High Cycle Fatigue Property Of Commercially Pure Zirconium Processed By ECAP And Rotary Swaging At Room Temperature

In order to further expand the application field of pure zirconium,the microstructure,mechanical properties and especially high-cycle fatigue performance of ultrafine grained commercially pure zirconium（UFG CP Zr）fabricated by Equal Channel Angular Pressing（ECAP）+Rotary Swaging（RS）at room temperature were investigated systematically by transmission electron microscopy（TEM）,scanning electron microscopy（SEM）,tensile test and high-cycle fatigue test.The main research contents and conclusions are as follows.The microstructure of UFG CP Zr shows that the combined processing of ECAP+RS can refine the grains with average grain size of about 250 nm.There are a high density of dislocations,a large number of dislocation cells and dislocation tangles zones and a small amount of subgrains.The tensile test results show that the tensile strength and yield strength of pure zirconium are significantly increased by ECAP+RS at room temperature,which are increased by 114%and 190%,respectively.The experimental results of high-cycle fatigue of CP Zr at room temperature show that the fatigue limit of CP Zr（cycle base is 10⁷cyc.）is greatly improved after the ECAP+RS.The S-N curve of UFG CP Zr is fitted by Basquin relationship,which isσ_a=750N_f^-0.06,The conditional fatigue limit is 285 MPa corresponding to 10⁷cyc.,which is 70%higher than that of coarse grain commercially pure zirconium（CG CP Zr）.The relationship between the static tensile strength and the fatigue limit of CG and UFG CP Zr isσ_-1=0.54σ_b andσ_-1=0.39σ_b,respectively.There are great differences in the strain response characteristics of UFG and CG CP Zr with different stress amplitudes under high-cycle fatigue test.The CG CP Zr undergoes strain softening at the early stage and then strain hardening during the cycle deformation,while UFG CP Zr shows the strain saturation from the beginning.The fatigue microstructure of CG CP Zr shows that the dislocation density increases,and a small amount of twins form after fatigue,indicating that dislocation slip is the main fatigue damage mechanisms,while the twinning are secondary damage mechanism.For UFG CP Zr,when the stress amplitude is low,the grains are broken and the grain boundaries are blurred.With the increase of the stress amplitude,the dislocation configuration in the UFG CP Zr transforms from the dislocation beam to the dislocation cell,and the grain size is slightly larger than that of the UFG CP Zr before fatigue.Therefore,the dislocation slip is the main damage mechanism of UFG CP Zr.Fatigue fracture analysis shows that fatigue fracture of both UFG and CG CP Zr is typical fatigue fracture morphology.Fatigue cracks initiate on the surface of the specimen,and a large amount of fatigue striations occur in the crack propagation zone.With the propagation of the crack,the fatigue striation spacing increases,followed by the instantaneous fracture.The larger the stress amplitude is,the rougher the fatigue fracture is,and the faster the crack propagation rate is.In addition,fatigue cracks of the material tend to the multiple sources with increasing the stress amplitude.However,the fatigue striation spacing in fatigue cracks metastability zone of UFG CP Zr is finer and straighter than that of CG CP Zr,and its fatigue crack propagation rate is slower.