The AZ31 Mg alloy was hot torsion tested from 180 to 450C and from 0.01 to 1.0/s. The flow curves exhibited a peak and a decline towards a steady-state regime which were lower as the temperature increased and the strain rate decreased. However, the fracture strain increased to about 1.9 at 0.1/s. In transmission electron microscopy, twins were observed from 180 to 360C (in decreasing numbers). At low temperatures, they had sharp walls and contrasting transverse bands while the matrix exhibited indistinct linear streaks. As the temperature increased, the twin bands developed cells with tangled walls and finally sub-grains (~360C), while the twin walls became tangles of dislocations and finally serrated boundaries. The matrix developed elongated dislocation walls and sub-grains at higher temperatures. The twin intersections at 180 and 240C consisted of diamond-shaped cells with a duplex set of orientations but, at 300 and 360C, these developed into polygonal cells with high misorientations in dark field. The first very small dynamically recrystallized grains were observed at these intersections; slightly larger than the cells. At 360 to 450C, as observed by optical microscopy, small dynamically recrystallized grains formed at the original grain boundaries; probably related to multiple slip. Since twinning and other features described at low temperatures were also found at high temperatures, albeit with decreasing frequency, the microstructures exhibited a severe heterogeneity which accounted for a limited ductility.

Twinning, Dynamic Recovery and Recrystallization in Hot Worked Mg–Al–Zn Alloy. M.M.Myshlyaev, H.J.McQueen, A.Mwembela, E.Konopleva: Materials Science and Engineering A, 2002, 337[1-2], 121-33