On the hardening of friction stir processed Mg-AZ31 based composites with 5-20% nano-ZrO2 and nano-SiO2 particles

Mg-AZ31 based composites with 10-20 vol% nano-sized ZrO2 and 5-10 vol% nano-sized SiO2 particles were fabricated by friction stir processing (FSP). The clusters of the nano-ZrO2 and nano-SiO2 particles, measuring 180-300 nm in average, were relatively uniformly dispersed. The average grain size of the Mg matrixes of the composites varied within 2-4 mu m after four FSP passes. No evident interfacial product between the ZrO2 particles and Mg matrix was found during the FSP mixing ZrO2 into Mg-AZ31. However, significant chemical reactions occurred at the Mg/SiO2 interface to form the Mg2Si phase. The mechanical responses of the resulting nano-composites in terms of hardness and tensile properties of these Mg/nano-ZrO2 and Mg/nano-SiO2 composites were examined and compared. The grain refinements and the corresponding hardening mechanisms are also analyzed and discussed.