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 allo ys was studied at 700 degrees C in air. Both alpha(2) and gamma lamellae we re altered markedly after 1000- and 3000-h exposure. Three types of decompo sition were found: (a) parallel decomposition of coarse alpha(2) into bunch es of very fine alpha(2) + gamma lamellae, (b) perpendicular decomposition of fine gamma (or gamma + alpha(2)) lamellae into short-ranged gamma domain s 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 ther mal 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 foun d to vary from alloy to alloy. (C) 2000 Elsevier Science Ltd. All rights re served.