Study On The Coupling Effect And Mechanism Of Irradiated Oxidation Of Iron-based Nuclear Structural Materials

Fe-based materials not only have a wide range of applications in the industrial field,but also are candidates for the structure material of the Generation IV nuclear power.Therefore,their physical properties under specific environments have attracted great attention from related researchers.The study found that in the process of the nuclear system operation,the water and liquid metal containing certain oxygen concentration severely affected the physical properties of the material,such as the decrease of the heat conductivity of the material,and the loss of the oxidation layer would block the channel of the component and affect the safety of the system.Moreover,the experiment found that the material oxidation corrosion was more serious in the irradiation environment,the most intuitive performance was the increase of the thickness of the oxide layer,which greatly shortened the service life of the material.Therefore,how to reduce the damage caused by oxidation corrosion in the process of material service has become one of the key problems to limit the development of nuclear energy.Theoretical studies have also shown that vacancy defects produced by irradiation accelerate oxidative nucleation and growth,but the physical and microscopic processes and images of oxidation corrosion promoted by irradiation are still unclear.For example,the microscopic migration process of interstitial oxygen atoms in the matrix during the initial oxidation stage,and how the vacancy accelerates the nucleation and growth of oxidation,etc.Therefore,we use the first principles calculation method from atomic scale to analyze properties of dissolution and diffusion of oxygen atoms and vacancies as well as the interaction of the two,enhancing understanding of the iron base structure materials of the micro process of oxidation corrosion,explain the experimental phenomena,and for investigation of long time large-scale oxidation corrosion provide necessary energetics and dynamics parameters,which provide suggestions for the corrosion protection of the material.The main contents and results are as follows:Firstly,the dissolution and diffusion properties of interstitial oxygen atoms in the grain boundary of body-centered cubic iron((50)3(111)、(50)5(310)and(50)5(210)),and the effects of different grain boundary structures on the interstitial oxygen energy property is investigated by constructing grain boundary oxidation model.The role of grain boundaries in the oxidation process is determined.It is found that the dissolved energies of interstitial oxygen atoms in the grain boundary region are less than those in the transition region and the bulk-like region,indicating that interstitial oxygen atoms are easy to segregate and migrate to the vicinity of grain boundaries.At the same time,the dissolution energy of oxygen atoms in grain boundaries depends on the structure of grain boundaries,and is closely related to the number of neighboring atoms,bond length and space,and the amount of charge transfer.When interstitial oxygen segregates to the grain boundary,strong charge transfer occurred with the neighboring iron atoms,and the interaction was mainly ion interaction.Subsequently,we use the Climbing Image Nudged Elastic Band method to investigate the migration characteristics of interstitial oxygen atoms in grain boundary,transition zone and bulk.For the(50)3(111)grain boundary,oxygen atoms overcome the lower energy barrier(less than the migration barrier in the bulk)to converge to the grain boundary,but need to overcome the higher energy barrier to reach the(50)5(310)and(50)5(210)grain boundary.At the same time,for(50)5(310)and(50)5(210)grain boundary region,the diffusion energy barrier of interstitial oxygen atoms is higher than that in the bulk,which indicates that interstitial oxygen is not easy to diffuse along the grain boundaries,but the diffusion energy barrier at the(50)5(210)grain boundaries is close to that of the bulk.Based on the dissolution and diffusion characteristics of interstitial oxygen atoms at grain boundaries without defects,grain boundary oxidation model under irradiation was established to further study the segregation and diffusion characteristics of defects near grain boundaries and their effects on the dissolution and diffusion of oxygen atoms.It is found that the vacancy formation energy in the grain boundary region is less than that in the transition region and the bulk-like region,indicating that the vacancy is also easy to segregate and migrate to the grain boundary.The dissolution energy of interstitial oxygen atoms in grain boundary,transition zone and bulk decreases when there are iron vacancies in the adjacent sites around interstitial oxygen atoms in grain boundary,transition zone and bulk,indicating that the existence of vacancies can promote the dissolution of interstitial oxygen atoms,and the dissolution process is a process of electron liberalization of interstitial oxygen atom p orbital and iron atom d orbital.At the same time,the interstitial oxygen atoms in the transition zone and the bulk have a large binding energy with the first nearest vacancy,forming a stable complex structure,which reduces the segregation and migration ability of interstitial oxygen atoms and vacancies to the grain boundary.At variance,in the grain boundary zone ability of binding between interstitial oxygen and near vacancy is relatively weak.Within the scope of the vacancy and interstitial oxygen atoms interaction,interstitial oxygen can only need to overcome the low base(less than no defect in the grain boundary migration),showed that the vacancy can accelerate the diffusion of oxygen atoms of grain boundary zone,moreover,the vacancy formed in the region where the iron atoms are arranged and distributed more densely promotes the migration of oxygen atoms more obviously.The above studies demonstrated that oxides and nano-channels preferentially form along high-energy defects such as grain boundaries in the oxidation process,and verified the observed distribution of oxides near grain boundaries in the experiment.In the initial stage of oxidation corrosion,the interstitial oxygen atoms are mainly in the form of bulk diffusion in the nonirradiation environment,and the diffusion of interstitial oxygen atoms is difficult to be promoted by grain boundaries.In the irradiation environment,the oxygen atoms in the interstitial are mainly diffused along grain boundary,and it is easy to form the vacancy and oxygen aggregates in the bulk zone,which can indicate that the corrosion resistance of polycrystalline materials is obviously decreased under the irradiation condition.