Influence of grain boundaries with dispersed nanoscale Al2O3 particles on the strength of Al for a wide range of homologous temperatures

The deformation and strengthening behavior of an ultra-fine grained (UFG) Al fabricated via powder metallurgy was investigated over a wide range of homologous temperatures (TH). Our results reveal that the presence of a high density of γ-Al2O3 nanoparticles, located primarily at high angle grain boundaries (HAGBs), promoted remarkable stabilization of the Al grain structure up to TH = 0.94. The 0.2% strain offset yield stress (YS0.2) of the materials annealed at 600 °C for 24 h with grain sizes (d3D) of 0.57, 1.67, 2.33 and 2.99 μm was systematically studied from room temperature (RT) to 600 °C. Our study reveals that with decreasing d3D the strain hardening ability of the materials gradually decreased, and this behavior became pronounced at elevated T, which was attributed to the onset of plastic instability. The YS0.2 of the materials at all T followed the relation YS0.2 = a + k d3D−0.5. As determined on the basis of data interpolation, a positive transition from an established Hall-Petch (HP) relation at RT occurred at d3D = ∼8 μm. As the parameter a was negative at RT and 300 °C the documented strength-structure relationship cannot be explained on the basis of a Hall-Petch relation. The γ-Al2O3 particles located at HAGBs did not contribute notably to RT strengthening, whereas GBs played a significant role in the overall strength. The materials showed a high YS0.2 up to a TH of 0.94. The YS0.2 was observed to be linearly correlated with the reciprocal square root of the d3D and the coefficient k followed k = 224.4-0.376 T relation. The HP model overestimated the YS0.2 values at elevated T namely for the materials with a smaller d3D. It is proposed that softening occurred as a result of the γ-Al2O3 particles densely distributed within a 3D network, which were included in Hansen's strengthening model; particle strengthening played a role at elevated T.