The formation of grain boundary gamma during cooling of Ti46Al8Nb

Samples of Ti46Al8Nb have been cooled from the alpha phase field at 4 °C/min and subsequently oil quenched; others have been end-quenched. The microstructures have been examined using optical and analytical scanning and transmission electron microscopy. It has been found that cooling to only 30 °C below the transus results in the formation of significant amounts of grain boundary gamma. This grain boundary gamma can grow into the alpha matrix; it can nucleate alpha/gamma lamellae, or massive gamma, or gamma/gamma lamellae; it can influence the fracture behaviour in massively transformed samples that have been heat treated to refine the microstructure. The factors that influence which structure is associated with the grain boundary gamma are investigated and it is clear that the cooling rate is the dominant factor. The significance of these observations is discussed in terms of the properties of cast samples which have been heat treated by precipitating alpha within the massive gamma.