The effects of long-term air exposure on the stability of lamellar TiAl alloys

The thermal stability of fine-grained and coarse-grained lamellar TiAl alloys was studied at 700°C in air. Both α2 and γ lamellae were altered markedly after 1000- and 3000-h exposure. Three types of decomposition were found: (a) parallel decomposition of coarse α2 into bunches of very fine α2+γ lamellae, (b) perpendicular decomposition of fine γ (or γ+α2) lamellae into short-ranged γ domains or (γ+α2) grains and (c) perpendicular decomposition of fine α2 lamellae into short-ranged α2/γ grains. All three types are still at an intermediate stage after 3000-h exposure and neither break-up of lamellae nor globularisation of segments was observed. The thermal stability of microstructural constituents in increasing order was found to be: thick α2, thin α2, thin γ, and thick γ lamellae. Tensile properties were affected by these changes and the extent was found to vary from alloy to alloy.