Formation of nano-twin domains by nucleation and multiplication of twins during fracture of a magnesium alloy

Abstract

Transmission electron microscopic study of crack propagation in a fine-grained Mg–Zn alloy shows a complex interplay of slip and twinning, leading to formation of twin-related nano-sized domains of 50 nm size related by {1 0 1} and {1 0 2} twin orientations. Subgrain formation with low angle boundaries is followed by formation of {1 0 1} twins and twin multiplication by nucleation and growth. Basal and prismatic slip play an important role in the growth and formation of new domains of twins and boundaries. These basal and prismatic slip occur in the matrix as well as inside the {1 0 1} twins. Steps on these twin boundaries indicate interaction with slip, leading to their growth. Evidence of nucleation of twins on grain boundaries is found. Most interesting is the indication of nucleation of {1 0 1}-type twins. String or stacks of nano-sized domains related to each other by {1 0 1} twinning were found in the matrix, where the matrix contained basal, prismatic as well as pyramidal dislocations. It appears that a localization of slip or formation of low angle boundaries to accommodate stress from surrounding domains leads to nucleation of these twins.