Phase transformations studies of TiAl-based alloys solidifying through the β phase

Following alloy development programs undertaken by ONERA with the aim to reduce the L->alpha columnar as-cast structure, a new beta-solidification TiAl (G4) alloy has been developed as a casting material with equiaxed microstructure for high temperature gas turbine applications. Since then, such a multi-phase material was found to induce a limited influence of aluminum content oil mechanical properties. Further microscopic insights were however needed for a better understanding in the phase transformations mechanisms for such beta solidification alloys. This paper highlights what are the structural factors, either related to the beta solidification or to the subsequent cooling, that can be used to identify the precise step reactions from the liquid for this category of TiAl-based alloys. Then, emphasis is given to the various solid-state transformations involving the beta phase such as beta->gamma, beta>alpha(2) and beta->omega those depending oil the alloy chemistry and the beat treatment conditions.