Microstructural evolution of pre-twinned Mg alloy with annealing temperature and underlying boundary migration mechanism

This study investigates the variations in the microstructural characteristics of a pre-twinned Mg alloy with the temperature of the subsequent annealing treatment. To this end, a rolled AZ31 alloy is compressed to 3% plastic strain along the rolling direction (RD) to activate {10-12} twinning and is subsequently annealed at 200, 250, 300, 350, and 400 °C. Numerous {10-12} twins are formed throughout the compressed material, leading to the formation of a RD-oriented texture. At an annealing temperature of 200 °C, no microstructural variations occur during annealing. As the annealing temperature increases from 250 to 400 °C, the residual strain energy and remaining twin boundaries of the annealed material decrease owing to the promoted static recovery and the increased area fraction of twin-free grown grains. Consequently, an increase in the annealing temperature results in a gradual microstructural transition from a fully twinned grain structure to a completely twin-free grain structure. The microstructural evolution during annealing is predominantly governed by the movement of high-angle grain boundaries via a strain-induced boundary migration mechanism, and a few twin boundaries migrate above 350 °C because of their lower boundary energy. The boundary migration behavior and resultant microstructural evolution are discussed in detail based on the variations in boundary mobility and driving force for boundary migration with annealing temperature.