Microstructural evolution and mechanical property of isothermally forged BT25y titanium alloy with different double-annealing processes

In order to eliminate the residual stress and inhomogeneous microstructure caused by hot deformation and achieve preferable mechanical performance, double annealing was carried out on the isothermally forged BT25y alloy. Microstructural evolution with different annealing conditions was analyzed by means of SEM and TEM observation. Quantitative relationships between microstructure characteristics and mechanical properties were obtained. Results show that peanut morphology, short acicular projections at rod-like α terminals or special spine feature at β grain boundaries generate during first annealing at different temperatures. Lamellae globularization through boundary splitting occurs during isothermal forging and initial stage of first annealing, and microstructure coarsening through termination migration and Ostwald ripening plays the dominant role in prolonged annealing. Secondary lamellar α precipitated on the β matrix and grain refinement of equiaxed α result in increased yield stress. Microstructure characteristics of α phase morphology, α colony size and β grain size determine the effective slip length and thus affect the tensile ductility. The disorderly distributed lamellar α, β grain size and continuity of grain boundary α layer are three main factors influencing the fracture toughness of BT25y alloy.