Thermomechanical properties and microstructure of fcc and bcc high-entropy alloys

该合作方案的目的是通过实验、理论和模拟相结合的方法，研究晶格结构和微观组织对单相fcc Cantor(CoCrFeMnNi)和bcc Senkov(HfNbTaTiZr)合金族热力学性能的影响。因此，我们独特的方法是基于这些稳定的面心立方和体心立方晶格，以及对这些模型合金之间的化学参数空间以及子集组成方向的持续探索，从而有助于对HEA的总体理解。我们想要关注的是性质从稀释的固溶体到浓缩的高熵合金的转变，在这种情况下，传统的混合物规则，如维加德定律不适用。对于体心立方合金，我们将在HfNbTaTiZr(Senkov)及相关系统上应用化学扩散和示踪扩散等实验手段，分析其相稳定性以及固溶强化和互扩散系数。对于面心立方合金，我们将重点研究Ni-CoCrFeMnNi，Al-CoCrFeNi，以及Cr-CoCrFeMnNi混合系统，研究晶界在这些面心立方合金中的作用。在HEA中，晶界对扩散、偏析、相分离和力学性能的具体贡献大多是未知的。面心立方合金的加工将涉及合金的严重塑性变形(SPD)以及随后的热处理和热分析。SPD后的纳米结构HEAs非常适合于研究相稳定性、互扩散过程以及位错晶界相互作用。在中等温度下，晶界有望成为相分解或结构相形成的形核中心。因此，晶界研究可以作为分析HEA结构不稳定性早期阶段的一面镜子。示踪剂扩散将为分析块体HEAs(主要是bcc系统)中的原子迁移率，特别是沿晶界(fcc系统)的原子迁移率提供一个重要工具。从原子级(TEM)到微米级(高分辨率EBSD)的所有相关长度尺度上的微观结构研究都将揭示缺陷结构、晶格应变以及内部同质(晶界)和异相界面的特征，如偏析、位错堆积或应变场。在不同应变率下，使用宏观压缩和纳米压痕测试来分析合金的机械性能，测试范围从RT到1100°C。实验研究与原子模拟密切相关，原子模拟提供了对HEA中晶格稳定性、基本变形机制、扩散机制、缺陷结构，特别是缺陷形成和缺陷相互作用的更多洞察力。

The goal of this collaborative proposal is to study the influence of lattice structure and microstructure on the thermomechanical properties of the single phase fcc Cantor (CoCrFeMnNi) and bcc Senkov (HfNbTaTiZr) alloy families in a concerted approach including experiment, theory and simulation. Our unique approach is thereby based on these stable fcc and bcc lattices and a continuous exploration of the chemical parameter space in between these model alloys as well as in the direction of subset compositions, thus contributing to the understanding of HEAs in general. We want to focus on transitions in the properties going from a diluted solid solution to a concentrated high entropy alloys, where conventional rules of mixtures, like Vegards law do not apply. For the bcc alloys we will apply experimental approaches like chemical diffusion and tracer diffusion on the HfNbTaTiZr (Senkov) and related system and analyze the phase stability as well as the solid solution strengthening and interdiffusion coefficients.For the fcc systems, we will focus on Ni-CoCrFeMnNi, Al-CoCrFeNi, as well as the Cr-CoCrFeMnNi mixed systems studying the role of grain boundaries in these fcc HEAs. In HEAs, the specific contribution of grain boundaries with respect to diffusion, segregation, phase separation and mechanical properties are mostly unknown. Processing of the fcc-alloys will involve severe plastic deformation (SPD) of the alloys with subsequent heat treatment and thermal analysis. The nanostructured HEAs after SPD are ideally suited to study phase stability, interdiffusion processes as well as the dislocation grain boundary interaction. At intermediate temperatures, grain boundaries are expected to act as nucleation centers for phase decomposition or structural phase formation. Thus, grain boundary investigations can be utilized as a looking glass for analyzing the early stages of structural instabilities in HEAs. Tracer diffusion will present an important tool for analyzing the atomic mobilities in the bulk HEAs (mainly bcc system) but also specifically along the grain boundaries (fcc systems). Microstructure investigations on all relevant length scales from the atomic (TEM) to the micron-scale (high resolution EBSD) shall reveal defect structures, lattice strain and the characteristics of internal homo- (grain boundaries) and hetero-phase interfaces such as segregation, dislocation pile-ups or strain fields. The alloys will be analyzed with respect to their mechanical properties, using macroscopic compression and nanoindentation testing from RT up to 1100°C at various strain rates. The experimental studies are closely interlinked to atomistic modelling, which provides additional insight into the lattice stability, basic deformation mechanism, diffusion mechanisms, defect structures, and particularly defect formation and defect interactions in HEAs.