制约高强度金属结构材料发展的瓶颈是其低的均匀拉伸应变和韧性，常常通过牺牲强度来换取塑性和韧性。利用晶粒尺寸跨尺度分布形成异构，变形时在异构界面引入应变梯度，实现异质变形诱导加工硬化，从而在高强度下获得满意的塑性。本文主要研究了一种异构单相面心立方高熵合金的准静态/动态变形行为与微结构演化，阐明了异构诱导强韧化的微结构机制。

　　通过冷轧后进行不同温度的退火处理，制备了有着不同非均匀晶粒结构的高熵合金CrMnFeCoNi。在一个大的应变率范围(2×10^-3-5×10⁴ s^-1)内，使用帽形试样对这些不同微结构的高熵合金的剪切变形行为进行了研究。获得了不同微结构试样对应的应变硬化指数(n)和应变率敏感性系数(m)。在较低应变率(2×10^-3-1×10¹ s^-1)，所有微结构在剪应变高达8时，均未观察到剪切局部化，而在动态剪切加载(5×10⁴ s^-1)时，不同微结构均在不同的临界剪应变观察到了应力下降和剪切局部化的发生。考虑到应变硬化、应变率硬化与热软化之间的相互竞争，提出了一个新的公式用来估计动态剪切加载下的临界剪应变，得出的结果与实验值有着较好的一致性。基于显微硬度测试，在相同的中断应变，动态剪切加载时，微结构演化带来的应变硬化，远大于准静态加载时。这可能是由于动态剪切变形时，更加充分的晶粒细化以及诱导出的多级变形纳米孪晶。

    It is well known that there exists a trade-off between strength and ductility/toughness in metals and alloys, and high strength metals with homogeneous structures usually have a low ductility/toughness. While, hetero-structures can be obtained by deploying grain sizes spanning the nanometer to micrometer range, strain gradients can be induced at the hetero-interfaces, resulting in extra hetero-deformation induced hardening and excellent synergy of strength and ductility. In the present study, the quasi-static/dynamic deformation behaviors and microstructural evolutions of a single-phase face-centered cubic high-entropy alloy (HEA) with hetero-structures have been studied, and the corresponding strengthening/toughnening mechanisms by hetero-structures have been revealed.

    CrMnFeCoNi high-entropy alloys (HEA) with various heterogeneous grain structures have been produced using cold rolling followed by critical annealing at various temperatures. Shear deformation behaviors of this HEA with various microstructures at a wide range of strain rates (2×10^-3-5×10⁴ s^-1) have been characterized using hat-shaped specimens. Strain hardening exponent and strain rate sensitivity have been obtained for various microstructures. No shear localization was observed up to shear strain of 8 for all microstructures under lower strain rates (2×10^-3-1×10¹ s^-1), while stress drop and shear localization were found to occur at various critical shear strains for various microstructures under dynamic shear loading (5×10⁴ s^-1). A new formula, considering competition of strain hardening and strain rate hardening against thermal softening, was proposed to estimate the critical shear strains under dynamic shear loading and the predicted results were found to be in a fairly good agreement with the experimental data. Based on micro-hardness testing, the strain hardening due to the microstructure evolution was found be much stronger under dynamic shear loading than that under quasi-static loading at the same interrupted shear strain, which can be attributed to the more efficient grain refinement and the triggered hierarchical deformation nanotwins under dynamic shear deformation.