In-situ observation microstructure evolution and growth kinetics of lamellar γphases in Ti44Al alloy during heat treatment

To study mechanism of microstructure evolution in TiAl based alloys during heat treatment, Ti44Al alloy was heat treated at 1240 °C for 0.5/1.0/2.0/2.5/3.0/5.0/7.0 h before being quenched in water. Chemical composition, phase component and microstructure are characterized by XRF, XRD, OM, SEM and EDS. In addition, microstructure evolution of Ti44Al alloy was in-situ observed through laser scanning confocal microscopy (LSCM) coupled with digital image correlation techniques during heat treatment at 1240 °C. Results show that both as-cast and heat-treated Ti44Al alloys contain two phases. As-cast alloy is fully lamellar microstructure. γphases will dissolve, re-precipitate and grow up in turns with extension of heat treatment time. Microstructure evolution of Ti44Al alloy obtained through water quenching is consistent with the results through in-situ observation during heat treatment. Lamellar microstructure becomes more uniform, due to sufficient precipitation of γphases during heat treatment in (α + γ) phase domain. Micro-hardness reaches the highest value about 440 HV after heat treatment for 2.0 h. Besides, a model of "diffusion and thickening"is proposed in this study, namely that morphology evolution of γphases in gains is achieved through directional diffusion of Al and Ti atoms during heat treatment. In this process, lamellar γphases grow up and thicken due to some adjacent γphases dissolving. Based on the model, diffusion coefficient (Dv = 2.94 × 10-3 μm2/s) of Ti atoms at 1240 °C is calculated in this study, which provides basic data for controlling morphology of lamellar phases during subsequent heat treatment.