The role of oxygen content and cooling rate on transformations in TiAl-based alloys

Samples of Ti46Al8Nb (at.%) with oxygen contents of 500 or 1500 wt ppm have been cooled at various rates in order to further our understanding of the factors which influence the phase transformations in these alloys. In agreement with earlier work it has been found that increase of oxygen can suppress the massive transformation. At high cooling rates retained alpha (ordered alpha 2) is found, with a very limited amount of massively transformed grain boundary gamma in both alloys. At lower cooling rates the response to quenching is very different in the different oxygen content samples. Retained alpha, heavily faulted alpha or fine lamellae are formed at grain boundaries as the cooling rate is decreased in the higher oxygen samples with limited massive gamma in grain centres, whereas in the 500 ppm sample the microstructure is virtually fully massive at these cooling rates. The amount of massive gamma which appears to be nucleated away from the original alpha boundaries is also strongly influenced by cooling rate and oxygen content. These results are interpreted in terms of the overall oxygen level, the extent of the segregation of oxygen during quenching, the nucleation of massive gamma in grain centres and the instability of retained alpha during slow cooling and at saltbath temperatures.