Analysis of the Stability of High Entropy Alloys by Dewetting of Thin Films

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High-Entropy Alloys (HEAs) are a new class of materials, with the potential to realize exceptional combinations of mechanical, electrical and thermal properties unachievable by conventional alloys. They contain about equal amounts of at least five elements, and can surprisingly crystallize as single fcc or bcc solid solutions. A better knowledge of the metallurgical and physical behavior of those materials is a prerequisite to understand their property combinations. Eventually it will foster the development of reliable HEA thin films for innovative technology drivers such as the microelectronic industry, and of bulk HEAs. Since HEA stability is suggested to depend strongly on material defects, a consortium of experts will address HEA thin film phase stability by systematically manipulating 1D and 2D defects. The defect density in the films will be controlled by (i) nano-/microscale deformation to introduce gradients in the dislocation density, (ii) altering the interface structure by employing amorphous and single crystalline substrates and (iii) varying film growth and processing conditions to manipulate the grain size. The joint French-German team merges the recognized expertise in different fields of materials science of four complementary partners (combinatorial thin film synthesis, microstructure physics, thermodynamics and mechanics of materials) to investigate the following fundamental issues on HEA films: (i) HEA phase stability, (ii) phase evolution and corresponding kinetics including 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-stress evolution, (vi) plastic deformation, (vii) thermo-mechanical fatigue mechanisms and lifetimes. AHEAD will focus on thin films of bcc-AlCrFeCoNi and fcc-MnCrFeCoNi as generic examples for two different crystal structures. Film synthesis, a key issue, will be performed by combinatorial deposition by one partner. The three other partners will address the mechanisms controlling the phase, microstructure and morphological stability of the HEA films during isothermal and cyclic thermo-mechanical annealing as a function of their thickness, composition and defect structure. Advanced experimental tools - from combinatorial thin film deposition and high-throughput characterization to multiscale quantitative microstructure analysis and miniaturized mechanical techniques - will be used and combined with the complementary expertise of the four partners, to provide for the first time a large and consistent data set on the thermodynamic, mechanical, microstructural and morphological stabilities of two types of HEA films.Within the AHEAD project, 2 PhD students and 2 postdoctoral fellows will benefit from a multidisciplinary working conditions, and international exchanges.