Analysis of the Stability of High Entropy Alloys by Dewetting of Thin Films
Mechanical Properties of Metallic Materials and their Microstructural Origins
Final Report Abstract
High-Entropy Alloys (HEAs) are a recently recognized class of materials, different to conventional alloys in that they contain approximately equal amounts of at least five elements, and can surprisingly crystallize as single phase solid solutions. They have the potential to realize exceptional, otherwise unachievable combinations of mechanical and functional properties. The concept has since been broadened to more generally Compositionally Complex Alloys (CCAs), since the single phase maybe metastable, or multiple phases may lead to even better properties. The AHEAD project systematically investigated fundamental issues governing the formation and properties of CCA films: (i) phase stability, (ii) phase evolution and corresponding kinetics including the influence of composition, defects (dislocations, interfaces, grain boundaries) and dimensional constraints (film thickness, patterning) on phase stability, (iii) grain growth and texture, (iv) dewetting kinetics and morphologies, (v) temperature and microstructure-caused stress evolution, (vi) plastic deformation, (vii) thermo-mechanical fatigue mechanisms and lifetimes. Thin films were chosen to investigate phase stability because kinetic barriers are easier to overcome compared to in bulk materials. Combinatorial synthesis and high-throughput characterization were used to efficiently explore a largely extended compositional space in the Co-Cr-Fe-Mn-Ni system, rather than one or two single compositions using the alternative bulk materials approach. Kinetics driving the phase transformations of CoCrFeMnNi was manipulated by introducing controlled 1-dimensional defects (dislocations), 2-dimensional defects (grain boundaries, heterointerfaces), and dimensional constraints (film thickness). At 250°C, there is decomposition into 4 phases, with atom probe tomography results pointing to the role of grain boundaries as nucleation sites. Below 700°C an excess of Cr leads to two phases (fcc and sigma) while an excess in Ni stabilizes the fcc phase. A slight depletion of Cr from the iso-concentration destabilizes the sigma phase, achieved either by tuning the nominal composition or by oxidizing the Cr after deposition. MD calculations and thermodynamic modelling of surfaces and grain boundaries have produced new results, finding preferential adsorption of different species. Microstructure and morphological stability investigation of annealed films showed that the quaternary fcc phase behaves like singlecomponent fcc metals, in terms of the orientation relationship of the film on the sapphire substrate. Annealing produces abnormal grain growth within the film, leading to a microstructure with very large, mm-scale grains. The kinetics of the grain boundary motion is much faster and the grains are much bigger than for single-component films at the same equivalent temperature. Mechanical testing reveals that the films behave globally elastically with a stress increase of 1.3 GPa upon cooling from 500°C to room temperature. Repeated thermal cycling leads to a change in texture, twinning, and local damage evolution (especially surface roughening) due to accumulated microplasticity after 700 cycles between 25 and 500°C. Hardness measurements of pristine and Cr-oxide dispersion strengthened CoCrFeNi thin films revealed that the lack in high temperature strength of this alloy can be improved by dispersoids. Thermal fatigue studies show surfaces with a combination of facets and slip steps, and heavily deformed regions that recrystallized into grains of the order of a few microns.
Publications
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Accelerated atomic-scale exploration of phase evolution in compositionally complex materials. The Royal Society of Chemistry, Materials Horizon 5, 86-92 (2018)
YJ. Li, A. Savan, A. Kostka, HS. Stein, A. Ludwig
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Modeling grain boundary and surface segregation in multicomponent high entropy alloys. Physical Review Materials, 3(5), 054004 (2019)
P. Wynblatt, D. Chatain
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Correlative chemical and structural investigations of accelerated phase evolution in a nanocrystalline high entropy alloy. Scripta Materialia 183, 122–126 (2020)
YJ. Li, A. Kostka, A. Savan, A. Ludwig
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Design of complex solid solution electrocatalysts by correlating configuration, adsorption energy distribution patterns and activity curves. Angew. Chem. Int. 59, 5844 (2020)
T. Löffler, A. Savan, H. Meyer, M. Meischein, V. Strotkötter, A. Ludwig, W. Schuhmann
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Microstructure evolution and thermal stability of equiatomic CoCrFeNi thin films on (0001) alpha-Al2O3. Acta Materialia, 200, 908–921 (2020)
Y. Addab, M.K. Kini, B. Courtois, A. Savan, A. Ludwig, N. Bozzolo, C. Scheu, G. Dehm, D. Chatain
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Phase decomposition in a nanocrystalline CrCoNi Alloy. Scripta Materialia, 188 259-263 (2020)
YJ. Li, A. Kostka, A. Savan, A. Ludwig
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Structure Zone Investigation of Multiple Principle Element Alloy Thin Films as Optimization for Nanoindentation Measurements. Materials 13, 2113 (2020)
A. Savan, T. Allermann, X. Wang, D. Grochla, L. Banko, Y. Kalchev, A. Kostka, J. Pfetzing-Micklich, A. Ludwig
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Nanocrystalline equiatomic CrFeCoNi alloy thin films: Are they single phase fcc?. Surface and Coatings Technology 410, 126945 (2021)
M.K. Kini, S. Lee, A. Savan, B. Breitbach, Y. Addab, W. Lu, M. Ghidelli, A. Ludwig, N. Bozzolo, C. Scheu, D. Chatain, G. Dehm
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Structure and hardness of in situ synthesized nano-oxide strengthened CrFeCoNi high entropy alloy thin films. Scripta Materialia 203, 114044 (2021)
S. Lee, D. Chatain, C.H. Liebscher, G. Dehm
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Surface segregation in multicomponent high entropy alloys: relation between atomistic simulations and simple analytical models. Computational Materials Science 187, 110101 (2021)
D. Chatain, P. Wynblatt
