Remarkable age-hardening response and microstructural evolution of Mg–7Sn alloy

The poor age-hardening response of Mg alloys limited their development and applications. In this study, the age-hardening response of Mg–7Sn alloy has been improved significantly by a 40 MPa compressive stress aging at 220 °C. Phase transition from metastable βˈ (Mg3Sn) phase to β (Mg2Sn) phase occurs during the stress aging. The βˈ with L12 structure has an orientation relationship with the Mg matrix of (11¯0)β'//(112¯0)α and [1¯1¯0]β'//⌈0001¯⌉α. The precipitated sequence in the stress-aged alloy is S.S.S. (supersaturated solid solution)→GP→βˈ(Mg3Sn)→β(Mg2Sn). The stress-aged alloy exhibits a high peak-aged hardness (∼71 HV) and short time to reach the peak hardness (12 h) as compared with the aged alloy without the stress. Interestingly, a long-time aging platform with 71 HV for 96 h is observed during stress aging. The formation of the platform is related to a balance of hardening and softening arisen from microstructure evolution during stress aging. The stress aging provides a promising heat treatment way to improve aging hardening response of Mg alloys.