随着人类社会的不断进步，生产力日新月异的发展，当我们探寻未知的脚步不断向前迈进时，对结构材料使用环境的要求也越发苛刻。人们追求着同时拥有更高的强度与更好韧性的晶体材料，但这两个指标往往被认为是此消彼长难以调和的。而近年来高熵合金——一类拥有完全全新设计思路材料的出现让研究者看到了进一步提升晶体材料承载能力的可能性。相较于传统晶体材料单一主元的设计方式，高熵合金需要至少五种元素作为主元，并以大致相等比例的分布，元素数目的增加使得即便在同一晶体结构下材料性质也会因为局部元素组合的不同而出现差异。原子排列的随机性将会带来层错能等一系列物理性质的随机涨落，产生丰富的变形机制和独特的力学性能。

在本工作中，我们将试图揭示多晶体系中晶粒尺寸和层错结构之间的关系，及其控制CoNiCrFeMn高熵合金屈服强度和流动应力的能力。我们将在相同应变率不同温度下对不同晶粒尺寸的CoNiCrFeMn纳米多晶高熵合金进行的单轴拉伸分子动力学模拟，模拟当中涉及到的平均晶粒尺寸从最小3.0 nm到最大48.6 nm不等。

该高熵合金材料在晶粒尺寸小于48.6 nm的情况下，屈服应力符合反常霍尔佩奇效应，而流动应力从15.0 nm起到48.6 nm都和霍尔佩奇效应相吻合，即流动应力随着晶粒尺寸的缩减而上升，但幅度异乎寻常的小。晶粒尺寸在15.0 nm以下的时候流动应力满足反常霍尔佩奇效应，塑性变形主要由晶界的膨胀与滑移变形为主导，应变增加到10%，晶粒内部只出现了极其有限的位错与层错结构，亦没有发生位错堆积的现象。当晶粒尺寸超过15.0 nm之后模型内部由晶界运动为主的缺陷主导形式逐渐转变为扩展位错及层错结构的生成和滑移为主的缺陷主导形式。残余在晶粒内部的层错结构既提供了塑性，同时也能够阻碍其他方向运动上分位错的运动。

为了更为清晰的认识层错阻碍作用对宏观性能的影响，我们测算了平均层错间距以及位错密度随着应变增加的变化过程，晶粒尺寸的增加将会带来平均层错间距的下降，这弥补了位错密度下降所带来的影响，使得该体系下平均流动应力同传统金属材料分子动力学模拟的结果相比有显著差异，其平均流动应力并不会随着晶粒尺寸从15.0 nm上升到48.6 nm就出现激烈的下降，我们认为其原因在于温和的层错强化硬化机制与晶界的强硬化机制相耦合所导致的。我们预计该层错强化机制能够在FCC结构的纳米多晶高熵合金中普遍的存在。本文的结果可能为以层错能随机分布为特征的高熵合金强化和增韧设计提供理论基础和工程指导。

With the continuous progress of human society and the rapid development of productivity, when we explore the unknown and move forward, the requirements for the use environment of structural materials are becoming more and more stringent. People pursue crystal materials with higher strength and better toughness at the same time, but these two indicators are often considered to be irreconcilable. In recent years, the emergence of high entropy alloys (HEAs), a kind of materials with completely new design ideas, has made researchers see the possibility of further improving the bearing capacity of crystal materials. Compared with the traditional design method of single principal element of crystal materials, high entropy alloys need at least five elements as principal elements, which are distributed in roughly equal proportion. The increase of the number of elements makes the material properties different even under the same crystal structure due to the different combination of local elements. The randomness of atomic arrangement will bring random fluctuations of a series of physical properties such as stacking fault energy, resulting in rich deformation mechanism and unique mechanical properties.

In this work, we will try to reveal the relationship between grain size and stacking fault structure in polycrystalline system and its ability to control the yield strength and flow stress of CoNiCrFeMn HEA. We will conduct uniaxial tensile molecular dynamics simulation of CoNiCrFeMn nano polycrystalline high entropy alloys with different grain sizes at different temperatures at the same strain rate. The average grain size involved in the simulation ranges from 3.0 nm to 48.6 nm.

Under the grain size below 48.6 nm, the yield stress of the HEA material conforms to the reverse Hall-Petch effect, while the flow stress is consistent with the Hall-Petch effect from 48.6 nm, that is, the flow stress increases with the reduction of grain size, but the range is unusually small until the grain size reaches 15.0 nm. When the grain size is below 15.0 nm, the model is mainly dominated by the expansion and slip deformation of grain boundary. When the strain increases to 10%, there are only very limited dislocation and stacking fault structures in the grain, and there is no dislocation accumulation. When the grain size exceeds 15.0 nm, the dominant form of defects in the model gradually changes from grain boundary movement to extended dislocation, and a large number of residual stacking faults. The residual stacking fault structure in the grain not only provides plasticity, but also acts as a defect to hinder the advance of partial dislocations moving in other directions.

In order to better understand the impact of stacking fault hindrance on the macro properties, we measured the change process of average stacking fault spacing and dislocation density with the increase of strain. The increase of grain size will lead to the decrease of average stacking fault spacing, which makes up for the impact of the decrease of dislocation density, making the average flow stress in this system significantly different from the results of traditional metal molecular dynamics simulation, The average flow stress does not decrease sharply with the increase of grain size from 15.0 nm to 48.6 nm. We believe that the reason is due to the coupling of mild stacking fault strengthening and hardening mechanism and strong hardening of grain boundary. We expect that the stacking fault strengthening mechanism can be widely existed in FCC structured nano polycrystalline high entropy alloys. The results of this paper may provide basis and engineering guidance for the strengthening and Toughening Design of high entropy alloys characterized by random distribution of stacking fault energy.