Molecular Dynamics Simulation Of Irradiation Resistance Of NiFe Single-phase Concentrated Solid Solution Alloys

The safety of reactor structural materials in long-term service under extreme environment has always been one of the main factors restricting the development of nuclear energy.Therefore,it is very important to improve the irradiation resistance of reactor structural materials.Compared with traditional alloys,the single-phase concentrated solid solution alloys(SP-CSAs)developed in recent years have better irradiation resistance properties.The formation of nanocrystals by grain refinement is a traditional strategy to enhance the irradiation resistance of materials.The research on the irradiation resistance of nanocrystalline SP-CSAs is helpful to further enhance the understanding of the irradiation resistance mechanism of SP-CSAs,which is of great significance for promoting the popularization and application of SP-CSAs in reactor structures.In this dissertation,the possible role of grain boundaries(GBs)in the irradiation resistance of SP-CSAs is studied by molecular dynamics(MD)method with Ni Fe solid solution alloys as the research object.The main research contents and innovative research results of the dissertation include:(1)For the influence of GB on the formation and evolution of irradiated point defects,the interactions between cascade collisions and GBs of Ni Fe solid solution alloys are studied.The effects of Fe atoms concentration,GB types(including symmetric tilt GBs,twin boundaries,twist GBs and asymmetric tilt GBs),primary knock-on atoms(PKA)energy and PKA distance to GBs on the interactions between cascade collisions and GBs are analyzed.The results show that the number of residual point defects in Ni Fe is less than that in pure Ni after the cascade collisions.When the concentration of Fe atoms is higher than30 at.%,the absorption capacity of the symmetric tilt GBs in Ni Fe bicrystals to the vacancies increases significantly.Compared with the corresponding GBs in pure Ni,the annihilation effect of symmetric tilt GBs and twist GBs on irradiated point defects can be enhanced in Ni Fe solid solution alloys,and the annihilation effect of asymmetric tilt GBs is weakened,while twin boundaries have no effect on the annihilation of point defects.These phenomena and laws can be explained from two aspects: the formation energy distribution of vacancies and interstitials,and the GB sink strength to vacancies and interstitials.(2)For the healing of the vacancy-type irradiated defect clusters by GBs,the interactions between large angle GBs,as well as low angle GBs,and the stacking fault tetrahedra(SFTs)in Ni Fe solid solution alloys are studied.The GB-SFT interactions mechanisms are explored from the perspectives of atomic configuration evolutions and dislocation reactions,respectively.The effects of Fe atoms concentration,temperature,and the size of SFTs are discussed.The results show that compared with the corresponding GBs in pure Ni,the large angle GBs in Ni Fe solid solution alloys are roughening during the migration process,and the number and distribution of transitional structural units are constantly adjusted,so that the vacancies of SFT are more easily absorbed and diffused in the GBs.The dislocation reactions between the perfect dislocations of low angle GBs and the stair-rod dislocations of SFTs are more diverse,so that the stair-rod dislocations are more likely to be annihilated.With the increase of Fe atoms concentration,the elevating of temperature and the decrease of the size of SFT,the ability of the two kinds of migrating GBs to heal the SFT is gradually enhanced.(3)For the irradiation-induced Helium(He)embrittlement,the intergranular fracture of Ni Fe solid solution alloy containing He bubbles segregation is studied.The effects of Fe atoms concentration,GB type and the number of He atoms in He bubbles are investigated.The results show that intergranular fracture is difficult to occur at the twin boundary.Compared with pure Ni,Ni Fe solid solution alloys have stronger anti-cracking ability of symmetric tilt GBs,twist GBs and asymmetric tilt GBs due to the disordered atomic blockages.Increasing the concentration of Fe atoms can enhance the ability to resist intergranular fracture.(4)For the GB stability under irradiation conditions,the GB migration behavior in nanocrystalline Ni and Ni Fe under constant shear loading parallel to GBs is investigated.It is found that the GBs of nanocrystalline Ni may further migrate during constant shear loading,but that of nanocrystalline Ni Fe does not,indicating that the GBs of nanocrystalline Ni Fe are stable under shear loading conditions.The internal mechanisms of GB migration in nanocrystalline Ni and GB stability in nanocrystalline Ni Fe are revealed by the redistribution of atomic shear stress.The research results are of great significance for deepening the understanding of irradiation resistance and mechanism of SP-CSAs,realizing the engineering regulation of irradiation resistance,and promoting their application in reactor structures.