.2 Xxx Aerospace Aluminum Alloy Fatigue And Fracture Behavior Of Research

By the axial fatigue properties testing, the fatigue crack propagation rate (FCP) testing,fracture toughness testing,and use of Optical microscopy(OM), XRD, Transmission electron microscopy (TEM),Scanning electron microscopy (SEM) and EBSD observations, the crack initiation and propagation of 2197,2524 alloys with fracture toughness of 2197 alloy were systematicly studied in this paper; effects of orientation and ageing treatments were also investigated.The main conclusions are as follows:(1) Fatigue crack initiation of 2197 and 2524 alloys mainly occured on the sample surface,with initiation from the second phase particles,the interface between the second phase particles and matrix, the surface defects and the grain boundary.(2) Fracture of 2197 and 2524 alloys showed a tortuous fatigue crack path at the early stage.Crack prioritily went along the favorable slip plane,and it did not deflect if favorable slip planes in the adjacent grains are the same,but deflected if not.As a result, crystallography feature was formed on the fracture surface.(3) 2197 alloy has a very good planar-slip feature.The crack mainly transcrystallinely went along the (111) plane, but also along the small-angle grain boundaries. The coherentδ'precipitates, can be cut by dislocations repeatedly,causing the tortuous crack path in the grain.(4) Small particles almost have no effect on the fatigue strips; but when running into large particles and then bypassing them, the crack was also hinderred by these large particles.(5) Only theδ' precipitations and GP zones existed in the alloy underaged in T6;T1 precipitates and lessδ' precipitates with PFZ appeared in the alloy peakaged in T6;the lestδ' precipitates and the most diffuse T1 precipitates appeared in the alloy peakaged in T8.(6) FCP rates of the L-T,T-L,L+45°orientions in the near-threshold region were:L+45°>T-L>L-T, with little difference;FCP rates in Paris region of the three were similar; sample in T-L oriention entered the third stage with fastest fracture rate atΔK of 23.7 Mpa×m1/2;sample in L +45°riention atΔK of 26.5 Mpa×m1/2 with the slowest rate; sample in L-T oriention atΔK=23.8 Mpa×m1/2, with rate between the two above.(7) Closure levels in the L-T,T-L,L+45°orientions showed little difference, indicating the oriention of samples makes little effect on crack propagation and closure.(8) FCP rates of the three aging treatment in the near-threshold region were:underaged T6