Morphological changes of the Ti3Al5 phase formed by phase-decomposition of TiAl intermetallics

When TiAl intermetallics containing Al of 56, 58 and 60 at.% are aged at 973 K, the TiAl phase decomposes into the two-phase state of Ti3Al5 and TiAl. Transmission electron microscopy observations showed the following microstructure changes because of ageing: (1) in Ti-56Al, first Ti3Al5 precipitate particles are thin plates having (001) surfaces and then they coalesce with each other to form large plates whose surfaces are not flat but undulating and consist of {113} planes; and (2) in Ti–58Al and Ti–60Al the tweed structure first appears, then Ti3Al5 particles grow and, finally, the coalescence takes place to form large rafts like folded strata. The computer simulations based on the Cahn–Hilliard non-linear diffusion equation can successfully reproduce such microstructure changes.