The spatial dependence of heterogeneous microstructure is crucial for additive manufacturing (AM) of high-temperature titanium (Ti) alloys rich in β-stabilizing element. However, the mechanism of phase and microstructure evolution remains unclear during the non-isothermal process, especially in the heating stage of AM technology. The present study focuses on the phase transition and microstructure evolution of a near-β Ti alloy fabricated by direct energy deposition during continuous heating, while the effects of different initial microstructures and heating rates on them are also investigated, systematically. The results demonstrate that the phase transition sequence of Ti alloy is β → α″ + α′ + β → α + β → β, starting from the initial single-β phase. The characteristic of nanoscale martensite phase varies with heating rate, leading to the formation of homogeneously distributed fine α lath. For the microstructure of fully intragranular Widmannstatten α phase (α WI ), its phase transition sequence during the heating process is α + β → β. The extent of dissolution for α phase lags behind the equilibrium value, since its transition is diffusion-driven, and the secondary α phase transforms into the β phase followed by the α WI lath. This work is of great significance for understanding the microstructure evolution of near-β Ti alloy fabricated by the AM technology, and thus guiding the design of new materials.
Zhang et al. (Sun,) studied this question.