Effects of reheating temperature and time on microstructure and tensile properties of thixoformed in situ Si_{p/ZA27 composites

The effects of reheating temperature and time on microstructure and tensile properties of thixoformed in situ Sip/ZA27 composites have been investigated. Simultaneously, the effect of the Si particles on the tensile properties has also been studied. The results indicate that the two parameters have similar effects on the composition, fraction and segregation degree of the liquid phase and the size and shape of the Si particles, and thus the solidification behaviour, the resulting microstructure, the fracture regime and the tensile properties. But in contrast, the effects of the reheating temperature are larger than those of the reheating time; the effects of the changes in the Si particle size and shape are smaller than those of the changes in the other factors. The optimum tensile properties, the ultimate tensile strength of 363 MPa and the elongation of 0.97%, can be obtained after being thixoformed under reheating for 70 min at 465°C. The Si particles exhibit two behaviours during tensile testing; namely fragmentation and interfacial de-bonding. This indicates that the load transfer mechanism is the main strengthening mechanism of the Si particles.