A Dilatometer High-Energy X-Ray Study—Quenching from the α Phase Field in a Niobium Rich Titanium Aluminide Alloy

Intermetallic γ-TiAl based alloys have been successfully introduced as structural materials for low-pressure turbine blades in civil aero engines during the last decade. A possibility to expand the range of their use is the introduction of novel production methods like additive manufacturing (AM). The manufacturing process of TiAl alloys with a subsequent heat treatment has a large impact on the properties of the material. A challenge of AM processes are the very fast cooling and heating rates that enforce phase transformations far away from thermodynamic equilibrium. These transformation pathways are difficult to understand using conventional characterisation and analysis methods solely based on the resulting microstructures.<br/>In this study, <i>in situ</i> high-energy synchrotron X-ray diffraction was used to investigate the phase transformation pathway of a niobium rich TiAl alloy which was quenched from the α phase field and subsequently reheated. The alloy had a nominal composition of Ti-46Al–9Nb (in atomic %). First, the sample was heated to 1350 °C and quenched. In the subsequent reheating process the sample was slowly heated to 1250 °C with 10 °C per minute and cooled at the same rate. During these processes the X-ray diffraction patterns were continuously measured with frame rates up to 10 Hz.<br/>The quenched sample showed an incomplete massive transformation, with large remaining α grains. During reheating the analysis of the collected data showed two significant changings in the diffraction patterns starting at 425 °C and 750 °C. The width and intensity of the diffraction peaks of both the γ and α₂phase started changing at these temperatures. We suggest that at the temperatures of about 425 °C and 750 °C short- and long-range diffusion start to occur, respectively, leading to an equalizing of the chemical disequilibrium in the sample. For the reheating process, the c/a ratio of the γ and α₂phase was analyzed. The c/a ratio at room temperature of the γ phase increased from 1.005 before the reheating process to 1.011 at the end. For the α₂phase, the ratio decreased from 0.803 to 0.797 at the end. The different room temperature c/a ratios indicate a chemical disequilibrium in the material introduced during the quenching process, which is not the case during slow cooling.