In-situ formation of Al2O3 nanoparticles during friction stir processing of AlSiO2 composite

Aluminum matrix composites reinforced with nanometer-sized Al2O3 particles were produced from AlSiO2 powder mixture by using friction stir processing (FSP). In the composites, Al2O3 particles were produced in situ by oxide-aluminum displacement reactions. This approach combined the hot working nature of FSP and an exothermic reaction between Al and SiO2. The Al2O3 particles were formed at the Al/SiO2 interface, and dispersed in the aluminum matrix by the stirring action of the rotating tool during FSP. Microstructural examinations revealed the Al2O3 particles present mainly in the form of clusters of nano-sized (approximately 20 nm) particles, which were identified as α-Al2O3. Because of the fine dispersion of nano-sized particles in an ultrafine-grained aluminum matrix, the composites exhibit superior tensile strength and ductility. Quantitative phase analysis indicated that the extent of the AlSiO2 reaction increased in conjunction with an increasing tool rotation rate and a decreasing tool traverse speed in FSP.