In-situ study of the time-temperature-transformation behaviour of a multi-phase intermetallic β-stabilised TiAl alloy

Intermetallic beta-stabilised TNM alloys with a nominal composition of Ti-43.5Al-4Nb 1Mo-0.1B (in at.%) exhibit excellent processing characteristics due to a high amount of disordered beta-phase present at hot-working temperatures. Balanced mechanical properties can be tailored by adjusting the material's microstructure in a post-forging multi-step heat treatment. In the present work, a TNM alloy with an increased content of beta-stabilising alloying elements (Nb, Mo) was studied by means of in-situ high-energy X-ray diffraction. Employing a dilatometer setup, forged and homogenised specimens were annealed in the (alpha + beta + gamma)-phase field region and subsequently subjected to cooling rates ranging from 35 to 1200 K min(-1). The evolution of phase fractions as a function of time and temperature was correlated with the resulting microstructure. Thereby, the focus was laid on the evolution of the gamma-TiAl phase, for which a continuous cooling transformation diagram was derived. In addition, in-situ heating experiments close to thermodynamic equilibrium along with quantitative metallography on heat-treated and water-quenched specimens gave ancillary information needed for temperature calibration. The performed in-situ diffraction experiments applying synchrotron radiation offered a deeper insight into the phase transformation behaviour of the investigated type of multi-phase alloy, which is not accessible with conventional characterisation techniques. (C) 2014 Elsevier Ltd. All rights reserved.