The effect of boron and alpha grain size on the massive transformation in Ti–46Al–8Nb–xB alloys

Samples of Ti–46Al–8Nb containing up to 1 at.% B have been examined using optical microscopy after cooling over a wide range of cooling rates from the α phase field in order to understand the influence of boron and grain size on the massive transformation. The grain size of the samples was controlled either by varying the boron level or by appropriate processing of B-free and B-containing alloys. The results show that the addition of boron suppresses the feathery and the Widmanstätten transformation. The massive transformation and the lamellar transformation are strongly influenced by prior α grain size independent of whether the grain size was achieved by heat treatment or by addition of boron. In fine-grained samples the range of cooling rates over which the massive transformation occurs is restricted by formation of the lamellar microstructure at high cooling rates. These observations are discussed in terms of the factors controlling the nucleation and the progression of these transformations.