Twinning in magnesium alloy AZ31B under different strain paths at moderately elevated temperatures

Abstract

Twinning activity and deformation limits were investigated for wrought and cast magnesium at room and moderately elevated temperatures and varying strain paths. In situ compression and tension tests, combined with high-resolution electron backscatter diffraction (HR-EBSD) techniques, demonstrated that twin formation and geometrically necessary dislocation (GND) density strongly depend on initial microstructure and load directions. It was also shown that GND content was lower inside a twin than outside, in the parent grain. This explains the observation of a leveling off of GND density, while twin fraction continued to increase with plastic strain. Plane strain and biaxial tension experiments at room temperature resulted in compression twin shear bands, which began to appear at effective strains of about 4%, compared to the uniaxial tension case, where shear band formation had been shown to occur at effective strains closer to failure. This behavior was attributed to the strong basal texture of the AZ31B sheet, where thinning of the sheet requires c-axis compression. While thinning strains were accommodated by compression twinning at room temperature, compression twin shear bands were not observed in plane strain and biaxial tension specimens at 75 and 125°C, at effective strains between 5% and 12%. This provides evidence of a transition from compression twinning to 〈c+a〉 slip in order to accommodate thinning strains at these mildly elevated temperatures, resulting in significant formability increases, especially in biaxial tension.