Inverse Hall–Petch Behaviour in an AZ91 Alloy and in an AZ91–Al₂O₃ Composite Consolidated by High‐Pressure Torsion

High‐pressure torsion (HPT) is a significant procedure for achieving substantial grain refinement but it may be used also to consolidate metallic particles to form bulk samples or composites where two (or more) different phases are mixed and consolidated. Herein, the consolidation of particles of the magnesium AZ91 alloy and a composite with an AZ91 matrix combined with 1% alumina powder is investigated. The results show that it is possible to fully consolidate this alloy after a large number of turns. As a consequence of the severe plastic deformation, the grain structure is significantly refined, with average grain sizes of ≈116 and ≈98 nm in the unreinforced alloy after 20 or 50 HPT turns and ≈76 nm in the composite after 50 HPT turns, respectively. This grain refinement is associated with a decrease in hardness and an increase in the strain rate sensitivity due to the onset of a grain boundary diffusion–assisted creep mechanism at room temperature. The results are consistent with the theoretical prediction of a breakdown in the Hall–Petch relationship at very small grain sizes.