Effects of artificial aging on microstructure and mechanical properties of the Mg–4.5Zn–4.5Sn–2Al alloy

• Effects of double aging on the Mg–Zn–Sn–Al alloy are firstly studied.
• The steady hardness over 87 HV is achieved with second aging from 5 h to 50 h.
• Double aging refines precipitates and enhances the formation of T-shaped particles.
• Double aging can improve both strength and plasticity for the Mg–Zn–Sn–Al alloy.

Effects of various aging treatments on microstructure and mechanical properties of the Mg–4.5Zn–4.5Sn–2Al alloy are comparatively studied by OM, SEM, TEM, tensile testing and micro-hardness measurement. Two hardness peaks are discernible in all the age hardening curves, corresponding with the precipitation of β1′ followed by the precipitation of Mg2Sn. The first peak hardness is lower than the second peak by single aging at both 150 °C and 200 °C. The peak hardness can increase to more than 90 HV by single aging at 150 °C and double aging. Double aging brings about the relatively steady hardness beyond 87 HV with the second aging at 200 °C from 5 h to 50 h. The long rod-like β1′ phase, the short rod-like Mg2Sn phase and the two-phase T-shaped particles are dominant in the double aged sample. Double aging brings about the obvious refinement of precipitates and the enhanced the nucleation of the Mg2Sn particle on the tip of the β1′ long rod. The double aged alloy exhibits the highest strength and the biggest elongation among all the as-aged alloys, with the strength comparable to that of single aging at 150 °C for 70 h and the plasticity about 27% higher. The strength improvement is attributable to the synergy effect of the refinement of precipitates, the formation of the short rod-like Mg2Sn precipitates with larger aspect ratios and the enhanced formation of the two-phase T-shaped particles.