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 degrees C in air. Both alpha(2) and gamma lamellae were altered markedly after 1000- and 3000-h exposure. Three types of decomposition were found: (a) parallel decomposition of coarse alpha(2) into bunches of very fine alpha(2) + gamma lamellae, (b) perpendicular decomposition of fine gamma (or gamma + alpha(2)) lamellae into short-ranged gamma domains or (gamma + alpha(2)) grains and (c) perpendicular decomposition of fine alpha(2)/gamma lamellae into short-ranged alpha(2)/gamma 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 alpha(2), thin alpha(2), thin gamma, and thick gamma lamellae. Tensile properties were affected by these changes and the extent was found to vary from alloy to alloy. (C) 2000 Elsevier Science Ltd. All rights reserved.