Influence Of Microstructure On Fatigue Damage Behavior Of Ti-55531 Alloy With High Strength And High Toughness

The fatigue failure,which is the common fracture mode in manufactured products and very important factor for safety of the airplane or spacecraft,always is the research focus in the field of microstructure-properties of materials.In recent decades,high strength & toughness titanium alloys have been successfully developed as the structural materials to make many significant components in aircraft industries,for instance,landing gear forgings,truck beams and springs in the aircraft,owing to their high special strengths,favorite corrosion resistance,and excellent fatigue properties,et al.With the rapid development of the aerospace industry,components in aircraft industry should own excellent comprehensive properties with higher fatigue limit,strength and plasticity to meet complicate conditions.However,it is very difficult that studying on fatigue damage and failure mechanisms of high strength & toughness titanium alloys,because of the diversity of influential factors on fatigue failure and the complexity of microstructureproperties relationships of high strength & toughness titanium alloys.Therefore,it is an essential and challenging problem that how to reveal the effect of microstructure features on fatigue damage behavior of high strength & toughness titanium alloys,and to tail the fatigue property of high strength & toughness titanium alloys,which should be urgently solved in the field of engineering and theory.Thus,the new high strength & toughness alloy Ti-5Al-5Mo-5V-3Cr-1Zr(Ti-55531)with lamellar microstructure(LM)and bimodal microstructure(BM)were chosen to be the researched subject.The deformation and fracture behavior of the alloy under situ loading were carefully studied.The high cycle fatigue(HCF)and low cycle fatigue(LCF)cracks initiation and propagation mechanisms of the alloy were also investigated by cyclic loading.The main conclusions can be summarized as follows:The tensile and torsion deformation behavior of the alloy were systematically investigated to reveal the critical microstructure and deformation mechanism.Results indicate that both loading type and microstructure have a great influence on deformation behavior of the alloy.The tensile strength of BM is higher than that of LM,while the elongation of BM is slightly lower than that of LM.The deformation of BM is mainly affected by globular ?p,and the primary deformation mechanism of ?p is prismatic slip.The higher strength of BM is casued by the fine ?trans laths.However,the deformation of LM is primarily controlled by coarsening ?s lamellae,and the main deformation mechanisms of ?s lamellae are slip and twinning.Twinning promotes the elongation of LM is slightly larger than that of BM.Both strength and ductility of BM are slightly lower than those of LM under torsion loading.The main microstructure and deformation mechanism of BM during torsion deformation are similar to those under tensile loading.The primary microstructure of LM is also similar to that during tensile deformation.However,the mechanism of LM is just slip under torsion loading,which leads to the torsion strength of LM is higher than that of BM.LM and BM were easier to be damaged under torsion loading than those of under tensile loading.Based on the in-situ SEM tensile test,deformation and fracture behavior of Ti-55531 alloy were observed under situ loading.The results show that ? phase with different characteristics owns different hardness and strength,which results in different fracture behavior of LM and BM.Microcracks mainly initiated at ?/? interphases and propagated along ?/? interphases in LM,which resulted from that ?/? interphases are the weakest microstructures of LM.However,the weakest microstructure of BM is the ?p,which results in microcracks primarily nucleated at ?p/?trans interfaces and at ?p particles interiors in BM,and then propagated along ?p/?trans interfaces.According to the results of HCF test,the effect of microstructure on HCF property and deformation mechanism of Ti-55531 alloy can be revealed.The schemic of HCF microcracks mechanism of the alloy with LM and BM were also drawn.BM presents much higher yield strength,lower ductility and slightly higher HCF strength than that of LM.It seems that the HCF limit of the alloy is mainly dependent on its yield strength.The Goodman fatigue programs of the two microstructures were drawn according to their HCF limits.In order to reveal fatigue crack initiation mechanisms of LM and BM,the fracture morphology and dislocation structures of HCF fatigued samples were analyzt.The cyclic deformation of LM was mainly controlled by slip and {101—1}? twinning of ?s lamellae.Whereas,the cyclic deformation of BM was primarily depended on prismatic slip of ?p,and slip and {1—011}? twins of ?s platelets.Fractographs display that all HCF cracks nucleated at subsurface of LM and BM fatigued specimens.Both ?GB and smallscale(?20 ?m)heterogeneous microstructure regions are the weakest microstructures of LM,which promote the initiation of fatigue microcracks.Fatigue microcracks primarily nucleated at ?p/?trans interfaces and at ?p particles interiors in BM.The grain boundary ? plays a more obvious role in promoting crack initiation in LM than that in BM.The results of LCF show that the cyclic response of alloy is greatly controlled by the strain amplitude during LCF loading.When the strain amplitude is less than 0.8%,both LM and BM present to cyclic hardening during first stage of cyclic deformation,and the cyclic hard ratio of BM is higher than LM,then present to cyclic saturation.With increasing of strain amplitude,as the strain amplitude is more than 0.8%,both BM and LM start to cyclic softening from the first stage of cyclic deformation.The cyclic soft ratio of LM is slightly higher than that of BM under the strain amplitude ranging from 0.8% to 1.0%.However,when the strain amplitude ranges from 1.0% to 1.5%,the cyclic soft ratio of LM is much lower than that of BM.In order to reveal fatigue crack initiation mechanisms of LM and BM under LCF loading,the fatigued fracture morphology and dislocation structures of LCF fatigued samples were analyzt.The cyclic deformation of LM was mainly controlled by slip and twinning of coarse ?s lamellae during LCF deformation.The cyclic deformation of BM was primarily contributed by slip and a little twinning of globular ?p duing LCF deformation.The thoughness(KIC)and the fatigue crack growth rate(da/dN)of LM and BM were investigated comparatively.Results display that microstructure has much influence on the damage tolerant performance of Ti-55531 alloy.Furthermore,this effect is so different when the loading type is different.The KIC value of LM(KIC=67.2 MPa·(?))is slightly higher than that of BM(KIC=63.2 MPa·(?))during situ loading.The da/dN value of LM is much lower than that of BM during the stage of the ?K >26.47 MPa·(?) under cyclic loading.However,the da/dN value of LM is slightly higher than that of BM during the?K<26.47 MPa·(?).In order to study crack propagation mechanisms of LM and BM,the fracture morphology and crack front profiles of HCF,LCF,KIC and da/dN specimens were thoroughly observed.Results indicate that crack propagation mechanisms of LM and BM are so different due to its different crack initiation mechanisms.Cracks propagated along GB and ?s/? interphases caused by cracks initiated at ?GB and ?s/? interphases in LM.The cracks mainly nucleated at ?p/?trans interfaces and at ?p particles interiors in BM,which results in the cracks propagated along ?p/?trans interfaces and transferred across ?p particles in BM.The direction of crack change greatly with high steps during transferring across GB or coarsen ?s/? interfaces,which leads to high crack steps of LM.Therefore,the crack front profile of LM is rougher than that of BM.