Microstructure, thermal behavior and mechanical properties of squeeze cast SiC, ZrO2 or C reinforced ZA27 composites

Abstract

ZA27 alloy based composites were synthesized by stirring method, followed by squeeze casting. Stir casting was employed successfully to incorporate 5vol.% of various reinforcement particulates, namely, SiC, ZrO2 or C. The porosity in the composites was decreased by squeeze pressure. The presence of particles and/or application of squeeze pressure during solidification resulted in considerable refinement in the structure of the composites. The microstructures, X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDXA) results indicated that no significant reactions occurred at the interface between the SiC or C particles and ZA27 alloy. However, in case of ZrO2 reinforced ZA27, the ZrO2 reacted with Cu present in the molten ZA27 alloy, forming Cu5Zr. Thermal analysis showed that both α and β nucleation and growth temperatures of the composites were lower than those of the ZA27 alloy. The presence of particles in the as-cast or squeezed composites led to not only an accelerated age hardening response, but also an increase in the peak hardness of the composites. The values of coefficient of thermal expansion (CTE) of the composites were drastically lower as compared to those of the ZA27 alloy. The tensile properties of the composites decreased as a result of the addition of the particles. Scanning electron microscope (SEM) pictures of the composites indicated that cracks mainly initiated at particle–matrix interface, propagated through the matrix and linked up with other cracks leading to failure of the composites.