Microstructure and texture development during friction stir processing of Al-Mg alloy sheets with TiO2 nanoparticles

Aluminum matrix nanocomposites were fabricated by friction stir processing of Al-Mg alloy sheets with pre-placed TiO2 nanoparticles at a concentration of 2 to 6 vol%. Microstructural studies showed that solid state reactions between the metal matrix and TiO2 particles caused in situ formation of MgO and Al3Ti nanophases with an average size ~50 nm. These nanophases were homogenously distributed in an ultra-fine grain structure (0.2-2 µm) of the base metal. The results of pole figures evaluation obtained by electron back scattered diffraction studies revealed that the random orientation of initial annealed sheet was changed to components near to shear and silver texture in the friction stir processed alloy without and with pre-placed powder, respectively. The concentration of TiO2 particles affected the preferred texture orientation as the ceramic inclusion restricted the severe plastic deformation and dynamic recrystalization of the metal matrix. Hardness and tensile yield strength of the Al-Mg alloy sheet were also significantly improved by employing friction stir processing in the presence of TiO2 nanoparticles (up to ~3.1 vol%). Fractographic studies showed a mixture of ductile-brittle fracture modes with an increase in the content of catastrophic manner at higher TiO2 fractions.