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

考虑到高熵合金的多主元素性质和可能的缓慢扩散，对其相分解和蠕变行为的可靠控制是一个巨大的挑战。目前的DIFFINITIO项目旨在通过开发和应用一种综合实验从头算方法来确定HEAs中精确的扩散系数，从基本角度解决这一挑战。该提案依赖于申请人在放射性示踪剂扩散测量和有限温度从头计算领域的领先和独特的专业知识。通过我们的研究，我们将对HEAs中扩散的基本原子机制提供基本的见解，量化多元素环境的影响，并仔细审查假设的概念作为缓慢扩散之一。我们专注于一种特殊的非磁性材料系统，AlHfScTiZr HEA。AlHfScTiZr在hcp晶格上结晶，并根据Al的浓度形成有序的亚晶格。在选定的AlHfScTiZr HEA中，所有主要元素（以Zn代替Al）的自扩散速率的温度依赖关系将通过基于dft的势垒计算和基于簇扩展的动力学蒙特卡罗模拟进行实验确定和评估，从而直接量化相关性和短程有序效应。子晶格的有序是一个令人着迷的特性，因为它强烈地影响自扩散速率和溶质扩散速率。初步研究表明，Ni等小的过渡金属元素在该合金中是超快的间隙扩散器。它们的扩散速率比预期的自扩散速率高4个数量级。这是违反直觉的，超快扩散器背后的机制到目前为止还不清楚。作为提案的一部分，我们建议开发一个独特的和极其敏感的实验工具，以解决早期阶段的相分解和有序的形成和演变，利用超快扩散现象。随着DIFFINITIO项目的完成，对HEA基本概念的基本理解有望取得重大进展，特别是考虑到目前文献中缺乏可靠的扩散数据。

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.