Microstructure and texture studies on twin-roll cast AZ31 (Mg–3 wt.%Al–1 wt.%Zn) alloy and the effect of thermomechanical processing

Abstract

The microstructure and texture of the twin-roll cast (TRC) AZ31 (Mg–3wt.%Al–1wt.%Zn) sheet, with a thickness of 6mm, have been investigated. The TRC AZ31 exhibits a dendritic microstructure with columnar and equiaxed grains. These contain Al–Mn and Mg–Al–Zn second-phase particles that are approximately 1μm in size. This heterogeneous structure is attributed to the effect of the cooling rate, which varies from 325°C/s on the surface to ∼150°C/s in the mid-thickness of the sheet. No surface segregation, but a certain degree of macrosegregation is observed in the mid-thickness which persists after annealing and rolling. Recrystallization at 420°C leads to a bimodal grain-size distribution, while a fine-grain structure is obtained after rolling and annealing. The TRC AZ31 sheet exhibits basal textures in the (i) as-received, (ii) rolled and (iii) rolled-annealed conditions. However, post-annealing of the TRC AZ31 at 420°C produces a relatively random texture that has not been previously observed in the conventional AZ31 sheet. The texture randomization is attributed to the particle-stimulated nucleation of new grains in the TRC structure. The preliminary evaluation of mechanical properties indicates that such annealing treatment slightly increases the ultimate tensile strength (UTS), but significantly improves elongation.