  Grain boundary (GB) mobility has been conventionally computed as a single
value; however, a recent study has suggested that GB mobility should be
expressed as a tensor. In this work, by using atomistic simulations, the
concept of GB mobility being applied to the shear direction was re-examined and
it is found that it follows the same physical rule as the conventionally
defined GB mobility based on the normal direction. The interface random walk
method was then used to compute the intrinsic GB mobility tensor at the
zero-driving force limit. In order to compute the off-diagonal elements of the
intrinsic GB mobility tensor, a shear coupling strength S is introduced in this
study, which we believe can better reflect the intrinsic characteristics of a
GB for its coupling trend between the normal and shear motion than the widely
used shear coupling factor. Furthermore, the effect of temperature and external
driving force on the GB mobility tensor, especially on its symmetry, was
systematically investigated. It is shown that the GB mobility in either the
normal or shear direction can show a non-Arrhenius type dependence on the
applied driving force, which is similar to the widely reported non-Arrhenius
(or anti-thermal) dependence of GB mobility on temperature. Accordingly, the
classical GB migration equation was adapted to describe the diverse variation
of GB mobility due to changes in both temperature and driving force.
