. and Frotscher, M. and Bei, H. and Simon, T. and George, E.P. and Eggeler, G.
    International Journal of Materials Research 103 (2012)
    Depending on the processing method, pseudoelastic NiTi alloys can have small, lenticular Ni4Ti3 precipitates; however, the mechanical properties of these precipitates are not well understood. By performing nanoindentation with a spherical indenter, Ni4Ti 3 precipitates within a pseudoelastic NiTi alloy were examined. Scanning electron microscopy was used to examine the indents after nanoindentation. After unloading, the hardness and remnant depth ratios of the indents in the Ni4Ti3 precipitates, the NiTi matrix, and the "average" NiTi alloy were compared. To decouple the effects of elasticity from those of pseudoelasticity, similar nanoindentation experiments were performed on an NiAl sample and compared with results from the NiTi sample. © 2012 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.110792
  • On the effect of grain boundary segregation on creep and creep rupture
    Otto, F. and Viswanathan, G.B. and Payton, E.J. and Frenzel, J. and Eggeler, G.
    Acta Materialia 60 (2012)
    The present work investigates the effect of grain boundary chemistry and crystallography on creep and on creep damage accumulation in Cu-0.008 wt.% Bi and Cu-0.92 wt.% Sb at stresses ranging from 10 to 20 MPa and temperatures between 773 and 873 K. Small additions of Bi and Sb significantly reduce the rupture strain and rupture time during creep of Cu. High stress exponents (Cu-Bi) and high apparent activation energies for creep (Cu-Bi and Cu-Sb) are obtained. Sb promotes creep cavitation on random high-angle grain boundaries. Bi, on the other hand, causes brittle failure when small crack-like cavities cause decohesion. Both elements suppress dynamic recrystallization, which occurs during creep of Cu at high stresses and temperatures. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.02.004
  • On the nature of internal interfaces in a tempered martensite ferritic steel and their evolution during long-term creep
    Payton, E.J. and Aghajani, A. and Otto, F. and Eggeler, G. and Yardley, V.A.
    Scripta Materialia 66 (2012)
    Orientation relationships between ferrite micrograins in a 12% Cr tempered martensite ferritic steel are characterized before and at intermediate times during long-term creep (120 MPa at 550 °C) up to ∼140,000 h. Orientations inherited from the martensitic microstructure during tempering deviate significantly from the well-known/idealized orientation relationships. The observed relationships between micrograins persist throughout creep. The spatial densities of former sub-block, block, and packet boundaries all decrease during creep, suggesting that coarsening takes place on all microstructural scales. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2012.02.042
  • 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 (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 abstract10.1016/j.actamat.2012.07.042
  • Orientation dependence of stress-induced phase transformation and dislocation plasticity in NiTi shape memory alloys on the micro scale
    Pfetzing-Micklich, J. and Ghisleni, R. and Simon, T. and Somsen, C. and Michler, J. and Eggeler, G.
    Materials Science and Engineering A 538 (2012)
    NiTi shape memory alloys can be used as micro actuators and small scale pseudoelastic components. Therefore there is a need to characterize their mechanical properties on the micro scale. In several previous studies, such tests (nanoindentation, pillar compression) were performed for different NiTi alloys. However, no consistent results concerning the coupling between plastic deformation and martensitic transformation were obtained. Moreover it is unclear whether the material's response to loading on the micro scale reflects its large scale mechanical anisotropy. In this study, we investigate a binary, solution annealed precipitate free NiTi alloy and compress small pillars in <0. 0. 1>-, <1. 0. 1>- and <1. 1. 1>-directions. Mechanical results are analyzed in the light of SEM and post-mortem TEM investigations. We identify deformation mechanisms and show that there is deformation anisotropy. We show that micro pillar testing yields results which are in good qualitative agreement with previous work from macroscopic investigations. © 2012 Elsevier B.V.
    view abstract10.1016/j.msea.2012.01.042
  • Pseudoelastic deformation and size effects during in situ transmission electron microscopy tensile testing of NiTi
    Manchuraju, S. and Kroeger, A. and Somsen, C. and Dlouhy, A. and Eggeler, G. and Sarosi, P.M. and Anderson, P.M. and Mills, M.J.
    Acta Materialia 60 (2012)
    The stress-induced B2-B19′ transformation in aged 51 at.% NiTi was investigated using in situ straining transmission electron microscopy (TEM). Increased applied strain along [1 1 0] B2 transforms B2 into plates containing B19′ variants that are related by a (1 1 0) B2 compound twin plane. This atypical twin plane is explained by relaxing the invariant plane constraint in the crystallographic theory of martensite (CTM) to an invariant line constraint. The relaxation is rationalized from the thin foil geometry. The relaxed CTM approach, coupled with conditions to maximize transformation strain along the loading axis and minimize elastic energy, predicts the observed twin interface, diffraction patterns, and interface with the B2 austenite. These results demonstrate subtleties in interpreting thin foil TEM results regarding martensitic transformations, and translating those results to bulk response. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.01.043
  • The effectiveness of coincidence site lattice criteria in predicting creep cavitation resistance
    Otto, F. and Payton, E.J. and Frenzel, J. and Eggeler, G.
    Journal of Materials Science 47 (2012)
    The coincidence site lattice (CSL) concept is often used in microstructural characterization by researchers studying grain boundary engineering as a method for improving the performance of polycrystalline materials. It is assumed that a high degree of shared lattice sites in the boundary between two grains will result in improved mechanical properties. For practical application of the CSL concept to experimental results, a maximum deviation from ideal CSL orientation relationships must be defined to distinguish potential CSL boundaries from random boundaries that are not likely to exhibit "special" properties. Several different maximum deviation criteria have been proposed in the literature. In this study, four of these criteria are investigated for their effectiveness in predicting the creep cavitation resistance of boundaries of different CSL character in three model alloys: pure Cu, Cu-Bi, and Cu-Sb. Bi and Sb strongly segregate to Cu grain boundaries and are detrimental to creep life. The experimental observations are compared to simulation results for a non-textured polycrystal. It is observed that only boundaries related to cubic annealing twins (∑3 and ∑9) exhibit special resistance to creep cavitation, that boundaries with ∑ > 3 are affected by the presence of segregants, and that the fraction of non-∑(3,9) boundaries tracks closely with what would be expected from a random polycrystal. It is shown that more restrictive criteria result in more reliable characterization of the fraction of cavitation-resistant boundaries only because they exclude more non-∑(3,9) boundaries from the analysis. © 2011 Springer Science+Business Media, LLC.
    view abstract10.1007/s10853-011-6124-1
  • The influence of secondary phase carbide particles on the passivity behaviour of NiTi shape memory alloys
    Neelakantan, L. and Monchev, B. and Frotscher, M. and Eggeler, G.
    Materials and Corrosion 63 (2012)
    The current investigation aims at studying the passivity behaviour of NiTi shape memory alloys with different levels of secondary phase titanium carbide (TiC) particles in an electrolyte of 0.9% sodium chloride at 37 °C. The influence of carbides and thermo-mechanical treatment/cold working on the passivity breakdown is highlighted. The polarisation studies on the as-cast and cold worked NiTi with high (0.05 wt%) and low (0.005 wt%) carbon levels show a significant difference in oxide stability. The alloy with extremely low carbon content shows a higher breakdown potential. Higher carbon levels result in higher density of larger TiC and these carbide/matrix interfaces are more susceptible to pitting. The qualitative behaviour of passive layer formed at 0.5 V on the cold worked NiTi alloy with different carbon levels was ascertained by electrochemical impedance spectroscopy (EIS). The oxide on the NiTi alloy with high (0.05 wt%) carbon levels showed lower resistance and poor stability at this condition. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/maco.201106402
  • Achieving small structures in thin NiTi sheets for medical applications with water jet and micro machining: A comparison
    Frotscher, M. and Kahleyss, F. and Simon, T. and Biermann, D. and Eggeler, G.
    Journal of Materials Engineering and Performance 20 (2011)
    NiTi shape memory alloys (SMA) are used for a variety of applications including medical implants and tools as well as actuators, making use of their unique properties. However, due to the hardness and strength, in combination with the high elasticity of the material, the machining of components can be challenging. The most common machining techniques used today are laser cutting and electrical discharge machining (EDM). In this study, we report on the machining of small structures into binary NiTi sheets, applying alternative processing methods being well-established for other metallic materials. Our results indicate that water jet machining and micro milling can be used to machine delicate structures, even in very thin NiTi sheets. Further work is required to optimize the cut quality and the machining speed in order to increase the cost-effectiveness and to make both methods more competitive. © ASM International.
    view abstract10.1007/s11665-010-9789-8
  • Can human mesenchymal stem cells survive on a NiTi implant material subjected to cyclic loading?
    Habijan, T. and Glogowski, T. and Kühn, S. and Pohl, M. and Wittsiepe, J. and Greulich, C. and Eggeler, G. and Schildhauer, T.A. and Köller, M.
    Acta Biomaterialia 7 (2011)
    Nickel-titanium shape memory alloys (NiTi-SMAs) exhibit mechanical and chemical properties which make them attractive candidate materials for various types of biomedical applications. However, the high nickel content of NiTi-SMAs may result in adverse tissue reactions, especially when they are considered for load-bearing implants. It is generally assumed that a protective titanium oxide layer separates the metallic alloy from its environment and that this explains the good biocompatibility of NiTi. Cyclic loading may result in failure of the protective oxide layer. The scientific objective of this work was to find out whether cyclic dynamic strain, in a range relevant for orthopedic implants, diminishes the biocompatibility of NiTi-SMAs. In order to analyze the biocompatibility of NiTi-SMA surfaces subjected to cyclic loading, NiTi-SMA tensile specimens were preloaded with mesenchymal stem cells, transferred to a sterile cell culture system and fixed to the pull rods of a tensile testing machine. Eighty-six thousand and four hundred strain cycles at 2% pseudoelastic strain were performed for a period of 24 h or 7 days. Cytokines (IL-6, IL-8 and VEGF) and nickel ion release were determined within the cell culture medium. Adherent cells on the tensile specimens were stained with calcein-AM and propidium iodide to determine cell viability. Dynamic loading of the tensile specimens did not influence the viability of adherent human mesenchymal stem cells (hMSCs) after 24 h or 7 days compared with the non-strained control. Dynamic cycles of loading and unloading did not affect nickel ion release from the tensile specimens. The release of IL-6 from hMSCs cultured under dynamic conditions was significantly higher after mechanical load (873 pg ml -1) compared with static conditions (323 pg ml-1). The present work demonstrates that a new type of mechanical in vitro cell culture experiment can provide information which previously could only be obtained in large animal experiments. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actbio.2011.02.022
  • Cell type-specific responses of peripheral blood mononuclear cells to silver nanoparticles
    Greulich, C. and Diendorf, J. and Geßmann, J. and Simon, T. and Habijan, T. and Eggeler, G. and Schildhauer, T.A. and Epple, M. and Köller, M.
    Acta Biomaterialia 7 (2011)
    Silver nanoparticles (Ag-NP) are increasingly used in biomedical applications because of their remarkable antimicrobial activity. In biomedicine, Ag-NP are coated onto or embedded in wound dressings, surgical instruments and bone substitute biomaterials, such as silver-containing calcium phosphate cements. Free Ag-NP and silver ions are released from these coatings or after the degradation of a biomaterial, and may come into close contact with blood cells. Despite the widespread use of Ag-NP as an antimicrobial agent, there is a serious lack of information on the biological effects of Ag-NP on human blood cells. In this study, the uptake of Ag-NP by peripheral monocytes and lymphocytes (T-cells) was analyzed, and the influence of nanosilver on cell biological functions (proliferation, the expression of adhesion molecules, cytokine release and the generation of reactive oxygen species) was studied. After cell culture in the presence of monodispersed Ag-NP (5-30 μg ml -1 silver concentration), agglomerates of nanoparticles were detected within monocytes (CD14+) but not in T-cells (CD3+) by light microscopy, flow cytometry and combined focused ion beam/scanning electron microscopy. The uptake rate of nanoparticles was concentration dependent, and the silver agglomerates were typically found in the cytoplasm. Furthermore, a concentration-dependent activation (e.g. an increased expression of adhesion molecule CD54) of monocytes at Ag-NP concentrations of 10-15 μg ml -1 was observed, and cytotoxicity of Ag-NP-treated monocytes was observed at Ag-NP levels of 25 μg ml -1 and higher. However, no modulation of T-cell proliferation was observed in the presence of Ag-NP. Taken together, our results provide the first evidence for a cell-type-specific uptake of Ag-NP by peripheral blood mononuclear cells (PBMC) and the resultant cellular responses after exposure. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actbio.2011.05.030
  • Comparative studies on thermomechanical behavior of veriflex, a shape memory polymer, for a low strain (m = 70%): Laser experiments
    Chowdhury, A.M.S. and Schmidt, C. and Neuking, K. and Eggeler, G.
    Journal of Macromolecular Science, Part A: Pure and Applied Chemistry 48 (2011)
    An interesting comparative case study on thermomechanical cycles including programming, cooling, unloading and heating to trigger the 1WE was done using Veriflex at 62C (T Tg close to and below 5C of Tg) and also at 72C (T Tg, close to and above 5C of Tg) for slightly low strains (m = 70%) and the recovery time of 10 min. Accumulation of strain was estimated during the thermomechanical treatments for using both 70% strains at 62C (T Tg), as well as at 72C (T Tg). Recovery ratios for 70% strains at 62C (T Tg), as well as for 72C (T Tg) were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE, in particular, during the first thermomechanical cycles. A LSCM (Laser Scanning Confocal Microscopic) study shows very little change in surface structure which evolved during cycling up to 70% strains at 72C (T Tg). © Taylor & Francis Group, LLC.
    view abstract10.1080/10601325.2011.596049
  • Elementary deformation and damage mechanisms during fatigue of pseudoelastic NiTi microstents
    Frotscher, M. and Wu, S. and Simon, T. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Advanced Engineering Materials 13 (2011)
    In the present study, we investigate the fatigue behavior of Nickel Titanium (NiTi) microstents at 22°C (room temperature) and 37°C up to 30×10 6 load cycles. We briefly describe our test procedure, which applies displacement-controlled pull-pull fatigue cycling between displacements corresponding to apparent strains of 5 and 7.5%. The response of the microstents to mechanical loading indicates cyclic softening during 30×10 4 cycles. Subsequently, the maximum load remains constant throughout the remainder of the test. We use transmission electron microscopy (TEM) to clarify the microstructural reasons for cyclic softening. A focused ion beam (FIB) technique is used to take out thin foil specimens from critical microstent locations. Our TEM results show that the dislocation density increases during cycling. We also find that microstructural regions with stabilized stress-induced B19 martensite can be detected. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201180001
  • Finite-element simulation of the anti-buckling-effect of a shape memory alloy bar
    Richter, F. and Kastner, O. and Eggeler, G.
    Journal of Materials Engineering and Performance 20 (2011)
    Shape memory alloys (SMA) are characterized by an intricate stress-strain curve modified by temperature, posing thermomechanically coupled problems. A model able to address this feature is the Mü ller-Achenbach- Seelecke (MAS) model which had been ported into the user material interface in the finite-element (FEM) simulation software ABAQUS. The literature on this model mainly focuses on pseudo-elasticity of SMA at elevated temperature. We address a numerical investigation in the low-temperature pseudo-plastic regime. The present publication deals with the little-known anti-buckling effect which occurs in de-twinned and pre-bent martensitic bars under axial compression. It was experimentally demonstrated by Urushiyama et al. (JSME (The Japan Society of Mechanical Engineers) Int. J. Ser. A, Solid Mech. Mater. Eng., 2003, 46(1), p 60-67). This study reveals that the origin of this effect roots in an interplay of inhomogeneous stress states and mechanically induced twin-twin phase transformations. The proper explanation of the anti-buckling effect can be inferred from the explicit knowledge of the martensitic phase composition of the bar during the process.We show that the MAS model is capable to resolve this matter in detail, hence addressing the reliability of this particular model also in the pseudo-plastic regime of SMA. The study thereby implies that the MAS model is an excellent modeling tool for the analysis of complex, thermomechanically coupled processes. © ASM International.
    view abstract10.1007/s11665-010-9797-8
  • High temperature test rig for inert atmosphere miniature specimen creep testing
    Peter, D. and Otto, F. and Depka, T. and Nörtershäuser, P. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 42 (2011)
    A creep test rig is described which allows to perform creep experiments with miniature tensile creep specimens under an inert gas atmosphere at temperatures up to 1150°C. Previous work on the development of a miniature tensile creep test procedure is described. The key elements of the new inert gas system are then described. The test rig yields reliable creep data. Three examples for applications of the new system addressing grain boundary sliding in a TiAl alloy, creep cavitation and damage accumulation in copper and to provide high temperature creep data for a Co-Re-Cr alloy are given. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201100682
  • Improvement of NiTi shape memory actuator performance through ultra-fine grained and nanocrystalline microstructures
    Frenzel, J. and Burow, J.A. and Payton, E.J. and Rezanka, S. and Eggeler, G.
    Advanced Engineering Materials 13 (2011)
    Ultra-fine grain sizes have been shown to enhance some key mechanical and functional properties of engineering materials, including shape memory alloys. While the effect of ultra-fine and nanocrystalline grain sizes on pseudoelastic shape memory materials is well-appreciated in medical device engineering, the effect of such microstructures on actuators has not been sufficiently characterized. In the present work, it is demonstrated that NiTi spring actuators with ultra-fine grained microstructures can be obtained by conventional wire drawing in combination with heat treatments and that the final grain size can be controlled by varying the final annealing temperature. Annealing at 400deg;C for 600s allows for the evolution of microstructures with median grain sizes of about 34nm, while annealing at 600deg;C for the same length of time results in median grain sizes of about 5 μm. It is observed that the grain size strongly affects the elementary processes of the martensitic phase transformation. Small austenite grain sizes inhibit twinning accommodation of transformation strains, such that a higher driving force is required to nucleate martensite. This increase in the martensite nucleation barrier decreases the martensite transformation temperatures such that only partial transformation to martensite is possible upon cooling to room temperature. The incomplete martensitic transformation reduces the exploitable actuator stroke; however, a reduction in grain size is shown to improve the functional stability of the material during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity. NiTi spring actuators with ultra-fine grained and nanocrystalline microstructures can be obtained by conventional wire drawing in combination with heat treatments. Grain size refinements into this range improve the functional stability during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity. The improvement in functional stability comes at the cost of exploitable actuator stroke, however, because very fine grain sizes result in only a partial transformation to martensite upon cooling to room temperature. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/adem.201000285
  • Length-scale modulated and electrocatalytic activity enhanced nanoporous gold by doping
    Wang, X. and Frenzel, J. and Wang, W. and Ji, H. and Qi, Z. and Zhang, Z. and Eggeler, G.
    Journal of Physical Chemistry C 115 (2011)
    In the present paper, we have investigated the dealloying of Pt- and/or Pd-doped Al2Au intermetallic compounds and the formation of ultrafine nanoporous Au (np-Au) alloys through a chemical dealloying strategy. The microstructural characterization confirms that these doping atoms enter into crystal lattices of the precursors and then transmit into the as-obtained np-Au, both existing in the form of solid solutions. When dealloying in the 20 wt % NaOH solution is performed, a certain amount of Pt and/or Pd addition shows a superior refining effect and the ligament/channel sizes of the as-doped np-Au can be facilely modulated below 10 nm. When dealloying in the 5 wt % HCl solution is performed, however, the anticoarsening capacity of Pt doping is more remarkable compared with that of Pd doping. In addition, the amount of doping also has an important influence on the ligament resistance to coarsening. Apart from causing the refinement of ligaments/channels, the introduction of Pt and/or Pd into np-Au has generated novel bi- or trimetallic functionalized nanoporous alloys. These as-doped np-Au alloys with an appropriate amount of Pt and/or Pd exhibit excellent electrocatalytic activities toward methanol and formic acid oxidation and will find promising applications in the catalysis-related areas. © 2011 American Chemical Society.
    view abstract10.1021/jp110011w
  • Martensitic transformation in rapidly solidified Heusler Ni 49Mn39Sn12 ribbons
    Zheng, H. and Wu, D. and Xue, S. and Frenzel, J. and Eggeler, G. and Zhai, Q.
    Acta Materialia 59 (2011)
    In the present work, the microstructure evolution and kinetics of the martensitic transformation are investigated in as-spun and annealed ribbons of Heusler Ni49Mn39Sn12 using electron microscopy, X-ray diffraction and differential scanning calorimetry. Both ribbons undergo a reversible martensitic transformation during thermal cycling and the low-temperature martensite is confirmed to be a modulated four-layered orthorhombic (4O) structure through in situ cooling transmission electronic microscopy investigation. The annealing effect on the martensitic transformation behavior is discussed from the viewpoints of electron concentration, Mn-Mn interatomic distance, atomic order degree and grain size. A strong cooling-rate dependence of phase transition kinetics is found and the mechanism is analyzed. The satisfactory reproducibility obtained during thermal cycling test of this alloy ribbons offers great potential for practical applications. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.05.044
  • Mechanical behavior of shape memory polymers by 1we method: Application to tecoflex®
    Schmidt, C. and Chowdhury, A.M.S. and Neuking, K. and Eggeler, G.
    Journal of Thermoplastic Composite Materials 24 (2011)
    Thermomechanical cycles including programming, cooling, unloading, and heating to trigger the 1WE were examined for a shape memory polymer, Tecoflex® (TFX EG - 72D). Cycles were performed at 60°C with 300% strain and a recovery time of 10 min. Strains evolving with time were estimated during the thermomechanical treatments for the total 50 cycles using 300% strain. Recovery ratios for the 300% strain were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE in particular during the first thermomechanical cycles. © 2011 The Author(s).
    view abstract10.1177/0892705711407785
  • Micro-shear deformation of pure copper
    Pfetzing-Micklich, J. and Brinckmann, S. and Dey, S.R. and Otto, F. and Hartmaier, A. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 42 (2011)
    In this paper a new micro-shear experiment is introduced using a double shear specimen machined by a focused ion beam technique. The micro-shear specimen is structured from pure copper promoting (111) [101] slip. Comparing scanning electron microscopy images before and after deformation provides evidence for localized shear. Load-displacement data identify a load plateau and characterize the localized shear process (critical shear-stress for activation of (111) [101] slip: 170 MPa). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/mawe.201100715
  • Modification of the bilateral sagittal split osteotomy (BSSO) in a study using pig mandibles
    Schoen, P. and Frotscher, M. and Eggeler, G. and Kessler, P. and Wolff, K.-D. and Boeckmann, R.
    International Journal of Oral and Maxillofacial Surgery 40 (2011)
    In a bilateral sagittal split osteotomy (BSSO) mechanical irritation of the inferior alveolar nerve (IAN) (e.g. by chiselling) should be avoided to prevent neural damage. A modification of the Obwegeser-Dal Pont operation technique was studied by splitting 100 pig mandibles ex vivo. An additional osteotomy at the caudal border of the mandible was used to facilitate the sagittal split by means of a locus of minor resistance. The chisel was inserted distal to the second molar and far away from the IAN. The mandible was split by torque. The modified technique reduced the required torque to split the mandible about 30% compared with the original technique (paired t-test, t(69) = -12.89; p < 0.05). 75% of all mandibles split by the modified technique were classified as bad splits compared with 100% using the original technique using the same protocol without the additional osteotomy. © 2011 International Association of Oral and Maxillofacial Surgeons.
    view abstract10.1016/j.ijom.2010.09.028
  • Molecular dynamics simulation study of microstructure evolution during cyclic martensitic transformations
    Kastner, O. and Eggeler, G. and Weiss, W. and Ackland, G.J.
    Journal of the Mechanics and Physics of Solids 59 (2011)
    Shape memory alloys (SMA) exhibit a number of features which are not easily explained by equilibrium thermodynamics, including hysteresis in the phase transformation and reverse shape memory in the high symmetry phase. Processing can change these features: repeated cycling can train the reverse shape memory effect, while changing the amount of hysteresis and other functional properties. These effects are likely to be due to formations of localised defects and these can be studied by atomistic methods. Here we present a molecular dynamics simulation study of such behaviour employing a two-dimensional, binary LennardJones model. Our atomistic model exhibits a symmetry breaking, displacive phase transition from a high temperature, entropically stabilised, austenite-like phase to a low temperature martensite-like phase. The simulations show transformations in this model material proceed by non-diffusive nucleation and growth processes and produce distinct microstructures. We observe the generation of persistent lattice defects during forward-and-reverse transformations which serve as nucleation centres in subsequent transformation processes. These defects interfere the temporal and spatial progression of transformations and thereby affect subsequent product morphologies. During cyclic transformations we observe accumulations of lattice defects so as to establish new microstructural elements which represent a memory of the previous morphologies. These new elements are self-organised and they provide a basis of the reversible shape memory effect in the model material. © 2011 Elsevier Ltd.
    view abstract10.1016/j.jmps.2011.05.009
  • On the effect of superimposed external stresses on the nucleation and growth of Ni 4Ti 3 particles: A parametric phase field study
    Guo, W. and Steinbach, I. and Somsen, C. and Eggeler, G.
    Acta Materialia 59 (2011)
    The effect of a superimposed stress on the coarsening of interacting Ni 4Ti 3 particles is studied using the multi-phase field method. It is found that the thickness/diameter ratio of a Ni 4Ti 3 particle in a (1 1 1) B2 plane increases with an increasing [1 1 1] B2 stress component. The particle shape can change from a disk to a sphere with increasing applied stress. It is also found that diffusional and mechanical interactions between two Ni 4Ti 3 particles can promote the nucleation of new particles. This provides an explanation for the autocatalytic nature of nucleation reported previously. Compressive stresses along [1 1 1] B2 increase the volume fraction and growth velocity of the Ni 4Ti 3 particles of the (1 1 1) B2 plane. Misoriented particles disappear during particle growth. The simulation results are discussed in the light of previous experimental results. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.02.002
  • On the evolution of microstructure in oxygen-free high conductivity copper during thermomechanical processing using rotary swaging
    Otto, F. and Frenzel, J. and Eggeler, G.
    International Journal of Materials Research 102 (2011)
    In the present work, the processing parameters which govern the evolution of microstructure during rotary swaging and intermediate/subsequent heat treatments in copper rods were studied. Copper ingots with an initial diameter of 40 mm were reduced to a final diameter of 11.7 mm by rotary swaging. Processing sequences were applied with different intermediate anneals and various final heat treatments. The resulting microstructures were characterized using orientation imaging microscopy, optical microscopy and hardness measurements. Special emphasis was placed on the evolution of microstructure with respect to the radial and longitudinal position in the rod. Most importantly, microstructural evidence for torsional loading during swaging was found, and a spiral grain morphology was observed. Moreover, localized deformation events were identified and evidence for abnormal grain growth was found. Finally, a combination of swaging and heat treatment parameters was identified which allowed a homogeneous grain structure to be produced. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.110501
  • 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 (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 abstract10.1016/j.actamat.2010.11.023
  • On the influence of small quantities of Bi and Sb on the evolution of microstructure during swaging and heat treatments in copper
    Otto, F. and Frenzel, J. and Eggeler, G.
    Journal of Alloys and Compounds 509 (2011)
    In the present work, the influence of small amounts of Bi and Sb on the microstructural evolution of Cu during an ingot metallurgy processing route is investigated. Both elements are known to segregate to grain boundaries in Cu. Cu ingots with an outer diameter of 40 mm containing 0.008 wt.% Bi and 0.92 wt.% Sb, respectively, were vacuum induction melted, cast, and gradually swaged down to a final diameter of 11.7 mm with several intermediate annealing steps. Subsequent annealing treatments were conducted to investigate the microstructural evolution of the swaged bars. Optical microscopy, hardness testing and orientation imaging microscopy were used to characterize the deformation and recrystallization behavior, as well as the evolution of texture in the alloys. The results are then compared to those obtained for pure Cu. It is shown that even small amounts of alloying elements significantly alter the hardening behavior and suppress recrystallization at low temperatures. At higher temperatures, recrystallization in Cu, Cu-Bi and Cu-Sb leads to different textures. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2010.12.178
  • On the stress-induced formation of R-phase in ultra-fine-grained Ni-rich NiTi shape memory alloys
    Olbricht, J. and Yawny, A. and Pelegrina, J.L. and Dlouhy, A. and Eggeler, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 42 (2011)
    Phase transformations in binary ultra-fine-grained (UFG) pseudoelastic NiTi wires were studied in a wide temperature range using mechanical loading/unloading experiments, resistance measurements, differential scanning calorimetry (DSC), thermal infrared imaging, and transmission electron microscopy (TEM). The formation of R-phase can be detected in the mechanical experiments. It is shown that the stress-strain response of the R-phase can be isolated from the overall stress-strain data. The R-phase always forms prior to B19' when good pseudoelastic properties are observed. The stress-induced B2 to R-phase transition occurs in a homogeneous manner, contrary to the localized character of the B2/R to B19' transformations. The temperature dependence of the critical stress values for the formation of the martensitic phases shows a Clausius Clapeyron type of behavior with constants close to 6 MPa/K (B19') and 18 MPa/K (R-phase). A stress-temperature map is suggested that summarizes the experimentally observed sequences of elementary transformation/deformation processes. © The Minerals, Metals & Materials Society and ASM International 2011.
    view abstract10.1007/s11661-011-0679-y
  • Phase transformations and functional properties of NiTi alloy with ultrafine-grained structure
    Prokofiev, E. and Burow, J. and Frenzel, J. and Gunderov, D. and Eggeler, G. and Valiev, R.
    Materials Science Forum 667-669 (2011)
    Severe plastic deformation (SPD) processes, are successfully employed to produce ultra fine grain (UFG) and nanocrystalline (NC) microstructures in Ni50.7Ti49.3 shape memory alloy. The effect of grain size on phase transformations during annealing is investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The results of comparative studies of phase transformations in coarse-grained, UFG and NC alloys after SPD and subsequent long-term (up to 100 hours) annealing at 400°C is presented. The functional properties and the innovation potential of UFG NiTi alloys is considered and discussed.
    view abstract10.4028/www.scientific.net/MSF.667-669.1059
  • Pilot study of modification of the bilateral sagittal split osteotomy (BSSO) in pig mandibles
    Böckmann, R. and Schön, P. and Frotscher, M. and Eggeler, G. and Lethaus, B. and Wolff, K.-D.
    Journal of Cranio-Maxillofacial Surgery 39 (2011)
    Objective: This study investigated a new technique for the bilateral sagittal split osteotomy (BSSO) by adding a new osteotomy line at the inferior border of the mandible in the Obwegeser-Dal Pont operation. Material and methods: For this purpose a test system was designed and 100 pig mandibles were split to assess the test's reliability, to compare the torque necessary to split the mandible in both techniques and to record the fracture lines. The splitting technique was standardized, avoiding any contact with the inferior alveolar nerve. All outcomes were statistically examined by paired t-tests. Results: By using the new technique, we demonstrated a decrease in the torque force required to split the mandible of 29.7% (t(69) = -12.68; p < 0.05, paired t-test) compared to the Obwegeser-Dal Pont technique. The fracture lines were close to ideal. Conclusion: The additional osteotomy facilitates the BSSO technique and it reduces the likelihood of bad splits and damage to the inferior alveolar nerve in pig mandibles. © 2010 European Association for Cranio-Maxillo-Facial Surgery.
    view abstract10.1016/j.jcms.2010.04.002
  • Processing and characterization of braided NiTi microstents for medical applications
    Frotscher, M. and Schreiber, F. and Neelakantan, L. and Gries, T. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 42 (2011)
    In the present exploratory study we investigate the processing of NiTi microstents, which were produced by braiding. We give a short description of the braiding of microstents using 0.1 mm pseudoelastic NiTi wires. We compare our braided microstents with microstents which were produced, using the traditional tube making and laser cutting technology. We outline why braiding may avoid some of the inherent problems related with the traditional procedure and highlight its cost effectiveness. We report on the thermal, chemical and mechanical properties of braided microstents. Most importantly, we provide information on the elementary damage processes which characterize displacement controlled fatigue loading up to 30·10 6 cycles. The results presented in this work provide information on a research field, which links the engineering manufacturing field braiding (as a potential new processing technology for NiTi stents) with state-of-the-art materials science techniques for the characterization of pseudoelastic NiTi shape memory alloys. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201100796
  • Stress-strain behavior of shape memory polymers by 1 WE method: Application to tecoflex®
    Schmidt, C. and Chowdhury, A.M.S. and Neuking, K. and Eggeler, G.
    Journal of Macromolecular Science, Part A: Pure and Applied Chemistry 48 (2011)
    Thermomechanical cycles including programming, cooling, unloading and heating to trigger the 1WE were examined for a shape memory polymer (SMP), Tecoflex® (TFX EG-72D). Cycles were performed at 60°C with 50% and 225% strains and the recovery time of 10 min. Strains evolving with time were estimated during the thermomechanical treatments for the total 44 cycles using 50% strains and the total 50 cycles using 225% strains. Recovery ratios for 50% strains and 225% were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE in particular during the first thermomechanical cycles. In parallel scanning electron microscopic study using secondary electron imaging shows a very slight wavy surface structure evolved during cycling. Copyright © Taylor & Francis Group, LLC.
    view abstract10.1080/10601325.2011.544630
  • Structural and magnetic characterization of self-assembled iron oxide nanoparticle arrays
    Benitez, M.J. and Mishra, D. and Szary, P. and Badini Confalonieri, G.A. and Feyen, M. and Lu, A.H. and Agudo, L. and Eggeler, G. and Petracic, O. and Zabel, H.
    Journal of Physics Condensed Matter 23 (2011)
    We report about a combined structural and magnetometric characterization of self-assembled magnetic nanoparticle arrays. Monodisperse iron oxide nanoparticles with a diameter of 20nm were synthesized by thermal decomposition. The nanoparticle suspension was spin-coated on Si substrates to achieve self-organized arrays of particles and subsequently annealed at various conditions. The samples were characterized by x-ray diffraction, and bright and dark field high resolution transmission electron microscopy. The structural analysis is compared to magnetization measurements obtained by superconducting quantum interference device magnetometry. We can identify either multi-phase FexO/γ-Fe2O3 or multi-phase Fe xO/Fe3O4 nanoparticles. The Fe xO/γ-Fe2O3 system shows a pronounced exchange bias effect which explains the peculiar magnetization data found for this system. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/23/12/126003
  • Studies on the cycling, processing and programming of an industrially applicable shape memory polymer Tecoflex® (or TFX EG 72D)
    Schmidt, C. and Chowdhury, A.M.S. and Neuking, K. and Eggeler, G.
    High Performance Polymers 23 (2011)
    The present investigations were undertaken to find out whether and how often cycling, processing and programming can be repeated, whether repeated programming affects the one way effect and how much irreversible strain the shape memory polymeric material accumulates at a particular temperature. The effect was investigated in dependence of different stress levels, and the effect of both recovery temperature and recovery time was considered. As a model material the commercially and industrially applicable amorphous shape memory polymer Tecoflex® was examined and subjected to 50 programming cycles. Tecoflex® is characterized by a glass transition temperature, Tg, of 74 °C, above which it looses all its strength. During tensile testing at 20 °C (T < Tg), stresses a steady increase to 26 MPa as strains approached the rupture strain of 25%. It is observed that at 60 °C (T < Tg, but near Tg) the material can be strained to more than 2500% before rupture occurs while stresses slowly increase to values less than 0.3 MPa. It turns out that programming, cooling, unloading and heating to trigger the one way effect causes an increase of irreversible strain that is associated with a corresponding decrease of the intensity of the one way effect during the first thermomechanical cycles. © The Author(s) 2011.
    view abstract10.1177/0954008311405245
  • Synthesis and hydriding/dehydriding properties of Mg2Ni-AB (AB = TiNi or TiFe) nanocomposites
    Zlatanova, Z. and Spassov, T. and Eggeler, G. and Spassova, M.
    International Journal of Hydrogen Energy 36 (2011)
    Mg2Ni-TiFe and Mg2Ni-TiNi nanocomposites were prepared by milling for a short time of two preliminary milled to a nanocrystalline state hydrogen absorbing phases, Mg2Ni and TiFe or Mg2Ni and TiNi. The milling results in a sufficient density of contacts between the fine powder particles with different composition. The presence of a large amount of such inter-particles contacts leads to lowering of the initial temperature of the composites gas phase hydriding, as in the same time the temperature range of hydriding is enlarged, compared to the composites components. On the grounds of the proved low temperature hydriding (≤200 °C) of the nanocomposites, taking place with appropriate kinetics, the possibility for improved electrochemical hydriding was checked, exploiting the idea for charging Mg2Ni particles through the contacts with TiFe/TiNi. In this way we are supposed to achieve more complete electrochemical hydriding of the Mg2Ni particles, which are usually only superficially hydrogenated at room temperature, mainly due to the low diffusion coefficient of hydrogen in the Mg2Ni crystal lattice and corrosion processes in strong alkaline solutions. The achieved discharge capacity for the Mg2Ni-TiFe composite is essentially higher compared to that of the mechanical mixture of the two composite's components. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights.
    view abstract10.1016/j.ijhydene.2011.03.092
  • Thermo-mechanical behaviour of Shape Memory Polymers, e.g., Tecoflex® by 1WE method: SEM and IR analysis
    Schmidt, C. and Chowdhury, A.M.S. and Neuking, K. and Eggeler, G.
    Journal of Polymer Research 18 (2011)
    Thermomechanical cycles including programming, cooling, unloading and heating to trigger the 1WE were examined for a shape memory polymer (SMP), Tecoflex® (TFX EG-72D). Cycles were performed at 60 °C with 65% strains and the recovery time of 10 min. Strains evolving with time were estimated during the thermomechanical treatments for the total 50 cycles using 65% strains. Recovery ratios for 65% strains were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE in particular during the first thermomechanical cycles. Gold coated scanning electron microscopic study using secondary electron imaging (120 times magnified) shows a very little wavy surface structure evolve during cycling up to 300% strains. IR study accurately features the chemical nature after cycling, processing and programming of TFX EG-72D. © 2011 Springer Science+Business Media B.V.
    view abstract10.1007/s10965-011-9587-5
  • Tuning the magnetic properties of Co nanoparticles by Pt capping
    Ebbing, A. and Hellwig, O. and Agudo, L. and Eggeler, G. and Petracic, O.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We show that by capping Co nanoparticles (NPs) with small amounts of Pt, strong changes of the magnetic properties can be induced. The Co NPs have a mean diameter of 2.7 nm. From magnetometry measurements we find that for zero and for small amounts of Pt (nominal thickness t Pt < 0.7 nm) the NPs behave superparamagnetic- (SPM)-like. With increasing t Pt the blocking temperature is enhanced from 16 up to 108 K. Capping with Pd yields comparable results. However, for values t Pt &gt; 1 nm a strongly coupled state is encountered resembling a ferromagnet (FM) with a T c ∼ 400 K. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.012405
  • Uptake and intracellular distribution of silver nanoparticles in human mesenchymal stem cells
    Greulich, C. and Diendorf, J. and Simon, T. and Eggeler, G. and Epple, M. and Köller, M.
    Acta Biomaterialia 7 (2011)
    Silver nanoparticles (Ag-NP) are widely used due to their well-known antibacterial effects. In medicine Ag-NP have found applications as wound dressings, surgical instruments and bone substitute biomaterials, e.g. silver-containing calcium phosphate cements. Depending on the coating technique, during resorption of a biomaterial Ag-NP may come into close contact with body tissues, including human mesenchymal stem cells (hMSC). Despite the widespread uses of Ag-NP, there is a serious lack of information concerning their biological effects on human cells. In this study the uptake of Ag-NP into hMSC has been analyzed and the intracellular distribution of Ag-NP after exposure determined. Non-agglomerated (dispersed) Ag-NP from the cell culture medium were detected as agglomerates of nanoparticles within the hMSC by combined focused ion beam/scanning electron microscopy. The silver agglomerates were typically located in the perinuclear region, as determined by light microscopy. Specific staining of cellular structures (endo-lysosomes, nuclei, Golgi complex and endoplasmatic reticulum) using fluorescent probes showed that the silver nanoparticles occurred mainly within endo-lysosomal structures, not in the cell nucleus, endoplasmic reticulum or Golgi complex. Quantitative determination of the uptake of Ag-NP by flow cytometry (scattergram analysis) revealed a concentration-dependent uptake of the particles which was significantly inhibited by chlorpromazine and wortmannin but not by nystatin, indicating clathrin-dependent endocytosis and macropinocytosis as the primary uptake mechanisms. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actbio.2010.08.003
  • 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 abstract10.1063/1.3292685
  • An ultrafine nanoporous bimetallic Ag-Pd alloy with superior catalytic activity
    Ji, H. and Frenzel, J. and Qi, Z. and Wang, X. and Zhao, C. and Zhang, Z. and Eggeler, G.
    CrystEngComm 12 (2010)
    An ultrafine nanoporous Ag80Pd20 alloy can be fabricated by chemical dealloying of a rapidly solidified Mg60Ag 32Pd8 alloy. The addition of the third element Pd into Mg-Ag realizes the design and functionalization of a nanoporous bimetallic structure, which exhibits superior catalytic activity towards electro-oxidation of ethanol. © 2010 The Royal Society of Chemistry.
    view abstract10.1039/c0ce00293c
  • Analysis of local microstructure after shear creep deformation of a fine-grained duplex γ-TiAl alloy
    Peter, D. and Viswanathan, G.B. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 58 (2010)
    The present work characterizes the microstructure of a hot-extruded Ti-45Al-5Nb-0.2B-0.2C (at.%) alloy with a fine-grained duplex microstructure after shear creep deformation (temperature 1023 K; shear stress 175 MPa; shear deformation 20%). Diffraction contrast transmission electron microscopy (TEM) was performed to identify ordinary dislocations, superdislocations and twins. The microstructure observed in TEM is interpreted taking into account the contribution of the applied stress and coherency stresses to the overall local stress state. Two specific locations in the lamellar part of the microstructure were analyzed, where either twins or superdislocations provided c-component deformation in the L10 lattice of the γ phase. Lamellar γ grains can be in soft and hard orientations with respect to the resolved shear stress provided by the external load. The presence of twins can be rationalized by the superposition of the applied stress and local coherency stresses. The presence of superdislocations in hard γ grains represents indirect evidence for additional contributions to the local stress state associated with stress redistribution during creep. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2010.08.005
  • Axial-torsional thermomechanical fatigue of a near-γ TiAl-alloy
    Brookes, S.P. and Kühn, H.-J. and Skrotzki, B. and Klingelhöffer, H. and Sievert, R. and Pfetzing, J. and Peter, D. and Eggeler, G.
    Materials Science and Engineering A 527 (2010)
    The uniaxial, torsional and axial-torsional thermomechanical fatigue (TMF) behavior of the near-γ TiAl-alloy TNB-V5 was investigated. TMF tests were performed at 400-800°C with mechanical strain amplitudes ranging from 0.15% to 0.7%. The tests were conducted thermomechanically in-phase (IP) and out-of-phase (OP).For the same lifetimes, uniaxial IP tests required the highest strain amplitudes, while OP test conditions were most damaging and needed the lowest strain amplitudes. The Mises equivalent mechanical strain amplitudes of pure torsional tests were found in between uniaxial in-phase and out-of-phase tests for the same lifetimes. The non-proportional multiaxial out-of-phase test showed a lower lifetime at the same equivalent mechanical strain amplitude compared to the other types of tests.The microstructure has been characterized applying electron microscopy and microstructural parameters such as fraction of twinned grains, grain size, lamellar distance and dislocation density have been quantified. © 2010 Elsevier B.V.
    view abstract10.1016/j.msea.2010.03.073
  • Creep properties beyond 1100°C and microstructure of Co-Re-Cr alloys
    Brunner, M. and Hüttner, R. and Bölitz, M.-C. and Völkl, R. and Mukherji, D. and Rösler, J. and Depka, T. and Somsen, C. and Eggeler, G. and Glatzel, U.
    Materials Science and Engineering A 528 (2010)
    The melting point of a novel Co-17Re-23Cr alloy (numbers given in at.%) could be increased by 250 °C as compared to established Ni-based superalloys, by optimising the content of Re. Samples were produced by vacuum arc-melting in order to evaluate the creep behaviour at temperatures beyond 1100 °C and for microstructural analysis. Three alloys (the Co-17Re-23Cr-based material, and the carbide strengthened alloys Co-17Re-23Cr-2.6C and Co-17Re-23Cr-2.6C-1.2Ta) were investigated. Creep properties, especially the minimum creep rate and the Larson-Miller plots, were compared. The Co-17Re-23Cr-2.6C-1.2Ta alloy has a higher minimum creep rate than Co-17Re-23Cr at 1200 °C but it has a lower minimum creep rate than Co-17Re-23Cr at 1100 °C. TaC coarsening, detected via transmission electron microscope (TEM) measurements may explain this effect. The overall creep behaviour of Co-17Re-23Cr-2.6C at 1200 °C is better than that of Co-17Re-23Cr-2.6C-1.2Ta, but worse than that of Co-17Re-23Cr.Microstructural investigations by scanning electron microscopy and TEM reveal a hexagonal closed-packed (hcp) matrix and σ-phases. The microhardness of the σ-phase was about 1570. HV (load: 1. g) and around 800. HV for the matrix. Pores and cracks occur along the brittle σ-phases and grain boundaries in the Co-Re-Cr alloys. A Norton exponent n in between 1.4 and 3.0 points to grain boundary dominated creep mechanisms. © 2010 Elsevier B.V.
    view abstract10.1016/j.msea.2010.09.035
  • Effect of low-temperature precipitation on the transformation characteristics of Ni-rich NiTi shape memory alloys during thermal cycling
    Wagner, M.F.-X. and Dey, S.R. and Gugel, H. and Frenzel, J. and Somsen, C. and Eggeler, G.
    Intermetallics 18 (2010)
    Thermal cycling of NiTi shape memory alloys is associated with functional fatigue: the characteristic phase transformation temperatures decrease with increasing number of cycles, and the transformation behavior changes from a single- to a two-stage martensitic transformation involving the intermediate R-phase. These effects are usually attributed to a gradual increase of dislocation density associated with micro-plasticity during repeated cycling through the transformation range. Here, these changes are shown to increase at a higher maximum temperature (in the fully austenitic state) during differential scanning calorimetric cycling of a Ni-rich alloy. Additional thermal cycling experiments without repeated phase transformations, and post-mortem microstructural observations by transmission electron microscopy, demonstrate that a relevant portion of functional fatigue is due to the formation of nano-scale Ni-rich precipitates of type Ni4Ti3 even at temperatures relatively close to the austenite finish temperature. These results show that both dislocation generation during the diffusion-less phase transformation, and diffusion-controlled nucleation and growth of Ni4Ti3 precipitates, can interact and contribute to the evolution of functional properties during thermal cycling of Ni-rich NiTi. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2010.02.048
  • Effect of Ni4Ti3 precipitation on martensitic transformation in Ti-Ni
    Zhou, N. and Shen, C. and Wagner, M.F.-X. and Eggeler, G. and Mills, M.J. and Wang, Y.
    Acta Materialia 58 (2010)
    Precipitation of Ni4Ti3 plays a critical role in determining the martensitic transformation path and temperature in Ni-Ti shape memory alloys. In this study, the equilibrium shape of a coherent Ni 4Ti3 precipitate and the concentration and stress fields around it are determined quantitatively using the phase field method. Most recent experimental data on lattice parameters, elastic constants, precipitate-matrix orientation relationship and thermodynamic database are used as model inputs. The effects of the concentration and stress fields on subsequent martensitic transformations are analyzed through interaction energy between a nucleating martensitic particle and the existing microstructure. Results indicate that R-phase formation prior to B19′ phase could be attributed to both direct elastic interaction and stress-induced spatial variation in concentration near Ni4Ti3 precipitates. The preferred nucleation sites for the R-phase are close to the broad side of the lenticular-shaped Ni4Ti3 precipitates, where tension normal to the habit plane is highest, and Ni concentration is lowest. © 2010 AWE and Crown Copyright. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2010.08.033
  • Effects of annealing on the microstructure and the mechanical properties of EB-PVD thermal barrier coatings
    Zotov, N. and Bartsch, M. and Chernova, L. and Schmidt, D.A. and Havenith, M. and Eggeler, G.
    Surface and Coatings Technology 205 (2010)
    The effects of thermal annealing at 1000°C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4mol% Y2O3 partially stabilized ZrO2 top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.07.008
  • Focused ion beam/scanning electron microscopy tomography and conventional transmission electron microscopy assessment of Ni4Ti3 morphology in compression-aged Ni-rich Ni-Ti single crystals
    Cao, S. and Somsen, C. and Croitoru, M. and Schryvers, D. and Eggeler, G.
    Scripta Materialia 62 (2010)
    The size, morphology and configuration of Ni4Ti3 precipitates in a single-crystal Ni-Ti alloy have been investigated by two-dimensional transmission electron microscopy-based image analysis and three-dimensional reconstruction from slice-and-view images obtained in a focused ion beam/scanning electron microscopy (FIB/SEM) dual-beam system. Average distances between the precipitates measured along the compression direction correlate well between both techniques, while particle shape and configuration data is best obtained from FIB/SEM. Precipitates form pockets of B2 of 0.54 μm in the compression direction and 1 μm perpendicular to the compression direction. © 2009 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2009.11.040
  • How dislocation substructures evolve during long-term creep of a 12% Cr tempered martensitic ferritic steel
    Pešička, J. and Aghajani, A. and Somsen, C. and Hartmaier, A. and Eggeler, G.
    Scripta Materialia 62 (2010)
    We document the evolution of dislocation densities in tempered martensite ferritic steels during long-term aging and creep. Scanning transmission electron microscopy in combination with a high-angle annular dark-field detector is used to study dislocations in a 12% Cr steel. During aging, the dislocation density quickly decreases by a factor 2 and then remains constant. Long-term creep results in an initial decrease by a factor 10, and after this sharp drop, the dislocation density continues to decrease. © 2009 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2009.10.037
  • Identification of quaternary shape memory alloys with near-zero thermal hysteresis and unprecedented functional stability
    Zarnetta, R. and Takahashi, R. and Young, M.L. and Savan, A. and Furuya, Y. and Thienhaus, S. and Maaß, B. and Rahim, M. and Frenzel, J. and Brunken, H. and Chu, Y.S. and Srivastava, V. and James, R.D. and Takeuchi, I. and Eggeler, G. and Ludwig, Al.
    Advanced Functional Materials 20 (2010)
    Improving the functional stability of shape memory alloys (SMAs), which undergo a reversible martensitic transformation, is critical for their applications and remains a central research theme driving advances in shape memory technology. By using a thin-film composition-spread technique and high-throughput characterization methods, the lattice parameters of quaternary Ti-Ni-Cu-Pd SMAs and the thermal hysteresis are tailored. Novel alloys with near-zero thermal hysteresis, as predicted by the geometric nonlinear theory of martensite, are identified. The thin-film results are successfully transferred to bulk materials and near-zero thermal hysteresis is observed for the phase transformation in bulk alloys using the temperaturedependent alternating current potential drop method. A universal behavior of hysteresis versus the middle eigenvalue of the transformation stretch matrix is observed for different alloy systems. Furthermore, significantly improved functional stability, investigated by thermal cycling using differential scanning calorimetry, is found for the quaternary bulk alloy Ti50.2Ni34.4Cu12.3Pd3.1 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.200902336
  • Influence of heat treatment and microstructure on the tensile pseudoelastic response of an Ni-rich NiTi shape memory alloy
    Bujoreanu, L.-G. and Young, M.L. and Gollerthan, S. and Somsen, C. and Eggeler, G.
    International Journal of Materials Research 101 (2010)
    The influence of microstructure on the stress-strain behavior of an Ni-rich NiTi shape memory alloy is examined. Specimens cut from a large-diameter bar of Ni50.7Ti49.3 shape memory alloy were analyzed in two states: (i) annealed and (ii) annealed and aged. The annealed state shows a fully austenitic structure with no precipitates and no distortions caused by residual stresses. The annealed and aged state has coherent Ni 4Ti3 particles precipitated in the proximity of the austenite grain boundaries. The size of the precipitates increases moving away from the grain boundaries toward the grain interiors. The evolution of the two states in the stress-strain-temperature space has been analyzed using tensile specimens with special geometry. Due to the complex effects of the coherent precipitates, the specimens in the aged state exhibited lower stress plateaus in the tensile loading-unloading curves, which enabled the occurrence of transformation pseudoelasticity from room temperature to 333 K. © 2010 Carl Hanser Verlag.
    view abstract10.3139/146.110317
  • Influence of Ni on martensitic phase transformations in NiTi shape memory alloys
    Frenzel, J. and George, E.P. and Dlouhy, A. and Somsen, C. and Wagner, M.F.-X. and Eggeler, G.
    Acta Materialia 58 (2010)
    High-precision data on phase transformation temperatures in NiTi, including numerical expressions for the effect of Ni on MS, MF, AS, AF and T0, are obtained, and the reasons for the large experimental scatter observed in previous studies are discussed. Clear experimental evidence is provided confirming the predictions of Tang et al. 1999 [19] regarding deviations from a linear relation between the thermodynamic equilibrium temperature and Ni concentration. In addition to affecting the phase transition temperatures, increasing Ni contents are found to decrease the width of thermal hysteresis and the heat of transformation. These findings are rationalized on the basis of the crystallographic data of Prokoshkin et al. 2004 [68] and the theory of Ball and James [25]. The results show that it is important to document carefully the details of the arc-melting procedure used to make shape memory alloys and that, if the effects of processing are properly accounted for, precise values for the Ni concentration of the NiTi matrix can be obtained. © 2010 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2010.02.019
  • Localization events and microstructural evolution in ultra-fine grained NiTi shape memory alloys during thermo-mechanical loading
    Schaefer, A. and Wagner, M.F.-X. and Pelegrina, J.L. and Olbricht, J. and Eggeler, G.
    Advanced Engineering Materials 12 (2010)
    Subjecting a thin NiTi specimen to uniaxial tension often leads to a localized martensitic transformation: macroscopic transformation bands form and propagate through the specimen, separating it into regions of fully transformed martensite and original austenite. In the present study, the alternating current potential drop (ACPD) technique is used to analyze the change in electrical resistance of ultra-fine grained NiTi wires subjected to a broad range of thermo-mechanical load cases: (i) uniaxial tensile straining at constant temperatures (pseudoelastic deformation); (ii) cooling and heating through the transformation range at constant load (actuator load case); (iii) a combination of mechanical and thermal loading. We monitor the ACPD signals in several zones along the gauge length of specimens, and we demonstrate that a localized type of transformation is a generic feature of pseudoelastic as well as of shape memory deformation. Moreover, the ACPD signals allow to differentiate between temperature-induced martensite (formed during cooling at no or relatively small loads), stress-induced martensite, and reoriented martensite (formed under load at low temperatures). © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201000063
  • Magnetic coupling mechanisms in particle/thin film composite systems
    Confalonieri, G.A.B. and Szary, P. and Mishra, D. and Benitez, M.J. and Feyen, M. and Lu, A.H. and Agudo, L. and Eggeler, G. and Petracic, O. and Zabel, H.
    Beilstein Journal of Nanotechnology 1 (2010)
    Magnetic Γ-Fe 2O 3 nanoparticles with a mean diameter of 20 nm and size distribution of 7% were chemically synthesized and spin-coated on top of a Si-substrate. As a result, the particles self-assembled into a mono layer with hexagonal close-packed order. Subsequently, the nanoparticle array was coated with a Co layer of 20 nm thickness. The magnetic properties of this composite nanopar-ticle/thin film system were investigated by magnetometry and related to high-resolution transmission electron microscopy studies. Herein three systems were compared: i.e. a reference sample with only the particle monolayer, a composite system where the particle array was ion-milled prior to the deposition of a thin Co film on top, and a similar composite system but without ion-milling. The nanoparticle array showed a collective super-spin behavior due to dipolar interparticle coupling. In the composite system, we observed a decoupling into two nanoparticle subsystems. In the ion-milled system, the nanoparticle layer served as a magnetic flux guide as observed by magnetic force microscopy. Moreover, an exchange bias effect was found, which is likely to be due to oxygen exchange between the iron oxide and the Co layer, and thus forming of an antiferromagnetic CoO layer at the Γ-Fe 2O 3/Co interface. © 2010 Confalonieri et al.
    view abstract10.3762/bjnano.1.12
  • Microstructural characterization of lamellar features in TiAl by FIB imaging
    Peter, D. and Eggeler, G. and Wagner, M.F.-X.
    Advanced Engineering Materials 12 (2010)
    A novel experimental procedure is introduced to determine phase fractions and the distribution of individual phases of TiAl-based two-phase alloys using the focused ion beam (FIB) technique. Two γ-titanium aluminide alloys with a fine-grained duplex and a nearly lamellar microstructure are examined. The special FIB-based preparation procedure results in high contrast ion beam-induced images for all investigated alloys and allows to quantify the phase contents easily by automated microstructural analysis. Fine two-phase structures, e.g. lamellar colonies in γ-TiAl, can be imaged in high resolution with respect to different phases. To validate the FIB-derived data, we compare them to results obtained with another method to determine phase fractions, electron back-scatter diffraction (EBSD). This direct comparison shows that the FIB-based technique generally provides slightly higher α2-fractions, and thus helps to overcome the limited lateral resolution near grain boundaries and interfaces associated with the conventional EBSD approach. Our study demonstrates that the FIB-based technique is a simple, fast, and more exact way to determine high resolution microstructural characteristics with respect to different phase constitutions in two-phase TiAl alloys and other such materials with fine, lamellar microstructures. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.200900339
  • Molecular dynamics simulations of the shape memory effect in a chain of Lennard-Jones crystals
    Kastner, O. and Eggeler, G.
    Multidiscipline Modeling in Materials and Structures 6 (2010)
    Purpose - Shape memory alloys are a fascinating class of materials because they combine both structural and functional properties. These properties strongly depend on temperature. One consequence of this dependency yields the characteristic shape-memory effect: shape memory alloys can recover processed reference configurations after significant plastic deformations simply upon a change of temperature. For real materials, such processes incorporate characteristic hysteresis. This paper aims at an understanding of these materials from an atomistic point of view. Design/methodology/approach - 2D molecular-dynamics (MD) simulations describing a chain consisting of 32 linked Lennard-Jones crystals are presented. The crystals consist of nested lattices of two atom species. Distinct lattice structures can be identified, interpreted as austenite and (variants of) martensite. Temperature and/or load-induced phase transitions between these configurations are observed in MD simulations. Previously, the thermal equation of state of one isolated crystal was investigated and its phase stability was discussed in detail. In the multi-crystal chain considered in the present paper, individual crystals contribute collectively to the thermo-mechanical behavior of the assembly. Findings - The paper presents the results of numerical experiments with this polycrystalline chain under strain-, load- and/or temperature-control. The results show that with the assumption of simple Lennard-Jones potentials of interaction between atoms in individual crystals and linking these crystals allows to reproduce the features associated with the fascinating behavior of shape memory alloys, including pseudo-plasticity, pseudo-elasticity and the shape memory effect. Originality/value - Owing to the special setup chosen, interfaces are missing between adjacent crystals in the chain assembly. The paper shows that in this situation load-induced austenite/ martensite transitions do not exhibit hysteresis in tension/compression cycles. This observation indirectly supports mesoscopic-level work in the literature which explicitly introduces interface energy to model such hysteresis. © Emerald Group Publishing Limited.
    view abstract10.1108/15736101011055275
  • Nanoindentation of a pseudoelastic NiTiFe shape memory alloy
    Pfetzing-Micklich, J. and Wagner, M.F.-X. and Zarnetta, R. and Frenzel, J. and Eggeler, G. and Markaki, A.E. and Wheeler, J. and Clyne, T.W.
    Advanced Engineering Materials 12 (2010)
    Nanoindentation is a suitable tool for characterizing the local mechanical properties of shape memory alloys (SMA) and to study their pseudoelastic behavior. There is a special interest in indenting with different indenter tips (as not all tips are associated with strain states that predominantly induce the martensitic transformation) and in indenting at different temperatures, where different phases are present. In this study, we perform nanoindentation on a ternary NiTiFe SMA with different indenter tips and at various testing temperatures. For nanoindentation with spherical tips, load-displacement hystereses clearly indicate pseudoelastic behavior, whereas indentation with Berkovich tips results in more pronounced plastic deformation. Testing at different temperatures is associated with different volume fractions of austenite, martensite, and R-phase. The corresponding nanoindentation responses differ considerably in terms of pseudoelastic behavior. Best pseudoelastic recovery is found at testing temperatures close to the R-phase start temperature, even though this temperature is below the austenite finish temperature, which is a well-known lower temperature bound for full recovery in macroscopic tests. Our results are discussed considering micromechanical aspects and the interaction between stress-induced phase transformation and dislocation plasticity. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.200900266
  • On the characterization of recrystallized fraction using electron backscatter diffraction: A direct comparison to local hardness in an IF steel using nanoindentation
    Dziaszyk, S. and Payton, E.J. and Friedel, F. and Marx, V. and Eggeler, G.
    Materials Science and Engineering A 527 (2010)
    Recrystallized fraction was characterized in a Ti-stabilized interstitial-free (IF) steel by electron backscatter diffraction (EBSD), optical metallography, and hardness-based techniques. EBSD and nanoindentation were performed on overlapping areas to assess the agreement between standard methods of EBSD analysis of deformation microstructures and local hardness. The results of the study indicate that carefully implemented misorientation-based techniques may be used to effectively determine recrystallization fraction, to better agreement with the local recrystallization state than EBSD image quality, optical metallography, or microhardness-based techniques. © 2010 Elsevier B.V.
    view abstract10.1016/j.msea.2010.08.063
  • On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys
    Simon, T. and Kröger, A. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 58 (2010)
    In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0}B2 planes parallel to {0 0 1}B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys. © 2010.
    view abstract10.1016/j.actamat.2009.11.028
  • Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading
    Young, M.L. and Wagner, M.F.-X. and Frenzel, J. and Schmahl, W.W. and Eggeler, G.
    Acta Materialia 58 (2010)
    An ultrafine-grained pseudoelastic NiTi shape-memory alloy wire with 50.9 at.% Ni was examined using synchrotron X-ray diffraction during in situ uniaxial tensile loading (up to 1 GPa) and unloading. Both macroscopic stress-strain measurements and volume-averaged lattice strains are reported and discussed. The loading behavior is described in terms of elasto-plastic deformation of austenite, emergence of R phase, stress-induced martensitic transformation, and elasto-plastic deformation, grain reorientation and detwinning of martensite. The unloading behavior is described in terms of stress relaxation and reverse plasticity of martensite, reverse transformation of martensite to austenite due to stress relaxation, and stress relaxation of austenite. Microscopically, lattice strains in various crystallographic directions in the austenitic B2, martensitic R, and martensitic B19′ phases are examined during loading and unloading. It is shown that the phase transformation occurs in a localized manner along the gage length at the plateau stress. Phase volume fractions and lattice strains in various crystallographic reflections in the austenite and martensite phases are examined over two transition regions between austenite and martensite, which have a width on the order of the wire diameter. Anisotropic effects observed in various crystallographic reflections of the austenitic phase are also discussed. The results contribute to a better understanding of the tensile loading behavior, both macroscopically and microscopically, of NiTi shape-memory alloys. © 2009 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2009.12.021
  • Recent Developments and Challenges in Shape Memory Technology
    Frotscher, M. and Eggeler, G.
    Nanotechnological Basis for Advanced Sensors (2010)
    Nickel-Titanium-based alloys (NiTi) represent structural materials with actor and sensor functions due to their shape memory properties. This is especially useful for actuator applications, e.g. in valves or switches. A temperature sensor is no longer needed because the material can be set to exhibit the shape memory effect at a given temperature in a certain temperature range. High actuator forces and strains can be generated in a small material volume, which is ideal for the miniaturization of technical devices. Recent developments and technological challenges in shape memory technology with a focus on the research of the interdisciplinary Center for Shape Memory Technology (SFB 459 - Formgedachtnistechnik) at the Ruhr-University Bochum are discussed.
    view abstract10.1007/978-94-007-0903-4_56
  • Suppression of Ni4Ti3 precipitation by grain size refinement in Ni-rich NiTi shape memory alloys
    Prokofiev, E.A. and Burow, J.A. and Payton, E.J. and Zarnetta, R. and Frenzel, J. and Gunderov, D.V. and Valiev, R.Z. and Eggeler, G.
    Advanced Engineering Materials 12 (2010)
    Severe plastic deformation (SPD) processes, such as equal channel angular pressing (ECAP) and high pressure torsion (HPT), are successfully employed to produce ultra fine grain (UFG) and nanocrystalline (NC) microstructures in a Ti-50.7 at% Ni shape memory alloy. The effect of grain size on subsequent Ni-rich particle precipitation during annealing is investigated by transmission electron microscopy (TEM), selected area electron diffraction (SAD, SAED), and X-ray diffraction (XRD). It is observed that Ni4Ti3 precipitation is suppressed in grains of cross-sectional equivalent diameter below approximately 150 nm, and that particle coarsening is inhibited by very fine grain sizes. The results suggest that fine grain sizes impede precipitation processes by disrupting the formation of selfaccommodating particle arrays and that the arrays locally compensate for coherency strains during nucleation and growth. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201000101
  • Surface chemistry and topographical changes of an electropolished NiTi shape memory alloy
    Neelakantan, L. and Valtiner, M. and Eggeler, G. and Hassel, A. W.
    Physica Status Solidi A-applications and Materials Science 207 (2010)
    Electropolishing and its mechanism for NiTi shape memory alloy in methanolic sulfuric acid have been investigated recently. In the current study, X-ray photoelectron spectroscopy was carried out to characterize the chemical nature of the electropolished surface. The electropolished surface is covered by a thin layer of TiO(2). For an electropolished specimen, the Ni:Ti ratio at surface (similar to 1 nm) was 0.31, whereas for a mechanically polished specimen the ratio was 0.70. The second aspect of this work is the attempt to characterize the salt film formed during electropolishing. The characterization of salt-film showed sulfide and sulfate compounds of Ni and/or Ti. The crystallographic and topographical changes caused due to thermally induced phase transformation on an electropolished NiTi was followed by in situ XRD and AFM. AFM studies revealed significant changes in the surface features due to phase transformation. The surface roughness value increased by a factor of 2. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssa.200983312
  • Where Does the Lithium Go? - A Study of the Precipitates in the Stir Zone of a Friction Stir Weld in a Li-containing 2xxx Series Al Alloy
    Rao, J.C. and Payton, E.J. and Somsen, C. and Neuking, K. and Eggeler, G. and Kostka, A. and Dos Santos, J.F.
    Advanced Engineering Materials 12 (2010)
    The main strengthening precipitates of aluminum alloy 2198-T8, which are of the T1 phase, dissolve during friction stir welding, sending many Li atoms into solid solution. The stir zone precipitates are characterized using high-resolution transmission electron microscopy, energy dispersive spectroscopy, and selected area diffraction techniques to begin answering questions about the microstructural evolution and the relationship between microstructure and mechanical properties in friction stir welding of the next generation of lightweight Li-containing Al alloys. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.200900284
  • alloys

  • creep

  • martensite

  • mechanical properties

  • microstructure

  • scanning electron microscopy

  • shape-memory alloys

  • superalloys

  • transmission electron microscopy

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