In situ study of the continuous cooling transformation behaviour of Ti-Al alloys within the two-phase (alpha+gamma) region

Abstract

A fully computer-controlled real-time resistivity-temperature-time measurement apparatus was constructed to monitor in situ a wide range of cooling rates from slow (0.2 degreesC s(1)) to fast (3700 degrees C s(1)) and employed to measure quantitatively at various cooling rates the continuous cooling transformation behaviour of two Ti-Al alloys, namely Ti-45.5 at.% Al and Ti-46. 5 at.% Al. The alpha --> gamma transformation in a Ti-45.5 at.% Al alloy predominantly occurs through a lamellar reaction. At relatively fast cooling rates, the alpha --> alpha (2) ordering reaction directly competes with the lamellar reaction with little intervention of massive reaction. The measurement of ordering kinetics showed that the ordering reaction temperatures are almost constant, irrespective of the cooling rates. The ordering start temperature was measured to be 1075 degreesC, which is in good agreement with the metastable alpha-(alpha+alpha (2)) boundary line. The alpha --> gamma transformation in a Ti-46.5 at.% Al alloy occurs through a lamellar reaction at low cooling rates but through a massive reaction at high cooling rates which abruptly stops at the composition containing 45.5 at.% Al. The presence of this critical composition for the massive transformation suggests that the metastable gamma -solvus line is the natural limit for the massive transformation in this alloy system.