Study On Relationship Between Fatigue Fracture Behavior And Crystal Orientation Of AA2099 Al-Li Alloy

In this paper,multiple characterization and analysis techniques including metallographic microscope,scanning electron microscopy?SEM?,X-ray diffraction?XRD?and electron backscatter diffraction?EBSD?were jointly employed to investigate a domestic rolled and extruded AA2099 Al-Li alloy sheets?T83?.And the relationship between crystal orientation and fatigue fracture behavior of the rolled and extruded AA2099 Al-Li alloy sheets was systematically studied.The main conclusions are as follows:?1?In the near-threshold region of fatigue crack growth,a large number of slip bands parallel to the path can be observed on the surface of specimens near the fatigue crack growth path.The phenomenon of fatigue crack propagating along the slip band in gains and deflecting in gain boundaries will lead to the formation of wedge fracture face,which will enhance the crack closure effect caused by fracture roughness,improving the fatigue crack growth resistance of specimens.The formation of slip bands is related to crystallographic orientation:the areas with larger Schmidt factor tend to form more dense slip bands;when the two largest Schmidt factors of a grain with a specific orientation have no obvious difference,alternating slip bands will be generated.As to T-L samples,grain with₁,,₂=220°,30°,65°orientation,the two largest Schmidt factors of the grain have no obvious difference?0.45?0.42?,alternating slip bands will be generated.When the fatigue crack propagates in the orientation grains with small difference between the two largest Schmidt factors,the fatigue cracks propagate along these alternating slip zones,forming a joint fracture.?2?Compared with traditional symmetrical extrusion sheets,AA2099 Al-Li alloy sheets extruded by asymmetrical means has partial recrystallization in the surface layer,forming Goss recrystallization texture,which enhance the crack closure effect caused by roughness improving the fatigue crack growth performance.Reducing the Brass texture component in the sheets can improve the plastic-induced crack closure effect in the?stage of fatigue crack growth,and reduce the mechanical anisotropy of AA2099 Al-Li alloy sheets in the plane.?3?Microstructure analysis of fracture apperance shows that the delamination of AA2099 Al-Li alloy changes the characteristics of intergranular cracking and forms free surface,which results in the deflection of cracks in the cross section.At the same time,partial recrystallization occurred during asymmetric extrusion sheets,which restrained the phenomenon of delamination fracture.Delamination often occurs at high angle grain boundaries with the misorientation exceed 50 degrees.The results of finite element simulation of crystal plasticity show that the stress concentration at the grain boundary adjacent to Brass oriented grains is the main reason for the delamination.