Synergistic Effects of Cerium-Based Rare Earth Addition and Hot Deformation on the Microstructure and Mechanical Properties of Mg-0.5Zn-0.5Zr Magnesium Alloy

The effects of cerium-based rare earth (RE) addition and hot deformation via extrusion on the microstructure and mechanical properties of Mg-0.5Zn-0.5Zr alloy were studied. It was found that RE addition could effectively refine the as-cast microstructure up to 2 wt% RE with a notable grain refining effect up to 1 wt% RE. High RE additions resulted in a less effective grain refinement and the appearance of coarse dendritic microstructure. While the as-cast strength and ductility were enhanced by increasing RE content up to 1 wt% due to grain refinement, alloys with high RE content faced a remarkable drop in total elongation and also showed lower ultimate tensile strength (UTS) values, which was related to the interconnected intergranular brittle intermetallics (Mg12RE) with high volume fractions. For the extruded alloy with 1 wt% RE, besides the fragmentation and dispersion of intermetallics, a remarkable grain refinement due to the recrystallization processes was observed. Accordingly, the UTS of 415 MPa, total elongation of 17% and tensile toughness of 65.9 MJ/m(3) were obtained after extrusion, which are quite superior to the corresponding values of 210 MPa, 12% and 22.1 MJ/m(3) for the as-cast counterpart, respectively. Regardless of the processing condition, the grain size effect was found to be the dominant factor in determining the yield stress (Hall-Petch law), where the deviation of this trend due to the presence of hard Mg12RE phase was discussed.