The Irradiation Resistance Mechanism Of Typical Nickel-containing High-and Medium-Entropy Alloys Via Computational Simulations

Under strong neutron irradiation,material in nuclear reactor will produce a large number of lattice defects,which further grow to large dislocation loops or voids,resulting in the degradation of material performance,and therefore threatening the safety of the reactor.It is very important for the design and development of advanced nuclear materials to understand the kinetic behavior of radiation-induced defects in materials and to reveal the corresponding radiation-resistance mechanism.High-and Medium-entropy alloys(HEAs and MEAs)are single-phase solid solution materials,consisting of multiple principal elements with equal or nearly equal atomic ratio.These alloys exhibit many outstanding performance,such as high fracture toughness,high yield strength,high temperature stability,high resistance to corrosion,and high swelling resistance,which however cannot be compatible in traditional metals.Therefore,HEAs and MEAs have been considered to be promising structural materials for fusion and generation Ⅳ fission reactors to address the increasing demands for nuclear energy.However,the mechanisms of irradiation resistance in these alloys are still unclear,owing to the complex relationships between their alloy composition and irradiation responce.In present study,pure nickel(Ni)and different types of Ni-containing HEAs and MEAs are studied in different stages of radiation damage by molecular dynamics(MD)and object probability-based long-time dynamics(OPLD)to investigate the generation and distribution of defects in the primary damage stage after displacement cascade and the kinetic behavior of different defect types in the stage of thermal defect migration.By analyzing a series of physical phenomena,such as thermal spike,trajectory,interaction and long-time evolution of defects,the swelling resistance mechanism of Ni-based HEA at different stages of radiation damage was revealed.The main research works are as follows:(1)The generation and evolution of irradiation-induced defects in the NiCoCrFe HEA were investigated by MD simulations to understand the mechanisms of its irradiation tolerance compared with bulk Ni.The displacement cascades were simulated for the energies of primary knock-on atoms(PKA)ranging from 10 to 50 keV to understand the irradiation resistance in HEAs.In general,there are more displaced atoms produced in the thermal spike phase,but fewer defects survived at the end of the cascades in the NiCoCrFe alloy than in Ni.Both interstitial and vacancy clusters increase in size or number with increasing PKA energy in both materials,but they do so more slowly in the NiCoCrFe HEA.The delayed damage accumulations in the NiCoCrFe HEA are attributed to the high defect recombination caused by the following two mechanisms.First,the enhanced thermal spike and the low thermal conductivity of HEAs for heat dissipation result in the higher efficiency of defect recombination.Furthermore,the substantially small binding energies of interstitial loops in the NiCoCrFe HEA,as compared with those in Ni,are responsible for the delayed interstitial clustering in the NiCoCrFe HEA.(2)By simulating the thermal annealing of interstitial clusters,the trajectory and evolution of interstitial clusters in AlxNiCoCrFe HEA with different Al concentrations during the stage of thermal defect migration were studied.The results show that the trajectory of interstitial clusters in HEA converts from one-dimensional mode to threedimensional mode with increaseing Al concentration,and the mean free path decreases gradually.In addtion,the interstitial cluster formed into a single 1/2<110>dislocation loop when Al concentration is low.As A1 concentration is increasing,the interstitial dislocation loop transformed into a complex dislocation structure.It is found that the lattice distortion and migration energy barrier both increase with with increasing Al concentration,which results in these different behaviors of interstitial clusters.(3)The interaction and evolution of Frenkel pairs in NiCoFe and NiCoCr MEAs during the stage of thermal defect migration were studied by simulating the thermal annealing of cascade-induced defects.The results show that the total number of Frenkel pairs decreases during annealing.By comparison,the decrease of Frenkel pairs in NiCoFe was larger than that in NiCoCr.The vacancy migration barrier in NiCoFe is lower than that in NiCoCr,while the interstitial migration barrier in NiCoFe is higher than that in NiCoCr.This leads to a smaller difference between vacancy and interstitial migration in NiCoFe than that in NiCoCr,which increases the reaction probability between vacancies and interstitials,thus enlarging the recombination efficiency.(4)The formation and evolution of Frenkel pairs as well as the nucleation and growth of stacking fault tetrahedron(SFT)and voids in NiCoCr MEAs and Ni have been studied by simulating both displacement cascade and long-time dyanimics with millisecond scale.The results show that the interstitials migrate first due to its low migration barrier,but the vacancies remain nearly immobile until that most of interstitials recombine with vacancies and escape from the cascade volume.While the vacancies in Ni grow into a large void,the vacancies in NiCoCr grow into a SFT.In addition,the SFT in Ni disappeared and evolved into a void after interacting with the displacement cascade,but the SFT in NiCoCr coexisted with the void.This is closely related to the relative formation energies of the SFTs and voids in the two materials,as well as the higher binding energy of SFTs than that of voids in the NiCoCr MEA.To summarize,by simulating the behavior of different defect types at different stages of radiation damage in Ni,different Ni-based HEAs and MEAs,it is found that recombination efficiency of Frenkel pairs and relative stability of SFT in HEAs are much higher than that in Ni.Besides,the migration behavior of interstitials and vacancies in the HEAs and MEAs are sensitive to their alloy composition.The corresponding radiation resistance mechanisms of HEAs and MEAs have been revealed and the correlation between microstructure and swelling resistance has been established by comparing with experimental results,providing important insights and strategic guidance for developing a new generation of swelling-resistant materials.