Microstructural Evolution of Rheo-Diecast AZ91D Magnesium Alloy with Gadolinium Addition

The rheo-diecasting (RDC) process, a novel semi-solid processing technology, was used to produce cast components with high integrity, fine and uniform microstructure, and therefore enhanced performance. AZ91D samples with 1-3 mass%Gd were solidified by RDC process. It was found that under intensive forced convection, the primary α-Mg phase produced inside the twin-screw slurry maker had fine particle size, spherical morphology and uniform distribution throughout the samples. Microstructure observations showed that Gd addition resulted in the formation of Al2Gd which dispersed in the α-Mg matrix. The size and amount of β-Mg17Al12 phase was reduced and its continuity was broken, which was the main reason for improving mechanical properties of the AZ91D alloy at high temperatures. The amount of Al2Gd particles increased with increasing Gd addition. From EPMA quantitative analysis, almost all Gd reacted with Al, leading to the low concentration of Gd in the α-Mg matrix. The Vickers hardness increased monotonously from HV=50.4 to HV=67.3 with increasing amount of Gd. This improvement was attributed to the consummation of aluminum in melt by precipitation of the Al2Gd phases.