Combined effect of non-equilibrium solidification and thermal annealing on microstructure evolution and hardness behavior of AZ91 magnesium alloy

摘要

Non-equilibrium solidification of commercial AZ91 magnesium alloy was performed by copper mold spray-casting technique and the thermal stability property of as-formed meta-stable microstructure was investigated by subsequent annealing at different temperatures and times. Remarkable grain refinement appears with increasing cooling rate during solidification process, which is accompanied by a visible cellular/dendrite transition for the grain morphology of primary phase. Moreover, the non-equilibrium solidified alloy exhibits obvious precipitation hardening effect upon annealing at 200 °C, and the precipitation mode of β-Mg_(17)Al_(12) phase changes from discontinuous to continuous with extending isothermal time from 4 h to 16 h, which generates an increase of resultant micro-hardness value. After solid solution treatment at the elevated temperature of 420 °C, the volume fraction of β-Mg_(17)Al_(12) phase decreases and a notable grain growth phenomenon occurs, which give rise to a reduction of hardness in comparison with that of as-quenched alloy.