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 and Ti-44Al-4Nb-4Zr-0.2Si alloys cooled from the (α + β) phase field using TEM and SEM. The morphology of the transformation products is shown to be sensitive to cooling rate and the results of the microstructural observations are used to construct a schematic continuous cooling transformation (CCT) diagram for these alloys. It is shown that under slow cooling, the beta phase is mainly consumed by the neighboring alpha grains via α/β interfacial migration. During the subsequent α → α + γ transformation which occurs at lower temperatures and produces a lamellar microstructure, the retained beta phase can transform partially to γ by a process analogous to the discontinuous coarsening of γ lamellae. It has also been shown that, at higher cooling rates, the beta phase can decompose to alpha via secondary lath formation and/or a Widmanstaetten transformation although transformation by α/β interfacial migration still occurs. The decomposition of beta directly to faceted gamma has also been observed both in slowly cooled and rapidly cooled materials. In the slowly cooled materials this is attributed to direct nucleation of gamma grains, whereas in the rapidly cooled materials they are thought to be the result of discontinuous coarsening. Furthermore, a B2 to ω transformation occurs readily in these alloys at temperatures higher than 700 C in both cases. Possible mechanisms for each of these transformations are discussed