Jun.-Prof. Dr. Guillaume Laplanche

Institute for Materials
Ruhr-Universität Bochum


  • Data compilation regarding the effects of grain size and temperature on the strength of the single-phase FCC CrFeNi medium-entropy alloy
    Schneider, M. and Laplanche, G.
    Data in Brief 34 (2021)
    In the present article, we present a data compilation reflecting recrystallized microstructures and the corresponding mechanical properties of an equiatomic, single-phase face-centered cubic (FCC) CrFeNi medium-entropy alloy (MEA). For the analysis, interpretation, and discussion of the data, the reader can refer to the original research article entitled “Effects of temperature on mechanical properties and deformation mechanisms of the equiatomic CrFeNi medium-entropy alloy”, see Ref. (Schneider and Laplanche, Acta Mater. 204, 2020). The data related to recrystallized microstructures comprise raw backscatter electron (BSE) micrographs (tif-files) obtained using a scanning electron microscope (SEM) for six grain sizes in the range [10–160 µm], optical micrographs of the alloy with the largest grain size (d = 327 µm), pdf-reports and tables presenting the corresponding grain-size distributions (d, accounting for grain boundaries only) and crystallite-size distributions (c, which accounts for both grain and annealing twin boundaries), the annealing twin thicknesses (t), the average number of annealing twin boundaries per grain (n), and the average Taylor factor (M) of each recrystallized microstructure. These are benchmark datasets that may serve to develop new algorithms for the automated evaluation of microstructural parameters. Such algorithms would help to speed up the analyses of microstructures and improve their reliability. Furthermore, several groups pointed out that in addition to the mean grain size, other microstructural parameters such as the grain size distribution (Raeisinia et al., Model. Simul. Mater. Sc. 16, 2008) and the average number of twins per grain (Schneider et al., Int. J. Plasticity, 124, 2020) may affect some material properties (e.g. Hall-Petch strengthening). Therefore, an effort was made here to determine and report almost all the microstructural parameters describing recrystallized microstructures of FCC alloys. The mechanical-properties data are provided as excel-sheets in which the raw stress-strain curves can be found. Compression tests for alloys with different grain sizes were performed at room temperature. Additional compression tests and tensile tests for the grain size d = 160 µm were performed at temperatures between 77 K and 873 K. Characteristic mechanical properties, such as yield stresses at 0.2% plastic strain (σ0.2%) and Hall-Petch parameters (σ0 and ky) are given for all temperatures in the tables below. Moreover, the Hall-Petch parameters as well as the mechanical data reported in the present study could be used for data mining and implemented in programs used for alloy design. © 2021 The Author(s)
    view abstract10.1016/j.dib.2020.106712
  • Effects of temperature on mechanical properties and deformation mechanisms of the equiatomic CrFeNi medium-entropy alloy
    Schneider, M. and Laplanche, G.
    Acta Materialia 204 (2021)
    An equiatomic CrFeNi medium-entropy alloy (MEA) that constitutes a cornerstone of austenitic stainless steels and Fe-based superalloys is investigated. Anneals at various temperatures revealed that CrFeNi forms a stable face-centered cubic (FCC) solid solution above ~1223 K. Based on this result, this alloy was cold-worked and recrystallized between 1273 K and 1473 K to produce different grain sizes. Compression tests were carried out at 293 K to investigate grain boundary strengthening (Hall-Petch slope: 966 MPa µm1/2) and this contribution was then subtracted from the overall strength to reveal the intrinsic uniaxial lattice strength (80 MPa). Additional compression and tensile tests were performed between 77 K and 873 K to study the effect of temperature on mechanical properties and deformation mechanisms. Ductility, yield and ultimate tensile strengths increased with decreasing temperature. To reveal the active deformation mechanisms in CrFeNi with the coarsest grain size (160 µm), tensile tests at 77 K and 293 K were interrupted at different strains followed by transmission electron microscopy analyses. In all cases, the deformation was accommodated by dislocation glide at low strains, while twinning additionally occurred above a critical resolved shear stress of 165 MPa, which was roughly temperature independent. This value compares well with predictions (180 MPa) based on the Kibey's model for twin nucleation. Moreover, the fact that this value is roughly temperature-independent is also consistent with the Kibey's model since the twin nucleation barrier (unstable twin stacking fault energy) of FCC metals and alloys does not vary significantly with temperature. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.11.012
  • High-Temperature Oxidation in Dry and Humid Atmospheres of the Equiatomic CrMnFeCoNi and CrCoNi High- and Medium-Entropy Alloys
    Stephan-Scherb, C. and Schulz, W. and Schneider, M. and Karafiludis, S. and Laplanche, G.
    Oxidation of Metals 95 (2021)
    Abstract: Surface degradation phenomena of two model equiatomic alloys from the CrMnFeCoNi alloy system were investigated in 2% O2 and 10% H2O (pO2 = 0.02 and 10−7 atm, respectively) at 800 °C for times up to 96 h. The crystallographic structures, morphologies, and chemical compositions of the corrosion layers developing on CrMnFeCoNi and CrCoNi were comparatively analyzed by mass gain analysis, X-ray diffraction, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The oxidation resistance of CrMnFeCoNi is relatively poor due to the fast growth of porous Mn-oxide(s). CrCoNi forms an external chromia layer that is dense and continuous in a dry 2% O2 atmosphere. This layer buckles and spalls off after exposure to 10% H2O atmosphere. Beneath the chromia layer, a Cr-depleted zone forms in the CrCoNi alloy in both environments. As the oxide scale spalls off in the H2O-containing atmosphere, a secondary chromia layer was observed and correspondingly enlarges the Cr-depleted zone. In contrast, as the chromia layer remains without significant spallation when CrCoNi is exposed to a dry oxidizing atmosphere, the region depleted in Cr is narrower. Graphic Abstract: [Figure not available: see fulltext.]. © 2020, The Author(s).
    view abstract10.1007/s11085-020-10014-7
  • Superior low-cycle fatigue properties of CoCrNi compared to CoCrFeMnNi
    Lu, K. and Chauhan, A. and Walter, M. and Tirunilai, A.S. and Schneider, M. and Laplanche, G. and Freudenberger, J. and Kauffmann, A. and Heilmaier, M. and Aktaa, J.
    Scripta Materialia 194 (2021)
    We report on the low-cycle fatigue behavior of single-phase, face-centered cubic CoCrNi and CoCrFeMnNi at room temperature. Both alloys manifest cyclic hardening followed by softening and a near steady state until failure. CoCrNi exhibits higher strength, lower inelastic-strain, and longer lifetime than CoCrFeMnNi. For both alloys, microstructural investigations reveal no noticeable changes of texture, grain size and twin fraction. Nevertheless, CoCrNi exhibits planar dislocation structures, while CoCrFeMnNi shows well-defined wavy dislocation structures. This is due to CoCrNi lower stacking fault energy, which enhances planar slip and delays deformation localization leading to its superior fatigue resistance, compared to CoCrFeMnNi. © 2020
    view abstract10.1016/j.scriptamat.2020.113667
  • Tracer diffusion in the σ phase of the CoCrFeMnNi system
    Zhang, J. and Muralikrishna, G.M. and Asabre, A. and Kalchev, Y. and Müller, J. and Butz, B. and Hilke, S. and Rösner, H. and Laplanche, G. and Divinski, S.V. and Wilde, G.
    Acta Materialia 203 (2021)
    A single Cr-rich σ-phase alloy with a composition of Co17Cr46Fe16.3Mn15.2Ni5.5 (at.%) and a tetragonal lattice structure was produced. The tracer diffusion coefficients of Ni and Fe were measured by secondary electron mass spectroscopy using the highly enriched 64Ni and 58Fe natural isotopes. On the homologous temperature scale, Ni and Fe diffuse in the σ phase faster as compared to the corresponding diffusion rates in the equiatomic and face-centered cubic CoCrFeMnNi alloy. In contrast, on the absolute temperature scale, these elements diffuse roughly at the same rates in both materials. Factors influencing element diffusion and phase stability of the σ phase compared to the equiatomic alloy are discussed. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.116498
  • Analysis of strengthening due to grain boundaries and annealing twin boundaries in the CrCoNi medium-entropy alloy
    Schneider, M. and George, E.P. and Manescau, T.J. and Záležák, T. and Hunfeld, J. and Dlouhý, A. and Eggeler, G. and Laplanche, G.
    International Journal of Plasticity 124 (2020)
    CrCoNi exhibits the best combination of strength and ductility among all the equiatomic single-phase FCC subsets of the CrMnFeCoNi high-entropy alloy. Here, its yield strength was determined in compression as a function of grain size and temperature. Yield strength was also plotted as a function of "crystallite" size, which takes into account both annealing twin boundaries and grain boundaries. The resulting Hall-Petch slopes were straight lines but with different slopes that depend on the number of twin boundaries per grain. Scanning transmission electron microscopy of deformed specimens revealed the formation of dislocation pile-ups at grain and annealing twin boundaries indicating that the latter also act as obstacles to slip and contribute to strength. Using a simple pile-up model, the strengths of the grain and twin boundaries were estimated to lie in the range 900-1250 »MPa. Assuming that they have the same strength, in the case of twin boundaries this strength corresponds roughly to the stress required to constrict Shockley partials, which suggests that dissociated dislocations have to become compact before they can cross the annealing twin boundaries. © 2019 The Authors.
    view abstract10.1016/j.ijplas.2019.08.009
  • Comparison of cryogenic deformation of the concentrated solid solutions CoCrFeMnNi, CoCrNi and CoNi
    Tirunilai, A.S. and Hanemann, T. and Reinhart, C. and Tschan, V. and Weiss, K.-P. and Laplanche, G. and Freudenberger, J. and Heilmaier, M. and Kauffmann, A.
    Materials Science and Engineering A 783 (2020)
    The current work compares the deformation behavior of CoCrFeMnNi and CoCrNi in the temperature interval between 295 K and 8 K through a series of quasi-static tensile tests. Temperature-dependent yield stress variation was found to be similarly high in these two alloys. Previous investigations only extended down to 77 K and showed that a small amount of ε-martensite was formed in CoCrNi while this phase was not observed in CoCrFeMnNi. The present study extends these investigations down to 8 K where similar low levels of ε-martensite were presently detected. Based on this result, a rough assessment has been made estimating the importance of deformation twinning to the strength. The relative work hardening rates of CoCrFeMnNi and CoCrNi were comparable in value despite the differences in ε-martensite formation during deformation. CoCrFeMnNi deforms by dislocation slip and deformation twinning while deformation in CoCrNi is also accommodated by the formation of ε-martensite at cryogenic temperatures. Additionally, CoNi, a solid solution from the Co–Cr–Fe–Mn–Ni system with low strength, was used for comparison, showing contrasting deformation behavior at cryogenic temperatures. © 2020 Elsevier B.V.
    view abstract10.1016/j.msea.2020.139290
  • Data compilation on the effect of grain size, temperature, and texture on the strength of a single-phase FCC MnFeNi medium-entropy alloy
    Schneider, M. and Werner, F. and Langenkämper, D. and Reinhart, C. and Laplanche, G.
    Data in Brief 28 (2020)
    This data article presents a compilation of microstructural and mechanical data regarding the ternary single-phase FCC MnFeNi medium-entropy alloy (MEA). For the analysis, interpretation, and comparison of the data to literature values, the reader can refer to the original related research article entitled “Effect of Temperature and Texture on Hall-Petch Strengthening by Grain and Annealing Twin Boundaries in the MnFeNi Medium-Entropy Alloy”, see Schneider et al. (Metals 9, 2019, 84). The microstructural data reported here include: (i) raw backscatter electron (BSE) micrographs (tif-files) obtained using a scanning electron microscope (SEM) for nine different grain sizes with four images for each grain size and (ii) pdf reports and tables shown below presenting the distributions of the grain- (d, accounting for grain boundaries only) and crystallite- (c, which accounts for both grain and annealing twin boundaries) sizes and of the annealing twin thicknesses (t). These datasets may be useful to develop new algorithms for the automated evaluation of microstructural parameters in recrystallized alloys, i.e. with these benchmark data, an algorithm for image analysis could be trained to assess the above mentioned microstructural parameters. This would help to speed up the analysis of microstructures and improve its reliability. Additional tables describing the recrystallized microstructures and texture include the average number of annealing twin boundaries per grain (n), and the average Taylor factors (M). Raeisinia et al. (Model. Simul. Mater. Sc. 16, 2008, 025001) recently used a viscoplastic model to show that differences in the distribution of microstructural parameters affect the Hall-Petch parameters, but no attempt has been carried out so far to experimentally investigate this possibility since grain size distributions are rarely reported. Here, our benchmark data (e.g. distribution in grain/crystallite sizes, annealing twins per grain, distribution of annealing twin thicknesses) could be used to address these issues. The data describing the mechanical properties reported here are excel-sheets of raw stress-strain curves for temperatures ranging from 77 K to 873 K and different grain sizes. The yield stress (σ0.2%) and the normalized Hall-Petch parameters (σ0/G and ky/Gb2) are given for all temperatures. The normalized Hall-Petch parameters are reported here since they allow to better compare the strength and the magnitude of grain boundary strengthening of different alloys with the same crystallographic structure, see Cordero et al. (Int. Mater. Rev. 61, 2016, 495–512). Moreover, the Hall-Petch parameters as well as the mechanical data reported here could be used for data mining and implemented in programs used for alloy design. © 2019 The Author(s)
    view abstract10.1016/j.dib.2019.104807
  • Data related to the growth of σ-phase precipitates in CrMnFeCoNi high-entropy alloys: Temporal evolutions of precipitate dimensions and concentration profiles at interfaces
    Laplanche, G.
    Data in Brief 33 (2020)
    A data compilation related to the growth kinetics of a topologically closed-packed (TCP) phase is reported. A high-entropy alloy (HEA) with a composition of Cr26Mn20Fe20Co20Ni14 in at.%, a mean grain size of 50 µm and initially single-phase face-centered cubic (FCC) was annealed at temperatures ranging from 600 °C to 1000 °C for times between 0.05 h and 1000 h. These heat treatments resulted in the formation of tetragonal σ precipitates that formed heterogeneously at different elements of the microstructure. The raw data of the present article include backscattered electron (BSE) micrographs where σ precipitates can be observed within grains, at grain boundaries, and triple points of the FCC matrix. From these images, the dimensions of the five largest precipitates observed within grains and those of the five largest allotriomorphs are provided for different times and temperatures in tables. As the σ precipitates are more enriched in Cr and depleted in Ni than the surrounding matrix, Cr-depleted (Ni-enriched) zones form in the FCC matrix next to the precipitates and widen at rates determined by diffusion. To document the evolution of the corresponding concentration profiles with time, electron dispersive X-ray spectroscopy (EDX) was used and the data are reported in Excel files. From these concentration profiles, the widths of the diffusion affected zones for Ni and Cr were systematically determined at different temperatures and times, apparent diffusion coefficients were deduced and all these data are provided in tables. The research data reported here have a fundamental value and document the growth kinetics of σ precipitates within grains and at grain boundaries. These data may help to establish a model able to predict how the precipitation kinetics of σ particles in FCC HEAs is affected by the alloy grain size and how the microstructure (volume fraction, size and distribution of σ precipitates) evolves with time and temperature. This approach may also be extended to austenitic steels and superalloys. © 2020 The Author(s)
    view abstract10.1016/j.dib.2020.106449
  • Deformation mechanisms in a superelastic NiTi alloy: An in-situ high resolution digital image correlation study
    Polatidis, E. and Šmíd, M. and Kuběna, I. and Hsu, W.-N. and Laplanche, G. and Van Swygenhoven, H.
    Materials and Design 191 (2020)
    An in-situ high resolution digital image correlation investigation during uniaxial tensile deformation reveals the recoverable and the non-recoverable strain mechanisms in a Ni51Ti49 alloy with a mean grain size of 35 μm. Recoverable strain is due to the martensitic transformation, for which more than one variant per grain can be activated. The majority of the activated variants exhibit high Schmid factor. The variant selection can be influenced by shear transmission across grain boundaries, when the geometrical compatibility between the neighboring habit plane variants is favourable; in these cases variants that do not have the highest Schmid factor, with respect to the macroscopically applied load, are activated. The experimentally determined transformation strains agree well with theoretical calculations for single crystals. The non-recoverable strain is due to deformation slip in austenite, twinning in martensite and residual martensite. The results are discussed in view of possible twinning modes that can occur in austenite resulting in significant non-recoverable strain. © 2020 The Authors
    view abstract10.1016/j.matdes.2020.108622
  • Effects of cryogenic temperature and grain size on fatigue-crack propagation in the medium-entropy CrCoNi alloy
    Rackwitz, J. and Yu, Q. and Yang, Y. and Laplanche, G. and George, E.P. and Minor, A.M. and Ritchie, R.O.
    Acta Materialia 200 (2020)
    CrCoNi-based high-entropy alloys have demonstrated outstanding mechanical properties, particularly at cryogenic temperatures. Here we investigate the fatigue-crack propagation properties of the equiatomic, single-phase, face-centered cubic, medium-entropy alloy (MEA), CrCoNi, that displays exceptional strength, ductility and toughness, all of which are enhanced at cryogenic temperatures. Fatigue-crack growth is examined, at a load ratio of 0.1 over a wide range of growth rates, from ~10−11 to >10−7 m/cycle, at room (293 K) and cryogenic (198 K, 77 K) temperatures for two grain sizes (~7 and 68 µm), with emphasis on near-threshold behavior. We find that the ΔKth fatigue thresholds are increased with decreasing temperature and increasing grain size: from 5.7 MPa√m at 293 K to 8 MPa√m at 77 K in the fine-grained alloy, and from 9.4 MPa√m at 293 K to 13.7 MPa√m at 77 K in the coarse-grained alloy. Mechanistically, transgranular cracking at 293 K transitions to a mixture of intergranular and transgranular at cryogenic temperatures, where the increased propensity of nano-twins appears to inhibit growth rates by deflecting the crack path. However, the main factor affecting near-threshold behavior is roughness-induced crack closure from interference between the crack flanks, which is enhanced by the rougher fracture surfaces at low temperatures, particularly in the coarser-grained microstructure. Fatigue-crack propagation behavior in CrCoNi is comparable to nickel-based superalloys but is superior to that of the high-entropy CrMnFeCoNi (Cantor) alloy and many high-strength steels, making the CrCoNi alloy an excellent candidate material for safety-critical applications, particularly involving low temperatures. © 2020
    view abstract10.1016/j.actamat.2020.09.021
  • 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
  • Growth kinetics of σ-phase precipitates and underlying diffusion processes in CrMnFeCoNi high-entropy alloys
    Laplanche, G.
    Acta Materialia 199 (2020)
    Key mechanisms and elementary diffusion processes that control the growth kinetics of σ precipitates in high-entropy alloys were investigated in the present study. For this purpose, an off-equiatomic Cr26Mn20Fe20Co20Ni14 alloy with an initially single-phase FCC structure was subjected to isothermal heat treatments, which are known to promote the formation of σ phase, i.e., aging between 600 °C and 1000 °C for times ranging from 0.1 h to 1000 h. The growth kinetics of σ precipitates at grain boundaries of the FCC matrix and those located within the interior of the grains were analyzed separately. The latter precipitates are found to grow through direct substitutional diffusion of Cr-solutes towards and Mn, Fe, Co, and Ni away from them and the growth rate of the allotriomorphs can be rationalized by the collector plate mechanism of interfacial diffusion-aided growth. From the growth-kinetics data obtained in the present study, lattice interdiffusion coefficients as well as diffusivities along crystalline defects were obtained. Above 800 °C, the growth kinetics are dominated by lattice interdiffusion of Cr in the FCC matrix described by DL = 9.8 × 10-4 exp[(-300 kJ/mol)/(RT)] m2/s. At lower temperatures, the growth kinetics are enhanced by fast interdiffusion along dislocation pipes, which temperature dependence is given by DD = 5.0 × 10-3 exp[(-205 kJ/mol)/(RT)] m2/s. The Cr-diffusivity along σ/FCC interphase boundaries deduced from the thickening kinetics of grain boundary precipitates can be represented by the Arrhenius relationship DI = 0.5 × 10-4 exp[(-145 kJ/mol)/(RT)] m2/s, which is similar to that found for grain boundary interdiffusion in metals and alloys. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.08.023
  • Interdiffusion in Cr–Fe–Co–Ni medium-entropy alloys
    Durand, A. and Peng, L. and Laplanche, G. and Morris, J.R. and George, E.P. and Eggeler, G.
    Intermetallics 122 (2020)
    Diffusion in multi-component alloys is attracting renewed attention because of the worldwide interest in high- and medium-entropy alloys (HEAs/MEAs). In the present work, we used diffusion multiples made from MEAs of the quaternary Cr–Fe–Co–Ni system arranged as six distinct pseudo-binary diffusion couples (Cr29Fe13Co29Ni29–Cr29Fe29Co29Ni13, Cr29Fe29Co13Ni29–Cr29Fe29Co29Ni13, and so on, where the interdiffusing elements are italicized for clarity). In the two halves of each couple, the starting concentrations of the interdiffusing elements (Fe,Ni and Co,Ni in the above examples) were different while those of the background elements (Cr,Co and Cr,Fe in the above examples) were the same. The diffusion multiples were annealed at temperatures between 1153 and 1355 K at times from 100 to 900 h, after which the concentrations of the different elements were measured as a function of distance across each couple. Interdiffusion coefficients were derived from such concentration profiles using the standard Sauer-Freise method and compared with literature data as well as with published tracer diffusion coefficients. Although the background elements were homogeneously distributed initially, some of them developed distinct sine-wave shaped concentration gradients near the interfaces after annealing, implying that uphill diffusion of these elements had occurred. We show using a kinetic model for substitutional diffusion via vacancy hopping that such uphill diffusion can occur even in the absence of thermodynamic interactions, i.e. in ideal solid solutions in which the thermodynamic factor Φ of each element is equal to one (Φi=1+∂lnfi/∂lnci where fi and ci are the activity coefficient and mole fraction of element i, respectively). The model accounts for all elemental fluxes and also rationalizes the diffusion profiles of the major interdiffusing elements. © 2020
    view abstract10.1016/j.intermet.2020.106789
  • Laser metal deposition of refractory high-entropy alloys for high-throughput synthesis and structure-property characterization
    Dobbelstein, H. and George, E.P. and Gurevich, E.L. and Kostka, A. and Ostendorf, A. and Laplanche, G.
    International Journal of Extreme Manufacturing 3 (2020)
    Progress in materials development is often paced by the time required to produce and evaluate a large number of alloys with different chemical compositions. This applies especially to refractory high-entropy alloys (RHEAs), which are difficult to synthesize and process by conventional methods. To evaluate a possible way to accelerate the process, high-throughput laser metal deposition was used in this work to prepare a quinary RHEA, TiZrNbHfTa, as well as its quaternary and ternary subsystems by in-situ alloying of elemental powders. Compositionally graded variants of the quinary RHEA were also analyzed. Our results show that the influence of various parameters such as powder shape and purity, alloy composition, and especially the solidification range, on the processability, microstructure, porosity, and mechanical properties can be investigated rapidly. The strength of these alloys was mainly affected by the oxygen and nitrogen contents of the starting powders, while substitutional solid solution strengthening played a minor role. © 2020 The Author(s). Published by IOP Publishing Ltd
    view abstract10.1088/2631-7990/abcca8
  • 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
  • Benchmark dataset of the effect of grain size on strength in the single-phase FCC CrCoNi medium entropy alloy
    Schneider, M. and George, E.P. and Manescau, T.J. and Záležák, T. and Hunfeld, J. and Dlouhý, A. and Eggeler, G. and Laplanche, G.
    Data in Brief 27 (2019)
    This data article presents the microstructural data as well as the mechanical properties of the CrCoNi medium-entropy alloy (MEA). The data presented in this article are related to the research article entitled “Analysis of strengthening due to grain boundaries and annealing twin boundaries in the CrCoNi medium-entropy alloy”, see Ref. Schneider et al., 2019. This article can be referred to for the analysis and interpretation of the data, as well as their comparison to other datasets in literature. Microstructural data available in the present paper are backscattered electron micrographs for sixteen different grain sizes. Also available are pdf reports of grain size analysis (annealing twin boundaries were neglected) and crystallite sizes (including annealing twin boundaries) as well as data describing the number of annealing twin boundaries per grain (n), corresponding Taylor factors (M) and average annealing twin thicknesses (t). Additionally, raw data of stress-strain curves at five different temperatures [77 K, 293 K, 473 K, 673 K and 873 K] are given for all sixteen grain sizes, which may be used for further research, e.g. data mining, machine learning and other analytical methods. Mechanical data such as yield stresses (σ0.2%), Hall-Petch parameters (σ0 and ky) and critical boundary strengths (τc) are provided along with a 1D discrete dislocation dynamics (1-D DDD) simulation results concerning the different boundary strengths. © 2019 The Author(s)
    view abstract10.1016/j.dib.2019.104592
  • Columnar to equiaxed transition and grain refinement of cast CrCoNi medium-entropy alloy by microalloying with titanium and carbon
    Liu, X.W. and Laplanche, G. and Kostka, A. and Fries, S.G. and Pfetzing-Micklich, J. and Liu, G. and George, E.P.
    Journal of Alloys and Compounds 775 (2019)
    Thermomechanical processing has been used to control the grain size/shape of the equiatomic CrCoNi medium-entropy alloy (MEA) and obtain excellent strength and ductility. However, in the cast state, the alloy has coarse columnar grains with average widths and lengths of approximately 120 and 1000 μm, respectively, resulting in inferior mechanical properties. To overcome this deficiency, here we microalloyed with Ti and C and successfully changed the grain shape (from columnar to equiaxed) and refined the grain size. The degree to which the microstructure changes depends on the amount of Ti and C added, with the best results obtained at 0.4 at.% each. In the optimal alloy [(CrCoNi)99.2Ti0.4C0.4], the as-cast grains were nearly equiaxed with a uniform size of ∼75 μm. Associated with this change in grain shape/size was a significant improvement of yield strength, ultimate tensile strength and elongation to fracture at both 293 and 77 K. The columnar to equiaxed transition is attributed to the strong mutual affinity of C and Ti, which leads to their build-up ahead of the solid-liquid interface and, in turn, to enhanced constitutional undercooling. © 2018 Elsevier B.V.
    view abstract10.1016/j.jallcom.2018.10.187
  • Data regarding the influence of Al, Ti, and C additions to as-cast Al0.6CoCrFeNi compositionally complex alloys on microstructures and mechanical properties
    Asabre, A. and Pfetzing-Micklich, J. and Stryzhyboroda, O. and Kostka, A. and Hecht, U. and Laplanche, G.
    Data in Brief 27 (2019)
    This brief paper contains raw data of X-ray diffraction (XRD) measurements, microstructural characterization, chemical compositions, and mechanical properties describing the influence of Al, Ti, and C on as-cast Al0.6CoCrFeNi compositionally complex alloys (CCAs). The presented data are related to the research article in reference [1] and therefore this article can be referred to as for the interpretation of the data. X-ray diffraction data presented in this paper are measurements of 2θ versus intensities for each studied alloy. A Table lists the obtained lattice parameters of each identified phase determined by Rietveld analysis. Microstructural-characterization data reported here include backscattered electron (BSE) micrographs taken at different magnifications in a scanning electron microscope (SEM) of Widmanstätten and dendritic microstructures and microstructural parameters such as phase volume fractions, thickness of face-centered cubic (FCC) plates, and prior grain sizes. The compositions of the identified individual phases determined by energy-dispersive X-ray spectroscopy (EDX) in the transmission electron microscope (TEM) are listed as well. Finally, mechanical data including engineering stress-strain curves obtained at different temperatures (room temperature, 400 °C, and 700 °C) for all CCAs are reported. © 2019 The Authors
    view abstract10.1016/j.dib.2019.104742
  • Effect of Al, Ti and C additions on Widmanstätten microstructures and mechanical properties of cast Al0.6CoCrFeNi compositionally complex alloys
    Asabre, A. and Kostka, A. and Stryzhyboroda, O. and Pfetzing-Micklich, J. and Hecht, U. and Laplanche, G.
    Materials and Design 184 (2019)
    The cast microstructure of the Al0.6CoCrFeNi compositionally complex alloy was successfully refined with small additions of Al, Ti and C and its mechanical properties were optimized. In the as-cast state, this alloy has a Widmanstätten microstructure with coarse grains (∼110 μm) of a strong BCC/B2 matrix and soft FCC plates (∼65 vol.%) with large widths (∼1.3 μm). The addition of 0.25 at.% C to Al0.6CoCrFeNi stabilizes the FCC phase and favors the formation of a coarse dendritic microstructure making this alloy unsuitable for structural applications. In contrast, alloying of either 3 at.% Al, Ti, or 3% Ti and 0.25% C to Al0.6CoCrFeNi refined its Widmanstätten microstructure, i.e. the thickness of the FCC plates and/or the size of the prior BCC/B2 grains were significantly reduced. As a result of these microstructural changes, Al and Ti containing alloys show an outstanding strength (twice higher than that of Al0.6CoCrFeNi) and ductilities ≤5% at 20 °C. These properties are retained at 400 °C but at 700 °C, the strength and ductility of almost all alloys decrease. However, Ti containing alloys exhibit much larger ductilities (∼50%) at 700 °C due to their high density of grain boundaries which accommodate plastic deformation through grain boundary sliding. © 2019 The Authors
    view abstract10.1016/j.matdes.2019.108201
  • Effect of temperature and texture on Hall–Petch strengthening by grain and annealing twin boundaries in the MnFeNi medium-entropy alloy
    Schneider, M. and Werner, F. and Langenkämper, D. and Reinhart, C. and Laplanche, G.
    Metals 9 (2019)
    Among equiatomic alloys of the Cr-Mn-Fe-Co-Ni system, MnFeNi was shown to exhibit a strong anti-invar behavior but little is known regarding its mechanical properties. The objective of the present study is to investigate Hall–Petch strengthening by grain and annealing twin boundaries in MnFeNi. For this purpose, seven different grain sizes between 17 and 216 µm were produced. Mean grain sizes (excluding annealing twin boundaries) and crystallite sizes (including them) were determined using the linear intercept method. Overall, 25% of the boundaries were found to be annealing twin boundaries regardless of the grain size. In some cases, two twin boundaries can be present in one grain forming an annealing twin, which thickness represents one quarter of the mean grain size. Based on a comparison of the mean twin thickness of different alloys with different stacking fault energy (SFE), we estimated an SFE of 80 ± 20 mJ/m 2 for MnFeNi. Compression tests of MnFeNi with different grain sizes were performed between 77 and 873 K and revealed a parallel shift of the Hall–Petch lines with temperature. The interaction between dislocations and boundaries was investigated by scanning transmission electron microscopy (STEM) in a deformed specimen. It was found that a large number of dislocations are piling up against grain boundaries while the pile-ups at annealing twin boundaries contain much fewer dislocations. This indicates that annealing twin boundaries in this alloy are less effective obstacles to dislocation motion than grain boundaries. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met9010084
  • Laser metal deposition of compositionally graded TiZrNbTa refractory high-entropy alloys using elemental powder blends
    Dobbelstein, H. and Gurevich, E.L. and George, E.P. and Ostendorf, A. and Laplanche, G.
    Additive Manufacturing 25 (2019)
    In the present study, laser metal deposition (LMD) was used to produce compositionally graded refractory high-entropy alloys (HEAs) for screening purposes by in-situ alloying of elemental powder blends. A compositional gradient from Ti25Zr50Nb0Ta25 to Ti25Zr0Nb50Ta25 is obtained by incrementally substituting Zr powder with Nb powder. A suitable strategy was developed to process the powder blend despite several challenges such as the high melting points of the refractory elements and the large differences in melting points among them. The influence of the LMD process on the final chemical composition was analyzed in detail and the LMD process was optimized to obtain a well-defined compositional gradient. Microstructures, textures, chemical compositions and mechanical properties were characterized using SEM, EBSD, EDX, and microhardness testing, respectively. Compositions between Ti25Zr0Nb50Ta25 and Ti25Zr25Nb25Ta25 were found to be single-phase bcc solid solutions with a coarse grain microstructure. Increasing the Zr to Nb ratio beyond the equiatomic composition results in finer and harder multiphase microstructures. The results shown in the present study clearly show for the first time that LMD is a suitable processing tool to screen HEAs over a range of chemical compositions. © 2018 The Authors
    view abstract10.1016/j.addma.2018.10.042
  • On the onset of deformation twinning in the CrFeMnCoNi high-entropy alloy using a novel tensile specimen geometry
    Thurston, K.V.S. and Hohenwarter, A. and Laplanche, G. and George, E.P. and Gludovatz, B. and Ritchie, R.O.
    Intermetallics 110 (2019)
    Deformation-induced nanoscale twinning is one of the mechanisms responsible for the excellent combination of strength and fracture toughness of the single-phase, face-centered cubic CrMnFeCoNi (Cantor)alloy, especially at cryogenic temperatures. Here, we use a novel, modified dogbone geometry that permits the sampling of varying stress and strain regions within a single tensile specimen to characterize the onset of twinning in CrMnFeCoNi at 293 K, 198 K and 77 K. Electron backscatter diffraction (EBSD)and backscattered electron (BSE)imaging revealed the presence of deformation nano-twins in regions of the samples that had experienced plastic strains of ∼25% at 293 K, ∼16% at 198 K, and ∼8% at 77 K, which are similar to the threshold strains described by Laplanche et al. (Acta Mater. 118, 2016, 152–163). From these strains we estimate that the critical tensile stress for the onset of twinning in this alloy is on the order of 750 MPa. © 2019 Elsevier Ltd
    view abstract10.1016/j.intermet.2019.04.012
  • Precipitation Hardenable High Entropy Alloy for Tooling Applications
    Stryzhyboroda, O. and Hecht, U. and Witusiewicz, V.T. and Laplanche, G. and Asabre, A. and Wilms, M.B. and Weisheit, A.
    MRS Advances 4 (2019)
    We present a high entropy alloy (HEA) from the system Al-Co-Cr-Fe-Ni with small additions of W, Mo, Si and C which was designed to allow for precipitation hardening by annealing in the temperature range from 600 to 900°C. The alloy development was supported by thermodynamic computations using ThermoCalc software and the specimens were produced by arc melting. The microstructure of one selected sample in as-cast and annealed conditions was analysed using SEM/EDS, SEM/EBSD and TEM. The as-cast microstructure consists of spinodally decomposed BCC dendrites enveloped by FCC+Cr 23 C 6 eutectic. Upon annealing at 700°C for 24 h nanoscale precipitates form within the spinodal BCC as well as from FCC. Precipitation is exquisitely uniform leading to an increase in microhardness from 415 HV0.5 in the as-cast state to 560 HV0.5 after annealing. We investigated coarsening of this microstructure using varying holding time for a constant temperature of 700°C. The microstructure evolution during coarsening and the corresponding mechanical properties obtained from instrumented indentation experiments are presented in this work. © Materials Research Society 2019.
    view abstract10.1557/adv.2019.146
  • Temperature and load-ratio dependent fatigue-crack growth in the CrMnFeCoNi high-entropy alloy
    Thurston, K.V.S. and Gludovatz, B. and Yu, Q. and Laplanche, G. and George, E.P. and Ritchie, R.O.
    Journal of Alloys and Compounds 794 (2019)
    Multiple-principal element alloys known as high-entropy alloys have rapidly been gaining attention for the vast variety of compositions and potential combinations of properties that remain to be explored. Of these alloys, one of the earliest, the ‘Cantor alloy’ CrMnFeCoNi, displays excellent damage-tolerance with tensile strengths of ∼1 GPa and fracture toughness values in excess of 200 MPa√m; moreover, these mechanical properties tend to further improve at cryogenic temperatures. However, few studies have explored its corresponding fatigue properties. Here we expand on our previous study to examine the mechanics and mechanisms of fatigue-crack propagation in the CrMnFeCoNi alloy (∼7 μm grain size), with emphasis on long-life, near-threshold fatigue behavior, specifically as a function of load ratio at temperatures between ambient and liquid-nitrogen temperatures (293 K–77 K). We find that ΔKth fatigue thresholds are decreased with increasing positive load ratios, R between 0.1 and 0.7, but are increased at decreasing temperature. These effects can be attributed to the role of roughness-induced crack closure, which was estimated using compliance measurements. Evidence of deformation twinning at the crack tip during fatigue-crack advance was not apparent at ambient temperatures but seen at higher stress intensities (ΔK ∼ 20 MPa√m) at 77 K by post mortem microstructural analysis for tests at R = 0.1 and particularly at 0.7. Overall, the fatigue behavior of this alloy was found to be superior, or at least comparable, to conventional cryogenic and TWIP steels such as 304 L or 316 L steels and Fe-Mn steels; these results coupled with the remarkable strength and fracture toughness of the Cantor alloy at low temperatures indicate significant promise for the utility of this material for applications at cryogenic environments. © 2019
    view abstract10.1016/j.jallcom.2019.04.234
  • Temperature dependence of elastic moduli in a refractory HfNbTaTiZr high-entropy alloy
    Laplanche, G. and Gadaud, P. and Perrière, L. and Guillot, I. and Couzinié, J.P.
    Journal of Alloys and Compounds 799 (2019)
    The equiatomic HfNbTaTiZr solid solution is currently regarded as a model disordered body-centered cubic high-entropy alloy. Therefore, the temperature dependence of its elastic moduli is of prime importance to improve our understanding of the mechanical properties of this refractory alloy. In this study, the alloy was found to be single phase, fully recrystallized with a slight texture along the normal direction after thermomechanical processing at room temperature. Elastic moduli were determined over the temperature range [293 K-1100 K]. © 2019 The Authors
    view abstract10.1016/j.jallcom.2019.05.322
  • Elastic moduli and thermal expansion coefficients of medium-entropy subsystems of the CrMnFeCoNi high-entropy alloy
    Laplanche, G. and Gadaud, P. and Bärsch, C. and Demtröder, K. and Reinhart, C. and Schreuer, J. and George, E.P.
    Journal of Alloys and Compounds 746 (2018)
    Elastic moduli of a set of equiatomic alloys (CrFeCoNi, CrCoNi, CrFeNi, FeCoNi, MnCoNi, MnFeNi, and CoNi), which are medium-entropy subsystems of the CrMnFeCoNi high-entropy alloy were determined as a function of temperature over the range 293 K–1000 K. Thermal expansion coefficients were determined for these alloys over the temperature range 100 K–673 K. All alloys were single-phase and had the face-centered cubic (FCC) crystal structure, except CrFeNi which is a two-phase alloy containing a small amount of body-centered cubic (BCC) precipitates in a FCC matrix. The temperature dependences of thermal expansion coefficients and elastic moduli obtained here are useful for quantifying fundamental aspects such as solid solution strengthening, and for structural analysis/design. Using the above results, the yield strengths reported in literature for these alloys were normalized by their shear moduli to reveal the influence of shear modulus on solid solution strengthening. © 2018 The Author(s)
    view abstract10.1016/j.jallcom.2018.02.251
  • Laser metal deposition of a refractory TiZrNbHfTa high-entropy alloy
    Dobbelstein, H. and Gurevich, E.L. and George, E.P. and Ostendorf, A. and Laplanche, G.
    Additive Manufacturing 24 (2018)
    Refractory elements have high melting points and are difficult to melt and cast. In this study it is successfully demonstrated for the first time that laser metal deposition can be used to produce TiZrNbHfTa high-entropy alloy from a blend of elemental powders by in-situ alloying. Columnar specimens with a height of 10 mm and a diameter of 3 mm were deposited with a pulsed Nd:YAG laser. The built-up specimen has near-equiatomic composition, nearly uniform grain size, equiaxed grain shape, is bcc single phase and exhibits a high hardness of 509 HV0.2. © 2018 The Authors
    view abstract10.1016/j.addma.2018.10.008
  • On Shear Testing of Single Crystal Ni-Base Superalloys
    Eggeler, G. and Wieczorek, N. and Fox, F. and Berglund, S. and Bürger, D. and Dlouhy, A. and Wollgramm, P. and Neuking, K. and Schreuer, J. and Agudo Jácome, L. and Gao, S. and Hartmaier, A. and Laplanche, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science (2018)
    Shear testing can contribute to a better understanding of the plastic deformation of Ni-base superalloy single crystals. In the present study, shear testing is discussed with special emphasis placed on its strengths and weaknesses. Key mechanical and microstructural results which were obtained for the high-temperature (T ≈ 1000 °C) and low-stress (τ ≈ 200 MPa) creep regime are briefly reviewed. New 3D stereo STEM images of dislocation substructures which form during shear creep deformation in this regime are presented. It is then shown which new aspects need to be considered when performing double shear creep testing at lower temperatures (T < 800 °C) and higher stresses (τ > 600 MPa). In this creep regime, the macroscopic crystallographic [11−2](111) shear system deforms significantly faster than the [01−1](111) system. This represents direct mechanical evidence for a new planar fault nucleation scenario, which was recently suggested (Wu et al. in Acta Mater 144:642–655, 2018). The double shear creep specimen geometry inspired a micro-mechanical in-situ shear test specimen. Moreover, the in-situ SEM shear specimen can be FIB micro-machined from prior dendritic and interdendritic regions. Dendritic regions, which have a lower γ′ volume fraction, show a lower critical resolved shear stress. © 2018 The Author(s)
    view abstract10.1007/s11661-018-4726-9
  • On the influence of crystallography and dendritic microstructure on micro shear behavior of single crystal Ni-based superalloys
    Laplanche, G. and Wieczorek, N. and Fox, F. and Berglund, S. and Pfetzing-Micklich, J. and Kishida, K. and Inui, H. and Eggeler, G.
    Acta Materialia 160 (2018)
    An in-situ SEM micromechanical test technique is used to investigate the response of a Ni-based single crystal superalloy to double shear loading. The present work shows that micro double shear testing can detect mechanical differences between interdendritic and dendritic regions with γ′-volume fractions of 77% and 72%, respectively, i.e., the interdendritic regions exhibit a larger flow stress than the dendritic regions. These micromechanical differences are apparent when micro double shear specimens are oriented for single-slip while they appear to be overshadowed by dislocation interactions, when multiple-slip is promoted. Sudden deformation events are observed to occur concomitantly with the formation of shear steps (localized plastic deformation) at the surface of the shear zones during single-slip. The micro double shear specimens oriented for single-slip show very low work-hardening. In contrast, much higher stresses are required to plastically deform micro double shear specimens oriented for multiple-slip which exhibit stronger work-hardening. No sudden deformation events could be detected for multiple-slip which results in a more homogeneous deformation of the shear zones (absence of localized plastic deformation). © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.08.052
  • Phase stability and kinetics of σ-phase precipitation in CrMnFeCoNi high-entropy alloys
    Laplanche, G. and Berglund, S. and Reinhart, C. and Kostka, A. and Fox, F. and George, E.P.
    Acta Materialia 161 (2018)
    Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600 °C and 1000 °C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.09.040
  • Thermal activation parameters of plastic flow reveal deformation mechanisms in the CrMnFeCoNi high-entropy alloy
    Laplanche, G. and Bonneville, J. and Varvenne, C. and Curtin, W.A. and George, E.P.
    Acta Materialia 143 (2018)
    To reveal the operating mechanisms of plastic deformation in an FCC high-entropy alloy, the activation volumes in CrMnFeCoNi have been measured as a function of plastic strain and temperature between 77 K and 423 K using repeated load relaxation experiments. At the yield stress, σy, the activation volume varies from ∼60 b3 at 77 K to ∼360 b3 at 293 K and scales inversely with yield stress. With increasing plastic strain, the activation volume decreases and the trends follow the Cottrell-Stokes law, according to which the inverse activation volume should increase linearly with σ−σy (Haasen plot). This is consistent with the notion that hardening due to an increase in the density of forest dislocations is naturally associated with a decrease in the activation volume because the spacing between dislocations decreases. The values and trends in activation volume agree with theoretical predictions that treat the HEA as a high-concentration solid-solution-strengthened alloy. These results demonstrate that this HEA deforms by the mechanisms typical of solute strengthening in FCC alloys, and thus indicate that the high compositional/structural complexity does not introduce any new intrinsic deformation mechanisms. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.10.014
  • 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
  • Effect of temperature on the fatigue-crack growth behavior of the high-entropy alloy CrMnFeCoNi
    Thurston, K.V.S. and Gludovatz, B. and Hohenwarter, A. and Laplanche, G. and George, E.P. and Ritchie, R.O.
    Intermetallics 88 (2017)
    Near-equiatomic multi-component high-entropy alloys (HEAs) have engendered much attention of late due to the remarkable mechanical properties of some of these new metallic materials. In particular, one of the first reported HEAs, the equiatomic, single-phase, face-centered cubic (fcc) alloy CrMnFeCoNi, often termed the Cantor alloy, has been shown to display an exceptional combination of strength, ductility and fracture toughness, i.e., damage tolerance, at room temperature, properties that are further enhanced at cryogenic temperatures. Despite this alloy being the most studied HEA to date, its resistance to crack growth under cyclic fatigue loading has not yet been characterized. Here, we examine its fatigue-crack propagation behavior, primarily at lower, near-threshold, growth rates, both at room temperature (293 K) and at 198 K. At 293 K, the alloy shows a fatigue threshold, ΔKTH, of ∼4.8 MPa√m, which increases by more than 30% to ΔKTH ∼6.3 MPa√m with decrease in temperature to 198 K; additionally, the Paris exponent m was found to increase from roughly 3.5 to 4.5 with decreasing temperature. Examination of the fracture surfaces and crack paths indicate a transition from predominantly transgranular crack propagation at room temperature to intergranular-dominated failure at the lower temperature. Such a change in crack path is generally associated with an increasing degree of physical contact between the two fracture surfaces, i.e., roughness-induced fatigue crack closure, which is likely to be the main reason for the difference in the measured thresholds. Additionally, we believe that the higher thresholds found at 198 K are associated with the alloy's higher strength at lower temperatures, which both reduces the crack-tip opening displacements at a given stress-intensity range and prevents plastic deformations of the grains in the wake of the crack. At room temperature, such plastically deformed grains can be associated with a loss of contact shielding of the crack-tip through closure, resulting in a lower threshold compared to 198 K. © 2017
    view abstract10.1016/j.intermet.2017.05.009
  • Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi
    Laplanche, G. and Kostka, A. and Reinhart, C. and Hunfeld, J. and Eggeler, G. and George, E.P.
    Acta Materialia 128 (2017)
    The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77 K) and room temperature (293 K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2&lt;110&gt; dislocations dissociated into 1/6&lt;112&gt; Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22 ± 4 mJ m−2, which is ∼25% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16 μm grain size is 260 ± 30 MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.02.036
  • Assessment of strain hardening in copper single crystals using in situ SEM microshear experiments
    Wieczorek, N. and Laplanche, G. and Heyer, J.-K. and Parsa, A.B. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 113 (2016)
    The effect of a pre-strain on the plasticity of copper single crystals subjected to in situ microshear deformation in a scanning electron microscope (SEM) is investigated. Pre-strains of 6.5 and 20% are imposed using [1 0 0] tensile testing. During tensile pre-deformation, several slip systems are activated and irregularly spaced slip bands form. A trace analysis revealed the presence of several slip bands on the tensile specimen near the grips while one family of slip bands parallel to the (1 1 1) crystallographic plane were detected in the middle of the tensile specimen. From the middle of the pre-deformed tensile specimens double microshear samples were prepared using focused ion beam (FIB) machining such that the [0 -1 -1] (1 -1 1) slip system could be directly activated. The results show how microshear behavior reacts to different levels of tensile pre-deformation. Sudden deformation events (SDEs) are observed during microshear testing. The critical stress associated with the first SDE is shown to increase with increasing pre-deformation as a result of an increasing number of slip bands introduced during pre-deformation per shear zone. The results allow also to obtain information on the interaction between dislocations activated during microshearing ([0 -1 -1] (1 -1 1)) and those which were introduced during tensile pre-deformation ([1 0 -1] (1 1 1) and [1 -1 0] (1 1 1)). When these slip systems interact glissile junctions and Lomer-Cottrell locks are likely to form. In the light of this analysis, we rationalize the occurrence of sudden deformation events based on piled up dislocation assemblies which overcome Lomer-Cottrell lock barriers. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.04.055
  • Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy
    Laplanche, G. and Kostka, A. and Horst, O.M. and Eggeler, G. and George, E.P.
    Acta Materialia 118 (2016)
    At low homologous temperatures (down to cryogenic temperatures), the CrMnFeCoNi high-entropy alloy possesses good combination of strength, work hardening rate (WHR), ductility, and fracture toughness. To improve understanding of the deformation mechanisms responsible for its mechanical properties, tensile tests were performed at liquid nitrogen and room temperature (77 K and 293 K) and interrupted at different strains to quantify the evolution of microstructure by transmission electron microscopy. Dislocation densities, and twin widths, their spacings, and volume fractions were determined. Nanotwins were first observed after true strains of ∼7.4% at 77 K and ∼25% at 293 K; at lower strains, deformation occurs by dislocation plasticity. The tensile stress at which twinning occurs is 720 ± 30 MPa, roughly independent of temperature, from which we deduce a critical resolved shear stress for twinning of 235 ± 10 MPa. In the regime where deformation occurs by dislocation plasticity, the shear modulus normalized WHR decreases with increasing strain at both 77 K and 293 K. Beyond ∼7.4% true strain, the WHR at 77 K remains constant at a high value of G/30 because twinning is activated, which progressively introduces new interfaces in the microstructure. In contrast, the WHR at room temperature continues to decrease with increasing strain because twinning is not activated until much later (close to fracture). Thus, the enhanced strength-ductility combination at 77 K compared to 293 K is primarily due to twinning starting earlier in the deformation process and providing additional work hardening. Consistent with this, when tensile specimens were pre-strained at 77 K to introduce nanotwins, and subsequently tested at 293 K, flow stress and ductility both increased compared to specimens that were not pre-strained. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.07.038
  • Oxidation Behavior of the CrMnFeCoNi High-Entropy Alloy
    Laplanche, G. and Volkert, U.F. and Eggeler, G. and George, E.P.
    Oxidation of Metals 85 (2016)
    Oxidation of the Cr20Mn20Fe20Co20Ni20 (at%) high-entropy alloy (HEA) was investigated at 500–900 °C in laboratory air. At 600 °C the oxide was mainly Mn2O3 with a thin inner Cr2O3 layer; at 700 and 800 °C it was mainly Mn2O3 with some Cr enrichment; at 900 °C it was Mn3O4. The oxidation rate was initially linear but became parabolic at longer times with an activation energy of 130 kJ/mol, comparable to that of Mn diffusion in Mn oxides but much lower than that for sluggish diffusion of Mn in the HEA. The diffusion of Mn through the oxide is considered to be the rate-limiting process. © 2016, Springer Science+Business Media New York.
    view abstract10.1007/s11085-016-9616-1
  • Plasticity of the ω-Al7Cu2Fe phase
    Laplanche, G. and Bonneville, J. and Joulain, A. and Gauthier-Brunet, V. and Dubois, S.
    Journal of Alloys and Compounds 665 (2016)
    Polycrystalline samples with the Al0.693Cu0.201Fe0.106 composition, corresponding to the tetragonal P4/mnc ω-Al7Cu2Fe crystallographic structure, were synthesised by spark plasma sintering and deformed in compression under constant strain-rate conditions, ε = 2 × 10-4 s-1, over the temperature range 650 K-1000 K. A brittle-to-ductile transition is evidenced between 700 K and 750 K. The stress-strain curves exhibit a yield point followed by softening or steady state conditions only. The upper yield stress, σUYS, shows a strong temperature dependence suggesting that the rate controlling deformation mechanisms are highly thermally activated. The strain-rate sensitivity of stress characterised either by stress exponents, nexp, or by activation volumes, Vexp, was measured by the load relaxation technique. High nexp values, i.e., larger than 7, associated with low Vexp, typically smaller than 1 nm3, are measured. The Gibbs free activation energy, ΔG, deduced by integrating Vexp with respect to stress varies from nearly 2 eV at 790 K to 4 eV at 1000 K. Because plasticity of the ω-Al7Cu2Fe phase takes place at temperatures at which diffusion processes are considered as dominant, the results are interpreted in the frame of dislocation climb models proposed to account for high temperature plasticity of crystalline phases. © 2016 Published by Elsevier B.V.
    view abstract10.1016/j.jallcom.2015.12.161
  • Microstructural evolution of a CoCrFeMnNi high-entropy alloy after swaging and annealing
    Laplanche, G. and Horst, O. and Otto, F. and Eggeler, G. and George, E.P.
    Journal of Alloys and Compounds 647 (2015)
    Abstract The processing parameters which govern the evolution of microstructure and texture during rotary swaging and subsequent heat treatments were studied in an equiatomic single-phase CoCrFeMnNi high-entropy alloy. After vacuum induction melting and casting, the diameter of the 40 mm cast ingot was reduced at room temperature to a final diameter of 16.5 mm by rotary swaging (diameter reduction of 60%/area reduction of 80%) and the alloy was then annealed at different temperatures for 1 h. The resulting microstructures were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction and correlated with results of microhardness measurements. It was found that the microhardness first increases slightly upon annealing below the recrystallization temperature but then drops steeply at higher annealing temperatures due to the onset of recrystallization. Special emphasis was placed on how the microstructure evolves with respect to the radial and longitudinal position in the rod. Finally, a combination of swaging and heat treatment parameters were identified that can produce CoCrFeMnNi high-entropy alloys with a homogeneous composition and grain size and almost no texture. © 2015 Published by Elsevier B.V.
    view abstract10.1016/j.jallcom.2015.05.129
  • Processing of NiTi shape memory sheets - Microstructural heterogeneity and evolution of texture
    Laplanche, G. and Kazuch, A. and Eggeler, G.
    Journal of Alloys and Compounds 651 (2015)
    In the present paper we study the evolution of microstructure and texture during processing of Ni<inf>51</inf>Ti<inf>49</inf> shape memory sheets using electron backscatter diffraction. Hot rolling results in a heterogeneous microstructure which reflects a temperature gradient in the sheet. Equiaxed and randomly oriented grains are observed close to the surface of the hot rolled sheet while the sheet interior shows a strong texture containing two main texture components {111}<110> and {110}<110> with grains elongated along the rolling direction. In contrast, cold rolling in combination with a recrystallization heat treatment produces a more homogeneous microstructure in terms of grain morphology and grain size. It also promotes a random grain orientation along the rolling and transverse directions while the normal direction shows a strong γ-fiber {111}<uvw> texture. To get a better understanding of the elementary deformation mechanisms which control the texture evolution during rolling, textures assessed in the present study are compared with simulations reported in the literature. © 2015, Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2015.08.127
  • Mechanical properties of Al-Cu-Fe quasicrystalline and crystalline phases: An analogy
    Laplanche, G. and Bonneville, J. and Joulain, A. and Gauthier-Brunet, V. and Dubois, S.
    Intermetallics 50 (2014)
    The mechanical properties of the ω-Al7Cu2Fe crystalline phase have been investigated over a large temperature range (650-1000 K). Despite of its antinomic structure with the icosahedral Al-Cu-Fe quasicrystalline phase, i.e. periodic vs non-periodic, its mechanical properties are very similar to those of the quasicrystalline phase, which strongly suggest similar deformation mechanisms. Consequently, as for the quasicrystalline structure, we propose that dislocation climb might control the plastic deformation of the ω-phase. However, in the present case, the specificities of the quasicrystalline structure cannot be invoked to justify the predominance of dislocation climb, which questions the role of quasiperiodicity on dislocation mobility. We suggest that this deformation mode certainly results from specific non-planar extensions of the dislocation core. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2014.02.004
  • Orientation dependence of stress-induced martensite formation during nanoindentation in NiTi shape memory alloys
    Laplanche, G. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 68 (2014)
    In the present work we used nanoindentation with a spherical indenter tip to study the formation of stress-induced martensite in NiTi shape memory alloys. Prior to nanoindentation, orientation imaging was performed to select austenite grains with specific crystallographic orientations, including the principal crystallographic directions [0 0 1], [1 0 1] and [1 1 1]. We studied a material where stress-induced martensite is stable at room temperature and found surface patterns with four-, two- and threefold symmetries for the [0 0 1], [1 0 1] and [1 1 1] crystallographic indentation directions, respectively. Atomic force microscopy investigations of the topography showed that the surface patterns were associated with sink-ins. The crystallographic sink-in patterns disappeared during heating, which proved their martensitic origin. Our results provide clear experimental evidence which shows that the crystallographic anisotropy of nanoindentation is governed by the crystallographic anisotropy of the stress-induced formation of martensite.©2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.01.006
  • Sudden stress-induced transformation events during nanoindentation of NiTi shape memory alloys
    Laplanche, G. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 78 (2014)
    This study investigates the stress-induced formation of martensite during nanoindentation of an austenitic NiTi shape memory alloy, where stress-induced martensite is stable at room temperature. An individual grain with a [1 1 1] surface normal was selected for spherical ex situ and in situ nanoindentation in a scanning electron microscope. The in situ load-displacement curves show several pop-ins which occur concomitantly with the formation of traces around the contact zone between the indenter tip and the sample. These traces exhibit a threefold symmetry around the remnant indent. A detailed study of the indentation-induced surface relief by atomic force microscopy before and after shape recovery allows to identify the formation of six twinned martensite plates. Post-mortem microstructural characterization shows that these twinned martensite plates are growing as the applied load is increasing. The activation of the experimentally observed twinned martensite plates is rationalized by analytical calculations of resolved shear stress and mechanical interaction energy density. Finally, the in situ nanoindentation results in combination with the post-mortem microstructural characterization show that the most likely deformation mechanism responsible for pop-in events corresponds to sudden increases of the thicknesses of twinned martensite plates. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.05.061
  • Temperature dependencies of the elastic moduli and thermal expansion coefficient of an equiatomic, single-phase CoCrFeMnNi high-entropy alloy
    Laplanche, G. and Gadaud, P. and Horst, O. and Otto, F. and Eggeler, G. and George, E.P.
    Journal of Alloys and Compounds 623 (2014)
    The equiatomic CoCrFeMnNi alloy is now regarded as a model face-centered cubic single-phase high-entropy alloy. Therefore, determination of its intrinsic properties such as the temperature dependencies of elastic moduli and thermal expansion coefficient are important to improve understanding of this new class of material. These temperature dependencies were measured over a large temperature range (200-1270 K) in this study. © 2014 Elsevier B.V.
    view abstract10.1016/j.jallcom.2014.11.061
  • Compressive behavior of Ti3AlC2 and Ti 3Al0.8Sn0.2C2 MAX phases at room temperature
    Bei, G.-P. and Laplanche, G. and Gauthier-Brunet, V. and Bonneville, J. and Dubois, S.
    Journal of the American Ceramic Society 96 (2013)
    In this study, we report on the compressive behavior of Ti 3AlC2 and Ti3Al0.8Sn 0.2C2 MAX phases at room temperature. We found that these two phases could be classified as Kinking Nonlinear Elastic (KNE) solids. The cyclic compressive stress-strain loops for Ti3AlC2 and Ti3Al0.8Sn0.2C2 are typical hysteretic and fully reversible. At failure, both compositions fracture in shear with maximum stresses of 545 MPa for Ti3AlC2 and 839 MPa for Ti3Al0.8Sn0.2C2. Consequently, the macroshear stresses for failure, τc, are 185 MPa and 242 MPa for Ti3AlC2 and Ti3Al0.8Sn 0.2C2, respectively. In addition to the grain size effects, the presence of a ductile TixAly intermetallic distributed in the grain boundaries plays an important role in the enhancement of the ultimate compressive and macroshear stresses for Ti3Al 0.8Sn0.2C2. SEM observations reveal that these two MAX phases exhibit crack deflections, intragranular fractures, kink band formation and delaminations, grain push-in and pull-out. © 2012 The American Ceramic Society.
    view abstract10.1111/jace.12092
  • Spark plasma sintering synthesis and mechanical spectroscopy of the ω-Al 0.7Cu 0.2Fe 0.1 phase
    Laplanche, G. and Gadaud, P. and Bonneville, J. and Joulain, A. and Gauthier-Brunet, V. and Dubois, S. and Jay, F.
    Journal of Materials Science 47 (2012)
    Starting from a mixture of Al-Cu-Fe quasicrystalline (QC) particles and Al powder, a fully dense and almost Al-Cu-Fe ω single-phase alloy was produced by spark plasma sintering. This technique allows synthesising large samples with sizes suitable for mechanical spectroscopy experiments. Mechanical spectroscopy was selected because it is a relevant tool for detecting the presence of structural defects at both nano and microscopic scales. Young's moduli were measured in the 15 kHz range as a function of temperature by the resonant frequency method. Young's moduli behave similarly for typical metals and exhibit values that are comparable to those of the Al-Cu-Fe QC phase. The damping coefficient Q -1 was determined at various temperatures between room temperature and 840 K over a large frequency range, i.e. between 10 -3 and 10 Hz. The results suggest that solid friction effects do occur. In addition, a relaxation peak is observed in the intermediate temperature range. © 2011 Springer Science+Business Media, LLC.
    view abstract10.1007/s10853-011-5784-1
  • Powder metallurgy processing and compressive properties of Ti3AlC2/Al composites
    Wang, W.J. and Gauthier-Brunet, V. and Bei, G.P. and Laplanche, G. and Bonneville, J. and Joulain, A. and Dubois, S.
    Materials Science and Engineering A 530 (2011)
    Al-matrix material composites are produced from pure Al and 40vol.% Ti3AlC2 powders using hot isostatic pressing technique. It is demonstrated that the nanocrystallized-Ti3AlC2 agglomerates, uniformly distributed in the Al matrix, form a hard continuous skeleton. The mechanical properties of the composites are evaluated over the temperature range of 20-500°C by performing compression tests at constant strain rate. The monotonic temperature dependence of the proof stress at 0.2% plastic strain suggests that the same thermally activated mechanism controls the composite plastic deformation over the entire temperature range. The yield stress of the composite, about twice as high as that of the Al matrix in the investigated temperature range proves that Ti3AlC2 particles constitute efficient reinforcement particles for Al matrix. SEM observations indicate that plastic deformation of 40Ti3AlC2/60Al composite takes place in the Al matrix while Ti3AlC2 particle agglomerates undergo substantial fracture. © 2011 Elsevier B.V.
    view abstract10.1016/j.msea.2011.09.068
  • Al-matrix composite materials reinforced by Al-Cu-Fe particles
    Bonneville, J. and Laplanche, G. and Joulain, A. and Gauthier-Brunet, V. and Dubois, S.
    Journal of Physics: Conference Series 240 (2010)
    Al-matrix material composites were produced using hot isostatic pressing technique, starting with pure Al and icosahedral (i) Al-Cu-Fe powders. Depending on the processing temperature, the final reinforcement particles are either still of the initial i-phase or transformed into the tetragonal ω-Al0 0.70Cu0.20Fe0.10 crystalline phase. Compression tests performed in the temperature range 293K - 823K on the two types of composite, i.e. Al/i and Al/ω, indicate that the flow stress of both composites is strongly temperature dependent and exhibit distinct regimes with increasing temperature. Differences exist between the two composites, in particul ar in yield stress values. In the low temperatureregime (T ≤ 570K), the yield stress of the Al/ω composite is nearly 75% higher than that of the Al/i composite, while for T &gt; 570K both composites exhibit similar yield stress values. The results are interpreted in terms of load transfer contribution between the matrix and the reinforcement particles and elementary dislocation mechanisms in the Al matrix. © 2010 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/240/1/012013
  • Microstructural and mechanical study of an Al matrix composite reinforced by Al-Cu-Fe Icosahedral particles
    Laplanche, G. and Joulain, A. and Bonneville, J. and Gauthier-Brunet, V. and Dubois, S. and El Kabir, T.
    Journal of Materials Research 25 (2010)
    In this study, we produced an Al matrix composite material reinforced by Al-Cu-Fe particles of the icosahedral phase. The composite material was prepared using a hot isostatic pressure technique at T = 673 K and P = 180 MPa. The mechanical properties were investigated by compression tests performed at constant strain rate over the temperature range 290-823 K. The results show a vigorous strengthening effect resulting from the reinforcement particles. Strengthening is attributed to two main contributions arising from load transfer between the Al matrix and the reinforcement particles and from plastic deformation of the Al grains. The present results are compared with those obtained in a previous work on an Al-based composite reinforced by Al-Cu-Fe particles of the ω-tetragonal phase. © 2010 Materials Research Society.
    view abstract10.1557/jmr.2010.0118
  • Microstructures and mechanical properties of Al-base composite materials reinforced by Al-Cu-Fe particles
    Laplanche, G. and Joulain, A. and Bonneville, J. and Schaller, R. and El Kabir, T.
    Journal of Alloys and Compounds 493 (2010)
    In this study, we produced four composite materials with Al-based matrix reinforced by Al-Cu-Fe particles initially of the quasicrystalline (QC) phase. The processing route was a gas-pressure infiltration of QC particle preforms by molten commercial Al and Al alloys. The resulting composites were investigated by scanning electron microscopy (SEM) working in the energy dispersive spectroscopy (EDS) mode and by X-ray diffraction (XRD). It is shown that such a synthesis technique leads to the formation of various phases resulting from specific diffusion processes. Compression tests were performed at constant strain rate in the temperature range 290-770 K. The stress-strain curves look similar to those of Al-Cu-Fe poly-quasicrystals and show the yield point, the origin of which is however of very different nature. Composite deformation is recognised to occur through the rupture of a hard phase skeleton and localised plastic deformation in the matrix. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2009.12.124
  • Synthesis and brittle-to-ductile transition of the ω-Al0.7Cu0.2Fe0.1 tetragonal phase
    Laplanche, G. and Joulain, A. and Bonneville, J. and Gauthier-Brunet, V. and Dubois, S.
    Materials Science and Engineering A 527 (2010)
    Synthesis of ω-Al-Cu-Fe single phase material is reported. Microhardness tests performed over the temperature range 293-898K indicate a brittle-to-ductile transition between 673K and 823K. Fracture toughness was determined from indentation cracks at 293K. Both the hardness and fracture toughness are comparable to that of the icosahedral Al0.635Cu0.240Fe0.125 material. © 2010 Elsevier B.V.
    view abstract10.1016/j.msea.2010.02.049
  • alloys

  • dislocations

  • high-entropy alloys

  • mechanical properties

  • microstructure

  • shape-memory alloys

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