Effect of initial grain sizes on hardness variation and strain distribution of pure aluminum severely deformed by compression tests

The effect of initial grain sizes on the evolution of microstructure and hardness with equivalent strain imposed in high-purity aluminum was studied through a combination of numerical simulation and experimental observations and measurements. Initial samples with four different grain sizes, ∼2, 6, 30 and 100 μm, were prepared by processing with equal-channel angular pressing followed by annealing. The cylindrical samples were compressed by 49% and 73% at a strain rate of 0.01 s^-1 in a single pass at room temperature. The strain imposed by the compression was evaluated quantitatively using finite element simulation. The microstructures were observed using electron backscatter diffraction analysis and transmission electron microscopy. The results show that the difference in the initial grain sizes gives rise to inhomogeneous distributions of microstructure and strain even under the same reduction rate. The hardness variations with the strain also depend on the initial grain sizes and the hardness maximum appears at a lower value of strain as the initial grain size decreases. It is concluded that not only the imposed strain but also the initial grain sizes play an important role for the grain refinement.