Time-resolved and In-situ Observation of Solidification in TiAl Alloys

Abstract

Time-resolved and in-situ observation of solidification by X-ray imaging is a powerful method to empirically understand the solidification process of metallic materials. Observations began with low-melting-point alloys such as Al, and were later applied to the observation of Fe-based alloys in the temperature range above 1750 K. In Ti–Al binary system, there is a γ phase (TiAl intermetallic compound) in addition to an α phase and a β phase, which results in a relatively complex phase diagram with multiple peritectic solidification. Therefore, although there are various possibilities for phase selection and microstructure formation during the solidification process, the solidification process has not been empirically clarified. Therefore, we used single crystal MgO as the sample holding container to suppress the reaction with the reactive molten Ti as much as possible, and for the first time, we successfully observed the solidification process of TiAl alloys. Time-resolved and in-situ observations showed that a solid state phase transformation with repeated nucleation and growth was selected instead of the peritectic solidification from α to γ phases shown in the Ti–Al binary phase diagram. The α/γ transformation should also be taken into account in TiAl alloys, since the α/γ transformation formed a fine γ phase and affected the microstructure formation. This understanding of the α/γ transformation was expected to provide new guidelines for controlling the solidification structure of TiAl alloys.