Grain size hardening versus texture softening: Mechanical anisotropy of an AZ31B magnesium alloy processed by equal-channel angular pressing

Abstract

Microstructural evolution of hexagonally close-packed (hcp) alloys subjected to severe plastic deformation differs from that of alloys with cubic crystal structure. While grain refinement is the main objective in conventional ECAP texture changes in hcp materials can result in lower strength but improved ductility depending on the deformation direction. This paper addresses the question of how the processes of grain size hardening and texture softening contribute to the highly anisotropic mechanical behavior of a severely deformed AZ31B magnesium alloy. ECAP is performed at different temperatures starting at 250 °C for the first 4 passes and subsequently reduced to 200 °C (C5, C6) and 150 °C (C7, C8). The mechanical properties of the resulting material conditions are investigated with tensile tests in four different deformation directions. While the yield strength increases up to 280 MPa in the transverse direction (TD), an opposite trend of low strength and good ductility can be observed for deformation in the extrusion (ED) and normal directions (ND). Additional microstructural investigations using EBSD show that the grain size is reduced by 95% during ECAP. Moreover, texture measurements using XRD reveal a strong fiber texture, with a high number of basal planes parallel to the shear plane of ECAP deformation, which results in a significantly improved ductility and lower strength in ED and ND. The results are discussed in the light of the competition between grain size hardening and texture softening. They highlight the potential of ECAP for the improvement of strength and ductility for magnesium alloys.

Keywords

• Severe plastic deformation
• Magnesium alloys
• Grain refinement
• ECAP
• Texture
• Mechanical behavior