Effect Of Fatigue Loading On Microstructure And Residual Stress Of Surface Nanocrystallized Industrial Zirconium

When industrial pure zirconium is applied in nuclear industry,it’s service life and engineering reliability are got great attention.Surface strengthening technology can effectively improve the reliability and increase service life of workpiece,but on the nanocrystallized Industrial zirconium in the service process,the external environment is complex,residual stress distribution and microstructure is not necessarily can maintain a stable state,residual stress will occur as the fatigue load for release,at the same time,the microstructure of the material change.The industrial pure zirconium was treated by high energy shot peening(HESP),and the metallographic structure was observed by polarizing microscope;the residual stress test and the calculation of dislocation density and grain size were carried out by X-ray diffractometer(XRD);the micro hardness of the sample surface was tested by Vickers hardness tester;the fatigue limit was tested by Lala fatigue sample,and the fatigue fracture was analyzed by scanning electron microscope(SEM),and the residual stress release and micro size in the process of fatigue were analyzed Organization evolution is analyzed and discussed.The results show that by HESP treatment with different parameters,the surface grain of industrial pure zirconium was refined to nanometer level,and residual compressive stress was introduced.The surface residual stress increased first and then decreased with the increase of shot peening treatment time,and the depth of maximum residual stress and maximum residual stress gradually increased with the increase of shot peening treatment time.At the same time,with the increase of shot peening time,the surface microcosmic distortion of the sample increases gradually,and the microcosmic distortion from the surface to the matrix decreases gradually under the same treatment parameters.Compared with the original sample,the fatigue limit of pure zirconium in surface nanometer industry was increased by 23%under fatigue loading.During the fatigue loading process,the twin fraction of the surface nanocrystallized industrial zirconium gradually increased with the increasing loading.At the loading stress of210MPa,the twin fraction increased from 5.8%to 9.4%after 10~3cycles to 5×10~5cycles under cyclic loading.At the loading stress of 320MPa,the twin fraction increased from 8.4%to14.0%after 10~3cycles to 5×10~4cycles under cyclic loading.The increase of twin fraction enhances the twin barrier effect and enhances the fatigue strength of surface nanocrystalline zirconium effectively.There are no twins in the matrix and only the low-angle or high-angle grain boundary changes.The release of residual stress of the surface nanocrystallized industrial zirconium in the process of fatigue was studied by fatigue experiment under two kinds of stress loading.At the loading stress of 210MPa,after 100,10~3,5×10~5and 8×10~5cycles of cyclic loading,the surface residual stress was released by 29.7%,40.0%,62.0%and 63.3%,respectively,and the release in the first 100 cycles accounts for 51.6%of the total release.At the loading stress of320MPa,after 100,10~3and 5×10~4cycles of cyclic loading,surface residual stress was released by 58.0%,80.0%and 84.7%,respectively,and the first 100 cycles release amount accounts for 70.8%of the total release amount,the release amount under high stress loading is greater.There is a significant linear logarithm relationship between the surface residual stress and the loading cycles.The release of residual stress is closely related to the coarsening of surface grain and the change of dislocation density during cyclic loading.