Twinning, dynamic recovery and recrystallization in hot worked Mg-Al-Zn alloy

The AZ31 Mg alloy was hot torsion tested from 180 to 450 degreesC and from 0.01 to 1.0 s(-1). The flow curves showed a peak and a decline towards a steady-state regime which were lower as temperature T rose and strain rate declined; however, the fracture strain increased to about 1.9 at 0.1 s(-1). In transmission electron microscopy, twins were observed from 180 to 360 degreesC (in declining numbers). At low T, they had sharp walls and contrasting transverse bands: while the matrix showed indistinct linear streaks. As T rose, the twin bands developed cells with tangled walls and finally subgrains (similar to 360 degreesC), while the twin walls became tangles of dislocations and finally serrated boundaries. The matrix developed elongated dislocation walls and subgrains at higher T. The twin intersections at 180 and 240 degreesC consisted of diamond-shaped cells with a duplex set of orientations but at 300 and 360 degreesC, these had 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 360-450 degreesC, 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 T were also found at high ones, albeit with decreasing frequency. the microstructures showed severe heterogeneity which accounted for the limited ductility. (C) 2002 Elsevier Science B.V. All rights reserved.