Effect of Gd on the microstructure and mechanical properties of high-pressure die-cast Mg-La-Ce alloys at ambient and elevated temperatures

Abstract

The microstructure and tensile properties of die-cast Mg-1.6 wt%La-1.0 wt%Ce alloys with Gd content ranging from 0 wt% to 3.0 wt% were investigated. The major intermetallic compound formed at the grain boundaries of the alloys was the skeleton-like Mg12RE phase, and the domains of {011} twins can be observed in Mg12RE. Three minor intermetallic compounds formed at the grain boundaries of the alloys with Gd addition, i.e., the irregular-shaped Mg2Gd phase, the needle-like Mg3Gd phase and the blocky Gd-Mn phase. The addition of Gd resulted in the increase of yield strength (YS) at both ambient and elevated temperatures. The YS of the Gd-free alloy was 135.3 MPa at ambient temperature and 57.3 MPa at 300 °C. The YS of the alloy with 3.0 wt% Gd was 162.1 MPa at ambient temperature and 97.3 MPa at 300 °C, which were increased by 19.8 % and 69.8 %, respectively, in contrast to the alloy without Gd addition. The addition of Gd increased the volume fraction of intermetallic compounds from 11.7 % to 18.5 %, while it had a very limited effect on the grain size of the Mg matrix phase. The increase in ambient YS can be mainly attributed to the increased secondary phase strengthening of intermetallic compounds at grain boundaries and the enhanced solid solution strengthening of Gd in the Mg matrix. The increased volume fraction of thermally stable intermetallic compounds Mg12RE, Mg2Gd and Mg3Gd contributed to the increase of YS at elevated temperatures. The fracture mechanism of the Gd-containing alloys at ambient temperature was a quasi-cleavage fracture.