600℃热暴露对Ti65合金疲劳性能的影响OA

[Objective]This study aims to investigate the effects of different thermal exposure time on the microstructure,surface oxidation behavior,and fatigue properties of Ti65 alloy under 600 ℃ thermal exposure conditions,and clarify the mechanism underlying the influences of precipitated phase evolution and surface oxide layer formation on the fatigue life of the alloy during thermal exposure.[Methods]The Ti65 alloy was thermally exposed at 600 ℃ for 55,100,500,1 000,and 2 000 h,and the microstructure and surface oxidation behavior of the Ti65 alloy before and after thermal exposure were characterized.The rotating bending fatigue tests were conducted on the Ti65 alloy before and after thermal exposure to obtain the fatigue life under the condition of 650 ℃ and 401 MPa.[Results]The microstructural composition of the Ti65 alloy before and after thermal exposure is basically consistent.With the increase of thermal exposure time,the size and content of the primary αp phase and the lamellar width of the secondary lamellar αs phase show no significant changes,while the size and content of the α2 phase and silicides increase to some extent.The thickness of the oxide layer is approximately 540 nm when the thermal exposure time reaches 2 000 h.Additionally,thermal exposure significantly increases the surface roughness of the specimen.Furthermore,as the thermal exposure time prolongs,the number of specimens capable of reaching the fatigue limit(107)exhibits a trend of increasing first and then decreasing,indicating that the fatigue stability initially improves and subsequently deteriorates.[Conclusions]Analysis indicates that during thermal exposure at 600 ℃,an oxide layer forms on the surface of the Ti65 alloy specimen,and oxygen elements diffuse into the matrix,creating an oxygen diffusion layer.On the one hand,the formation of oxide particles during thermal exposure increases the surface roughness of the specimen.On the other hand,the oxide layer is primarily composed of ceramic-like brittle oxides.Both factors contribute to stress concentration,making fatigue cracks more likely to initiate under cyclic loading,thereby reducing the fatigue life of the Ti65 alloy.Simultaneously,during thermal exposure,the size and content of the α2 phase and silicides increase,enhancing their hindrance to dislocation motion.Overall,the fatigue properties of the Ti65 alloy are primarily influenced by the combined effects of precipitated phase evolution and surface oxide layer formation during thermal exposure,with a competitive relationship between the two.In the early stage of thermal exposure,the formation of the precipitated phase plays a dominant role,increasing the number of specimens that can achieve the fatigue limit as thermal exposure time prolongs.When the thermal exposure time reaches 100 h,the number of specimens reaching the fatigue limit peaks.As the exposure time is further increased,the influence of the surface oxide layer gradually becomes dominant,leading to a gradual reduction in the number of specimens capable of achieving the fatigue limit.