Microstructural evolution and mechanical properties of a hot-rolled Ti6121 titanium alloy under various cooling rates post-solution treatment

The systematic solution treatment with various cooling rates followed by the identical aging treatment were performed to elucidate the effect of cooling rate post-solution treatment on microstructural evolution and mechanical properties of Ti6121 alloy. The crystalline structure and microstructure were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscope (TEM), respectively. The results indicated that the β phase could be adequately transformed into the primary α (α P ) phase precipitates accompanied by the alloying element partitioning during furnace cooling (FC), which enhanced the thermal stability of the β phase. With the increase of cooling rate, the β→α P phase transition was gradually suppressed, and a large amount of the fine acicular secondary α (α S ) phases were subsequently precipitated from the β phase during air cooling (AC). These nanoscale precipitates could effectively hinder the dislocation motion, inducing the substantial grain boundary strengthening. However, the complete inhibition of diffusion transformation resulted in the martensitic α' phase formation through shear-dominated mechanism during water quenching (WQ), and the 10 1 ̅ 1 nano-twins were activated by stress accommodation during the martensitic transformation. The tensile test results demonstrated a positive correlation between the cooling rate and the ultimate strength (UTS). The increment of the tensile strength was mainly attributed to the refinement of the critical α S phase. Moreover, the interface strengthening effect caused by the nano-twin boundaries further increased the strength of Ti6121 alloy. Meanwhile, a certain amount of the equiaxed α P phases with homogeneous dispersion characteristic could maintain the considerable ductility of Ti6121 alloy. • The morphology and content of α phases are regulated by controlling the cooling rate. • Strength was improved with increasing cooling rate, while the elongation had an opposite trend. • The improvement in yield strength was attributed to the refinement of acicular α S phase.