Microstructure and Mechanical Properties of Friction Stir Process Derived Al-TiO₂ Nanocomposite

Aluminum-based composites have many advantages over their conventional counterparts. A major problem in such composites is the clustering of particles in the matrix. Friction stir processing (FSP) can homogenize particle distribution in aluminum-based composites. In this study, unannealed TiO₂ particles were used to prepare Al-TiO₂ nanocomposite using FSP. The TiO₂ particles, about 1 µm, were dispersed into an aluminum matrix by 6 passes of FSP. The TiO₂ particles were fractured by multiple FSP passes, leading to a nano-size particle distribution in the matrix. Nanoscale dispersion was confirmed by scanning electron microscopy and transmission electron microscopy. The fractured TiO₂ particles reacted with the aluminum matrix to form Al₃Ti intermetallic and Al₂O₃ ceramic. The progression of the Al-TiO₂ reaction from the fourth to the sixth pass of FSP was revealed by x-ray diffraction. Due to the nanoscale dispersion, the yield and ultimate tensile strength of the composite increased to 97 and 145 MPa, respectively. Ductility of the composite decreased marginally compared to the as-received aluminum. As the dispersed particles pin dislocations, the strain-hardening rate of the composite was considerably increased and the same was seen in the Kocks-Mecking plot. The TiO₂ particles are mechanically activated due to their fracture during FSP, hence leading to reaction with the matrix. The particle refinement and dispersion lead to a homogeneous matrix with higher strength.