  We study the Grain Boundary (GB) migration based on the underlying
disconnection structure and mechanism. Disconnections are line defects that lie
solely within a GB and are characterized by both a Burgers vector and a step
height, as set by the GB bicrystallography. Multiple disconnection modes can
nucleate, as determined by their formation energy barriers and temperature, and
move along the GB under different kinds of competing driving forces including
shear stress and chemical potential jumps across the GBs. We present a
continuum model in two dimensions for GB migration where the GB migrates via
the thermally-activated nucleation and kinetically-driven motion of
disconnections. We perform continuum numerical simulations for investigating
the GB migration behavior in single and multi-mode disconnection limits in both
a bicrystal (under two types of boundary conditions) and for a finite-length GB
with pinned ends. The results clearly demonstrate the significance of including
the coupling and competing between different disconnection modes and driving
forces for describing the complex and diverse phenomena of GB migration within
polycyrstalline microstructures.
