Effect of α morphology on the diffusional β ↔ α transformation in Ti-55531 during continuous heating: Dissection by dilatometer test, microstructure observation and calculation

The α ↔ β phase transformation in dual-phase Ti-55531 alloy during the continuous heating was investigated by combining dilatometer test with microstructure characterization and local composition mapping. It was found that the α ↔ β phase transformation in individual temperature regions mainly involved either of two types of α phases, i.e. the lamellar α phase which forms during the continuous heating or the slab α phase raised in preliminary isothermal reaction. The corresponding transformation sequence was verified as β → αlamellae, αlamellae → β, and αslab → β. The pile-ups of alloying elements at the α/β interface were understood by either local equilibrium condition or differences among net diffusion fluxes of alloying elements. More important was that the growth and dissolution kinetics of both α phases were modeled and compared with experiments. With the calculations, the dissolution of slab α phase was demonstrated to follow the kinetics of planar interface without the Gibbs-Thomson effect. In contrast, the Gibbs-Thomson effect was proven to do contribution to lamellar α phase. It not only decreased the stability of α tip, such that the re-dissolution of lamellar α tips took place at 838 K below the equilibrium temperature, but also introduced a competition between the shrink of tips and thickening of broad face in α lamellae at 838-911 K. Moreover, the lengthening of lamellar α tips was proven to follow a kinetics controlled by a mixture of bulk and boundary diffusion, in comparison with the pure bulk diffusion of fastest Al governed the re-dissolution of both tips and broad faces.