Grain growth and recrystallization of nanocrystalline Al3Ti prepared by mechanical alloying

The grain sizes and lattice strains during mechanical alloying of Ti-75 at.% Al powder mixtures were studied using X-ray diffraction methods. Nanocrystalline L12-Al3Ti was obtained after a certain time period of ball milling. Minimum grain sizes of 17 nm for Al and 28 nm for Ti have been determined using XRD. During subsequent thermal annealing processing, an obvious recrystallization resulting in significant reduction of grain size was observed. The recrystallization in nanocrystalline Al3Ti was affected by both the temperature and the degree of order. The incubation period for recrystallization at 400°C was about 6 hours while those at 510 and 700°C were about 2 hours. The completion time of recrystallization in Al3Ti at 400 and 700°C was about 15 hours and 8 hours at 510°C. It is clear that the recrystallization at 700°C was retarded as a result of the higher degree of order structure which limited the mobility of the boundaries. Phase transformation occurring within the recrystallization temperature range was observed to have little influence on the recrystallization itself. However, transformation products do have significant effects on it which is originated from the degree of order in the products. The recrystallization in this alloy system provides an excellent means to maintain the nanocrystalline microstructure during the necessary consolidation thermal cycle by decreasing the processing temperature and increasing the hold time considerably.