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.
This work was supported by the National Natural Science Foundation of China (NSFC) under Grant No. 51271107 and the Shanghai Committee of Science and Technology, China, under Grant No. 16520721700. Support by the Instrumental Analysis and Research Center of Shanghai University is gratefully acknowledged.
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Conflict of interest
The authors declare that they have no conflict of interest.
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