Production and microstructural investigation of cast ZrO2/AZ91 nanocomposite

Application of ceramic reinforcements is one of the effective and well-known ways to refine the microstructure of brittle metals such as magnesium. In this research, the influence of nano/micro particles of zirconia on the microstructure of cast AZ91 alloy was studied. At the first stage, nano and micro ZrO2 powders were blended through mechanical alloying procedure. In five specimens, the total amount of nano and micro reinforcements in the final mixture was fixed at 5 wt%, whereas their ratio was varied. Two other composites were also produced using 5wt% of nano or micro particles of zirconia. These powder mixtures were then stirred in the molten AZ91 at 650°C by vortex method and finally cast in a sand mold at 615°C. For comparison, two monolithic castings including a conventionally cast specimen and a super heat-treated sample were also cast. The average grain sizes for all composites were decreased with respect to both monolithic castings. The best results in terms of grain size and microstructure improvement were obtained for AZ91/5wt% nano ZrO2 composite with remarkable improvement in comparison with monolithic castings.