Grain boundary sliding revisited : Developments in sliding over four decades

It is now recognised that grain boundary sliding (GBS) is often an important mode of deformation in polycrystalline materials. The developments in GBS over the last four decades, including the procedures available for estimating the strain contributed by sliding to the total strain in metals, ceramics and geological materials and the division into Rachinger GBS in conventional creep and Lifshitz GBS in diffusion creep, are reviewed. It is shown that Rachinger GBS occurs under two distinct conditions in conventional creep depending upon whether the grain size, d, is larger or smaller than the equilibrium subgrain size, lambda. A unified model is presented leading to separate rate equations for Rachinger GBS in power-law creep and superplasticity. It is shown that these two equations are in excellent agreement with experimental observations. There are additional recent predictions, not fully resolved at the present time, concerning the role of GBS in nanostructured materials. 98 refs.