Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104.
Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR, China.
Proc Natl Acad Sci U S A. 2020 Mar 3;117(9):4533-4538. doi: 10.1073/pnas.1920504117. Epub 2020 Feb 18.
The grain-boundary (GB) mobility relates the GB velocity to the driving force. While the GB velocity is normally associated with motion of the GB normal to the GB plane, there is often a tangential motion of one grain with respect to the other across a GB; i.e., the GB velocity is a vector. GB motion can be driven by a jump in chemical potential across a GB or by shear applied parallel to the GB plane; the driving force has three components. Hence, the GB mobility must be a tensor (the off-diagonal components indicate shear coupling). Performing molecular dynamics (MD) simulations on a symmetric-tilt GB in copper, we demonstrate that all six components of the GB mobility tensor are nonzero (the mobility tensor is symmetric, as required by Onsager). We demonstrate that some of these mobility components increase with temperature, while, surprisingly, others decrease. We develop a disconnection dynamics-based statistical model that suggests that GB mobilities follow an Arrhenius relation with respect to temperature T below a critical temperature [Formula: see text] and decrease as [Formula: see text] above it. [Formula: see text] is related to the operative disconnection mode(s) and its (their) energetics. For any GB, which disconnection modes dominate depends on the nature of the driving force and the mobility component of interest. Finally, we examine the impact of the generalization of the mobility for applications in classical capillarity-driven grain growth. We demonstrate that stress generation during GB migration (shear coupling) necessarily slows grain growth and reduces GB mobility in polycrystals.
晶界（GB）迁移率将 GB 速度与驱动力联系起来。虽然 GB 速度通常与垂直于 GB 面的 GB 法线运动有关，但在 GB 上经常存在一个晶粒相对于另一个晶粒的切向运动；即，GB 速度是一个向量。GB 运动可以由 GB 上化学势的跳跃或平行于 GB 面的剪切驱动；驱动力有三个分量。因此，GB 迁移率必须是张量（非对角分量表示剪切耦合）。我们在铜的对称倾斜 GB 上进行分子动力学（MD）模拟，证明 GB 迁移率张量的所有六个分量都不为零（迁移率张量是对称的，这是 Onsager 所要求的）。我们证明，其中一些迁移率分量随温度升高而增加，而另一些则令人惊讶地降低。我们开发了一种基于断开动力学的统计模型，表明在临界温度 [Formula: see text] 以下，GB 迁移率随温度 T 呈指数关系，在其之上则降低。[Formula: see text]与操作中的断开模式及其（它们）的能量有关。对于任何 GB，哪种断开模式占主导地位取决于驱动力的性质和感兴趣的迁移率分量。最后，我们研究了迁移率的推广对经典毛细驱动晶粒生长应用的影响。我们证明了在 GB 迁移过程中（剪切耦合）产生的应力必然会减缓晶粒生长并降低多晶体中的 GB 迁移率。
