Prof. Dr.-Ing. Jan Frenzel

Institute for Materials
Ruhr-Universität Bochum

Author IDs

  • X-radiography front tracking gradient furnace for directional solidification of bulk Al-alloys
    Jafarizadeh-Koohbanani, A. and Steinbach, S. and Drescher, J. and Frenzel, J. and Kargl, F.
    Review of Scientific Instruments 94 (2023)
    A unique gradient furnace for directional solidification experiments with bulk Al-alloy samples developed at German Aerospace Center is presented. It allows for in situ process control in solidifying samples by using x-radiography, and further insight into the solidification process is gained in combination with x-ray computational tomography on the solidified samples. Tracking of interfaces during directional solidification of bulk samples via in situ x-radiography (TIREX) enables the investigation of the melting process and observation of the movement of the entire mushy zone through the sample, tracing the solid-liquid interface during directional solidification and correlating the observations with the microstructure of the samples. Monitoring the temperature profile inside the sample by in situ observation of the length of the mushy zone is particularly important because the temperature gradient G and the rate of interfacial growth v determine the microstructure of solidification. The x-radiography setup offers temporal and spatial resolutions of 0.5 s and 70 μm, respectively, with a field of view of 10 × 50 mm2. Constant solidification velocities of up to 0.15 mm s−1 at a temperature gradient of up to 8 K mm−1 can be achieved in a temperature range of 537-1373 K. A flat solid-liquid interface inside a rod-like sample with 5 mm diameter is achieved by surrounding the sample by thermal isolating graphite foam. Performance tests with hypoeutectic Al-10 wt. % Cu alloy samples show the functionality of the furnace facility. © 2023 Author(s).
    view abstract10.1063/5.0151523
  • A Novel Thermo-Mechanical Processing Route Exploiting Abnormal Grain Growth in Heusler-Type Co–Ni–Ga Shape Memory Alloys
    Lauhoff, C. and Pham, T. and Paulsen, A. and Krooß, P. and Frenzel, J. and Eggeler, G. and Niendorf, T.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 53 (2022)
    view abstract10.1007/s11661-022-06827-7
  • Exploring MgO/HA ceramic nano-composites for biodegradable implants: Exploring biological properties and micromechanics
    Khalili, V. and Sengstock, C. and Kalchev, Y. and Pfetzing-Micklich, J. and Frenzel, J.
    Surface and Coatings Technology 445 (2022)
    view abstract10.1016/j.surfcoat.2022.128730
  • The role of electrons during the martensitic phase transformation in NiTi-based shape memory alloys
    Kunzmann, A. and Frenzel, J. and Wolff, U. and Han, J.W. and Giebeler, L. and Piorunek, D. and Mittendorff, M. and Scheiter, J. and Reith, H. and Perez, N. and Nielsch, K. and Eggeler, G. and Schierning, G.
    Materials Today Physics 24 (2022)
    view abstract10.1016/j.mtphys.2022.100671
  • A 3d analysis of dendritic solidification and mosaicity in ni-based single crystal superalloys
    Scholz, F. and Cevik, M. and Hallensleben, P. and Thome, P. and Eggeler, G. and Frenzel, J.
    Materials 14 (2021)
    Ni-based single crystal superalloys contain microstructural regions that are separated by low-angle grain boundaries. This gives rise to the phenomenon of mosaicity. In the literature, this type of defect has been associated with the deformation of dendrites during Bridgman solidification. The present study introduces a novel serial sectioning method that allows to rationalize mosaicity on the basis of spatial dendrite growth. Optical wide-field micrographs were taken from a series of cross sections and evaluated using quantitative image analysis. This allowed to explore the growth directions of close to 2500 dendrites in a large specimen volume of approximately 450 mm3. The application of tomography in combination with the rotation vector base-line electron back-scatter diffraction method allowed to analyze how small angular differences evolve in the early stages of solidification. It was found that the microstructure consists of dendrites with individual growth directions that deviate up to ≈4° from the average growth direction of all dendrites. Generally, individual dendrite growth directions coincide with crystallographic <001> directions. The quantitative evaluation of the rich data sets obtained with the present method aims at contributing to a better understanding of elementary processes that govern competitive dendrite growth and crystal mosa-icity. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14174904
  • 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
  • Laboratory-Scale Processing and Performance Assessment of Ti–Ta High-Temperature Shape Memory Spring Actuators
    Paulsen, A. and Dumlu, H. and Piorunek, D. and Langenkämper, D. and Frenzel, J. and Eggeler, G.
    Shape Memory and Superelasticity 7 (2021)
    Ti75Ta25 high-temperature shape memory alloys exhibit a number of features which make it difficult to use them as spring actuators. These include the high melting point of Ta (close to 3000 °C), the affinity of Ti to oxygen which leads to the formation of brittle α-case layers and the tendency to precipitate the ω-phase, which suppresses the martensitic transformation. The present work represents a case study which shows how one can overcome these issues and manufacture high quality Ti75Ta25 tensile spring actuators. The work focusses on processing (arc melting, arc welding, wire drawing, surface treatments and actuator spring geometry setting) and on cyclic actuator testing. It is shown how one can minimize the detrimental effect of ω-phase formation and ensure stable high-temperature actuation by fast heating and cooling and by intermediate rejuvenation anneals. The results are discussed on the basis of fundamental Ti–Ta metallurgy and in the light of Ni–Ti spring actuator performance. © 2021, The Author(s).
    view abstract10.1007/s40830-021-00334-1
  • Path to single-crystalline repair and manufacture of Ni-based superalloy using directional annealing
    Kalfhaus, T. and Schaar, H. and Thaler, F. and Ruttert, B. and Sebold, D. and Frenzel, J. and Steinbach, I. and Theisen, W. and Guillon, O. and Clyne, T.W. and Vassen, R.
    Surface and Coatings Technology 405 (2021)
    Advanced methods for the repair of single-crystalline (SX) Ni-based superalloys are of special interest for the gas turbine industry. Polycrystalline repair approaches show promising results, while the repair of SX materials is still challenging. Directional annealing experiments resulted in large columnar grains by imposing thermal gradients at the abnormal grain growth temperature of a specific Ni-based superalloy. A numerical model of the Bridgman process is applied to provide an insight into the temperature evolution during zone annealing of the Vacuum-Plasma-Spray (VPS) repair coatings with the aim of promoting grain growth from the SX substrate. The results presented here suggest that this is a promising approach to repair or manufacture SX turbine blades. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.126494
  • Plasticity induced by nanoindentation in a CrCoNi medium-entropy alloy studied by accurate electron channeling contrast imaging revealing dislocation-low angle grain boundary interactions
    Habiyaremye, F. and Guitton, A. and Schäfer, F. and Scholz, F. and Schneider, M. and Frenzel, J. and Laplanche, G. and Maloufi, N.
    Materials Science and Engineering A 817 (2021)
    In the present work, interactions of nanoindentation-induced dislocations (NIDs) with a low-angle grain boundary (LAGB) are investigated in a single-crystalline CrCoNi medium-entropy alloy (MEA). Microstructural evolutions before and after nanoindentation were examined using accurate electron channeling contrast imaging (A-ECCI). In the as-grown state, the alloy microstructure consists of subgrains separated by LAGBs. After nanoindentation on the (001) plane far away from LAGBs, the load-displacement curves exhibit the typical behavior of metals and alloys with a pop-in marking the elastic-plastic transition. This pop-in is related to the nucleation of NIDs that are observed to form pile-ups on {111} planes. In contrast, when indents are performed in the vicinity of a LAGB with a low misorientation angle of 0.24° and consisting of dislocations spaced ~60 nm apart, different micromechanical responses and deformation mechanisms are observed depending on the distance between the LAGB and the nanoindenter tip. When the distance between the LAGB and the nanoindenter tip is larger than four times the size of the indent (corresponding ratio: R > 4), the LAGB does not affect the micromechanical response nor interact with NIDs. In contrast, when the indenter comes in direct or indirect contact with the LAGB (R < 1), the load-displacement curve deviates at low loads from the elastic stage, and pop-ins are not observed. In this case, the continuous deformation is accommodated by the movement of the pre-existing LAGB dislocations. For intermediate cases with 1 < R < 4, the load of the initial pop-in is dependent on the local defect density. In this latter case, the pile-ups of NIDs directly impinge on the LAGB. Microstructural analyses reveal that the LAGB accommodates plasticity by blocking the NIDs, activating a dislocation nucleation site in the adjacent subgrain/emission of dislocation from the LAGB, and inducing slight motions of its constituent dislocations. © 2021 Elsevier B.V.
    view abstract10.1016/j.msea.2021.141364
  • 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
  • Burst-like reverse martensitic transformation during heating, cooling and under isothermal conditions in stabilized Ni-Ti-Nb
    Picornell, C. and Pons, J. and Paulsen, A. and Frenzel, J. and Kaminskii, V. and Sapozhnikov, K. and Van Humbeeck, J. and Kustov, S.
    Scripta Materialia 180 (2020)
    To reveal mechanisms of martensite stabilization by prestrain in shape memory alloys, details of reverse martensitic transformation (MT) in prestrained Ni45Ti46Nb9 alloy were studied using calorimetry, dilatometry and resistivity. The first reverse MT is burst-like and is shifted to higher temperatures by ~120 K as compared to the nominal MT. During thermal cycling approaching the first reverse MT, stabilized martensite shows “forbidden” behaviours like burst-like reverse MT not during heating, but during cooling and under isothermal conditions. These unusual effects are attributed to non-thermoelastic nature of the first reverse MT in stabilized martensite under high chemical driving force. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2020.01.018
  • 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
  • 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
  • On the Importance of Structural and Functional Fatigue in Shape Memory Technology
    Frenzel, J.
    Shape Memory and Superelasticity (2020)
    The present work provides a brief overview on structural and functional fatigue in shape memory alloys (SMAs). Both degenerative processes are of utmost technological importance because they limit service lives of shape memory components. While our fundamental understanding of these two phenomena has improved during the last two decades, there are still fields which require scientific attention. NiTi SMAs are prone to the formation of small cracks, which nucleate and grow in the early stages of structural fatigue. It is important to find out how these micro-cracks evolve into engineering macro-cracks, which can be accounted for by conventional crack growth laws. The present work provides examples for the complexity of short crack growth in pseudoelastic SMAs. The importance of functional fatigue has also been highlighted. Functional fatigue is related to the degeneration of specific functional characteristics, such as actuator stroke, recoverable strain, plateau stresses, hysteresis width, or transformation temperatures. It is caused by the accumulation of transformation-induced defects in the microstructure. The functional stability of SMAs can be improved by (1) making phase transformations processes smoother and (2) by improving the material’s resistance to irreversible processes like dislocation plasticity. Areas in need of further research are discussed. © 2020, The Author(s).
    view abstract10.1007/s40830-020-00281-3
  • 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 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
  • Orientation-dependent deformation behavior of 316L steel manufactured by laser metal deposition and casting under local scratch and indentation load
    Pöhl, F. and Hardes, C. and Scholz, F. and Frenzel, J.
    Materials 13 (2020)
    This study analyzes the local deformation behavior of austenitic stainless steel 316L, manufactured conventionally by casting and additively by laser metal deposition (LMD). We produced directionally solidified 316L specimens with most grains showing (001) orientations parallel to the longitudinal specimen axis. We conducted nanoindentation and scratch experiments for local mechanical characterization and topography measurements (atomic force microscopy and confocal laser scanning microscopy) of indentation imprints and residual scratch grooves for the analysis of the deformation behavior and, in particular, of the pile-up behavior. The local mechanical properties and deformation behavior were correlated to the local microstructure investigated by scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. The results show that the local mechanical properties, deformation behavior, and scratch resistance strongly depend on the crystallographic orientation. Nearly (001)-oriented grains parallel to the surface show the lowest hardness, followed by an increasing hardness of nearly (101)-and (111)-oriented grains. Consequently, scratch depth is the greatest for nearly (001)-oriented grains followed by (101) and (111) orientations. This tendency is seen independently of the analyzed manufacturing route, namely Bridgman solidification and laser metal deposition. In general, the laser metal deposition process leads to a higher strength and hardness, which is mainly attributed to a higher dislocation density. Under the investigated loading conditions, the cellular segregation substructure is not found to significantly and directly change the local deformation behavior during indentation and scratch testing. © 2020 by the authors.
    view abstract10.3390/MA13071765
  • 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
  • The Effect of Increasing Chemical Complexity on the Mechanical and Functional Behavior of NiTi-Related Shape Memory Alloys
    Hinte, C. and Barienti, K. and Steinbrücker, J. and Hartmann, J.-M. and Gerstein, G. and Herbst, S. and Piorunek, D. and Frenzel, J. and Fantin, A. and Maier, H.J.
    Shape Memory and Superelasticity 6 (2020)
    The introduction of high-entropy alloys (HEA) into the field of shape memory alloys offers enormous potential for improving their functional properties. It is shown how a successive increase in chemical complexity results in strictly monotonically enlarged and increasingly distorted lattices. With increasing the number of elements added to the alloy, the effect of solid solution strengthening appears to be curtailed and first insights into the contribution of additional mechanisms based on lattice distortions are possible. The alloys developed in this study, reaching from ternary NiTiHf to senary TiZrHfCoNiCu, show a great potential to exploit interatomic interactions regarding improvement of functional fatigue. Despite the absence of stress plateaus related to detwinning, recovery effects at loads above 1000 MPa and significant strain recoveries are shown. © 2020, ASM International.
    view abstract10.1007/s40830-020-00284-0
  • 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
  • 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
  • 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
  • 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 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
  • Reconciling Experimental and Theoretical Data in the Structural Analysis of Ti–Ta Shape-Memory Alloys
    Ferrari, A. and Kadletz, P.M. and Chakraborty, T. and Liao, K. and Langenkämper, D. and Motemani, Y. and Paulsen, A. and Lysogorskiy, Y. and Frenzel, J. and Rogal, J. and Ludwig, Al. and Somsen, C. and Drautz, R. and Schmahl, W.W.
    Shape Memory and Superelasticity 5 (2019)
    The structural characterization of the various phases that occur in Ti–Ta-based high-temperature shape-memory alloys is complicated by the presence of many competing phases as a function of composition. In this study, we resolve apparent inconsistencies between experimental data and theoretical calculations by suggesting that phase separation and segregation of undesired phases are not negligible in these alloys, and that finite temperature effects should be taken into account in the modeling of these materials. Specifically, we propose that the formation of the ω phase at low Ta content and of the σ phase at high Ta content implies a difference between the nominal alloy composition and the actual composition of the martensitic and austenitic phases. In addition, we show that temperature affects strongly the calculated values of the order parameters of the martensitic transformation occurring in Ti–Ta. © 2018, ASM International.
    view abstract10.1007/s40830-018-00201-6
  • 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
  • Strength of hydrogen-free and hydrogen-doped Ni 50 Ti 50 shape memory platelets
    Weiser, A. and Buršíková, V. and Jarý, M. and Dymáček, P. and Dugáček, J. and Frenzel, J. and Čermák, J. and Dlouhý, A.
    Scripta Materialia 162 (2019)
    Small-punch and nano-indentation tests were used for the first time to probe strength of 500 μm thin Ni 50 Ti 50 shape memory platelets in their hydrogen-free and hydrogen-doped states. Results show excellent reproducibility and suggest that hydrogen penetrates the alloy more efficiently during the cathodic charging at ambient temperatures as compared to heat treatments in a controlled hydrogen atmosphere. Hydrogen content exceeding 100 wtppm results in a retransformation from the B19′ martensite to the R lattice and causes a systematic drop of the rupture strength. The retransformation events in thin surface lamellae were documented by the transmission electron microscopy. © 2018 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2018.10.044
  • Strength of hydrogen-free and hydrogen-doped Ni50Ti50 shape memory platelets
    Weiser, A. and Buršíková, V. and Jarý, M. and Dymáček, P. and Dugáček, J. and Frenzel, J. and Čermák, J. and Dlouhý, A.
    Scripta Materialia 162 (2019)
    Small-punch and nano-indentation tests were used for the first time to probe strength of 500 μm thin Ni50Ti50 shape memory platelets in their hydrogen-free and hydrogen-doped states. Results show excellent reproducibility and suggest that hydrogen penetrates the alloy more efficiently during the cathodic charging at ambient temperatures as compared to heat treatments in a controlled hydrogen atmosphere. Hydrogen content exceeding 100 wtppm results in a retransformation from the B19′ martensite to the R lattice and causes a systematic drop of the rupture strength. The retransformation events in thin surface lamellae were documented by the transmission electron microscopy. © 2018 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2018.10.044
  • Development of Single-Crystal Ni-Base Superalloys Based on Multi-criteria Numerical Optimization and Efficient Use of Refractory Elements
    Markl, M. and Müller, A. and Ritter, N. and Hofmeister, M. and Naujoks, D. and Schaar, H. and Abrahams, K. and Frenzel, J. and Subramanyam, A.P.A. and Ludwig, Al. and Pfetzing-Micklich, J. and Hammerschmidt, T. and Drautz, R. and Steinbach, I. and Rettig, R. and Singer, R.F. and Körner, C.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 (2018)
    The development of new Ni-base superalloys with a complex composition consisting of eight or more alloying elements is a challenging task. The experimental state-of-the-art development cycle is based on the adaption of already existing compositions. Although new alloy compositions with potentially improved material properties are expected to be similar to already known superalloys, this procedure impedes efficiently finding these compositions in the large multi-dimensional design-space of all alloying elements. Modern alloy development combines numerical optimization methods with experimental validation to guide the development towards promising compositions. In this work, an improved numerical multi-criteria optimization tool using CALPHAD calculations and semi-empirical models for alloy development is presented. The model improvements to its predecessor are described and the successful application for the development of rhenium-free single-crystal Ni-base superalloys ERBO/13 and ERBO/15 is revisited. The optimization tool is described and the designed alloys are discussed regarding phase stability. Finally, a possible phase stability model extending the optimization tool and improving the alloy composition predictions is presented. © 2018, The Author(s).
    view abstract10.1007/s11661-018-4759-0
  • 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
  • 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 the Ni-Ion release rate from surfaces of binary NiTi shape memory alloys
    Ševčíková, J. and Bártková, D. and Goldbergová, M. and Kuběnová, M. and Čermák, J. and Frenzel, J. and Weiser, A. and Dlouhý, A.
    Applied Surface Science 427 (2018)
    The study is focused on Ni-ion release rates from NiTi surfaces exposed in the cell culture media and human vascular endothelial cell (HUVEC) culture environments. The NiTi surface layers situated in the depth of 70 μm below a NiTi oxide scale are affected by interactions between the NiTi alloys and the bio-environments. The finding was proved with use of inductively coupled plasma mass spectrometry and electron microscopy experiments. As the exclusive factor controlling the Ni-ion release rates was not only thicknesses of the oxide scale, but also the passivation depth, which was two-fold larger. Our experimental data strongly suggested that some other factors, in addition to the Ni concentration in the oxide scale, admittedly hydrogen soaking deep below the oxide scale, must be taken into account in order to rationalize the concentrations of Ni-ions released into the bio-environments. The suggested role of hydrogen as the surface passivation agent is also in line with the fact that the Ni-ion release rates considerably decrease in NiTi samples that were annealed in controlled hydrogen atmospheres prior to bio-environmental exposures. © 2017 Elsevier B.V.
    view abstract10.1016/j.apsusc.2017.08.235
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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 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
  • 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
  • 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
  • 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
  • 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 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
  • Reaction Network of Methanol Synthesis over Cu/ZnO Nanocatalysts
    Martinez-Suarez, L. and Siemer, N. and Frenzel, J. and Marx, D.
    ACS Catalysis 5 (2015)
    The efficiency of industrial methanol synthesis from syngas results from a complex scenario of surface chemical reactions in the presence of dynamical morphological changes of the catalyst material in response to the chemical and physical properties of the gas phase, which are believed to explain the superior performance of the Cu/ZnO catalyst. Yet, the applied conditions of elevated temperatures and pressures substantially hamper in situ experimental access and, therefore, detailed understanding of the underlying reaction mechanism(s) and active site(s). Here, part of this huge space of possibilities emerging from the structural and chemical configurations of both, adsorbates and continuously altering Cu/ZnO catalyst material, is successfully explored by pure computational means. Using our molecular dynamics approach to computational heterogeneous catalysis, being based on advanced ab initio simulations in conjunction with thermodynamically optimized catalyst models, the resulting mapping of the underlying free energy landscape discloses an overwhelmingly rich network of parallel, competing, and reverse reaction channels. After having analyzed various pathways that directly lead from CO2 to methanol, not only specific Cu/ZnO interface sites but also the near surface region over the catalyst surface were identified as key to some pivotal reaction steps in the global reaction network. Analysis of the mechanistic details and electronic structure along individual steps unveils three distinct mechanisms of surface chemical reactions being all at work, namely Eley-Rideal, Langmuir-Hinshelwood, and Mars-van Krevelen. Importantly, the former and latter mechanisms can only be realized upon including systematically the near surface region and dynamical transformations of catalyst sites, respectively, in the reaction space throughout all simulations.
    view abstract10.1021/acscatal.5b00442
  • 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
  • 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
  • [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
  • Cu/ZnO nanocatalysts in response to environmental conditions: surface morphology, electronic structure, redox state and CO2 activation
    Martinez-Suarez, L. and Frenzel, J. and Marx, D.
    Physical Chemistry Chemical Physics 16 (2014)
    Methanol synthesis is one of the landmarks of heterogeneous catalysis due to the great industrial significance of methanol as a clean liquid fuel and as a raw material for industry. Understanding in atomistic detail the properties of the underlying metal/oxide catalyst materials as a function of temperature and composition of the reactive gas phase is of utmost importance in order to eventually improve the production process. By performing extensive density functional theory based slab calculations in combination with a thermodynamic formalism we establish an atomistic understanding of gas phase-induced changes of surface morphology, redox properties and reactivity of ZnO supported Cu nanocatalysts. Extending our recent insights [Phys. Rev. Lett., 2013, 110, 086108], we explore surface stabilization mechanisms and site-dependent redox states of both catalyst components as well as the pronounced electronic charge transfer processes across the metal-support interface. Moreover, ab initio molecular dynamics simulations unveil the vital role played by dynamical shape fluctuations of the deposited Cu-8 cluster. The pronounced structural flexibility of the metal nanoparticle is found to enhance CO2 activation over Cu-8 at the elevated temperature conditions of the industrial methanol synthesis process, in addition to activation of CO2 via electronic charge transfer from the ZnO support.
    view abstract10.1039/c4cp02812k
  • 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
  • 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
  • Investigation of ternary subsystems of superalloys by thin-film combinatorial synthesis and high-throughput analysis
    König, D. and Pfetzing-Micklich, J. and Frenzel, J. and Ludwig, Al.
    MATEC Web of Conferences 14 (2014)
    A Co-Ti-W thin film materials library was fabricated by magnetron sputtering. By using automated high-throughput measurement techniques (resistance mapping, automated XRD measurements) and cluster analysis a yet unknown phase region was revealed. The existence region of the new ternary phase is close to the composition Co60Ti15W25. In order to transfer the results from thin film to bulk material, a bulk sample was prepared by arc melting and subsequent heat treatment. Scanning electron microscopy and chemical micro-analysis data support that a yet unknown ternary phase exists in the system Co-Ti-W. © 2014 Owned by the authors, published by EDP Sciences.
    view abstract10.1051/matecconf/20141418002
  • Investigation of the thin-film phase diagram of the Cr-Ni-Re system by high-throughput experimentation
    Janghorban, A. and Pfetzing-Micklich, J. and Frenzel, J. and Ludwig, Al.
    Advanced Engineering Materials 16 (2014)
    The Cr-Ni-Re system was investigated over the whole composition range using combinatorial fabrication methods combined with high-throughput characterization techniques in order to establish its thin film phase diagram. After annealing at 940 and 1100°C, the phase equilibrium was reached in the Ni-rich part of the ternary in agreement with the published bulk phase diagram. Annealing the materials library at 940°C is not sufficient to achieve the equilibrium state in the Re-rich part of the system, however by annealing the materials library at 1100°C the formation of expected phases (three solid-solutions and a topologically close packed compound) could be observed. As a result of this study, a thin film phase diagram of the complete Cr-Ni-Re at 1100°C was established, which is well comparable to the bulk phase diagram. This shows that the combinatorial thin film phase diagram approach is feasible and especially promising for materials systems with expensive and/or high melting point constituents. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201300430
  • 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
  • Methanol synthesis on ZnO(000(1)over-bar). IV. Reaction mechanisms and electronic structure
    Frenzel, J. and Marx, D.
    Journal of Chemical Physics 141 (2014)
    Methanol synthesis from CO and H-2 over ZnO, which requires high temperatures and high pressures giving rise to a complex interplay of physical and chemical processes over this heterogeneous catalyst surface, is investigated using ab initio simulations. The redox properties of the surrounding gas phase are known to directly impact on the catalyst properties and thus, set the overall catalytic reactivity of this easily reducible oxide material. In Paper III of our series [J. Kiss, J. Frenzel, N. N. Nair, B. Meyer, and D. Marx, J. Chem. Phys. 134, 064710 (2011)] we have qualitatively shown that for the partially hydroxylated and defective ZnO(000 (1) over bar) surface there exists an intricate network of surface chemical reactions. In the present study, we employ advanced molecular dynamics techniques to resolve in detail this reaction network in terms of elementary steps on the defective surface, which is in stepwise equilibrium with the gas phase. The two individual reduction steps were investigated by ab initio metadynamics sampling of free energy landscapes in three-dimensional reaction subspaces. By also sampling adsorption and desorption processes and thus molecular species that are in the gas phase but close to the surface, our approach successfully generated several alternative pathways of methanol synthesis. The obtained results suggest an Eley-Rideal mechanism for both reduction steps, thus involving "near-surface" molecules from the gas phase, to give methanol preferentially over a strongly reduced catalyst surface, while important side reactions are of Langmuir-Hinshelwood type. Catalyst re-reduction by H-2 stemming from the gas phase is a crucial process after each reduction step in order to maintain the catalyst's activity toward methanol formation and to close the catalytic cycle in some reaction channels. Furthermore, the role of oxygen vacancies, side reactions, and spectator species is investigated and mechanistic details are discussed based on extensive electronic structure analysis. (C) 2014 AIP Publishing LLC.
    view abstract10.1063/1.4896053
  • 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 development of high quality NiTi shape memory and pseudoelastic parts by additive manufacturing
    Haberland, C. and Elahinia, M. and Walker, J.M. and Meier, H. and Frenzel, J.
    Smart Materials and Structures 23 (2014)
    Additive manufacturing provides an attractive processing method for nickel-titanium (NiTi) shape memory and pseudoelastic parts. In this paper, we show how the additive manufacturing process affects structural and functional properties of additively manufactured NiTi and how the process parameter set-up can be optimized to produce high quality NiTi parts and components. Comparisons of shape recovery due to shape memory and pseudoelasticity in additively manufactured and commercial NiTi exhibit promising potential for this innovative processing method. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0964-1726/23/10/104002
  • 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
  • Polyethylene wear particles induce TLR 2 upregulation in the synovial layer of mice
    Paulus, A.C. and Frenzel, J. and Ficklscherer, A. and Roßbach, B.P. and Melcher, C. and Jansson, V. and Utzschneider, S.
    Journal of Materials Science: Materials in Medicine 25 (2014)
    A cellular and receptor mediated response to ultra-high-molecular-weight- polyethylene (UHMWPE) wear particles results in a release of proinflammatory cytokines and induces an inflammatory reaction causing osteolysis in total joint replacement. This investigation offers insight into the toll-like receptor (TLR) mediated activation by polyethylene wear particles in the synovial layer of mice. We hypothesized that, similar to recent in vitro results, UHMWPE particles lead to an upregulation of TLR 1 and 2 and TLR 4 in vivo in the synovial tissue of mice as well. Therefore, UHMWPE particles were generated in a common knee simulator according to the ISO standard, separated by acid digestion and determined by scanning electron microscopy. Endotoxin was removed using a method based on ultracentrifugation. A particle suspension (50 μl; 0.1 vol./vol.%) was injected into the left knee joint of female Balb/c mice (n = 8). In a control group, phosphate-buffered saline was injected into the left knee of Balb/c mice (n = 8). The mice were sacrificed after 7 days. Immunohistochemical staining was performed with TLR 1, 2 and 4 polyclonal antibodies for Balb/c mice and evaluated by light microscopy. The particle-stimulated group showed a thickened synovial layer, an increased cellular infiltration and a TLR 2-upregulation in the synovial layer compared to the control group. An increased expression of TLR 1 and TLR 4 could not be demonstrated. These results indicate a mainly TLR 2-induced inflammation to polyethylene wear debris in the synovial layer of mice. © 2013 Springer Science+Business Media New York.
    view abstract10.1007/s10856-013-5095-y
  • Possibilities to improve the antioxidative capacity of beer by optimized hopping regimes
    Kunz, T. and Frenzel, J. and Wietstock, P.C. and Methner, F.-J.
    Journal of the Institute of Brewing 120 (2014)
    Different hopping regimes were evaluated to investigate the effect on the oxidative stability of wort and beer. Compared with a single hop dosage at the beginning of wort boil, it was possible to increase the concentration of α-acids in pitching wort and beer by applying incremental hop dosage, dry hopping or the use of a pre-isomerized hop product in combination with an α-acid extract, which concomitantly resulted in lower iron concentrations and an enhanced flavour stability as indicated by standard wort and beer analyses, atomic absorption spectroscopy, electron spin resonance spectroscopy and sensory analysis of fresh and force-aged beers. The functional principle of hop dosage variations is explained by saving of α-acids throughout the wort production process, which yields an increased formation and precipitation of pro-oxidative acting transition metal ions (e.g. Fe) in α-acid-complexes during the whirlpool rest and fermentation. Consequently, fewer reactive oxygen species are generated. Additional laboratory trials simulating wort cooling and beer storage in buffered model solutions proved that un-isomerized α-acids are strong iron chelators and confirmed the functional principle of the applied hopping regimes. Negative effects of higher α-acid contents on fermentation performance and depletion of the zinc concentration, which is an essential nutrient for yeast, could be excluded. © 2014 The Institute of Brewing & Distilling.
    view abstract10.1002/jib.162
  • Site occupation of Nb atoms in ternary Ni-Ti-Nb shape memory alloys
    Shi, H. and Frenzel, J. and Martinez, G.T. and Van Rompaey, S. and Bakulin, A. and Kulkova, S. and Van Aert, S. and Schryvers, D.
    Acta Materialia 74 (2014)
    Nb occupancy in the austenite B2-NiTi matrix and Ti2Ni phase in Ni-Ti-Nb shape memory alloys was investigated by aberration-corrected scanning transmission electron microscopy and precession electron diffraction. In both cases, Nb atoms were found to prefer to occupy the Ti rather than Ni sites. A projector augmented wave method within density functional theory was used to calculate the atomic and electronic structures of the austenitic B2-NiTi matrix phase and the Ti2Ni precipitates both with and without addition of Nb. The obtained formation energies and analysis of structural and electronic characteristics explain the preference for Ti sites for Nb over Ni sites. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.03.062
  • Additive manufacturing of shape memory devices and pseudoelastic components
    Haberland, C. and Elahinia, M. and Walker, J. and Meier, H. and Frenzel, J.
    ASME 2013 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2013 1 (2013)
    Processing of Nickel-Titanium shape memory alloys (NiTi) is by no means easy because all processing steps can strongly affect the properties of the material. Hence, near-net-shaping technologies are very attractive for processing NiTi due to reduction of the processing route. Additive Manufacturing (AM) provides especially promising alternatives to conventional processing because it offers unparalleled freedom of design. In the last 5 years AM of NiTi received little attention from academics and researchers and, therefore, is far from being established for processing NiTi today. This work is to highlight the current state of the art of using the AM technique Selective Laser Melting (SLM) for processing high quality NiTi parts. For this reason, fundamentals for SLM processing of NiTi are described. It is shown in detail that a careful control of process parameters is of great importance. Furthermore, this work characterizes structural and functional properties like shape recovery, referring to the shape memory effect in Ti-rich SLM NiTi, or pseudoelasticy in Ni-rich SLM NiTi. It is shown that both types of shape memory effects can be adjusted in SLM NiTi by the choice of the raw material and processing strategy. By comparing the properties of SLM NiTi to those of conventionally processed NiTi, this work clearly shows that SLM is an attractive manufacturing method for production of high quality NiTi parts. Copyright © 2013 by ASME.
    view abstract10.1115/SMASIS2013-3070
  • Athermal nature of the martensitic transformation in Heusler alloy Ni-Mn-Sn
    Zheng, H. and Wang, W. and Wu, D. and Xue, S. and Zhai, Q. and Frenzel, J. and Luo, Z.
    Intermetallics 36 (2013)
    Martensitic transformations are generally classified into two groups, namely athermal and isothermal, according to their kinetics. In case of athermal transformations, the amount of the product phase only depends on temperature, and not on time. However, much debate rises about this issue due to unexpected experimental observations of isothermal effects in typically athermal transformations. Considering that the wide applications of Heusler Ni-Mn based materials are based on martensitic transformations, it is of importance to clarify the nature of their martensitic transformation. In this paper, we made an effort to study isothermal effects in a Ni-Mn-Sn alloy using differential scanning calorimetry (DSC). It is proposed that the martensitic transformation of Ni-Mn based materials is athermal in nature although a time-depending effect is observed through DSC interrupted measurements. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2013.01.012
  • 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
  • Composition-dependent crystal structure and martensitic transformation in Heusler Ni-Mn-Sn alloys
    Zheng, H. and Wang, W. and Xue, S. and Zhai, Q. and Frenzel, J. and Luo, Z.
    Acta Materialia 61 (2013)
    In the present work, modulated four- and five-layered orthorhombic, seven-layered monoclinic (4O, 10M and 14M) and unmodulated double tetragonal (L10) martensites are characterized in Heusler Ni-Mn-Sn alloys using X-ray diffraction, high-resolution transmission electron microscopy, electron diffraction techniques and thermal analysis. All modulated layered martensites exhibit twins and stacking faults, while the L10 martensite shows fewer structural defects. The substitution of Sn with Mn in Ni 50Mn37+xSn13-x (x = 0, 2, 4) enhances the martensitic transition temperatures, while the transition temperatures decrease with increasing Mn content for constant Sn levels in Ni50-yMn37+ySn13 (y = 0, 2, 4). The compositional dependence of the martensitic transition temperatures is mainly attributed to the valence electron concentration (e/a) and the unit-cell volume of the high-temperature phase. With increasing transition temperatures (or e/a), the resultant martensitic crystal structure evolves in a sequence of 4O → 10M → 14M → L10 in bulk Ni-Mn-Sn alloys. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.04.035
  • Em characterization of precipitates in as-cast and annealed Ni45.5Ti45.5Nb9 shape memory alloys
    Shi, H. and Frenzel, J. and Schryvers, D.
    Materials Science Forum 738-739 (2013)
    Nb-rich precipitates in the matrix of as-cast and annealed Ni45.5Ti45.5Nb9alloys are investigated by scanning and scanning transmission electron microscopy, including slice-and-view and geometric phase analysis (GPA). The Nb-rich bcc nano-precipitates in the as-cast alloy have a 10% lattice parameter difference with the B2 matrix and reveal compensating interface dislocations. The 3D reconstruction of the configuration of small Nb-rich precipitates in the annealed alloy reveals a wall-like distribution of precipitates, which may increase the thermal hysteresis of the material. © (2013) Trans Tech Publications, Switzerland.
    view abstract10.4028/
  • 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
  • Methanol synthesis on ZnO from molecular dynamics
    Frenzel, J. and Kiss, J. and Nair, N. N. and Meyer, B. and Marx, D.
    Physica Status Solidi B-basic Solid State Physics 250 (2013)
    This paper reviews our efforts to simulate methanol synthesis from CO and H2 on defective ZnO surfaces using advanced molecular dynamics techniques. This apparently simple chemical reaction occurring on a seemingly well-defined surface appears to be astonishingly complex. First of all, the preferred oxidation state of F centers at the polar oxygen terminated surface is found to be dictated by the chemical composition and the thermodynamic properties of the gas phase in contact with ${\rm ZnO}(000\overline {1} )$. Secondly, reaction intermediates and pathways along the catalytic cycle taking place at or close to these defects are found to depend in a sensitive way on their oxidation state. Thirdly, it is seen that the gas phase close to the catalytic surface might be transiently involved in some of the reaction steps in a non-trivial manner. Last but not least, the scenario is found to be greatly enriched upon involving copper clusters on polar ZnO surfaces in view of utmost strong metal-support interactions (SMSIs), which are directly related to the polar nature of ${\rm ZnO}(000\overline {1} )$. Taken together, an unexpectedly rich picture is unveiled by the molecular dynamics approach to computational heterogeneous catalysis when applied to methanol synthesis on bare ZnO.
    view abstract10.1002/pssb.201248446
  • Methanol synthesis on ZnO(000(1)over-bar). II. Structure, energetics, and vibrational signature of reaction intermediates
    Kiss, J. and Frenzel, J. and Meyer, B. and Marx, D.
    Journal of Chemical Physics 139 (2013)
    A rigorous characterization of a wealth of molecular species adsorbed at oxygen defects on ZnO(000 (1) over bar) is given. These defects represent the putative active sites in methanol synthesis from CO and H-2. The oxidation state of the ZnO catalyst and thus the preferred charge state and the reactivity of the oxygen vacancies depend on the gas phase temperature and pressure conditions. Considering charge states of oxygen vacancies relevant at the reducing conditions of the industrial process, i.e., F++/H-2, F-0, F-0/H-2, and F--, as well as the F++ center which is abundant at UHV conditions and therefore important to allow for comparison with surface science experiments, we have investigated the structure, energetics, and vibrational frequencies of an exhaustive catalog of reaction intermediates using electronic structure calculations. After having identified the characteristic adsorption modes of CO, formate, formic acid, hydroxymethylene, formyl, formaldehyde, dioxomethylene, hydroxymethyl, hydroxymethoxide, methoxide, as well as methanol itself, the thermodynamic stability of all species with respect to the charge state of the oxygen vacancy and their electronic stabilization is discussed in detail and summarized in an energy level diagram. (C) 2013 AIP Publishing LLC.
    view abstract10.1063/1.4813404
  • Nano- and microcrystal investigations of precipitates, interfaces and strain fields in Ni-Ti-Nb by various TEM techniques
    Schryvers, D. and Shi, H. and Martinez, G.T. and Van Aert, S. and Frenzel, J. and Van Humbeeck, J.
    Materials Science Forum 738-739 (2013)
    In the present contribution several advanced electron microscopy techniques are employed in order to describe chemical and structural features of the nano- and microstructure of a Ni45.5Ti45.5Nb9 alloy. A line-up of Nb-rich nano-precipitates is found in the Ni-Ti-rich austenite of as-cast material. Concentration changes of the matrix after annealing are correlated with changes in the transformation temperatures. The formation of rows and plates of larger Nb-rich precipitates and particles is described. The interaction of a twinned martensite plate with a Nb-rich nano-precipitate is discussed and the substitution of Nb atoms on the Ti-sublattice in the matrix is confirmed. © (2013) Trans Tech Publications, Switzerland.
    view abstract10.4028/
  • 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 biocompatibility of dense and porous Nickel-Titanium produced by selective laser melting
    Habijan, T. and Haberland, C. and Meier, H. and Frenzel, J. and Wittsiepe, J. and Wuwer, C. and Greulich, C. and Schildhauer, T.A. and Köller, M.
    Materials Science and Engineering C 33 (2013)
    Nickel-Titanium shape memory alloys (NiTi-SMA) are of biomedical interest due to their unusual range of pure elastic deformability and their elastic modulus, which is closer to that of bone than any other metallic or ceramic material. Newly developed porous NiTi, produced by Selective Laser Melting (SLM), is currently under investigation as a potential carrier material for human mesenchymal stem cells (hMSC). SLM enables the production of highly complex and tailor-made implants for patients on the basis of CT data. Such implants could be used for the reconstruction of the skull, face, or pelvis. hMSC are a promising cell type for regenerative medicine and tissue engineering due to their ability to support the regeneration of critical size bone defects. Loading porous SLM-NiTi implants with autologous hMSC may enhance bone growth and healing for critical bone defects. The purpose of this study was to assess whether porous SLM-NiTi is a suitable carrier for hMSC. Specimens of varying porosity and surface structure were fabricated via SLM. hMSC were cultured for 8 days on NiTi specimens, and cell viability was analyzed using two-color fluorescence staining. Viable cells were detected on all specimens after 8 days of cell culture. Cell morphology and surface topography were analyzed by scanning electron microscopy (SEM). Cell morphology and surface topology were dependent on the orientation of the specimens during SLM production. The Nickel ion release can be reduced significantly by aligned laser processing conditions. The presented results clearly attest that both dense SLM-NiTi and porous SLM-NiTi are suitable carriers for hMSC. Nevertheless, before carrying out in vivo studies, some work on optimization of the manufacturing process and post-processing is required. © 2012 Elsevier B.V.
    view abstract10.1016/j.msec.2012.09.008
  • Tuning the Reactivity of a Cu/ZnO Nanocatalyst via Gas Phase Pressure
    Martinez-Suarez, L. and Frenzel, J. and Marx, D. and Meyer, B.
    Physical Review Letters 110 (2013)
    By calculation of a thermodynamic phase diagram we provide an atomistic understanding of the morphological changes in ZnO-supported Cu nanocatalysts, which are subject to strong metal-support interactions, in response to the redox properties of the surrounding gas phase, i.e., depending on temperature and pressure. The reactivity, and thus the strong metal-support interactions, of this catalyst is traced back to a redox-state dependent occupation of delocalized ZnO substrate bands and localized Cu cluster states at the Fermi level. It is shown that at the conditions of industrial methanol synthesis complex electronic charge transfer processes across the metal-support interface, driven by morphological and electronic changes, explain the enhanced catalytic reactivity toward CO2. DOI: 10.1103/PhysRevLett.110.086108
    view abstract10.1103/PhysRevLett.110.086108
  • 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
  • Cast-replicated NiTiCu foams with superelastic properties
    Young, M.L. and Defouw, J.D. and Frenzel, J. and Dunand, D.C.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 (2012)
    Ni40Ti50Cu10 foams were replication cast into a porous SrF2 preform. This space holder is chemically stable in contact with liquid and solid Ni40Ti50Cu10, but can be removed by dissolution in nitric acid. A Ni40Ti50Cu10 foam with 60 pct porosity exhibits low stiffness (1 to 13 GPa) and large recoverable strains (~4 pct) during cyclical compression testing at 311 K (38C), within the superelastic range based on calorimetry results. This is the first time that replication casting is used to create an open foam of a NiTi-based shape-memory alloy, due to difficulties associated with the high reactivity and strong contamination tendency of the melt. Casting NiTi-based shape-memory alloy foams enable the economical production of porous actuators, energy absorbers, and biomedical implants with complex shapes. © The Minerals, Metals & Materials Society and ASM International 2012.
    view abstract10.1007/s11661-011-1060-x
  • 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
  • Do cement nanotubes exist?
    Manzano, H. and Enyashin, A.N. and Dolado, J.S. and Ayuela, A. and Frenzel, J. and Seifert, G.
    Advanced Materials 24 (2012)
    Using atomistic simulations, this work indicates that cement nanotubes can exist. The chemically compatible nanotubes are constructed from the two main minerals in ordinary Portland cement pastes, namely calcium hydroxide and a calcium silicate hydrate called tobermorite. These results show that such nanotubes are stable and have outstanding mechanical properties, unique characteristics that make them ideally suitable for nanoscale reinforcements of cements. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adma.201103704
  • Electronic-structure calculations of large cadmium chalcogenide nanoparticles
    Frenzel, J. and Joswig, J. O.
    Physica Status Solidi B-basic Solid State Physics 249 (2012)
    In this paper, we will review our studies of large cadmium chalcogenide nanoparticles and present some new results on cadmium telluride systems. All calculations have been performed using density-functional based methods. The studies deal with the structural properties of saturated and unsaturated nanoparticles where the surfactants generally are hydrogen atoms or thiol groups. We have focused on the investigation of the density of states, the Mulliken charges, the eigenvalue spectra, and the spatial distributions of the frontier orbitals. Optical excitation spectra of pure CdS and CdSe/CdS core-shell systems have been calculated using a linear-response formalism. The reviewed studies are compared to the state of the art of modeling large cadmium chalcogenide particles.
    view abstract10.1002/pssb.201100622
  • Evaluation of different methods to eliminate adherent endotoxin of polyethylene wear particles
    Paulus, A.C. and Schröder, C. and Sievers, B. and Frenzel, J. and Jansson, V. and Utzschneider, S.
    Wear 294-295 (2012)
    Wear particles have to be cleaned from any substances, which could eventually lead to confounding effects in studies concerning the in vitro or in vivo biological activity of wear debris. Several observations have demonstrated that lipopolysaccharide (LPS) as a component of the outer membrane of gramnegative bacteria can modulate the cell response to wear debris. There are numerous methods described in the literature for the removal of LPS. But there is an exception for polyethylene particles, they cannot be treated with radiation and heating processes. There remain four possible methods: Cleaning the particles with sodium hydroxide or with acetic acid, washing them with ethanol or via using a demanding ultracentrifugation procedure. Thus we decided to compare the different methods in consideration of the remaining LPS level and their effects on the particle numbers and morphology. During the study several problems regarding these methods appeared: Either they could not remove the LPS below the demanded detection level, or the methods had influence on amount and morphology of the particles. Due to these findings the authors developed a new method based on ultracentrifugation. With this method the LPS could be removed as required, amount and morphology of the particles were not affected. © 2012 Elsevier B.V.
    view abstract10.1016/j.wear.2012.07.018
  • 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 properties of Ni-rich NiTi shape memory parts produced by selective laser melting
    Haberland, C. and Meier, H. and Frenzel, J.
    ASME 2012 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2012 1 (2012)
    Processing of Nickel-Titanium (NiTi) shape memory alloys (SMAs) is challenging because smallest compositional variances and all types of microstructural features strongly affect the elementary processes of the martensitic transformation and thus the functional properties of the material. Against this background, powder metallurgical near net shape methods are attractive for the production of NiTi components. Especially additive manufacturing technologies (AM) seem to provide high potential, although they have received only little attention for processing NiTi so far. This work is the first to report on pseudoelastic properties of additive manufactured Ni-rich NiTi. We show how to establish pseudoelasticity in NiTi samples prepared by the additive manufacturing technique Selective Laser Melting (SLM). Therefore, we analyze phase transformation behavior, mechanical characteristics and functional properties of our materials subjected to different heat treatments. The obtained results are compared to the behavior of conventional NiTi. The presented results clearly indicate that SLM provides a promising processing route for the fabrication of high quality NiTi parts. Copyright © 2012 by ASME.
    view abstract10.1115/SMASIS2012-8040
  • Structural properties of metal-organic frameworks within the density-functional based tight-binding method
    Lukose, B. and Supronowicz, B. and St Petkov, P. and Frenzel, J. and Kuc, A. B. and Seifert, G. and Vayssilov, G. N. and Heine, T.
    Physica Status Solidi B-basic Solid State Physics 249 (2012)
    Density-functional based tight-binding (DFTB) is a powerful method to describe large molecules and materials. Metal-organic frameworks (MOFs), materials with interesting catalytic properties and with very large surface areas, have been developed and have become commercially available. Unit cells of MOFs typically include hundreds of atoms, which make the application of standard density-functional methods computationally very expensive, sometimes even unfeasible. The aim of this paper is to prepare and to validate the self-consistent charge-DFTB (SCC-DFTB) method for MOFs containing Cu, Zn, and Al metal centers. The method has been validated against full hybrid density-functional calculations for model clusters, against gradient corrected density-functional calculations for supercells, and against experiment. Moreover, the modular concept of MOF chemistry has been discussed on the basis of their electronic properties. We concentrate on MOFs comprising three common connector units: copper paddlewheels (HKUST-1), zinc oxide Zn4O tetrahedron (MOF-5, MOF-177, DUT-6 (MOF-205)), and aluminum oxide AlO4(OH)(2) octahedron (MIL-53). We show that SCC-DFTB predicts structural parameters with a very good accuracy (with less than 5% deviation, even for adsorbed CO and H2O on HKUST-1), while adsorption energies differ by 12 kJ mol(-1) or less for CO and water compared to DFT benchmark calculations.
    view abstract10.1002/pssb.201100634
  • 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
  • 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
  • Methanol synthesis on ZnO(000(1)over-bar). III. Free energy landscapes, reaction pathways, and mechanistic insights
    Kiss, J. and Frenzel, J. and Nair, N. N. and Meyer, B. and Marx, D.
    Journal of Chemical Physics 134 (2011)
    The interplay of physical and chemical processes in the heterogeneous catalytic synthesis of methanol on the ZnO(000 (1) over bar) surface with oxygen vacancies is expected to give rise to a complex free energy landscape. A manifold of intermediate species and reaction pathways has been proposed over the years for the reduction of CO on this catalyst at high temperature and pressure conditions as required in the industrial process. In the present study, the underlying complex reaction network from CO to methanol is generated in the first place by using ab initio metadynamics for computational heterogeneous catalysis. After having "synthesized" the previously discussed intermediates in addition to finding novel species, mechanistic insights into this network of surface chemical reactions are obtained based on exploring the global free energy landscape, which is refined by investigating individual reaction pathways. Furthermore, the impact of homolytic adsorption and desorption of hydrogen at the required reducing gas phase conditions is probed by studying such processes using different charge states of the F-center. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3541826]
    view abstract10.1063/1.3541826
  • 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 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
  • Optical Excitations in CdSe/CdS Core-Shell Nanoparticles
    Frenzel, J. and Thieme, S. and Seifert, G. and Joswig, J. O.
    Journal of Physical Chemistry C 115 (2011)
    In this paper, we present the excitation spectra of fully saturated CdSe/CdS and CdS/CdSe core shell nanoparticles with a size distribution ranging from 100 to 650 atoms. The spectra have been calculated using linear-response theory and a density-functional tight-binding method. We have investigated their dependence with respect to the size of the core and the shell as well as the underlying crystal structure (zincblende and wurtzite). The influence of the size of the material with the lower band gap (CdSe) is clearly visible in the spectra. Moreover, the frontier orbitals are localized in either the core or the shell part, leading to a moderate overlap only. In contrast, the influence of the symmetry reduction from cubic zincblende to hexagonal wurtzite is rather small and results in a splitting of the excitation peaks.
    view abstract10.1021/jp111474n
  • 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/
  • 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
  • Ancient technology/novel nanomaterials: Casting titanium carbide nanowires
    Zhang, Z. and Wang, Y. and Frenzel, J.
    CrystEngComm 12 (2010)
    We show here that novel nanomaterials can be fabricated by an ancient casting technology. Titanium carbide (TiC) nanowires have been synthesized by casting NiTi alloys containing a little amount of carbon. The morphology and structure of the TiC nanowires have been investigated using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The TiC nanowires have a single crystalline structure and grow along the <100> direction. The diameters of the TiC nanowires range from 50 to 500 nm, and their lengths vary from 10 to 100 μm. Moreover, the diameters and lengths of the TiC nanowires can be adjusted by simply changing applied cooling rates during casting. The TiC nanowires have high aspect ratios of 80-500, which are beneficial to their field emission performance. A eutectic reaction mechanism has been presented to explain the formation of the TiC nanowires. The ancient casting technology may be used to synthesize novel nanowires of other metal carbides, oxides or nitrides. Our findings can provide implications for fabricating novel nanomaterials using ancient or traditional technologies. © 2010 The Royal Society of Chemistry.
    view abstract10.1039/b925807h
  • 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
  • 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 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
  • 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 reticular construction concept of covalent organic frameworks
    Lukose, B. and Kuc, A. and Frenzel, J. and Heine, T.
    Beilstein Journal of Nanotechnology 1 (2010)
    The concept of reticular chemistry is investigated to explore the applicability of the formation of Covalent Organic Frameworks (COFs) from their defined individual building blocks. Thus, we have designed, optimized and investigated a set of reported and hypothetical 2D COFs using Density Functional Theory (DFT) and the related Density Functional based tight-binding (DFTB) method. Linear, trigonal and hexagonal building blocks have been selected for designing hexagonal COF layers. High-symmetry AA and AB stackings are considered, as well as low-symmetry serrated and inclined stackings of the layers. The latter ones are only slightly modified compared to the high-symmetry forms, but show higher energetic stability. Experimental XRD patterns found in literature also support stackings with highest formation energies. All stacking forms vary in their interlayer separations and band gaps; however, their electronic densities of states (DOS) are similar and not significantly different from that of a monolayer. The band gaps are found to be in the range of 1.7-4.0 eV. COFs built of building blocks with a greater number of aromatic rings have smaller band gaps.
    view abstract10.3762/bjnano.1.8
  • 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
  • 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
  • alloys

  • martensite

  • microstructure

  • scanning electron microscopy

  • shape-memory alloys

  • transmission electron microscopy

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