The transition from Hall-Petch (HP) to inverse Hall-Petch (IHP) behaviors associated with grain size reduction has been recognized for over two decades. However, the underlying mechanisms for such transition in high entropy alloys (HEAs) under dynamic loading, in which abundant deformation mechanisms could be activated either sequentially or simultaneously, remain unclear. Here, we investigate the HP to IHP transition in nanocrystalline CoCrFeMnNi HEAs under shock loading by examining their deformation mechanisms and flow stresses using large-scale molecular dynamics (MD) simulations. It is found that this transition is strongly dependent on the shock pressure as a result of the complex interplay among multiple competing deformation mechanisms, including the hardening mechanisms such as dislocations interactions and grain boundary (GB) blocking, as well as the softening mechanisms like phase formation, amorphization, GB thickening, and grain rotation. Moreover, there exists a critical shock pressure, which corresponds to the largest critical grain size for the HP-IHP transition. Below the critical shock pressure, the critical grain size increases with pressure due to a stronger hardening effect in grain interior (GIs), while above the critical pressure, the critical grain size first decreases and then undergoes a pressure-insensitive plateau before further decrease due to softening effects in GIs. A theoretical model that includes different deformation mechanisms is proposed for the first time to capture the shock pressure-dependent HP-IHP transition. Our work provides valuable guidance for optimizing the grain size of nanocrystalline HEAs for applications involving dynamic loadings.

中文翻译：

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揭示冲击载荷下纳米晶高熵合金中的 Hall-Petch 到逆 Hall-Petch 转变


二十多年来，人们已经认识到与晶粒尺寸减小相关的从霍尔-佩奇 (HP) 行为到逆霍尔-佩奇 (IHP) 行为的转变。然而，高熵合金（HEA）在动态载荷下的这种转变的基本机制仍不清楚，其中丰富的变形机制可以顺序或同时激活。在这里，我们通过使用大规模分子动力学 (MD) 模拟检查纳米晶 CoCrFeMnNi HEA 的变形机制和流动应力，研究了冲击载荷下纳米晶 CoCrFeMnNi HEA 的 HP 到 IHP 转变。研究发现，这种转变强烈依赖于冲击压力，这是多种竞争变形机制之间复杂相互作用的结果，包括位错相互作用和晶界（GB）阻塞等硬化机制，以及相变等软化机制形成、非晶化、晶界增厚和晶粒旋转。此外，存在临界冲击压力，它对应于 HP-IHP 转变的最大临界晶粒尺寸。在低于临界冲击压力时，由于晶粒内部（GI）的硬化效应更强，临界晶粒尺寸随着压力的增加而增加；而在高于临界压力时，临界晶粒尺寸首先减小，然后经历一个对压力不敏感的平台，然后由于晶粒内部（GI）的硬化效应而进一步减小。地理标志的软化效果。首次提出了包含不同变形机制的理论模型来捕获冲击压力相关的 HP-IHP 转变。我们的工作为优化涉及动态载荷的应用的纳米晶 HEA 晶粒尺寸提供了宝贵的指导。