Prominent role of a high volume fraction of Mg17Al12 particles on tensile behaviors of rolled Mg–Al–Zn alloys

Abstract

Fabricating magnesium alloy sheets exhibiting superplasticity >500% by conventional rolling remains a challenge due to the difficulty in achieving fine-grained structure. Even more puzzling is the role of softened Mg17Al12 particles locating at grain boundaries, which play a promoting or retarding effect on superplasticity at elevated temperatures, has not been understood so far. Here, fine-grained (∼3.0 μm) Mg–9Al–1Zn (AZ91) alloy with a high volume fraction of spherical Mg17Al12 particles was prepared by conventional rolling. The rolled AZ91 alloy exhibits a high tensile strength of ∼370 MPa and ductility of ∼13% at room temperature. More importantly, high superplasticity of ∼563% and ∼735% is achieved at 250 and 300 °C, respectively. It is recognized that the deformation mechanisms of the Mg–Al–Zn alloys at elevated temperatures depend significantly on the amount of Mg17Al12 particles. This work highlights the prominent role of a high volume fraction of Mg17Al12 particles segregating along grain boundaries at elevated temperatures, which on one hand can effectively pin grain boundaries to retard grain coarsening, and on the other hand, are beneficial to relax the intergranular stress incompatibility, and hence significantly facilitate GBS behavior. The current study promotes significantly the industry application of high superplastic rolled AZ91 alloy sheets.