Computational Study On The Effects Of Sn And Nb On The Phase Composition And Transformation Behavior Of Zirconium Alloy Oxide Films

The corrosion resistance of zirconium alloys in nuclear fuel cladding is a key factor determining the life of the cladding.The oxide film on the surface of the zirconium alloy has an important influence on the corrosion resistance.The addition of alloying elements usually causes the phase composition of the zirconium alloy oxide film to change,which in turn causes Changes in corrosion resistance.Sn and Nb are common alloying elements in commercial zirconium alloys.Therefore,Sn and Nb affect the change law of zirconium alloy oxide film phase composition and the mechanism of their effect on oxide film phase transformation,which are of great significance for understanding the corrosion of zirconium alloys.Based on the principles of thermodynamics and kinetics,the effects of Sn and Nb elements on the phase structure and phase transformation of zirconium alloy oxide films were studied by using phase diagram calculations,first-principles calculations,and Stochastic surface walking,and their effects on corrosion resistance were analyzed.The main results and conclusions are as follows:（1）The Zr-Nb-Sn-O phase diagram database is built by using the phase diagram calculation method,based on the existing computational phase diagram database and the phase transformation experimental data of alloys with related components.Then,the phase diagram was calculated using the quaternary phase diagram database,and the influence of Nb,Sn,Zr,O content,temperature and other factors on the phase composition of the zirconium alloy oxide film was revealed.The analysis showed that the calculation results were reasonable and consistent with the existing experiments better.（2）The stability of m-ZrO₂compared with t-ZrO₂was studied by first-principles calculation method.In pure ZrO₂system,m-ZrO₂was more stable than t-ZrO₂.In the Nb-containing system,the stability of t-ZrO₂is improved.When the content of Nb reaches 25 at.%,the relative stability of the two phases of t-ZrO₂and m-ZrO₂also reverses.In the Sn-containing system,Sn only improves the stability of t-ZrO₂.These results explain the experimental phenomenon that both Sn and Nb can lead to the increase of t-ZrO₂content in the oxide film.（3）The tetragonal-monoclinic（t-m）phase transitions of pure ZrO₂and ZrO₂systems containing Sn and Nb elements were studied by using the stochastic potential energy surface method.The t-m phase transition energy barriers of ZrO₂and ZrO₂containing 25 at.%Sn and Nb elements are 0.71 me V/atom,1.73 me V/atom and40.04 me V/atom,respectively.Comprehensive kinetic and thermodynamic calculation results show that although both Sn and Nb can increase the content of t-ZrO₂,Nb can prevent the phase transition from occurring,while Sn only slightly increases the difficulty of the phase transition,and has little hindering effect on the phase transition.Therefore,Nb can reduce the occurrence of phase transition in the oxide film,while Sn makes the phase transition more concentrated.At the same time,according to the phase structure in the phase transition path,the strain in the phase transition process is calculated and analyzed.The results show that during the phase transition process,there will be large lattice compression in some specific directions,resulting in tensile strain and inducing the formation of cracks.