Capillarity-driven shrinkage of grains with tilt and mixed boundaries studied by molecular dynamics

Abstract

Molecular dynamics simulations of isolated cylindrical grains in aluminum shrinking under capillary forces were carried out to systematically study the effect of grain boundary character on grain shrinkage behavior. Grains with pure tilt and various mixed tilt-twist 14.25°〈100〉 boundaries were examined. The simulation results confirmed that the shrinking grains with pure tilt boundary simultaneously rotated towards higher misorientations. However, the results also showed that even a minor change of the grain boundary character from pure tilt to mixed tilt-twist is enough to avoid grain rotation. The observed kinetics of the shrinking grains was strongly dependent on the character of grain boundaries. The mobility of the mixed boundaries was estimated to be much higher than that of the pure tilt boundary. Also, the shrinkage kinetics of computed grains was found to vary with a decreasing grain size. The advanced stage of shrinkage was characterized by the stagnation or decrease of the rate of grain area change. This behavior of small grains was attributed to a lack of mechanisms for the elimination of the dislocations (boundary structural units) in the grain boundary. It is hypothesized that such shrinkage stagnation of nanosized grains (≤6 nm) can contribute to the thermal stability of nanocrystalline materials.