Effects of step-quenching on the α″ martensitic transformation, α precipitation, and mechanical properties of multiphase Ti–10Mo alloy

The effects of step-quenching on the microstructure and mechanical properties of a binary Ti–10Mo (wt%) alloy were investigated by transmission electron microscopy. The step-quenching treatment, consisting of solution treatment in the β phase field at 850 °C followed by step-quenching to the α/β two-phase region at 650 °C and holding for 0.5 h, was employed before water-quenching the alloy to room temperature. Direct quenching from 850 °C to room temperature with or without an aging step at 650 °C was also conducted for comparison. Microstructural observation revealed that step-quenching favored the formation of α precipitates thinner than 20 nm in the absence of α″ or ω heterogeneous nucleation agents and effectively moderated the subsequent α″ martensitic transformation by increasing the stability of the β phase. Step-quenching generated a multiphase microstructure comprising α″, β, ω, and α phases by balancing the competitive martensitic α″ and diffusional α transformations. Only α″ martensite was formed in the β matrix after direct water-quenching; the mixture of α″ + β phases was transformed to a lamellar α + β microstructure with 5 min aging. The kinetics of α precipitation was calculated to illustrate the temperature dependence of α precipitation behavior during step-quenching. Direct water-quenching produced a tensile strength of 688 MPa and 36% ductility. After aging, the tensile strength was increased to 837–867 MPa, while the ductility was decreased to 5%. By step-quenching, the high tensile strength of 790 MPa and ductility of 23% were achieved.