PHASE TRANSITIONS OF GRAIN BOUNDARIES.

We report the results of an atomistic computer simulation study of the thermodynamic properties of a number of high angle grain boundaries. The technique employed was an isothermal-isostress formulation of molecular dynamics, and the boundaries studied were two and three-dimensional symmetric tilt boundaries in which the atoms were interacting through a Lennard-Jones potential. Excess thermodynamic properties of the boundaries were calculated through a large range of temperatures up to the bulk melting point using perfect crystal reference systems. Special attention was focussed on calculation of excess Gibbs free energies. All of the boundaries underwent phase transitions which appear to be first order, at temperatures significantly below the bulk melting point. In this transition the crystalline boundaries are replaced with highly disordered, liquid-like layers. The boundary transition temperature is inversely proportional to the boundary excess volume.