The influence of thermal activation on the nature of the interactions between an array of particles and a migrating grain boundary is considered, adapting concepts from the theory of thermally activated deformation. Thermally activated unpinning of boundaries is found to become significant at small particle sizes and high temperatures. For a given particle array, this leads to a time dependent average grain size which is coarser than the predictions of the traditional "Zener-Gladman" static equilibrium model. For grain sizes and temperatures of interest in ferrous alloys, substantial reductions in particle size and volume fraction of grain refining dispersions can be achieved before thermal activation interferes with the effectiveness of boundary pinning.
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