Molecular Dynamics Simulation Of Effects Of Grain Boundaries On Irradiated Defects And Mechanical Properties Of Fe-Ni-Cr Alloy

Fe-Ni-Cr alloy is widely used in nuclear reactor core structural materials,such as cladding and pressure vessels,due to its excellent mechanical properties,high-temperature stability,corrosion resistance and radiation resistance.During the service of Fe-Ni-Cr alloy,the atoms are bombarded by high energy particles to separate the atoms from their lattice and form a large number of vacancy,interstitial atoms,gases and other irradiation defects.The irradiation microstructure formed by the recombination,absorption and clustering of irradiation defects will affect the reliability and safety of nuclear reactor operation.The absorption of irradiation point defects at grain boundaries(GBs)is helpful to stabilize the irradiated microstructure and properties,but the absorption of point defects varies with the structural differences of different GBs.Understanding the interaction between GBs and irradiation point defects is helpful to the design of Fe-Ni-Cr Alloys.In this paper,the molecular dynamics method is used to study the radiation damage and mechanical properties of different symmetrical tilt GBs.The main results are as follows.The point defect concentration and distribution are related to the irradiation intensity.The concentration of defects in single crystal(SC)and bi-crystal increases with the increase of irradiation intensity.In the SC,the vacancy and interstitial atoms are produced by irradiation only recombination,and the residual defects are dispersed;the GB of the bi-crystal is an effective sink,which leads to the formation of vacancy clusters or stacking fault tetrahedron(SFT)near the GB.The results of the uniaxial tensile test perpendicular to the GB show that the SC materials exhibit an irradiation softening phenomenon.The higher the irradiation intensity is,the more obvious the irradiation softening phenomenon is.This is related to the fact that irradiation defects accelerate the dislocation propagation and slip of perfect SC.Compared with SC,the GB of bicrystal reduces the ultimate tensile strength(UTS),and the increase of irradiation intensity increases the concentration of residual vacancies in the crystal.Under high irradiation intensity,the vacancy clusters in the crystal aggregate seriously,and even form SFT,which further reduces the UTS.Taking 15keV irradiation intensity as an example,the irradiation defects,microstructure evolution and mechanical properties changes caused by the structural differences of ?3(111)??3(112),?9(114),?11(113),?17(223)and ?19(116)symmetrical tilt GBs were studied.Among them,the GB thickness of ?3(111),?11(113),?9(114),?17(223),?19(116)and ?3(112)increases in turn;the rotation angle ?3(111)>?17(223)>?3(112)>?11(113)>?9(114)>?19(116).The defect absorption capacity is positively related to the thickness of the GB.The thicker the GB,the stronger the absorption capacity.Except for the narrow GBs of ?3(111)and ?11(113),the biased absorption of the interstitial was observed in the rest of the GBs after irradiation,resulting in a large number of vacancies remain in the grains.The GBs ?3(111)and ?11(113)are narrow,and the absorption capacity is limited.the vacancies and interstitials are mainly recombined,and there are few residual defects.The evolution of the tensile structure shows that the GB hinders the propagation of dislocation.When there is no irradiation,the dislocation is always confined in a certain grain;after irradiation,the defects break the limitation of GB.The stress-strain analysis shows that under the same irradiation conditions,the rotation angle of symmetrical tilt grain boundary has the greatest influence on the maximum tensile strain(MTS),fracture toughness and Young's modulus,and the medium rotation angle GB usually has the characteristics of balanced MTS,fracture toughness and Young's modulus.The UTS can be measured by the ? value,and the higher value has a superior stability of UTS.The relationship between defect concentration and microstructure and mechanical properties was studied.The higher the point defect concentration is,the worse the tensile properties are;under the condition of equal interstitial and vacancy concentration,the tensile mechanical properties of interstitial type are worse than that of vacancy type,for example,the UTS of 0.4%vacancy concentration model is 11.45GPa,while that of interstitial type is 5.59GPa,and the UTS of interstitial type is about 104.8%lower than that of vacancy type.During the tensile process,the main deformation mechanism of the interstitial type micro structure is 1/2<110>Perfect dislocation slip,while that of vacancy microstructure is 1/6<112>Shockley dislocation slip.Compared with the 1/6<112>Shockley dislocation,thel/2<110>Perfect dislocation has higher energy,unstable structure and easy to decompose into a large number of partial dislocations,which increases the density of dislocations,promotes further slip and twinning,and reduces the UTS of the material.