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

高浓度合金，特别是所谓的多主元素合金（MPEAs）中的扩散，由于先进的技术应用而引起越来越多的关注。这样的合金表现出独特的特征，显著地缓慢扩散，其严重影响各种材料性质，例如，组织演变、相稳定性、蠕变行为、耐辐照性。MPEAs的独特性通常源于其化学结构的复杂性，同时也给实验和理论研究带来了巨大的挑战。从理论上讲，晶格热振动的作用，结合当地环境的强烈异质性和它们对MPEAs中扩散的影响还远未被理解，阻碍了定量，有时甚至是定性的扩散率预测。基于来自斯图加特（模拟）和明斯特（实验）的团队的共同努力，他们都在各自的研究领域建立了领导者，并通过长期的合作相互联系，本项目旨在开发准确和通用的方法来研究和从根本上理解复杂MPEAs中的扩散。利用先进的放射性示踪剂技术与几个高度准确的从头开始知情的模拟技术相结合，我们将集中在本项目的空缺介导的扩散BCC MPEAs的MoNbTaVW系统。由于组成元素的高熔点，据报道BCC MPEAs具有可能超过常规高温合金的优异高温强度。事实上，MoNbTaVW系统被认为是具有上级高温性能的下一代结构和功能合金。通过对一系列具有不同化学复杂性的BCC MPEAs的精确测量和模拟示踪剂扩散率的联合交互式分析，随机固溶体和合金与短期/长期秩序（B2），我们将不仅提供了一个广泛的扩散数据与前所未有的准确性，但也热振动，化学相互作用，元素替代和化学有序的影响，以及它们的相互作用的透彻的物理理解。一个显着的进步，在MPEAs的扩散和相关的有序/无序的趋势的基本理解。

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.