THE DECOMPOSITION OF THE BETA-PHASE IN TI-44AL-8NB AND TI-44AL-4NB-4ZR-0.2SI ALLOYS

The decomposition of the beta phase has been studied in Ti-44Al-8Nb an d Ti-44Al-4Nb-4Zr-0.2Si alloys cooled from the (alpha + beta) phase fi eld using TEM and SEM. The morphology of the transformation products i s shown to be sensitive to cooling rate and the results of the microst ructural observations are used to construct a schematic continuous coo ling transformation (CCT) diagram for these alloys. It is shown that u nder slow cooling, the beta phase is mainly consumed by the neighbouri ng alpha grains via alpha/beta interfacial migration. During the subse quent alpha --> alpha + gamma transformation which occurs at lower tem peratures and produces a lamellar microstructure, the retained beta ph ase can transform partially to gamma by a process analogous to the dis continuous coarsening of gamma lamellae. It has also been shown that, at higher cooling rates, the beta phase can decompose to alpha via sec ondary lath formation and/or a Widmanstatten transformation although t ransformation by alpha/beta interfacial migration still occurs. The de composition of beta directly to faceted gamma has also been observed b oth in slowly cooled and rapidly cooled materials. Tn the slowly coole d materials this is attributed to direct nucleation of gamma grains, w hereas in the rapidly cooled materials they are thought to be the resu lt of discontinuous coarsening. Furthermore, a B2 to omega transformat ion occurs readily in these alloys at temperatures higher than 700 deg rees C in both cases. Possible mechanisms for each of these transforma tions are discussed. (C) 1998 Acta Metallurgica Inc. Published by Else vier Science Ltd. All rights reserved.