Migration and Mobility of Low-Angle Grain Boundaries

Abstract

The stress-induced migration of low-angle grain boundaries (LAGBs) was investigated using a combination of discrete dislocation dynamics simulations and analytical arguments. The LAGBs were described using arrays of lattice dislocations, the parameters of which may be related to the macroscopic grain boundary parameters via Frank's formula. We examined the migration of low-angle tilt boundaries consisting of one and two sets of parallel dislocations, as well as low-angle mixed boundaries composed of two sets of intersecting dislocations. In addition to the migration of LAGBs comprising of only geometrically necessary (intrinsic) dislocations, we also investigated the migration of LAGBs containing extrinsic dislocations.

For all of these LAGBs, the simulations indicate that the migration velocity is proportional to the applied stress. The migration of low-angle tilt boundaries composed of one set of intrinsic dislocations may be dislocation-climb limited due to pinning by extrinsic dislocations that move with the boundary via dislocation climb mechanism. In agreement with experiments, the effective migration mobility for such boundaries is independent of boundary misorientation.

In general, low-angle tilt boundaries composed of two sets of parallel dislocations with dissimilar Burgers vectors are faceted. Although these boundaries can migrate via a combination of dislocation glide/climb mechanism at higher temperatures, their migration is dislocation-climb limited. In the absence of extrinsic dislocations, the migration mobility for these boundaries is inversely proportional to boundary misorientation.

Simple low-angle mixed boundaries migrate by a combination of dislocation glide/ climb mechanism, but their migration by no means dislocation-climb limited. Moreover, the migration (perpendicular to the boundary plane) of these boundaries was observed to be coupled to the motion parallel to the boundary plane (tangential motion). Depending on the constituent dislocation structure of the boundary, either the migration or tangential velocity of such boundaries can switch direction at sufficiently high dislocation climb mobility. Isolated from extrinsic dislocations, the mobility of simple low-angle mixed boundaries is inversely proportional to boundary misorientation, Finally, we show that not every component of the mobility tensor for such low-angle mixed boundaries can be successfully inferred from stress-induced migration.