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

鉴于高熵合金(HEAs)的多元素性质和缓慢扩散，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-从头算方法来从根本上解决这一挑战，以确定HEA中准确的扩散系数。该提案依赖于申请者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，我们将提供对HEA中扩散的基本原子机制的基本见解，量化多元素环境的影响，并仔细审查假设的概念作为一个缓慢的扩散之一。我们专注于一个特定的，非磁性材料系统，AlHfScTiZr HEA。AlHfScTiZr在HCP晶格上结晶，并根据Al浓度的不同可能形成亚晶格有序化。通过基于密度泛函理论的势垒计算和基于团簇展开的动力学蒙特卡罗模拟，将实验确定和评估所选AlHfScTiZRHEA中所有主要元素(以锌为Al替代)的自扩散速率的温度依赖性，从而直接量化关联效应和短程有序效应。亚晶格有序是一个吸引人的特征，因为它强烈地影响自扩散和溶质扩散速率。初步研究表明，Ni等过渡金属元素是该合金的超快间隙扩散。它们的扩散速度比预期的自扩散速度高出四个数量级。这是违反直觉的，超高速扩散器背后的机制到目前为止还没有弄清楚。作为建议的一部分，我们建议开发一种独特的、极其灵敏的实验工具，利用超快扩散现象来研究相分解和有序化形成和演化的早期阶段。随着DIFFINITIO项目的完成，预计在对HEA基本概念的基本理解方面将取得重大进展，特别是考虑到目前文献中缺乏关于这一材料类别的可靠扩散数据。

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.