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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 → α lamellae , α lamellae → β, and α slab → 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. [ABSTRACT FROM AUTHOR]

A sophisticated understanding of phase transformations and microstructure evolution is crucial in mechanical property optimization for the newly developed low-cost Ti-35421 (Ti-3Al-5Mo-4Cr-2Zr-1Fe wt.%) titanium alloy. The phase transformations in dual-phase Ti-35421 were studied by experiments and thermo-kinetic modeling. The phase transformation reactions and temperature ranges were determined as β→αlamellar [410–660 °C], αlamellar→β [660–740 °C], αlath→β [740–825 °C]. The Gibbs-Thomson effect and multicomponent diffusivities were proven to be responsible for the distinguishing behaviors of growth and dissolution between two α phases. The aging temperature of 540 °C was optimized based on calculations. It introduced a bimodal microstructure containing stubby α lamellae and β matrix. The mechanical properties of bimodal Ti-35421 were tested and compared with baseline alloy Ti-B19 and other near-β titanium alloys. The 540 °C aged alloy exhibits an optimal combination of mechanical properties with tensile strength of 1313 MPa, yield strength of 1240 MPa, elongation of 8.62%, and fracture toughness of 75.8 MPa·m1/2. The bimodal Ti-35421 shows comparable performance to Ti-B19 but has lower cost in raw materials and processing. The results also demonstrate that thermo-kinetic modeling can effectively be utilized in tailoring microstructure and enhancing mechanical properties. [ABSTRACT FROM AUTHOR]