| The basis of using nuclear energy safely depends on the safe performance of structural materials used in nuclear reactors.Among these materials,the cladding is not only the first important protection to ensure the safety of nuclear fuel in the core,but also a primary guarantee for preventing the radiant pollution caused by the leak of fission products and for the safe separation between the coolant and the nuclear fuel.After the Fukushima nuclear plant accident,the research and preparation of alternative nuclear cladding has become one of hottest topics in nuclear energy field.In this thesis,the properties of FeCrAl ternary alloy,which is suggested as one advanced cladding material applied in the accident tolerant fuel(ATF)system,have been studied with computational simulation method,especially,the influence of Cr and Al addition on the structure of dislocation loops produced by irradiation and the interaction between loops and dislocation lines was investigated.The understanding of the microstructure evolution of this candidate under irradiation and the influence on mechanical properties,provide the new insight for the research and development of a future ATF.In this thesis,for the FeCrAl ternary system,the segregation of solute atoms Cr&Al on the dislocation loop was firstly studied from the energy point of view.Through the comparison of calculation results,the conclusions can be made as followings:(1)The system with solute atoms substituted for the interstitial atoms of dislocation loops has a lower total energy.This indicates that the systems,where Cr and Al atoms segregate into the dislocation loops,have higher binding energies,and the driving force of solute atoms to segregate in dislocation loop is higher,which could explain the segregation phenomenon of solute atoms in loop observed in TEM images of irradiated Fe-based alloy.(2)Comparing to the solute atom Al,solute atom Cr has a higher binding energy on the interstitial atomic position,which explains that Cr atoms have a higher driving force to segregate in the dislocation loops observed experimentally.In addition,the effects of Cr and Al solute atoms on the interaction between a dislocation loop and a dislocation line were also studied with simulation method.The results indicated that the obstacle strength from sessile<100>dislocation loops decorated with both Cr and Al solute atoms to the mobility of a 1/2<111>straight dislocation line,has been increased.Furthermore,the hardening effect of Cr solute atoms is significantly higher than that of Al.Finally,the interaction between two dislocation loops with different sizes and types in FeCrAl alloy was also studied.It was found that the addition of Cr and Al would affect the interaction between dislocation loops,which resulted in the formation of dislocation networks easier than the case without solute atoms.These results indicated that the segregation of solute atoms improves the resistance of smaller dislocation loops to be absorbed by large loops.In summary,based on the simulations performed in this paper,the underlying mechanism of the segregation of Cr and Al in dislocation loops in the FeCrAl alloys is explored.Meanwhile,the configuration of dislocation loops after the segregation of solute atoms Cr and Al observed in the corresponding experiment can be explained with the present results.Furthermore,the influence of segregation of Cr and Al on the interaction of dislocation loops with dislocation lines,especially the potential hardening effect due to the addition of Cr and Al,was studied.Finally,the interaction between dislocation loops with the segregation of both solute atoms has also been studied.These results provide new understandings and energy data for further study of irradiation properties of FeCrAl alloys,and provide a new physical mechanism to predict mechanical properties.All these results in this work presented the key scientific basis for further applications of FeCrAl alloy in advanced ATF system used in future reactors. |
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