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

高熵合金(HEA)，宽泛地定义为含有近等摩尔组成的四种以上元素的金属固溶体，代表着一类令人兴奋的新合金。特别是，高强度和良好的延展性以及高硬度、耐磨性和耐腐蚀性的结合使HEAS成为高性能结构应用的理想候选材料。HEA优异的抗毁性通常归功于某种极端形式的固溶强化。然而，由于高能材料中“每个原子都是溶质原子”，传统的固溶体硬化理论不能直接应用，分析和预测高能材料中位错运动所需的新的理论概念还没有建立起来。本项目的目的是从根本上了解位错滑动运动以及位错塑性如何受到其独特的潜在原子结构的影响，并建立一个预测位错速度的应力和温度依赖性以及伴随的塑性变形行为的理论框架。为此，我们提出了一种多尺度模拟方法，其中原子模拟被用来描述位错在其中运动的能量格局。通过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。这些信息被用作使用离散位错动力学(DDD)方法对位错进行介观模拟的输入，其中关于位错-晶格相互作用的原子尺度信息以随机钉扎场的形式合并。这种原子模拟和介观模拟相结合的方法允许研究由复杂的、扩展的能垒控制的热激活位错运动，这是由于位错可能调整其形状以适应局部钉扎能量格局的结果。此外，蒙特卡罗模拟被用来研究短程扩散如何通过改变静止位错周围的局部能量来导致老化。能量的变化以及相关的长度和时间尺度随后被用在中尺度框架中来研究动态应变老化和类似PLC的现象，这是最近在HEA中观察到的。最终，对单相催化裂化高能气体燃料电池模型系统的研究将有助于开发一种方法框架，能够基于其原子结构和组成对高能气体燃料电池的塑性变形行为进行计算预测。这样的框架对于计算合金设计是至关重要的，这对于HEA系统尤其重要，在HEA系统中，大量的组成自由度使得通过实验试错法进行传统的合金优化特别具有挑战性。

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.