Effect of strain path on microstructure and mechanical properties of AZ31 magnesium alloy sheets processed by constrained groove pressing

Abstract

Constrained groove pressing (CGP) is a novel severe plastic deformation (SPD) method suitable for fabricating ultra-fine grained (UFG) sheet metals. In this work, two passes of CGP were conducted to AZ31 magnesium alloy sheets at 523 K, and three routes including traditional CGP, 180° cross-CGP and 90° cross-CGP were employed to investigate the effects of pressing number and strain path on microstructure, texture and mechanical properties of the alloy sheets. The results show that, irrespective of CGP routes, a bimodal microstructure is developed in the deformed materials and the capability of CGP in fabricating UFG magnesium alloy sheets is confirmed by TEM observation. The superior grain refinement and microstructure homogenization is achieved in traditional CGP route from the as-annealed 9.85 μm–5.04 μm after one pass while no further improvement is observed after two passes. The average grain sizes are 5.69 μm and 5.18 μm after two passes of 180° cross-CGP and 90° cross-CGP, respectively. The average misorientation angle is mainly determined by the fraction of very high angle grain boundaries and texture and microstructure mechanisms dominate the generation of high angle grain boundaries in traditional and cross-CGP routes, respectively. The coexistence of strain free recrystallized grains and substructured/deformed grains is observed after one pass of traditional CGP due to incomplete dynamic recovery and dynamic recrystallization and promotes the subsequent activation of strain induced grain boundary migration, which may be postponed by employing cross-CGP routes. A double-peak basal texture with the basal poles inclined from ND to RD is developed in traditional CGP route while an inclination of the basal poles from ND to TD accompanied by a redistribution of the prismatic intensity toward RD is observed in cross-CGP routes due to the change of shear deformation direction. The texture softening plays a more important role in decreased strength than grain refinement in traditional CGP route while both grain refinement and texture modification contribute to the improvement of tensile properties in cross-CGP routes. Due to the inherent nature of CGP, the reverse loading condition between successive pressings cannot be changed even in cross-CGP routes. The relatively high processing efficiency in 90° cross-CGP route should be attributed to the unique strain accumulation sequence.