Study On Fatigue Behavior Of Surface Nano-sized Industrial Zirconium Under Alternating Load

Surface nanocrystallization is the formation of nanocrystalline structure with a certain thickness on the surface of bulk metal by means of severe plastic deformation,and the surface strengthening technology with residual compressive stress and work hardening effect is introduced,which can effectively improve the overall properties of the material.In this paper,ultrasonic shot peening technology(USSP)was used to treat the dense hexagonal industrial pure zirconium(Zr-3),the microstructure and properties of the surface layer were studied and analyzed,and the evolution process of nano-structure under alternating load was characterized.The stress field characteristics under alternating loads were simulated and the fatigue behavior of pure zirconium nanoparticles was studied in order to provide a theoretical reference for the optimization of properties of zirconium alloys and the development of surface nanoscale technology.The industrial pure zirconium was treated with USSP for 45 minutes.The microstructure of Zr-3 treated with USSP was characterized by metallographic microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM).The residual stress distribution in the surface layer of USSP treated samples was measured by residual stress tester,and the microhardness of the samples was tested by nano indentation instrument.The fatigue limit of Zr-3 specimen before and after USSP treatment was tested by four-point bending fatigue test and the fatigue fracture was observed and analyzed.The stress distribution of four-point bending of Zr-3 was simulated by finite element method.The influence mechanism of surface nanoscale on the fatigue behavior of industrial pure zirconium was analyzed by means of nanostructure characteristics.The results show that the fatigue limit of Zr-3 specimen is significantly improved by surface nano-treatment,and the fatigue limit of 45 min sample treated by USSP is 23% higher than that of original sample.After treatment,the fatigue crack initiation of the specimen and the subsurface of the specimen are about 100 μ m away from the surface layer,and the fatigue source is multi-variable.Under the four-point bending fatigue load,although the residual stress of the surface nanocrystalline Zr-3 surface has been released,the actual stress is still lower than that of the untreated sample.At the same time,the high hardness of the nano-layer and the high distortion of the surface make it have higher deformation resistance,which makes the crack initiation occur in the sub-surface layer,which makes the surface nano-Zr-3 obtain higher fatigue properties.The microstructure of nanocrystalline layer will change during fatigue loading after nanosurface loading,and the nanocrystalline will grow up gradually under the action of fatigue load,and the larger the applied load is,the larger the nanocrystalline length will be.The nanocrystalline layer length of fatigue fracture specimen is much longer than that of non-fatigue fracture specimen.When the loading cycle is more than 100 w,the grain distribution tends to grow around the nano-grain,and there is delamination between the nanocrystalline and the grown-up grain.