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

Abstract

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. γ phases 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 γ phases 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 γ phases in gains is achieved through directional diffusion of Al and Ti atoms during heat treatment. In this process, lamellar γ phases grow up and thicken due to some adjacent γ phases 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.

Keywords

• TiAl alloy
• Microstructure evolution
• Heat treatment
• Diffusion
• Kinetic model
• In-situ observation