| Nickel-based single crystal superalloys,which have superior properties operating at elevated temperatures,are widely used as gas turbine blades and vans for aero-engines and industrial gas turbines.It is particularly important to research on fatigue properties,with great danger of single crystal superalloys causing by fatigue fracture in service.However,due to the orientation dependency,crystal orientations of single crystal superalloys are considered to be one of the significant factors influencing fatigue properties.Therefore,focusing on second generation single crystal superalloy containing 3Re,the present work aims to research the low cycle fatigue behavior of the superalloy with the[001],[Oil]and[111]orientations at 980 ?.The fatigue fracture surfaces,microstructures after deformation and dislocation configurations of the single crystal superalloy with the three crystal orientations have been observed using scanning electron microscopy(SEM)and transmission electron microscopy(TEM).Moreover,the fracture and deformation mechanisms of low cycle fatigue has been analyzed.The investigation of the influence of crystal orientations on the low cycle fatigue life of the single crystal superalloy suggests that the fatigue life on strain controlled low cycle fatigue is highly orientation depended,which is mainly related to variations in elastic modulus with orientation.The[001]direction specimens,the smallest in elastic modulus,show the longest fatigue life,while the[111]direction specimens,the largest in elastic modulus,show the shortest fatigue life.Furthermore,with increasing of total strain amplitude,the fatigue life of the three crystal orientations is reduced.The investigation of fatigue fracture mechanism of single crystal superalloys with various crystal orientations suggests that multiple crack sites initiate at surface or subsurface micropores or inclusions on the fracture surfaces,the noncrystallographic fracture mode mainly exhibits in the[001],[Oil]and[111]orientations,while some of[111]orientations crack along crystallographic plane.Cyclic plastic deformation is the main fatigue damage mechanism in low cycle fatigue,while some extent of creep damage is observed in fatigue steps.For the[001]specimens,the fracture surfaces and ?' rafting direction are almost perpendicular to the stress axis,and multiple slip initiate in different slip planes.While for the[011]specimens,the fracture surfaces are rough,the ?' rafted morphologies align along the 45 degrees direction of the stress axis,and one family of slip bands are easily started.As for the[111]specimens,the?' morphologies do not show specific rafting direction,and multiple slip and cross-slip are easily activated.Moreover,for the[001]and[011]specimens,secondary cracks propagate along the?' rafting direction.However,for the[111]specimens,secondary cracks are formed in the intersection interfaces between crack propagation planes,propagating along one of the crack growth planes.The investigation of dislocation configurations and deformation mechanism of the single crystal superalloy with various crystal orientations suggests that deformation with increased strain amplitude is characterized by cyclic softening at early stage.Under the condition of low strain amplitude,dislocations in the[001]specimens heterogeneously distribute in y channel normal to stress axis by cross-slip and climb.The main reasons of the cyclic softening are ?'coarsening and dislocation recovery process caused by dislocation annihilation and rearrangement.While for the[011]specimens,plenty of parallel aligned dislocations glide mainly in the roof channels,which reduces the probability of dislocation interactions and results in cyclic hardening.With respect to the[111]specimens,dislocation density increases,dislocations distribute homogeneously and arrange irregularly,and a considerable number of dislocations pile up in the y channels.Therefore,the resistance of the dislocation movement increases and cyclic hardening is resulted. |
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