Grain boundary (GB) migration plays a crucial role in the thermal and mechanical responses of polycrystalline materials, particularly in ultrafine-grained and nano-grained materials exhibiting grain size-dependent properties. This study investigates the migration behaviors of a set of GBs in Ni through atomistic simulations, employing synthetic driving forces and shear stress. Surprisingly, the displacements of some shear-coupling GBs do not follow the widely assumed linear or approximately linear relation with time; instead, they exhibit a noticeable acceleration tendency. Furthermore, as the bicrystal size perpendicular to the GB plane increases, the boundary velocity significantly decreases. These observations are independent of the magnitude and type of driving force but are closely linked to temperature, unique to shear-coupling GBs that display a rise in the kinetic energy component along the shear direction. By adopting a specific boundary condition, the acceleration in migration and size effect can be largely alleviated. However, the continuous rise in kinetic energy persists, leading to the true driving force for GB migration being lower than the applied value. To address this, we propose a technique to extract the true driving force based on a quantitative analysis of the work-energy relation in the bicrystal system. The calculated true mobility reveals that the recently proposed mobility tensor may not be symmetric at relatively large driving forces. These discoveries advance our understanding of GB migration and offer a scheme to extract the true mobility, crucial for meso- and continuum-scale simulations of GB migration-related phenomena such as crack propagation, recrystallization, and grain growth.

中文翻译：

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剪切耦合晶界迁移中的异常加速度和尺寸效应

晶界（GB）迁移在多晶材料的热和机械响应中起着至关重要的作用，特别是在表现出晶粒尺寸依赖性特性的超细晶粒和纳米晶粒材料中。本研究通过原子模拟，采用合成驱动力和剪切应力，研究了一组晶界在镍中的迁移行为。令人惊讶的是，一些剪切耦合 GB 的位移并不遵循广泛假设的线性或近似线性关系。相反，它们表现出明显的加速趋势。此外，随着垂直于晶界平面的双晶尺寸增加，边界速度显着降低。这些观察结果与驱动力的大小和类型无关，但与温度密切相关，这是剪切耦合 GB 所特有的，它显示沿剪切方向的动能分量增加。通过采用特定的边界条件，可以大大减轻迁移的加速和尺寸效应。然而，动能的持续上升仍然存在，导致GB迁移的真正驱动力低于应用值。为了解决这个问题，我们提出了一种基于双晶系统中功能关系的定量分析来提取真实驱动力的技术。计算出的真实迁移率表明，最近提出的迁移率张量在相对较大的驱动力下可能不对称。这些发现增进了我们对晶界迁移的理解，并提供了一种提取真实迁移率的方案，这对于晶界迁移相关现象（例如裂纹扩展、再结晶和晶粒生长）的细观和连续尺度模拟至关重要。