  The formation and migration of disconnections (line defects constrained to
the grain boundary (GB) plane with both dislocation and step character) control
many of the kinetic and dynamical properties of GBs and the polycrystalline
materials of which they are central constituents. We demonstrate that GBs
undergo a finite-temperature topological phase transition of the
Kosterlitz-Thouless (KT) type. This phase transition corresponds to the
screening of long-range interactions between (and unbinding of) disconnections.
This phase transition leads to abrupt change in the behavior of GB migration,
GB sliding, and roughening. We analyze this KT transition through mean-field
theory, renormalization group theory, and kinetic Monte Carlo simulations, and
examine how this transition affects microstructure-scale phenomena such as
grain growth stagnation, abnormal grain growth and superplasticity.
