Nucleation of massive gamma during air cooling of Ti46Al8Ta

Interrupted quenching has been used to investigate the factors influencing the nucleation of massive gamma during air cooling of Ti46Al8Ta from the alpha phase field; air cooling directly to room temperature results in virtually 100% massive gamma and water quenching results in virtually 100% retained alpha. If samples are air cooled to successively lower temperatures within the (α + γ) phase field, before being water quenched, it has been found that they contain increasing amounts of massive gamma. Measurements of the cooling rates during interrupted quenching and during air cooling to room temperature together with microstructural observations suggest that the undercooling required to nucleate massive gamma is reduced by the presence of “equilibrium” grain boundary gamma formed above T0. It has been shown that a {111} of the massive gamma is parallel with a {111} of the grain boundary gamma implying that the massive gamma is nucleated by the grain boundary gamma. These observations are discussed in terms of the factors which influence nucleation of massive gamma. The formation of significant amounts of grain boundary gamma above T0 (the temperature above which massive gamma cannot form) sets a lower limit to the cooling rate which can be used to generate virtually 100% massive gamma.