EVOLUTION OF MICROSTRUCTURE AND HARDENING, AND THE ROLE OF Al_3Ti COARSENING, DURING EXTENDED THERMAL TREATMENT IN MECHANICALLY ALLOYED Al-Ti-O BASED MATERIALS

摘要

Extruded rods of mechanically alloyed (MA) Al-0.25wt percent C-10vol percent TiO_2 (A1), Al-0.35wt percent Li-1wt percent Mg-0.25wt percent C-10vol percent TiO_2 (A2), and Al-0.35wt percent Li-1wt percent Mg-0.25wt percent C-7.5vol percent TiO_2 (A3) have been subjected to thermal treatments at 500 deg C, 550 deg C, 600 deg C, and 655 deg C for times up to 1500 h. At 550 deg C and 600 deg C, the materials displayed hardness increments, but the increase was highest for A1 (approx 30 kg/mm~2) due to fine Al_3Ti precipitation. The starting grain size of approx 0.5 #mu#m in Al showed virtually no increase even after 1500 h at 600 deg C. XRD and TEM indicated transformation of TiO_2 and TiO to Al_3Ti. Numerous "block-shaped" Al_2O_3 particles (#alpha#, #delta#, #eta#, and #gamma# polymorphs) were formed in A2 at 600 deg C. The #alpha#-Al_2O_3 particles exhibited some lattice matching with the #alpha#-Al matrix. An initially rapid Al_3Ti coarsening rate at 600 deg C in A1 was reduced significantly between 336 h and 1500 h. Dispersion strengthening by Al_3Ti and Al_2O_3 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.