Dislocation Motion in Single-Phase High-Entropy Alloys -- Theory and Simulation

高熵合金（High-entropy alloys， HEA）是一类令人兴奋的新型合金，被宽泛地定义为含有四种以上近等摩尔成分的金属固溶体。特别是高强度和良好的延展性以及高硬度、耐磨损和耐腐蚀的结合，使HEAs成为高性能结构应用的有前途的候选材料。HEAs优异的抗破坏性能通常归因于某种极端形式的固溶强化。然而，由于HEAs中“每个原子都是溶质原子”，传统的固溶硬化理论不能直接应用，分析和预测HEAs中位错运动所需的新颖理论概念尚未建立。该项目的目的是获得位错滑动运动的基本理解，从而获得位错塑性，在HEAs中，如何受到其独特的底层原子结构的影响，并建立一个理论框架来预测位错速度的应力和温度依赖以及伴随的塑性变形行为。为此，我们提出了一种多尺度建模方法，其中原子模拟用于表征位错移动的能量景观。利用统计物理学中发展的方法和理论概念对结果进行分析，以确定能源景观的相关特征和空间相关性。这些信息作为使用离散位错动力学（DDD）方法的位错介观模拟的输入，其中位错-晶格相互作用的原子尺度信息以随机钉住场的形式被纳入。这种原子和中尺度模拟方法的结合允许研究由复杂的、扩展的能量势垒控制的热激活位错运动，这是由于位错可能根据局部钉住的能量格局调整其形状而产生的。此外，蒙特卡罗模拟用于研究如何短程扩散可以通过改变局部能量景观的位错周围的静止导致老化。然后在中尺度框架中使用能量变化以及相关的长度和时间尺度来研究最近在HEAs中观察到的动态应变老化和plc样现象。最终，对单相fcc HEAs模型系统的研究将有助于开发一种方法框架，使基于其原子结构和组成的HEAs塑性变形行为的计算预测成为可能。这样的框架对于计算合金设计至关重要，这对于HEA系统尤其重要，因为在HEA系统中，大量的成分自由度使得通过实验试错方法进行传统合金优化变得特别具有挑战性。

High-entropy alloys (HEA), loosely defined as metallic solid solutions containing more than four elements in near-equimolar composition, represent an exciting new class of alloys. In particular the combination of high strength and good ductility as well as high hardness, wear and corrosion resistance makes HEAs promising candidate materials for high performance structural applications. The excellent failure resistance of HEAs is commonly attributed to some extreme form of solid solution strengthening. However, as in HEAs "every atom is a solute atom", conventional theories of solid solution hardening cannot be directly applied, and the necessary novel theoretical concepts for analyzing and predicting dislocation motion in HEAs have not yet been established. The aim of this project is to obtain a fundamental understanding of how dislocation glide motion, and hence dislocation plasticity in HEAs, is influenced by their unique underlying atomic structure, and to develop a theoretical framework for predicting the stress and temperature dependence of the dislocation velocity and the concomitant plastic deformation behavior. To this end, we propose a multi-scale modeling approach where atomistic simulations are used to characterize the energy landscape in which dislocations move. The results are analyzed by drawing on methods and theoretical concepts developed in statistical physics to identify the relevant features and spatial correlations of the energy landscape. This information serves as input for the mesoscopic simulation of dislocations using the discrete dislocation dynamics (DDD) method, where atomic-scale information on dislocation-lattice interactions is incorporated in the form of a stochastic pinning field. This combination of atomistic and mesoscale simulations methods allows for the study of thermally activated dislocation motion governed by complex, extended energy barriers, which result from the possibility of the dislocations to adjust their shape to the local pinning energy landscape. In addition, Monte Carlo simulations are used to study how short-range diffusion can lead to ageing by changing the local energy landscape around a dislocation at rest. The changes in energy and the associated length and time scales are then used in a mesoscale framework to investigate the dynamic strain aging and PLC-like phenomena, which have been recently observed in HEAs. Ultimately, this study on model systems for single-phase fcc HEAs will serve to develop a methodological framework which enables the computational prediction of the plastic deformation behavior of HEAs based on their atomic structure and composition. Such a framework is crucial for computational alloy design, which is of particular importance for HEA systems, where the large number of compositional degrees of freedom renders conventional alloy optimization by experimental trial-and-error approaches particularly challenging.