Chapter 5 - The Mobility and Migration of Boundaries

Abstract

The purpose of this chapter is to examine the models and mechanisms of boundary migration and in particular the mobilities of boundaries, in order to provide a foundation for the discussions of recovery, recrystallization, and grain growth. The migration of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB) plays a central role in the annealing of cold worked metals. Boundary migration involves atomistic processes occurring rapidly, at high temperatures, and under conditions that are far from equilibrium. Low angle boundary migration occurs during recovery and during the nucleation of recrystallization, and high angle boundary migration occurs both during and after primary recrystallization. Low angle and high angle grain boundaries migrate by means of atomistic processes, which occur in the vicinity of the boundary. The mechanism of boundary migration depends on several parameters, including the boundary structure, which, in a given material, is a function of misorientation and boundary plane. It also depends on the experimental conditions, in particular the temperature and the nature and magnitude of the forces on the boundary. Measurements of boundary mobility are made by determining the velocity of a boundary in response to a well-defined driving force.