Study On The Surface Irradiation And Surface Alloying With Copper Of Zircaloy-4 And N18 Alloy Induced By High Current Pulsed Electron Beam

Because of their outstanding nuclear properties,zirconium alloys are widely used as fuel element claddings and core structure materials for water-cooled power reactors.With the continuous development of society and economy,in order to continuously meet the development needs of nuclear power for safer,more reliable,more efficient and more economical,especially after the Fukushima nuclear accident in Japan,researchers from all over the world are working hard to adopt various means to continuously improve the surface comprehensive performance of various zirconium alloys,especially high temperature corrosion resistance,in the hope of reducing the probability of failure of related components and extending their service life.Cumulative research results show that high current pulsed electron beam?HCPEB?surface irradiation and surface alloying are simple but extremely effective methods to improve the surface hardness,wear resistance and corrosion resistance of metal materials under certain conditions.However,there are few researches on the surface modification of zirconium alloy by HCPEB at home and abroad,and surface alloying is almost blank.In view of this,this paper took Zircaloy-4 and N18 alloy as research objects,and direct surface irradiation treatment and surface Cu alloying treatment were carried out on this two alloys using HCPEB device.Afterwards,various characterization methods were used to detailedly characterize and analyze the microstructures,hardness and stress of sample surfaces before and after modification as well as the microstructures of oxide films grown on sample surfaces after high temperature and high pressure corrosion tests.Finally,the internal relationship between surface microstructures and properties was summarized.The main research results are as follows:After surface irradiation by HCPEB,the surface layers of Zircaloy-4 and N18 alloy were all melted,impurity elements such as Al,Co,and Mg in the surface layer were continuously removed with the eruption of volcano-like craters,realizing the purification of sample surface.After irradiated with low pulses,the melted layer was mainly composed of a large number of twin-martensites.With the increasing of pulses,martensitic structures were constantly refined.After irradiated with 25 pulses,the average thickness of the melted layers for two alloys reached to about 4.8?m?Zircaloy-4?and 6.5?m?N18 alloy?,respectively,inside which except for large numbers of ultra-fine or even nano-scaled twin-martensites,large quantities of ultra-fine grains were also formed.By comparison,the fine density of both structures in the surface layer of N18 alloy was relatively higher.Second phase particles were dissolved in both the melted layers and the heat affected zones of two alloys,alloying elements Fe,Cr,and Nb were all evenly dissolved into the zirconium matrix.In addition,after irradiation,high residual compressive stress was accumulated in the surface layer of both alloys.Performance test results showed that,after irradiated with 25 pulses,the surface hardness as well as the corrosion resistance at 500°C/10.3 MPa superheated steam of both two alloys were all improved significantly compared to their initial samples.The improvement of hardness was mainly due to the joint action of solution strengthening,fine-grained strengthening,phase transformation and deformation strengthening of the surface layer,and the improvement of corrosion resistance was mainly due to the high residual compressive stress,supersaturated alloying elements,selective purification effect and abundant structure defects in the surface layer.After comparing the two alloys,it was found that the enhance effect of surface hardness and corrosion resistance of N18 alloy was better than that of Zircaloy-4,which was mainly due to the addition of Nb element and the finer structures in the surface layer.After surface Cu-alloyed by HCPEB,alloying layers with average thickness of about2.8?m and 3.1?m were respectively formed in the surface layers of Zircaloy-4 and N18alloy,which was mainly composed of evenly distributed ultrafine?grains.Specifically,large quantities of nano-scaled grains were also observed in the Cu-alloyed layer of N18 alloy.But martensitic structures were only formed inside a small quantity of?grains.Most of the Cu elements successfully entered into the zirconium matrix were uniformly distributed in the?-Zr supersaturated solid solution,others insoluble or continuously removed from the sample surface with the eruption of volcano-like craters or existed in the form of fine dispersed ZrCu second phase particles.Different from the sample directly irradiated with 25pulses,after alloying with Cu,a special solid-state martensitic phase transformation layer appeared under the melted layer of both alloys.Similarly,Zr?Fe,Cr?₂/Zr?Fe,Cr,Nb?₂ second phase particles also dissolved in the modified layer.In addition,after alloying with Cu,a certain value residual compressive stress was also accumulated in the surface layer of both alloys.Performance test results showed that,after Cu-alloyed by HCPEB,the surface hardness as well as the corrosion resistance at 500°C/10.3 MPa superheated steam of both two alloys were further significantly improved compared to their directly 25-pulsed irradiation samples.The improvement of hardness was mainly due to the further enhancement of solution strengthening effect,the dispersion strengthening effect of ZrCu second phase particles,and the appearance of solid-state martensitic phase transformation layer in the subsurface layer after Cu-alloyed by HCPEB.The most critical factor for the further improvement of high temperature and high pressure corrosion resistance of the Cu-alloyed sample was those Cu elements that supersaturated in zirconium matrix.After comparing the two alloys,it was found that due to the higher Cu content in the surface layer,the enhance effect of surface hardness of Zircaloy-4 Cu-alloyed sample was relatively better,and due to the addition of Nb element and the finer structures in the surface layer,the enhance effect of corrosion resistance of N18 Cu-alloyed sample was relatively better.