Low Cycle Fatigue Behavior Of Ultrafine Grained CP-Zr Processed By ECAP And RS

The strain controlled low-cycle fatigue(LCF)tests of ultrafine-grained commercial purity zirconium(UFG CP-Zr)processed by equal channel angular pressing and rotary swaging(ECAP+RS)and coarse grained commercial purity zirconium(CG CP-Zr)were carried out at room temperature(RT)under the different total strain amplitudes.The LCF tests were performed under fully reversed tension/compression on a closed-loop servo-hydraulic fatigue testing machine.The different total strain amplitudes are in the range of 4×10-3~1.2×10-2.The characteristics of fatigue cycle stress and strain response(CSS),fatigue lives and hysteresis loop after cyclic loading were investigated.The fatigue fracture surfaces and fatigue microstructures were observed using scanning electron microscopy(SEM)and transmission electron microscope(TEM).The main research results are as follows.Compared with CG CP-Zr,the fatigue life of UFG CP-Zr are obviously enhanced.The fatigue life of the UFG CP-Zr is 2~4 times longer than that of the CG CP-Zr.The fatigue lives in all the strain amplitudes follow the relationship of Coffin-Manson: :??_p?2=?_f'(2N_f)~c.The number of cycles decreases with the total strain amplitude increasing.The strain amplitude of UFG CP-Zr is less than that of CG CP-Zr at the same cycle.The fatigue fracture characteristics of CP-Zr were studied by SEM.When the total strain amplitude of LCF is relatively lower,the cracks are initiated on the sample surface.While at the higher total strain amplitude,the crack initiation occurs inside the sample.The macroscopic fracture of CG CP-Zr is homogeneous and perpendicular to the normal stress,and the shear plane of UFG CP-Zr with uneven fracture is approximately 45° with the tensile direction.In the crack propagation zone,the cracksare transformed from the slow propagation to the unsteady propagation.And there are many radition lines consistented with the crack propagation direction,whichare tearing edges,and many obvious secondary crackscan be observed in local area.In the final fracture zone,a large number of dimples are found in UFG CP-Zr.The fatigue microstructures were observed by TEM.It is found that plastic deformation of the CP-Zr only occurs in some grains.The plastic deformation among the grains is in homogeneous.With increasing the total strain amplitude,plastic deformation of the grain increases and the proportion of the deformed grains increases correspondingly.Compared the low-cycle fatigue behavior of CP-Zr with different grain sizes,it is found that UFG CP-Zr has longer fatigue life,which shows an obvious "ratcheting behavior".