Microstructure evolution and quench sensitivity characterizations of Mg-9.5Gd-0.9Zn-0.5Zr alloy

Abstract

Mg-9.5Gd-0.9Zn-0.5Zr alloys were water quenched (WQ), air cooled (AC) and furnace cooled + water quenched (FC + WQ) after solution treatment. Effects of quenching conditions on the microstructure, mechanical properties and age hardening behavior were investigated, then the quench sensitivity and strengthening mechanism were analyzed. Results showed that only rod-like Zn2Zr3 phases were found in the WQ alloy, while Mg5Gd and 14H LPSO phases were also detected in the AC and FC + WQ alloys. The WQ alloy exhibited the most significant age hardening response, and the peak-aged alloy was consisted of densely distributed γ' + γ'' precipitates, Zn2Zr3 and Mg5Gd particles. In contrast, decreased amount of γ' + γ'' precipitates were observed in the AC and FC + WQ peak aged alloys. The increment of yield strength and ultimate tensile strength after aging treatment decreased with the decreasing quenching rate, and the elongation decreased sharply in the WQ and AC peak-aged alloys, while it only decreased slightly in the FC + WQ peak-aged alloy. The calculated strengthening contribution exhibited that the variation of precipitation strengthening in peak-aged alloys was more dramatic than that of solid solution strengthening in solution treated alloys, then lead to a higher quench sensitivity.