Effects of Ca, Ag addition on the microstructure and age-hardening behavior of a Mg-7Sn (wt%) alloy

The microstructure and age-hardening behaviors of Mg-7Sn, Mg-7Sn-lCa and Mg-7Sn-lCa-lAg alloys have been investigated in this study. The results show that the combined addition of Ca and Ag in Mg-Sn alloy has a positive effect on grains refinement from 113.5 mu m of Mg-7Sn alloy to 54.3 mu m of Mg-7Sn-lCa-lAg alloy. Meanwhile, the size of CaMgSn phase is refined due to suppressing the formation and growth of CaMgSn phase by Ag element. The average length of CaMgSn phase decreases from 27.2 to 14.9 mu m. Compared with Mg-7Sn and Mg-7Sn-lCa alloys, Mg-7Sn-lCa-lAg alloy exhibits the highest age-hardening response. The peak-aged hardness value increases from 61.1 HV to 80.4 HV by ageing treatment at 200 degrees C and the incubation period of age-hardening response also shortens from 80 h to 20 h with Ag addition. It is attributed to the effective promotion of dispersive precipitation and the volume fraction increasement of Mg2Sn precipitates in Mg-7Sn-1Ca-lAg alloy, which can lead to a strong precipitation strengthening effect in alpha-Mg matrix.