Microstructural evolution of extruded AZ31 alloy with bimodal structure during compression

Abstract

The twinning behavior and microstructural variations of extruded Mg alloy with a bimodal structure during compression along the extrusion direction are investigated using AZ31 alloy partially recrystallized during hot extrusion; this alloy consists of fine dynamically recrystallized (DRXed) grains (22 mu m) and coarse unDRXed grains (305 mu m). It is found that the stronger < 10 - 10 > texture and larger grain size of the unDRXed grains in comparison to those of the DRXed grains result in an increase in the Schmid factor for (10 - 12} twinning and a reduction in the twinning stress, respectively; this consequently leads to a much higher activation of {10 - 12} twinning during compression in the unDRXed grains. The size of the unDRXed grains decreases considerably to 84 mu m by the formation of a number of narrow twin bands at a compressive strain of 5%, and it then increases to 112 mu m by the growth and coalescence of the twin bands at a strain of 10%. However, despite the formation of twins, the size of the DRXed grains does not change during deformation, because twinning occurs throughout the entire grain owing to the low internal strain in these grains. These results indicate that the relative activity of {10 - 12} twinning and the microstructural and textural evolutions in the DRXed and unDRXed grains are considerably different, which is attributed mainly to the differences in their size, texture, and internal accumulated strain.