The relation between grain-boundary structure and sliding resistance

During sliding, the grain-boundary (GB) energy depends on the atomic structures produced during relative translation of the two grains. The variation in the GB energy within the two-dimensional boundary unit cell (BUC) constitutes the GB-gamma surface. Maxima in the slope of the gamma surface determines the sliding resistance, that is the stress required to move the system over the lowest saddle points along a particular path within the BUC. In this paper we present the results of an atomistic study of the gamma surfaces for two types of boundaries in a fcc metal. One of the boundaries is a Sigma = 11, (110) {131} which is a low-energy boundary and has a simple gamma surface with a single stable configuration located at the corners and centre of the BUC. The resistance to sliding was determined by chain-of-states methods along four shear vectors connecting equivalent states within the BUC and is found to be very high in all cases. The asymmetric, Sigma = 11, (110) {252}-{414} GB, has a higher GB energy and its gamma surface is much more complex, with distinctly different structures appearing at various locations in the BUC. At certain locations, more than one structure is found for the asymmetric GB. Although complex, a chain-of-states calculation along one path across the BUC suggests that the shear strength of this GB is also quite high. Extrinsic GB dislocations are found to lower the resistance to shear considerably and, therefore, to perform the same role in shear of GBs as do glide dislocations in slip of the lattice. The existence of multiple configurations has significant implications for the interaction of lattice dislocations with GBs, the core structure of GB dislocations, the temperature dependence of GB properties, and the GB sliding resistance, which we discuss.