Ti-6Al-4V (Ti64) wrought components are commonly used by the aerospace industry for their good fatigue resistance and high specific strength. Recently, additive-manufactured Ti64 has attracted much attention due to the reduction in component build time and machining costs. The Ti64 α phase has a hexagonal closed packed crystal structure at room temperature, but transforms to the body centred cubic β phase above ~980°C. Upon cooling from the β phase to room temperature, the α grains adopt one of twelve orientations directly related to the parent β phase through the Burgers orientation relationship (BOR), and so a small β-grain size is desirable to reduce texture strength and improve mechanical properties. It is thus of interest to understand the β recrystallisation (RX) mechanisms and subsequent grain growth seen in wrought or additively manufactured Ti64 components. Upon transformation, α laths in Ti64 tend to exhibit variant selection whereby a subset of the maximum possible twelve BOR-related orientations nucleate, often forming clusters of the same single orientation in colonies. Therefore it is also of interest to understand how the α phase nucleates during cooling and how colony formation and variant selection occur throughout the transformation, so that processing routes can be selected to favour α microstructures with the weakest textures.
The present work applied in-situ EBSD analysis to study a new Ti64 RX mechanism occurring during the transformation from the α to β phase, the subsequent growth of the β grains above 980°C, and the α-phase nucleation during cooling below the β transus temperature. The samples were observed in real time using an SEM Gatan heating stage and an Oxford Instruments Nordlys EBSD detector fitted with infra-red protection. The EBSD results revealed the possible link between twinning and the RX mechanism in the growing β phase during heating, and also showed the heating-rate sensitivity of the RX mechanism. During the β to α transition upon cooling, individual α lamellae were observed as they nucleated on the β-grain boundaries, and the α variant selection was shown to be dependent on the cooling rate.