Improvement heat resistance and thermal stability of Mg-7Y-3Zn-0.4Mn alloy by Al/Ca element addition

Elevated temperatures lead to a reduction in the strength of magnesium-based alloys. At elevated temperatures (200–300 °C), even rare-earth reinforced Mg alloys experience notable strength deterioration. In this paper, the additions of different composition (0.4 and 0.8 wt.%) of Ca or Al in Mg-7Y-3Zn-0.4Mn alloy is added to increase the service temperature. The strength of Mg-7Y-3Zn-0.4Mn is effectively improved by Al element and higher after more Al is added, particularly at 300 °C. Specifically, the ultimate tensile strength (UTS) of Mg-7Y-3Zn-0.4Mn alloy increases from 181 MPa to 213 MPa by adding 0.8 wt.% Al. Remarkably, the UTS declines merely by 36 MPa (from 249 to 213 MPa) between 200 °C and 300 °C. And at both RT and elevated temperature (300 °C), Al alloying effectively improves the EL of Mg-7Y-3Zn-0.4Mn alloy, with improvements from 5.4% to 14.4% and 8.2% to 23.1%, respectively. Al element has dramatically increased the thermal stability and more significant effect at higher temperature. After prolonged annealing at 475 °C for 48 h, the Mg-7Y-3Zn-0.4Mn-0.8Al alloy shows only a limited increase in grain size from 16.9 to 23 µm. Even after annealing at 525 °C for 48 h, the Mg-7Y-3Zn-0.4Mn-0.8Al alloy maintains a grain size of approximately 41 µm. However, the growth of grain in basic alloy and 0.4 wt.% Ca alloy exceeds 150 µm. For microstructure, Ca addition primarily promotes LPSO phase formation and subsequent coarsening. Alloying with Al leads to three distinct microstructural changes: the LPSO phase transitions from interconnected networks to discrete blocks, Al2Y particles precipitate homogenously, and second phases distribute more uniformly. And the texture is also weakened by Al element. Essential mechanisms of Ca/Al effects on mechanical properties, thermal stability, and microstructure in Mg-7Y-3Zn-0.4Mn alloys are investigated.