Abstract
The lamellar α₂(Ti₃Al) + γ(TiAl) microstructure of intermetallic Ti-46Al-8Ta (at.%) alloy is unstable and transforms to α₂ + γ + τ type at temperatures ranging from 700 to 800 °C during long-term ageing up to 10000 h. TEM observations and selected area diffraction patterns show that the ternary τ phase with B8₂ type structure (space group P6₃/mmc) is preferentially formed along the grain and lamellar colony boundaries. Volume fraction of the τ and γ phases increase with increasing ageing time at the expense of decreasing volume fraction of the α₂ phase at all ageing temperatures. The lattice parameters of the α₂ and γ phases change during ageing when compared with those of the as received alloy. The ageing temperatures have no measurable effect on the lattice parameters of the coexisting α₂, γ and τ phases after ageing for 10000 h. The Vickers microhardness decreases reaching a minimum after ageing for about 500 h and then increases to peak values with increasing ageing time. The ageing time longer than 1500 h leads to a continuous softening of the alloy at all ageing temperatures.

Key words
titanium aluminides, TiAl, phase identification, microstructure, diffraction

Full text (509 KB)