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

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%- 7.5vol%TiO2 (A3) have been subjected to thermal treatments at 500 degreesC, 550 degreesC, 600 degreesC, and 655 degreesC for rimes up to 1500 h. At 55 0 degreesC and 600 degreesC, tile materials displayed hardness increments, but the increase was highest for Al (similar to 30 kg/mm(2)) due to fine Al 3Ti precipitation. The starting grain size of similar to0.5 mum in A1 showe d virtually no increase even after 1500 h at 600 degreesC. XRD and TEM indi cated transformation of TiO2 and TiO to Al3Ti. Numerous "block- shaped" Al2 O3 particles (alpha, delta, eta, and gamma polymorphs) were formed in A2 at 600 degreesC. The alpha -Al2O3 particles exhibited some lattice matching w ith the ct-Al matrix. An initially rapid Al3Ti coarsening rate at 600 degre esC in Al was reduced significantly between 336 h and 1500 h. Dispersion st rengthening by Al3Ti and Al2O3 was mainly responsible for the maximum stren gth of Al after heat treatment, while the Hall-Fetch contribution was less significant, despite the! fine Slain size. (C) 2001 Acta Materialia Inc. pu blished by Elsevier Science Ltd. All rights reserved.