The role of the interface stiffness tensor on grain boundary dynamics

Grain boundary (GB) properties and associated anisotropies due to the boundary's geometric degrees of freedom (DOF) greatly influence many of the salient features of polycrystalline aggregates, and as a consequence the observable properties of the material. Through theoretical analysis and atomistic simulations, we show that when considering the GB plane normal DOF, the GB interface stiffness tensor plays a paramount role in a wide range of GB dynamical processes. As a demonstration, we examine the interface stiffness tensor of Σ3 GBs in nickel and show that the stiffness can be much larger in magnitude and more anisotropic than the GB energy itself. Moreover, it is found that a wide range of inclinations exhibit negative stiffness values, a signature of a structural instability. This is the first study to consider the complete spatial description of the GB stiffness tensor, where both angular variations describing the interface plane normal are explored. In broad terms, our results highlight the integral role that the stiffness tensor plays in GB interfacial phenomena, such as curvature-driven flow and faceting instabilities.