Coupling Grain Boundary Motion to Shear Deformation
Yuri Mishin, A Suzuki, John W. Cahn
Molecular dynamics (MD) simulations confirm that normal grain boundary (GB) motion must often be coupled to tangential translation of grains and will then produce shear deformation of the lattice traversed by the GB. Conversely, shear stresses applied to a GB can induce its normal motion. Using  symmetrical tilt GBs in copper as a model, the coupling factor Beta between the GB motion and grain translations has been calculated by MD simulations over the entire misorientation range and a wide range of temperatures. The coupling factor is multi-valued, can be positive or negative, and shows an abrupt switch from one branch to another at a tile angle of about 35 degrees. At high temperatures the response sliding is observed for low-angle GBs up to near the melting point.A geometric model of coupling proposed in this work predicts the misorientation dependence of Beta in excellent agreement with MD results and relates the multi-valued character of Beta to the point symmetry of the crystal. Two kinds of low-angle GBs with different dislocations occur when the tilt angle is small and again when it approaches 90 degrees. In these limits, the multiplicity of Beta is explained by different Burgers vectors of the dislocations. The results of this work are summarized as a temperature-misorientation diagram of mechanical responses of GBs.