Effect of silicon content on hot working, processing maps, and microstructural evolution of cast TX32-0.4Al magnesium alloy

The effect of silicon (0.2-0.8wt%) addition on the hot working behavior and deformation mechanisms of the Mg-3Sn-2Ca-0.4Al (TX32-0.4Al) alloy has been evaluated by generating processing maps in the temperature and strain rate ranges of 300-500°C and 0.0003-10s^-1. The processing map for the base TX32-0.4Al alloy exhibited two dynamic recrystallization (DRX) domains in the ranges (1) 300-360°C and 0.0003-0.001s^-1 and (2) 400-500°C and 0.003-0.7s^-1. While 0.2% Si addition did not result in any significant change in the processing map of the base TX32-0.4Al alloy, 0.4% Si addition has enhanced hot workability by widening the processing window(s) and by reducing flow instability. The rate controlling mechanism in Domain 1 is identified as climb, whereas it is cross-slip in Domain 2. When the Si content is increased to 0.6 and 0.8%, the volume fraction of hard intermetallic particles has increased nearly two fold. The processing map for the alloy with 0.6% Si addition exhibited an additional Domain 3 at higher temperatures and high strain rates (475-500°C and 0.01-10s^-1). However, cracking has occurred in Domain 1 due to void formation at hard particles. In Domains 2 and 3, DRX occurred predominantly by basal slip with climb as a recovery process, as confirmed by the resulting basal texture and tilt type sub-boundary structure. This is attributed to the large back stress generated by the increased volume fraction of intermetallic particles due to which the extensive activation of basal slip required considerably high temperatures. Increase in the volume fraction of hard particles due to higher Si content reduces the flow instability by generating a high rate of entropy production through increasing the nucleation sites for power dissipation and enhances the occurrence of void formation and/or ductile fracture. © 2014.