Contraction Twinning Dominated Tensile Deformation and Subsequent Fracture in Extruded Mg-1Mn (Wt Pct) at Ambient Temperature

Abstract

Due to their excellent strength-to-weight ratio, Mg alloys are attractive for applications where weight savings are critical. However, the limited cold formability of wrought Mg alloys severely restricts their widespread usage. In order to study the role that deformation twinning might play in limiting the elongation-to-failure ( $${\varepsilon} _{\text {f}}$$ ), in-situ tensile tests along the extrusion axis of Mg-1Mn (wt pct) were performed at 323 K, 423 K, and 523 K. The alloy exhibited a strong basal texture such that most of the grains experienced compression along their $$\langle {c}\rangle $$ -axis during deformation. At 323 K, fracture occurred at about 10 pct strain. Although basal, prismatic, and pyramidal $$\langle {c+a}\rangle $$ slip activity was observed along with extension twinning, contraction twinning significantly influenced the deformation, and such twins evolved into {10 $${\bar{1}}$$ 1}–{10 $${\bar{1}}$$ 2} double twins. Crystal plasticity simulation showed localized shear deformation within the contraction twins and double twins due to the enhanced activity of basal slip in the reoriented twin volume. Due to this, the twin–matrix interface was identified to be a potential crack initiation site. Thus, contraction twins were considered to have led to the failure of the material at a relatively low strain, suggesting that this deformation mode is detrimental to the cold formability of Mg and its alloys. With increasing temperature, there was a significant decrease in the activity of contraction twinning as well as extension twinning, along with a decrease in the tensile strength and an increase in the $${\varepsilon} _{\text {f}}$$ value. A combination of basal, prismatic, and pyramidal $$\langle {c+a}\rangle $$ slips accounted for a large percentage of the observed deformation activity at 423 K and 523 K. The lack of contraction twinning was explained by the expected decrease in the critical resolved shear stress values for pyramidal $$\langle {c+a}\rangle $$ slip, and the improved $${\varepsilon} _{\text {f}}$$ values at elevated temperatures were attributed to the vanishing activity of contraction twinning.