Physical metallurgy for wrought gamma titanium aluminides: Microstructure control through phase transformations

This paper characterizes the phase equilibria among β/α(α2)/γ phases and phase transformations involving β-Ti phase in Ti–Al–M (M is β stabilizer against α phase in pure Ti) ternary systems at elevated temperatures. Based on the knowledge, a novel method has been proposed to develop a new class of wrought gamma alloys having a good hot workability for processing and good toughness in service at around 900 K. The ternary systems exhibit the β+α(α2)+γ three-phase tie triangle moving toward the Ti–Al binary edge with decreasing temperature, which results in a unique composition range with the transformation pathway of β+α→α→β+γ. The high-temperature β phase is effective for hot forming processes, whereas the decomposition (β+α→α) and precipitation (α→β+γ) of the β phase make it possible to produce a fully lamellar microstructure with very fine β particles dispersed within the lamellae.