Evolutions of Microstructure and Mechanical Properties in Mg-5Li-1Zn-0.5Ag-0.5Zr-xGd Alloy

The evolution of the microstructure and mechanical properties of alloy system with nominally composition Mg-5Li-1Zn-0.5Ag-0.5Zr-xGd (x = 0, 1.2, 2.4, 3.6, 4.8, 6) is evaluated based on computational phase diagram and corresponding experimental studies. The results show that grains are significantly refined with the increase of Gd content. The main phases of as-cast alloys are alpha-Mg, beta-Li, AgLi2Mg, and Mg3Gd. With the increase of Gd content, the amounts of Mg3Gd phase and beta-Li phase have been increased. When the Gd content exceeds 3.6 wt%, Mg3Gd phase precipitates in a form of the network at the grain boundaries. The precipitation of beta-Li can be attributed to the competitive dissolution of Zn, Gd, and Li in Mg. Meanwhile, gamma & DPRIME; is formed after the addition of Gd, which grows and transforms into gamma & PRIME; with the increase of Gd content. In solidification process, stacking faults are formed by solid transformation of partial alpha-Mg and Mg3Gd. Eventually, with the synergistic effect of Mg3Gd, beta-Li, and gamma & DPRIME; (or gamma & PRIME;), as the Gd content increasing, the tensile strength of the alloy first increases, then decreases, and the elongation decreases. When the content of Gd is 4.8 wt%, the ultimate tensile strength and yield strength reach the maximum values of 227 MPa and 139 MPa, and the elongation is 18.1%, respectively.