| Metastable ?-titanium alloy has been the focus of the development and application of titanium alloy due to its excellent comprehensive properties.Unfortunately,the molybdenum equivalent of the commercial metastable ?-titanium alloys is relatively high,resulting in increased alloy costs,smelting difficulty,inhomogeneous precipitation,and a slow aging response.Therefore,it is of great industrial significance and scientific interest to study a new type of high strength metastable ?-titanium alloy Ti-3.5Al-5Mo-4V with near critical molybdenum equivalent and multi-component strengthening.In this paper,the ??? phase transition kinetics of the Ti-3.5Al-5Mo-4V alloy with two different grain sizes was investigated at the isothermal temperature of 500 ?.A method to estimate the function of the precipitate fraction of the ? phase with different aging times was developed based on X-ray diffraction analysis.On this basis,the microstructure and mechanical properties of Ti-3.5Al-5Mo-4V alloy during single and duplex aging were studied by adding low-temperature pre-aging treatment.The conclusions of this study can be summarized as follows:(1)The microstructure of the Ti-3.5Al-5Mo-4V alloy after solution treatment at 850? and 1050? presented a matrix of metastable ? grains of approximately equiaxed morphology.In the early stage of the aging treatment at 500?,the ? phase precipitated at the initial ? grain boundaries.The value of the ? precipitate fraction increased sharply at first and then increased slowly with increasing aging time,finally reaching equilibrium.The value of the ? precipitate fraction was higher in the alloy aged for the same time at a higher solution temperature,while the size of the ? precipitate was smaller in the alloy at a higher solution temperature.(2)The isothermal ??? phase transition kinetics under isothermal-aging treatments was modeled,and the kinetic parameters of JMAK deduced different transformation mechanisms in the process of the transition.The mechanism of the first-stage phase transition was dominated by mixed transformation mechanisms(homogeneously nucleated and acicular-grown ? structure,and grain boundarynucleated and grown ? precipitate),while the second stage involved the growth of fine ? precipitates controlled by slow diffusion.(3)The isothermal ? phase was precipitated by pre-aged at 250?,300? and 350? for different time after the Ti-3.5Al-5Mo-4V alloy solution-treated at 850? and 1050?.With the increase of the pre-aging time,the content of ? phase increased first and then decreased,and finally the ? phase disappears.At the same pre-aging temperature,the hardening curves of the alloy had the same trend.(4)The hardness of the Ti-3.5Al-5Mo-4V alloy increased sharply with increasing aging time under the condition of single aging-treated at 500?.After the hardness of the alloy reached the peak value,it began to decline.The hardness of the alloy was always higher at a higher solution temperature.However,under the duplex aging treatment,as the pre-aging temperature increased,the hardness of the alloy also gradually increased and remains basically stable in the later stage of the aging.(5)Under duplex aging treatment,the secondary ? phase changed from elongated needle shape to short needle shape with the extension of pre-aging treatment time,and finally to an irregular shape.Compared with single aging treatment,the secondary ? phase was refined obviously after duplex aging treatment,and as the pre-aging time is prolonged,the refinement of the secondary ? phase was more obvious.Under the duplex aging process of pre-aged at 300?,the secondary ? phase is the most uniform and fine.(6)The duplex aging treatment can effectively improve the strength of the alloy,but also reduce the plasticity of the alloy.At a higher solution temperature,the strength of the alloy is also higher.With the increase of pre-aging time,the strength and plasticity of the alloy increase gradually,the plasticity gradually decreases,and the fracture mechanism of the alloy changes from mixed fracture to brittle fracture. |
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