TEM and DTA study on the stability of Al 5Ti 3- and h-Al 2Ti-superstructures in aluminium-rich TiAl alloys

Abstract

The phases Al 5Ti 3 and h-Al 2Ti, which are superstructures of the L1 0 TiAl structure, are frequently observed in as-cast and low-temperature-annealed aluminium-rich TiAl alloys. The strong decrease of the solubility of aluminium in TiAl with decreasing temperature leads to a supersaturation of the solid solution with aluminium during cooling. The decomposition of the supersaturated TiAl results in the precipitation of the superstructure phases at low temperatures. The evolution of the Al 5Ti 3 and h-Al 2Ti phases and the resulting microstructures were studied as a function of time, temperature, and composition by TEM and DTA investigations on Ti–Al alloys with 55 to 64 at.% Al. Both superstructures were found not to be equilibrium phases. Al 5Ti 3 is metastable below a composition-dependent critical temperature in the range of about 750–900°C with a maximum value reached near the stoichiometric composition. Above this temperature, Al 5Ti 3 rapidly dissolves. Extended lamellar Al 5Ti 3+TiAl microstructures have been found in a Ti–60 at.% Al alloy after low-temperature annealing, whereas in Ti–62 at.% Al large single-phase domains of Al 5Ti 3 have grown. h-Al 2Ti is a metastable phase at least up to 1200°C. It slowly transforms into the equilibrium phase r-Al 2Ti during annealing.