Fracture micro-mechanisms in hot-rolled AZ31 and AZ31-Sr magnesium alloys

Abstract

Micro specimens of hot-rolled AZ31 and AZ31 + 0.8 wt% Sr magnesium alloys were tensioned in the rolling direction, and fracture micro-mechanisms were studied by in-situ real-time optical observations, electron microscopy, and DIC analysis. In-situ observations indicated that strain localization on the microstructural shear bands developed during tension was responsible for the fracture of the AZ31 specimens. DIC analysis demonstrated a substantial strain heterogeneity in the AZ31 sample, with the most intense concentration on the tension-induced shear bands. Neither rolling nor tension shear bands were observed in AZ31-Sr alloy. Instead, many cracks were formed on the specimen surface, whose propagation led to the final fracture. Compared to the AZ31 alloy, the Sr-containing AZ31 alloy experienced a more uniform strain distribution during deformation. Most of the twins inherited from the rolling process were perpendicular to the rolling direction, which were identified as the source of crack initiation, especially in the Sr-added alloy. Microstructural evidence showed that the contribution of precipitates in damage of Sr-added AZ31 alloy was higher than that in AZ31 alloy. However, cracks were mostly initiated from the twin boundaries, not the precipitate stringers. The variations of plastic anisotropy coefficient (R-value) were described as a function of applied strain using DIC analysis. It was found that incorporation of Sr into AZ31 alloy reduced its pronounced anisotropy.