Microstructural evolution of mechanically milled and hot isostatically pressed nanocrystalline Ti-47.5Al-3Cr alloy annealed at 1200°C and air cooled

Mechanical alloying and hot isostatic pressing techniques were employed to synthesize a fully dense Ti-47.5Al-3Cr alloy containing equiaxed nanocrystalline γ-TiAl grains with a mean grain size of 42 nm. Isothermal annealing was conducted at 1200°C using holding times of 5, 10, 25 and 35 hours, followed by air cooling. Analytical transmission electron microscopy investigations revealed the presence of γ grains which coarsened during annealing. Rapid grain growth at the beginning of annealing and a saturation stage for the growth at annealing times after 25 hours were observed. The mean grain size increased from 42 nm to about 850 nm after 10 hrs annealing, and the saturation size was about 1 μm. Twins were observed inside some grains, the thickness of which increased during annealing. An orientation relationship between the grain matrix and twins was identified as [1̄10](1̄1̄2)_M∥[11̄0] (112̄)_T, with a common twinning plane of (111).