Microstructure Evolution And Mechanical Property Decay In Ti-55511 Alloy During Fatigue And Stress Relaxation

Ti-55511（Ti-5Al-5Mo-5V-1Cr-1Fe）alloy has been widely used as structural materials in the fields of aerospace and petrochemical industry due to its low density,high specific strength and excellent corrosion resistance.As a main type of nearβTi alloy,the fatigue and stress relaxation of Ti-55511 alloy will lead to the change of its structure and the decay of its properties.Therefore,the morphology and distribution ofαphase have a great influence on the mechanical behaviour of Ti-55511 alloy.In this study,the effect of bimodal and lamellar microstructures,in whichαmorphology and distribution are totally different,on the mechanical behaviour of Ti-55511 alloys at room temperature and high temperatures using optical microscopy（OM）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and tensile and fatigue testing machine,etc.The main contents and conclusions are as following:（1）The microstructure evolution and fracture mechanism of bimodal and lamellar microstructures during mechanical tension at room temperature were investigated.Compared to lamellar microstructure,bimodal microstructure possessed higher yield strength and tensile strength while a lower elongation.During stretch,βmatrix bore the main plastic deformation in bimodal microstructure and the crack nucleated at the interface between the primary sphericalαphase（α_Pphase）andβphase.For lamellar microstructure,some coarse lamellarαphase（α_Lphase）at grain boundaries（GBs）took a severe plastic deformation and the crack were prone to nucleate and propagate along theα_Lphase at GBs which finally led to the intergranular fracture of alloy.（2）The fatigue crack initiation and propagation mechanisms of bimodal and lamellar microstructures were investigated.At the same stress level,lamellar microstructure possessed a longer fatigue lives and smaller fatigue crack growth rate than those of bimodal microstructure.At the late stage of fatigue,secondaryαphase（α_S）were cut and dissolved by the dislocation motion inβmatrix,which led to the obvious decrease ofα_S and the early decline in elastic modulus（E）.During fatigue crack propagation,the crack tended to nucleate at and propagated along the fracturedα_Pphase or its interface in bimodal microstructure while the crack were likely to grow along the coarseα_Lphase at GBs.（3）The effect ofαmorphology and distribution on the mechanical tensile behaviour and fatigue behaviour at high temperatures for bimodal and lamellar microstructure were investigated.Both microstructures exhibited an obivious strain hardening during stretch at 350℃,while they displayed an evident strain softing during stretch at 500℃.During fatigue loaded at high temperature,theα_Pphase were likely to fracture and evolved into crack in bimodal microstructure,while the crack were prone to initiate inα_Lphase at GBs.Besides,some secondaryα_Lphases(α_LS)with large misorientation might take place in theα_Lphase of lamellar microstructure.（4）The stress relaxation behaviour and microstructure evolution of bimodal and lamellar microstructures at 350°C,400°C,450°C and500°C were investigated.The relaxation limits（σ_∞）of both alloys decreased with the increase of temperature.At the same temperature,theσ_∞value of lamellar microstructure was obviously higher than that of bimodal microstructure.This is probably due to the fencing effect ofα_Lphase which impeded the dislocation motion inβand thus decreased the stress relaxation rate of alloy.