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 (T_H). 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 T_H = 0.94. The 0.2% strain offset yield stress (YS_0.2) of the materials annealed at 600 °C for 24 h with grain sizes (d_3D) 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 d_3D 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 YS_0.2 of the materials at all T followed the relation YS_0.2 = a + k d_3D^−0.5. As determined on the basis of data interpolation, a positive transition from an established Hall-Petch (HP) relation at RT occurred at d_3D = ∼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 YS_0.2 up to a T_H of 0.94. The YS_0.2 was observed to be linearly correlated with the reciprocal square root of the d_3D and the coefficient k followed k = 224.4–0.376 T relation. The HP model overestimated the YS_0.2 values at elevated T namely for the materials with a smaller d_3D. 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.