High temperature deformation mechanisms and processing map for hot working of cast-homogenized Mg-3Sn-2Ca alloy

The hot working behavior of Mg-3Sn-2Ca alloy has been investigated in the temperature range 300-500 °C and strain rate range 0.0003-10 s^-1, with a view to evaluate the mechanisms and optimum parameters of hot working. For this purpose, a processing map has been developed on the basis of the flow stress data obtained from compression tests. The stress-strain curves exhibited steady state behavior at strain rates lower than 0.01 s^-1 and at temperatures higher than 350 °C and flow softening occurred at higher strain rates. The processing map exhibited two dynamic recrystallization domains in the temperature and strain rate ranges: (1) 300-420 °C and 0.0003-0.003 s^-1, and (2) 420-500 °C and 0.003-1.0 s^-1, the latter one being useful for commercial hot working. Kinetic analysis yielded apparent activation energy values of 161 and 175 kJ/mole in domains (1) and (2) respectively. These values are higher than that for self-diffusion in magnesium suggesting that the large volume fraction of intermetallic particles CaMgSn present in the matrix generates considerable back stress. The processing map reveals a wide regime of flow instability which gets reduced with increase in temperature or decrease in strain rate.