Microstructure and texture development during friction stir processing of Al-Mg alloy sheets with TiO_2 nanoparticles

摘要

Aluminum matrix nanocomposites were fabricated by friction stir processing of Al-Mg alloy sheets with pre-placed TiO_2 nanoparticles at a concentration of 2 to 6 vol%. Microstructural studies showed that solid state reactions between the metal matrix and TiO_2 particles caused m situ formation of MgO and Al_3Ti nanophases with an average size ～50nm. 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 TiO_2 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 TiO_2 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 TiO_2 fractions.