  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.
