The structures and physical properties of 2 tilt grain boundaries at various temperatures were studied by using the molecular dynamics method. It was found that, with increasing temperature, the grain boundary structure changed in steps. Three characteristic temperatures were identified with the changes. This observation was in agreement with previous experimental conclusions. Anelastic measurements had shown that, at about 400K, a grain boundary internal friction began to appear. It attained a maximum at about 550K. Upon increasing the temperature, the internal friction had dropped rapidly at about 623K. At about 723K, the internal friction had almost disappeared. It was suggested that, at the lowest temperatures, the grain-boundary atoms vibrated harmonically around their equilibrium positions and that their behavior was like that of atoms in an elastomer; thus supporting no significant internal friction. At the lowest transition, the grain boundary structure was relaxed and some grain boundary atoms underwent a slow diffusive motion. The grain boundary could no longer be regarded as an elastomer. Thus non-zero relaxation stresses and the appearance of an internal friction peak could occur. The present results showed that, above the second transition, the grain-boundary atoms were rather mobile. The viscosity, and therefore the internal friction, of the grain boundary would then be very small. Detailed information on the grain-boundary structure at high temperatures was very limited. It was found that, when the first transition was passed, some structural units of grain boundaries were destroyed and the grain boundaries became disordered. Above the second transition, some point defects (such as interstitials) aggregated to form disordered clusters. Above the third transition, the number of clusters decreased but some large disordered clusters formed.

Molecular Dynamics Study of Structural Changes in the Tilt Grain Boundary of Aluminium. W.Fan, Y.He, X.G.Gong: Philosophical Magazine A, 1999, 79[6], 1321-33