| In pressurized water reactors,zirconium alloy cladding tubes react with the primary coolant in a zirconium-water reaction,and when the amount of hydrogen absorbed by the cladding tubes exceeds the limit of solid solubility,hydrides precipitate,which have a great impact on the mechanical properties of zirconium alloys.Most of the current research focuses on the macroscopic experimental aspects of the influence of hydrides on the mechanical properties of zirconium alloys,and lacks the analysis of the microscopic deformation mechanism.In order to study the diffusion mechanism of hydrogen atoms in zirconium after the zirconium-water reaction and the influence of hydrides on the mechanical properties of zirconium at the microscopic scale,this paper uses molecular dynamics to simulate the zirconium-water system reaction with the COMB3 potential function,and studies the diffusion law and influencing factors of hydrogen atoms,and establishes a hydride system to study the influence of hydride density,spacing distribution,size,and orientation on the mechanical properties of zirconium matrix.Based on this,the relationship between stress concentration caused by hydrides and fracture crack initiation and propagation is explored.In addition,macroscopic tensile and compressive experiments are performed on Zirlo alloy cladding tubes after zirconium-water reaction to verify the reliability of the simulation results.The following are the main conclusions:(1)Simulation of Zr-H2O reaction process shows that after H-O bond breaks,O atom adsorbs on Zr substrate surface.H atom diffuses into Zr substrate interior and diffuses violently along basal direction,which is consistent with zirconium-water reaction experimental phenomenon;temperature rise promotes H atom diffusion,COMB3 potential calculates activation energy of 5.9 kcal/mol,which is closer to experimental value than EAM/MEAM potential,verifying rationality of COMB3 potential.(2)The study of the effects of δ-ZrH1.5 precipitation density,spacing distribution,size,and orientation on Zr mechanical properties shows that hydrides cause lattice distortion in Zr matrix,produce stress concentration zones,and the stress at the hydride ends increases with the increase of hydride density,resulting in a decrease in tensile strength,compressive strength and fracture strain of the system(five-layer δ-ZrH1.5 precipitation reduces the tensile strength of pure Zr by 5.44 GPa,compressive strength by 1.89 GPa,tensile fracture strain by 5.35%,and compressive fracture strain by 2%);hydride spacing increase causes a decrease in tensile strength,compressive strength and fracture strain,and a stress concentration zone is formed between two hydrides.increasing the risk of fracture along the hydrides in the system;hydride size increase.increases the contact area with Zr matrix interface.aggravates stress concentration.and reduces tensile strength.compressive strength and fracture strain;radial hydrides have lower tensile strength than circumferential hydrides by 0.44 GPa.lower compressive strength by 0.055 GPa,lower tensile fracture strain by 0.7%,and lower compressive fracture strain by 0.1%.(3)The results of circumferential tensile and compressive tests on Zirlo alloy cladding tubes after zirconium-water reaction show that with the increase of hydrogen concentration,the overall trend is a decrease in tensile strength,compressive performance and ductility(The tensile strength of hydrogen concentration 701μg/g is 187.13MPa lower than that of 0μg/g,and the elongation after fracture is 4.43%lower).The reason is that the samples show a high density,large size and increased radial hydrides with the increase of hydrogen concentration,which verifies the trend of tensile and compressive performance decreasing with the increase of hydride density,size and radial hydrides in molecular dynamics calculations.By observing the fracture surface and fracture characteristics of the samples by SEM,it is found that cracks originate from the hydride ends,cracks propagate along the hydrides,and the increase of hydride content causes more cracks and more severe fracture,which verifies the molecular dynamics simulation of stress concentration zones at the hydride ends and interfaces,and the higher stress concentration value with the increase of hydride density increases the risk of fracture.The reasons for the difference between molecular dynamics simulation values and experimental data are mainly the limitation of simulation scale and experimental uncertainty factors such as Sn,Nb and other trace elements in Zirlo alloy cladding tubes.Molecular dynamics cannot fully fit experimental parameters,but can predict the overall trend of mechanical behavior and reveal the micro-mechanism of hydrides on zirconium mechanical behavior.This has important theoretical significance for optimizing the preparation process of zirconium alloys and improving their corrosion resistance and mechanical properties. |
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