Microstructural Evolution of Vacuum Die-Cast AZ91D Magnesium Alloy during Solution Treatment

抄録

The microstructural evolution in different regions on the cross section of AZ91D vacuum die castings during solution treatment, especially during the early stage, was investigated. Optical microscope and scanning electron microscope were employed for the microstructure observation. The results exhibited that the supersaturated α-Mg solution located in the grain boundaries first dissolved. The β-Mg₁₇Al₁₂ particles in the surface layer had a slower dissolution speed when compared to those in the central region, and the remaining β-Mg₁₇Al₁₂ particles in the surface layer could effectively restrain the grain growth during the early stage. In the central region, due to the mergence of neighbouring dendrite arms of the same ESC, most remaining β-Mg₁₇Al₁₂ particles were encapsulated in the relatively large near globular α-Mg grains evolved from the ESCs. The shrinkage porosities concentrated in the segregation band could effectively restrain the grain growth, causing the segregation band maintained a fine microstructure during the solution treatment.

Fig. 7 Optical micrographs taken from different regions on the casting cross section after solution treatment for 30 min: (a) the surface layer, (b) the central region.