| The second phase(? phase or ? phase)precipitated during the aging heat treatment after quenching is the main strengthening method of the ?+? two-phase titanium alloy and the metastable ? titanium alloy.Stress-induced martensitic transformation in titanium alloys has an important influence on the mechanical properties of titanium alloys.Therefore,the research on the influencing factors of the stress-induced martensitic transformation critical driving stress of titanium alloy and the regulation of stress-induced martensitic transformation by appropriate heat treatment process have been hot topics.In this paper,microstructures with and without ?" martensite were obtained by quenching the Ti-3Al-5Mo-4.5V alloy at 800? and 750? separately.The quenched samples were subjected to different aging conditions and the effects of different aging treatments on the microstructure evolution of the alloy were systematically investigated by using SEM,XRD,TEM and other advanced characterization techniques.Room temperature tensile test and hardness test were carried out to evaluate the effect of different aging treatments on the mechanical properties of the alloy.The effect of aging conditions on the triggering stress of stress-induced martensitic transformation was studied.The result indicates that stress-induced martensitic transformation occurred during tensile process in the samples quenched at both 750? and 800?.For the samples quenched at 750?,? precipitation completely inhibited the stress-induced martensitic transformation during deformation and significantly improved the strength and hardness of the alloy aging at temperatures below 400?.? phase precipitated aging at 450? and 500?.With the increasing aging temperature,the strength and hardness decreased and plasticity increased due to the increase of grain size of a precipitation.For the samples quenched at 800?,? precipitation was not observed after aging treatment and the athermal martensite obtained during quenching treatment decomposed aging at temperatures above 450?.Stress-induced martensitic transformation occurred aging at temperatures below 300? and the triggering stress increased with the increase of aging temperature or aging time.Aging at 350?,the strength and hardness increased greatly owing to the enhanced age-hardening effect,resulting in the loss of plasticity and the brittle fracture during tensile test.Aging at 450?,the strength and hardness increased further because of more a phase precipitated due to the decomposition of martensite.Aging at 500?,the strength and hardness decreased and plasticity increased due to the increase of grain size of a precipitation. |
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