Microstructure Of Pure Nickel After High Current Pulsed Electron Beam Treatment

In the present work,bulk pure nickel surface treatment was carried out with high current pulsed electron beam(HCPEB) equipment of type Nedazhda-2.Surface temperature field induced by HCPEB irradiation was simulated,temperature distribution profile gained.The changes of nickel surface morphologies and microstructure were characterized by optical microscope(OM),scanning electron microscope(SEM),X-ray diffractometry(XRD) and transmission electron microscope (TEM).Surface temperature field simulation shows that the temperature of near-surface of materials has exceeded the melting point of nickel.The depth of heat-affected zone of the nickel induced by HCPEB is about 30μm,the melting starts from 0.2μs and the site is at a sublayer about 0.4μm from the surface.The heating and coolong rate is approximately 10~9K/s and 10~7K/s,respectively.The surface melted after HCPEB treatment,the average depth of surface re-melted layer is about 1.6μm after HCPEB irradiation with 5 pulses.Numbers of craters formed on the irradiated surface,the quantity density of craters are close associate with that of surface defects.Meanwhile,Surface micropores with nano-size distributed on surface. Abundants of vacancies by means of dislocations and grain boundaries transferring to the surface,accumulated to micropores.Cross-sectional microstructures were divided into re-melted layer,heat-affected zone,stress-affected zone and substract along the direction of electron beam incidence. Nanocrystalline structures with the size of 70 nm homogeneously formed on the top re-melted layer.Melted layer suffered rapid solidification in the cooling process,the very fast cooling rate of melted layer is the main reason for the formation of nanocrystalline structures.In the heat-affected zone,thermal-stress induced by HCPEB energy deposition lead to severe plastic deformation.And grain refinement was occurred due to plastic deformation across the heat-affected zone.High density dislocation cells and dislocation walls were the main deformation structures with 1 pulse HCPEB irradiation.After 5 pulses bombardment,twinning deformation taken place in sub-surface,accompaniing with the extention of stacking faults.Deformation twins and stacking faults were the main deformation structures with 10 pulses HCPEB irradiation.In addition,we observed high density defects sturcture,for instance, dislocation cells,dislocation walls,even sub dislocation walls.The formation of those defects structure due to severe deformation.Meanwhile,dislocation loops and SFT were distributed on dislocation walls and grain boundaries.The analysis showed that vacancy clusters have a close associate with the formation of deformation twins.X-ray diffraction results showed that the high amplitude residual stresses were accumulated inside nickel samples.After 5 pulses irradiation,the value of residual stresses exceeded 1 GPa.Such a high residual stress results in materials modification of deeper zones.We also detect an evident broadening of Bragg peak.Surface nanocrystallization,grain refinement induced by severe plastic deformation and high amplitude residual stress is the main reason of diffraction peak broadening.