Grain size effects in aluminum processed by severe plastic deformation

We investigated the effects of the grain size on the yield strength of aluminum using a group of samples with grains successively refined by equal-channel angular pressing (ECAP) and another group of samples produced by 8 ECAP passes and subsequently annealed step by step to produce coarser grains. Tensile tests were performed on all the samples. Adopting a linear addition model for different strengthening contributions, the observed yield strength was resolved into dislocation-related and grain size-related strengthening contributions. The former was directly evaluated using the Taylor equation with dislocation densities determined by the Williamson-Hall method, and the latter was quantified by subtracting the former and the friction stress from the observed yield strength. It was found that for the as-ECAP-processed samples, the amount of the grain size-related strengthening was consistent with an extrapolation of the conventional Hall-Petch relation, and marked extra strengthening, which was clearly related to the grain refinement, appeared after annealing.