Microstructural evolution and grain growth mechanism of pre-twinned magnesium alloy during annealing

The microstructural evolution and underlying grain growth mechanism of a {10–12}-twin-containing Mg alloy during annealing are investigated through quasi in situ electron backscatter diffraction measurements of a rolled AZ31 alloy subjected to precompression along the rolling direction (RD). The precompressed material shows a partially twinned structure consisting of a twinned region and a residual matrix region, and this structure changes to an almost twin-free structure consisting of grown grains with serrated grain boundaries in twin- and matrix-originated regions after annealing at 250 °C for 1 h. In addition, the average grain size almost doubles and the internal strain energy decreases significantly after annealing. These microstructural variations are induced mainly by grain growth through the strain-induced migration of high-angle grain boundaries without the movement of twin boundaries. The twinned region of the precompressed material has higher stored strain energy than the residual matrix region because the crystallographic orientation of the former region is favorable for basal slip and because of the occurrence of the dislocation transmutation reaction in the twins. For reducing the total strain energy accumulated in the precompressed material, the residual matrix region—having lower stored strain energy—preferentially grows while consuming the twinned regions formed in the surrounding grains during annealing. As a result, the area fraction of grains with a matrix texture increases whereas that of grains with a twin texture decreases after annealing. The twin texture intensity increases significantly and this texture becomes more concentrated along the RD because the highly RD oriented twins tend to grow during annealing on account of their fairly low stored strain energy.