Effects of Gd Addition on the Microstructure and Tensile Properties of Mg–4Al–5RE Alloy Produced by Three Different Casting Methods

Abstract

This work deals with the effect of 0.67 wt% Gd addition on the microstructure and tensile properties of Mg–4Al–5RE (where RE represents La–Ce mischmetal) alloy produced by sand casting (SC), permanent mold casting (PMC), and high-pressure die casting (HPDC). The results show that Gd addition could refine the grains, but its efficiency decreases by increasing the cooling rate due to the shifting from SC to PMC and finally to the HPDC method. Meanwhile, the acicular Al11RE3 phase is modified into the short-rod or granular-like shape under the three casting conditions. Such refined and modified microstructures are due to the Al2(Gd, RE) phases, which act as the nucleation sites in both the α-Mg matrix and Al11RE3 phase. Also, the weakening grain refinement effect in the increased cooling rates can be attributed to the narrow constitutional undercooling zone. After Gd addition, the 0.2% proof strength of the SC and PMC alloys increases by about 16.9% and 12.7%, respectively, while in the HPDC alloy, it decreases by about 5.9%. The main factor in the strength increment of the SC and PMC alloys is the grain boundary strengthening due to grain refinement which is proved by modeling the related mechanisms, whereas weak secondary phases and grain boundary strengthening mechanisms in the HPDC alloy lead to strength reduction. After Gd addition, the elongation to failure of the SC, PMC, and HPDC alloys is significantly enhanced by about 34.8%, 20.2%, and 12.3%, respectively, due to the crack resistance nature of the modified short-rod/granular Al11(RE, Gd)3 phase compared to the acicular one.