Effects of solution treatment on microstructure and mechanical properties of thixoformed Mg2Sip/AM60B composite

The studies on heat treatment of thixoformed magnesium based composites are very scarce. To verify the heat treatment behaviors of this kind of materials, the effects of solution treatment on microstructure and mechanical properties of thixoformed Mg2Sip/AM60B composite have been investigated. The results indicate that the microstructure evolution includes two stages during solutionizing at 415 °C. The first stage (0-2 h) refers to the rapid dissolution of eutectic β (Mg17Al12) phases and the resulted fast coarsening of both the primary α-Mg particles and secondarily primary α-Mg grains. The second stage (after 2 h) is regarded as the normal growth of the two kinds of grains through mergence. Due to the adding of Mg2Si phase, the microstructure evolution is slowed. The tensile properties first increase as the time increases from 0 h to 6 h due to the decrease of β phase harmfulness, increased solid solution strengthening and composition homogenization, and then decrease because of the grain coarsening. Correspondingly, the fracture regime changes from intergranular regime in turn to transgranular and intergranular regime. The effects of solution temperature are similar to those with the solution time. After being solutionized at 430 °C for 6 h, the tensile properties are up to the highest values, UTS of 277 MPa and elongation of 18.5%, increased by about 20% and 135% respectively compared with those of the as-thixoformed composite.