In Situ Observation of Various Phase Transformation Paths in Nb-Rich TiAl Alloys during Quenching with Different Rates

In recent years intermetallic gamma-TiAl based alloys with additional amounts of the ternary beta phase have attracted attention due to their improved hot workability. Depending on alloy composition and heat treatment the ternary beta phase can transform to different ternary phases at lower temperatures. A few of them are assumed to be detrimental to ductility. However, the current phase diagrams of these multiphase materials are often quite uncertain. Thus, the possible transformations of third phases in the temperature range between 700 and 1 100 degrees C are studied by means of in situ high-energy X-ray diffraction (HEXRD). Depending on quenching rate reversible transformations of beta(o) to different to related phases are observed, indicating a stepwise diffusion controlled transformation mechanism of B2-ordered beta(o) to B8(2)-ordered omega(o) Apparently, the addition of Mo can hinder this transformation.