Microstructure and mechanical properties of Mg–Al based alloy with calcium and rare earth additions

Effects of calcium and rare earth additions to alloy AZ91 on the microstructure and mechanical properties were investigated. The results indicated that small amounts of calcium addition to AZ91 did not cause the formation of any new phases in the microstructure, but refined the as-cast microstructure and increased the thermal stability of the β phase so that the yield strength and creep resistance of the alloy were significantly improved. Additions of lanthanum-rich misch metal (MM) resulted in the formation of needle-shaped particles, which showed very high thermal stability and did not dissolve into the matrix after the solution treatment at 420 °C for 20 h. The strength as well as creep resistance of the alloy at elevated temperatures was remarkably increased when MM was added combined with calcium. The highest creep resistance was obtained from the alloy with 3% of MM and 0.3% of calcium addition and its steady state creep rate reached as low as 2.69×10−8 s−1, one order of magnitude lower than that of alloy AZ91 without MM and calcium additions.