Room-temperature tensile properties of a directionally solidified magnesium alloy and its deformation mechanism dominated by contraction twin and double twin

A Mg–4.78Zn–0.45Y–0.10Zr (wt%) alloy with specifically oriented columnar grain structures (the preferential growing plane was the prism (101̄2) plane and the growth direction was 〈011̄0〉) was prepared using a directional solidification technique. The columnar grain structures had parallel-growing primary arms and straight longitudinal grain boundaries. Room-temperature tensile tests were carried out along the growth direction of the columnar grains. The results showed that the tensile strength (σb) of the alloy was 198MPa and the elongation (δ) was 20.5%. The fractography results showed that the fracture surface had many dimples of different sizes, indicating its ductile nature. The electron backscatter diffraction (EBSD) analysis results showed that in the beginning of the tensile deformation, a bamboo leaf-shaped {101̄1} contraction twin was activated and then a {101̄2} secondary extension twin was activated within the {101¯1} contraction twin, forming a double twin. As the deformation continued, several bamboo leaf-shaped {101̄1} contraction twins joined together, forming a contraction twin band cluster. And because the {101̄2} secondary extension twinning, subgrains and recrystallized grains afterwards formed inside the contraction twin variants, boundaries of these contraction twins were not flat or smooth anymore. Twins were then gradually split to form fragmentation, and newly formed twins would nucleate and grow alongside the twin boundaries. The main reason for the good room-temperature plasticity and specific grain orientation of this alloy was the plastic deformation mechanism dominated by the contraction and double twins.