Diffusion in BCC multi-principal element alloys from experiment and ab initio: Impact of thermal vibrations and chemical complexity

由于先进的技术应用，高浓度合金，特别是所谓的多主元素合金（mpea）中的扩散问题越来越受到人们的关注。这种合金表现出独特的特点，即缓慢扩散，严重影响各种材料的性能，如微观结构演变、相稳定性、蠕变行为、辐射耐受性。mpea的独特之处在于其化学性质的复杂性，同时也给实验和理论研究带来了巨大的挑战。从理论上讲，晶格热振动与局部环境的强异质性及其对mpea中扩散的影响的作用还远远没有被理解，这阻碍了对扩散率的定量预测，有时甚至是定性预测。基于斯图加特（模拟）和德国<s:1>国家科学技术大学（实验）团队的共同努力，这两个团队都在各自的研究领域建立了领导者，并通过长期合作相互联系，本项目旨在开发准确和通用的方法来调查和从根本上理解复杂mpea中的扩散。利用先进的放射性示踪技术和几种高度精确的从头计算模拟技术的结合，我们将在本项目中重点研究MoNbTaVW系统的BCC mpea中空缺介导的扩散。由于组成元素的高熔点，BCC mpea具有优异的高温强度，可能超过传统的高温合金。事实上，MoNbTaVW系统被认为是具有优越高温性能的下一代结构和功能合金。通过对具有不同化学复杂性的BCC mpea的精确测量和模拟示踪剂扩散系数的联合、交互分析，例如随机固溶体和具有短/长顺序（B2）的合金，我们不仅将提供一套广泛的扩散数据，具有前所未有的准确性，而且还将对热振动、化学相互作用、元素取代的影响进行全面的物理理解。化学排序以及它们之间的相互作用。期望在mpea中扩散和相关有序/无序趋势的基本理解方面取得重大进展。

Diffusion in concentrated alloys, especially in so-called multi-principal element alloys (MPEAs), attracts increasing attention due to advanced technological applications. Such alloys exhibit unique features prominently sluggish diffusion that critically affect various materials properties, e.g., microstructure evolution, phase stability, creep behavior, radiation tolerance. The unique features of MPEAs generally originate from the significant chemical complexity which, at the same time, brings about tremendous challenges for both experimental and theoretical studies. Theoretically, the role of lattice thermal vibrations in combination with the strong heterogeneity of local environments and their impact on diffusion in MPEAs are far from being understood, impeding a quantitative, and sometimes even a qualitative, prediction of diffusivities. Based on a concerted effort of teams from Stuttgart (simulation) and Münster (experiment) both established leaders in their respective research fields and connected with each other through a long-standing collaboration—the present project aims at developing accurate and versatile approaches to investigate and fundamentally understand diffusion in complex MPEAs. Utilizing a combination of an advanced radiotracer technique together with several highly accurate ab initio-informed simulation techniques, we will focus in the present project on vacancy-mediated diffusion in BCC MPEAs of the MoNbTaVW system. Owing to the high melting points of the constituent elements, BCC MPEAs were reported to possess outstanding high-temperature strength that may surpass the conventional superalloys. Indeed, the MoNbTaVW system is considered as a next-generation structural and functional alloy with superior high-temperature performance. Through the joint, interactive analyses of accurately measured and simulated tracer diffusivities for a collection of BCC MPEAs featuring different chemical complexities, e.g., random solid solutions and alloys with short-/long-range order (B2), we will not only provide an extensive set of diffusion data with unprecedented accuracy but also a thorough physical understanding of the impact of thermal vibrations, chemical interactions, element substitutions, and chemical ordering as well as their interplay. A significant advance in the fundamental understanding of diffusion and related ordering/disordering tendencies in MPEAs is expected.