  Knowledge about grain boundary migration is a prerequisite for understanding
and ultimately modulating the properties of polycrystalline materials. Evidence
from experiments and molecular dynamics (MD) simulations suggests that the
formation and motion of disconnections is a mechanism for grain boundary
migration. Here, grain boundary migration is modeled using a multiphase field
model based on the principle of minimum dissipation potential with nonconvex
boundary energy, along with a stochastic model for thermal nucleation of
disconnection pairs. In this model, disconnections arise spontaneously in the
presence of an elastic driving force, and that their motion mediates boundary
migration. The effect is due to the fact that the formation of the
disconnections pairs results in a stress concentration, causing the elastic
driving force to exceed the threshold value and driving the propagation of the
disconnection along the interface. The model is applied to study the
propagation/annihilation of single disconnection pairs, the relaxation of a
perturbed interface, and shear coupling at various temperatures. The results
are consistent with the current understanding of disconnections, and capture
the effect of thermal softening.
