Abstract

For the alloy AZ91 (Mg-9.OAl-0.7Zn-0.13Mn) die cast specimens were subjected to torsion testing at 150, 180, 240, 300, 420 and 450C̕ at 0.05 0.5 and 5.0 s -1 The as-cast specimens exhibited hot shortness at 360C̕ and above; however in that domain, after prior thermomechanical processing (TMP) at 300C̕, they showed much improved properties (which were reported along with as-cast properties at 300C̕ and below). For AZ31-Mn (Mg-3.2Al-1-1Zn-0.34Mn), AZ31 (Mg-2-8Al-0-88Zn-0.01Mn), AZ63 (Mg-5-5Al-2.7Zn-0.34Mn) and ZK60 (Mg-5.7Zn-0.65Zr-O-O1A]), the specimens were subjected to hot torsion testing in the range 180 to 450C̕ and 0.01, 0.1, and 1.0 s -1 . In the temperature range below 300C̕ flow curves rise to a peak with failure occurring immediately thereafter. Above 300C̕ the flow curves exhibited a peak and a gradual decline towards steady state. The temperature and strain rate dependence of the strength is described by a sinh-Arrhenius equation with Q HW between 125 and 144 kJ/mol; this indicates control by climb in comparison with creep in the range 200-400C̕. The alloy strength and activation energy declined in the order AZ63, AZ31-Mn AZ91, AZ31 and ZK60, while ductility increased with decreasing strength. In working of Mg alloys from 150 to 450C̕, the flow curves harden to a peak and work soften to a steady state regime above 300C̕. At temperatures below 300C̕, twinning is observed initially to bring grains into more suitable slip orientations. At high T a substructure develops due to basal and prismatic slip, Forming cells of augmented misorientation first near the grain boundaries and later towards the grain cores. Near the peak, new grains appear along the old boundaries (mantle) as a result of dynamic recrystallization DRX but not in the core of the initial grains. As T rises, the new grains are larger and the mantle broader, enhanced DRX results in higher ductility. At intermediate T, shear bands form through alignment of mantle zones resulting in reduced grain sizes and higher misorientation. Dynamic recrystallization does not become general as in Cu or Ni, thus it does not markedly raise the ductility. Retention of the hot worked substructure, or the refined grains, improves the strength and ductility of the product.