Study On Fatigue Damage Behavior Of Powder Metallurgy FeCoCrNiN_0.07 High-Entropy Alloy

In this study,ultrafine-grained（UFG）and coarse-grained（CG）high-entropy alloys（HEAs）with average grain sizes of 0.83μm and10.51μm were prepared by annealing the cold-rolled sheets of powder metallurgy Fe Co Cr Ni N_0.07HEA at different temperatures.The tensile mechanical behavior and low-cycle fatigue behavior（R=-1）of HEAs with different grain sizes,and high-cycle fatigue behavior（R=0.1）of the UFG HEA,were investigated by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）,and electron backscatter diffraction（EBSD）.The fracture morphology and deformed microstructure were observed.The deformation and fracture mechanisms under unidirectional quasi-static tensile,low-cycle fatigue and high-cycle fatigue conditions were discussed.The main research results are as follows:（1）The yield strength of the UFG HEA is significantly higher than that of the CG HEA,but the elongation and work hardening rates are lower than those of the CG HEA.During unidirectional quasi-static tensile,Slip and twinning are the main deformation mechanisms of the CG HEA.Slip is the main deformation mechanism of the UFG HEA.（2）The UFG HEAs show cyclic stability,slight cyclic hardening,and cyclic softening,while the strain amplitude is 0.25%～0.35%.The UFG HEAs show cycle hardening first and then cycle softening,while the strain amplitude is 0.45%～0.65%.The CG HEAs undergo cyclic hardening first and then cyclic softening,while the strain amplitudes is0.35%～0.65%.The rate and extent of cyclic hardening of the CG HEA and UFG HEA increase with the increase of the applied strain amplitude.Under the same applied strain amplitude,the CG HEA has lower cyclic stress,higher cyclic hardening degree and longer fatigue life than the UFG HEA.（3）During low-cycle fatigue,the main deformation mechanism of the UFG HEA is slip,and the main deformation mechanism of the CG HEA changes from slip to twinning with the increase of the applied strain amplitude.（4）The fatigue strength and fatigue ratio of the UFG HEA are 519MPa and 0.53.After high-cycle fatigue,there is no grain coarsening or twinning.Slip is the main deformation mechanism of the UFG HEA during high-cycle fatigue.（5）During low-cycle and high-cycle fatigue,fatigue crack mainly expands in a transgranular manner.The spacing of the low-cycle fatigue striations increases with the increase of the applied strain amplitude,and the spacing of the high-cycle fatigue striations increases with the increase of the maximum stress.The low-cycle fatigue striations of the CG HEA are denser and longer than those of the UFG HEA.（6）The crack growth region of low-cycle fatigue fracture is characterized by cleavage fracture,and the transient fracture region is characterized by ductile fracture.The stable crack growth region and unstable crack growth region of high-cycle fatigue fracture are characterized by cleavage fracture and mixed fracture,respectively,and the transient fracture region is characterized by ductile fracture.