Evolution of microstructure and hardening, and the role of Al3Ti coarsening, during extended thermal treatment in mechanically alloyed Al-Ti-O based materials

Abstract Extruded rods of mechanically alloyed (MA) Al-0.25wt%C-10vol%TiO2 (A1), Al-0.35wt%Li-1wt%Mg-0.25wt%C-10vol%TiO2 (A2), and Al-0.35wt%Li-1wt%Mg-0.25wt%C-7.5vol%TiO2 (A3) have been subjected to thermal treatments at 500°C, 550°C, 600°C, and 655°C for times up to 1500 h. At 550°C and 600°C, the materials displayed hardness increments, but the increase was highest for A1 (∼30 kg/mm2) due to fine Al3Ti precipitation. The starting grain size of ∼0.5 μm in A1 showed virtually no increase even after 1500 h at 600°C. XRD and TEM indicated transformation of TiO2 and TiO to Al3Ti. Numerous “block-shaped” Al2O3 particles (α, δ, η, and γ polymorphs) were formed in A2 at 600°C. The α-Al2O3 particles exhibited some lattice matching with the α-Al matrix. An initially rapid Al3Ti coarsening rate at 600°C in A1 was reduced significantly between 336 h and 1500 h. Dispersion strengthening by Al3Ti and Al2O3 was mainly responsible for the maximum strength of A1 after heat treatment, while the Hall–Petch contribution was less significant, despite the fine grain size.