Research On Effects Of Grain Boundary Feature On The Pre-corroded Fatigue Crack Growth Behavior Of AA2099 Al-Li Alloy

The noval Al-Li alloy has the characteristics of high specific strength and specific stiffness,excellent damage tolerance and low temperature mechanical properties,and has been widely used in aerospace industry.In view of the characteristics of aviation components in service,the damage tolerance is an important performance index.The high temperature and high humidity salt-fog corrosion environment in the South China Sea puts forward higher performance requirements for aviation equipment in service here.It is of great significance to fully understand the influence mechanism of salt-fog environment on the damage tolerance of noval Al-Li alloy for ensuring the safe and efficient operation of aviation equipment.Based on the domestic third generation AA2099 Aluminum-lithium alloy,the effects of grain boundary feature in different heat treatment conditions(T8、T6)on the the fatigue crack growth behavior of salt-fog pre-corroded AA2099 Al-Li alloy compact tensile samples(C(T))were studied by means of Optical Microscope(OM),Scanning Electron Microscope(SEM),X-Ray Diffraction(XRD),Backscattered Electron(BSE),Electron Back Scattering Diffraction(EBSD)and other equipments or technology,the main conclusions are as follows:1.The distribution feature of salt-fog pitting corrosion was related to the distribution feature of precipitates at grain boundaries.The morphology of precipitates at grain boundaries of samples in T8 state were generally discontinuous particles,the corrosion micropits of samples in T8 state were usually arranged in chains along the extrusion direction.The size of precipitates at grain boundary of samples in T6 state increased significantly,so the continuity of precipitates distribution at grain boundary of samples in T6 state was reduced obviously,meanwhile,the corrosion tendency was increased,then the size and density of pitting was significantly increased.2.Distribution feature of pitting corrosion caused anisotropy of crack growth of C(T)sample.Under L-T loading,the direction of pitting corrosion of samples in T8 state was nearly perpendicular to the direction of crack growth,which induced crack closure effect,the crack growth rate of corroded sample was similar to that of normal sample.Under the loading of T-L,the direction of pitting corrosion of samples in T8 state was parallel to the crack propagation direction,the crack closure effect was suppressed,then the crack growth rate increased after corrosion.With the increased distribution density of pitting corrosion of samples in T6 state,the crack closure effect was more obvious under L-T loading.The crack growth rate of samples in T6 state after corrosion was significantly lower than that of samples without corrosion.3.The delamination of C(T)samples during fatigue loading was closely related to the grain boundary feature.Delamination was easy to occur at the straight and high angle grain boundary(HAGB)where the grain boundary precipitates were discontinuous.Salt-fog corrosion could promote the occurrence of delamination during fatigue loading.4.Both the degree of HAGB tortuosity and the continuity of precipitates at grain boundaries were decreased with the increased of pre-stretch deformation.Aging temperature would effect the above grain boundary features significantly.With the increase of aging temperature,the degree of HAGB tortuosity of the samples was significantly increased due to the increased proportion of HAGB,and the continuity of precipitates of grain boundary was decreased significantly due to the size coarsen of precipitates of grain boundary.