Achieving enhanced high-temperature mechanical properties in Mg-Nd-Sm-Zn-Ca-Zr alloy by Ag addition

Mg alloys have been attracting particular interests in industrial manufacture. Nevertheless, the poor high-temperature mechanical properties of Mg alloys limit their further practical applications. Thus, as-cast Mg-2.5Nd-1.0Sm-0.4Zn-0.1Ca-0.5Zr-xAg (x = 0, 0.5, 1.0, 1.5, 2.0 wt%) alloys were designed and fabricated to overcome this shortcoming in this study. The influences of Ag addition on the microstructures and mechanical properties of Mg alloys were analyzed in detail. The results indicated that the microstructures of the alloy were composed of equiaxed α-Mg grains and β phase. The content of the β phase increased from 5.56% to 8.91% upon increasing the content of Ag from 0 wt% to 2.0 wt%, indicating that Ag can promote the formation of the β phase significantly. Moreover, the first-principles calculations supported that the formation enthalpy for the β phase was reduced with the doping of Ag resulting in increased stability of the β phase. Ag also enhanced the bulk modulus and ductility of the β phase. Meanwhile, the Young's modulus and elastic anisotropy of the β phase was diminished by Ag addition. The addition of Ag increased the strength and decreased the elongation of the as-cast alloys. The as-cast alloy with 2.0 wt% Ag addition achieved comprehensive mechanical properties. Furthermore, the as-cast alloy with 2.0 wt% Ag addition showed the least decrease in ultimate tensile strength with increasing temperature compared to the other alloys. The ultimate tensile strength of the as-cast alloy with 2.0 wt% Ag addition decreased from 196.0 MPa to 188.8 MPa as the temperature changed from 25 °C to 275 °C. The increase in strength and heat resistance of the alloys is mainly attributed to the increased β phase content and stability. Overall, this work developed a new heat resistant Mg-Nd-Sm-Zn-Ca-Zr-Ag alloy with excellent high-temperature mechanical properties. This suggests that it helps to improve the high-temperature mechanical properties and reliability of new aero engines and promote applications of lightweight engines.