Unusual hardening in Ti/Al₂O₃ nanocomposites produced by high-pressure torsion followed by annealing

Nanocomposites of Ti-18vol.% Al₂O₃ were fully consolidated by applying severe plastic deformation to powder mixtures using high-pressure torsion (HPT). The HPT-processed composites exhibited an unusual increase in the hardness from 350Hv to 650Hv when they were annealed at 973K for 1h. Microstructural observations including elemental mappings were conducted using scanning electron microscopy with an electron probe X-ray micro-analyzer and scanning transmission electron microscopy with an energy dispersive X-ray spectrometer. It was shown that the Al₂O₃ particles are fragmented and the grain size of the Ti matrix is reduced to the nanometer level (<100nm) after processing with HPT. Following the annealing, the fragmented Al₂O₃ particles with particle sizes less than ∼400nm are reduced to Al in the Ti matrix and the average grain size of Ti is coarsened to ∼500nm in conflicting with the hardness increase. The hardness increase after annealing is attributed to the dissolution of Al and O atoms in the Ti-matrix and the formation of reaction product, Ti₃Al intermetallic, at the interface and improvement of mechanical connectivity. Enrichment of Al along grain boundaries confirmed that Al atoms diffuse fast through the grain boundaries and move towards the grain interior from the grain boundaries.