Diffusion in high entropy alloys: Development and application of an experiment-ab initio approach

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Reliable control over phase decomposition and creep behavior of high entropy alloys (HEAs) represents an enormous challenge in view of their multi-principal element nature and presumably slow diffusion. The present project DIFFINITIO aims at tackling this challenge from a fundamental perspective by developing and applying an integrated experiment-ab initio approach for the determination of accurate diffusion coefficients in HEAs. The proposal relies on the leading and unique expertise of the applicants in the fields of radiotracer diffusion measurements and finite temperature ab initio computations. With our investigations we will provide fundamental insights into the basic atomistic mechanisms of diffusion in HEAs, quantifying the impact of the multi-element environment, and scrutinizing postulated concepts as the one of sluggish diffusion.We focus on a specific, non-magnetic material system, the AlHfScTiZr HEA. AlHfScTiZr crystallizes on the hcp lattice and may develop sublattices ordering depending on the Al concentration. The temperature dependencies of the self-diffusion rates of all principal elements (with Zn as Al substitute) in the selected AlHfScTiZr HEA will experimentally be determined and evaluated from the DFT-based barrier calculations and the cluster expansion-based kinetic Monte carlo simulations enabling direct a direct quantification of the correlation and short-range ordering effects. The sublattice ordering is a fascinating feature because it affects strongly self-diffusion and solute diffusion rates. Preliminary investigations show clearly that small transition-metal elements like Ni are ultrafast interstitial diffusers in this alloy. Their diffusion rates are higher than those expected for self-diffusion by four orders of magnitude. This is counterintuitive and the mechanism behind the ultrafast diffusers is not clarified so far. As a part of the proposal, we suggest to develop a unique and extremely sensitive experimental tool for addressing the early stages of phase decomposition and formation and evolution of ordering, making use of the phenomenon of ultrafast diffusion. A significant advance in the basic understanding of fundamental HEA concepts is expected with the accomplishment of the DIFFINITIO project, especially in view of the present absence of reliable diffusion data for this material class in the literature.