Reactive mechanism and mechanical properties of in situ composites fabricated from an Al–TiO2 system by friction stir processing

In situ (Al3Ti + Al2O3)/Al composites were fabricated from powder mixtures of Al and TiO2 using hot pressing, forging and subsequent multiple-pass friction stir processing (FSP). The reactive mechanism and mechanical properties of the FSPed composites were investigated. Four-pass FSP with 100% overlapping induced the Al–TiO2 reaction, as a result of the enhanced solid diffusion and mechanical activation effect caused by the severe deformation of FSP. Decreasing the size of TiO2 from 450 to 150 nm resulted in the formation of more Al3Ti and Al2O3 particles. The formation mechanisms of Al2O3 and Al3Ti during FSP are understood to be a deformation-assisted interfacial reaction and deformation-assisted solution-precipitation, respectively, based on detailed microstructural observations. The microhardness, Young’s modulus and tensile strength of the FSPed composites were substantially enhanced compared with those of FSPed pure Al with the same processing history, and increased as the TiO2 size decreased from 450 to 150 nm. The strengthening mechanisms of the FSPed composites included load transferring, grain refinement and Orowan strengthening, among which Orowan strengthening contributed the most to the yield strength of the composites.