Research On Tensile And Fatigue Failure Of SiC_f/Ti17 Composites

Continuous SiC fiber reinforced Ti matrix composites（SiC_f/Ti）have great application prospects in the aerospace field due to its high strength,high stiffness,good creep resistance and excellent fatigue resisitance.In order to ensure the safety of SiC_f/Ti-based composite components in service,it is necessary to conduct a comprehensive and in-depth study on the mechanical properties of SiC_f/Ti composite.Among them,the longitudinal tensile properties and fatigue properties must be the focus of research for unidirectionally reinforced SiC_f/Ti matrix composites.However,the interfacial reaction,matrix microstructure and longitudinal tensile behavior of SiC_f/Ti composites controlled by miscrostructure of Ti coating still needs to be studied further.In addition,the fatigue behavior of SiC_f/Ti matrix composites is also mainly concentrated on the condition that the maximum value of cyclic stress S_max is below1000 MPa stress level and the fiber volume fraction is about 35%.The longitudinal fatigue behavior of SiC_f/Ti matrix composites with higher fiber volume fraction at higher cyclic stress levels remains unclear.Based on this,the precursor filament was obtained by depositing Ti17 coating on SiC fibers by magnetron sputtering technology,and then SiC_f/Ti17 composites were prepared by hot isostatic pressing.Firstly,Ti17coatings with different microstructures were prepared by changing the substrate bias,and their effects on the interface reaction,matrix structure and tensile failure behavior of SiC_f/Ti17 composites were explored.In addition,the longitudinal fatigue failure behavior of SiC_f/Ti17 composites in the high cyclic stress range of S_max=1100～1700MPa was studied.The following results were obtained:1.The interface reaction and matrix evolution of SiC_f/Ti17 composites dominated by bias voltage are discussed firstly.Two kinds of Ti17 coating(Ti₀ and Ti_-200,respectively)was deposited on the surface of SiC fiber using magnetron sputtering technology by changing substrate bias voltage:0V and-200V,respectively,and further two kinds of precursor wire was obtained.And then SiC_f/Ti₀ and SiC_f/Ti_-200 composites were prepared by hot isostatic pressing（HIP）.Both Ti17 coatings areαphase,however,compared with Ti₀,Ti_-200 coating has smllar grains,narrower columnar units,denser structure and better mechanical properties.The microstructure of Ti17 coating further affects the interface reaction and matrix microstructure evolution of SiC_f/Ti17composites.The SiC_f/Ti_-200 has a thinner interface reaction layer thickness than SiC_f/Ti₀composite.SiC_f/Ti₀ composite has an equiaxed structure matrix,while SiC_f/Ti_-200composite has a lamellar structure matrix.2.The tensile behavior of SiC_f/Ti₀ and SiC_f/Ti_-200 composites was studied by fracture analysis.The fracture of the two composites is caused by the unstable growth of the cluster of sveral broken fibers in different planes,where SiC_f/Ti₀ has a macro fracture with an angle of 45°to the stress axis and its matrix shows ductile fracture mode,while SiC_f/Ti_-200 has a macro fracture perpendicular to the stress axis and its matrix presents brittle fracture mode.The lower interfacial shear strength and stronger matrix in SiC_f/Ti₀ composites make it have higher tensile strength.3.The longitudinal fatigue behavior of SiC_f/Ti17 composites with 51%core fiber volume fraction was studied in the range of S_max=1100～1700MPa cyclic stress level.The fatigue life of SiC_f/Ti17 composites decreases with the increase of S_max.The fatigue fracture mechanism and fatigue damage behavior of SiC_f/Ti17 composites are closely related to the applied stress level.The fatigue fracture of SiC_f/Ti17 composites is mainly caused by interface damage under S_max=1100MPa and 1400MPa.The crack initiated in RL deflects into the C/RL interface,and the type II interface crack will propagate and cause interface debonding.When the interface debonding reaches a certain degree,the fibers break rapidly and connect to each other to form a propagation crack,and then the joint of multiple propagation cracks causes the fatigue fracture of the composites.Under the S_max=1400MPa cyclic stress level,the fiber pullout on the fatigue fracture is smaller and the number of flat areas is more.However,at S_max=1700MPa,the fatigue fracture of SiC_f/Ti17 composite is mainly caused by fiber cumulative fracture and interface damage together.