Atomistic Study Of The Effect Of Irradiation-induced Helium Bubble On The Mechanical Behavior Of FCC Cu And Cu/Nb Multilayers

The irradiation property of nuclear materials plays a significant role in the research and development of nuclear engineering.Irradiation effects including hardening,embrittlement,swelling and creep etc.seriously affect the safety and lifetime of nuclear engineering,which are inherently ascribed to irradiation-induced defects.Helium bubbles are one of the main defects of many irradiated materials.Therefore,the study on the microscopic deformation mechanism of the influence of helium bubbles on the mechanical properties of metal materials is one of the fundamental works for the design and development of irradiated materials.In this paper,the consequences due to He bubbles inside single crystal copper and Cu/Nb multilayer are systematically studied from the aspects of equilibrium internal pressure,size effect,temperature effect,interface shear strength and dislocation nucleation etc.by means of molecular simulation.The major works and contributions are as follows:The dependence of bubble internal pressure on the ratio of He/V is systematically investigated with considering different He bubble size under different temperature.Based on energy principle and stress criterion,a method is proposed to determine the equilibrium internal pressure of He bubbles.It is found: 1)An abnormal size effect of helium bubble on the equilibrium pressure is obtained,i.e.when the size of the He bubble is less than a critical one(3nm),the equilibrium pressure does not increase but drops slightly with the decrease of bubble size,which can be ascribed to the geometrical effects of edges and corners at Cu-He interface and physical characteristics of the Cu-Nb potential.2)Compared with molecular simulation results,the traditional Young-Laplace equation overestimates the equilibrium internal pressure of helium bubbles,and the error increases significantly with the decrease of the size of helium bubbles.For instance,when the aperture is 3nm,the relative error exceeds 71%.3)The equilibrium pressures obtained from the minimum energy criterion and that from the stress equilibrium criterion are coincided.The helium bubble-induced residual stress and its influence on the microscopic deformation progress of copper under simple tension are investigated.The simulation results show that: 1)Due to the polyhedron characteristics of the nanoscale helium bubble and the anisotropy of the material,there exists an additional stress field in the surrounding copper matrix of the bubble even under the action of the equilibrium internal pressure.However,this residual stress decreases rapidly as the increase of the distance from the center of the bubble.2)The time and spatial averaging method is proposed to reduce the effect of thermal-induced fluctuation on the stress field and the obtained residual stress field is more like the sense of that in continuum mechanics.3)The stress concentration exists at the edges and corners of the matrix around the helium bubble,and the corresponding Mises equivalent stress at the bubble surface increasing significantly with the rise of He/V ratio,which makes the He bubble becoming a strong dislocation source.Furthermore,the competition between dislocation sources of the bubble and surface is highly related to the ratio of He/V.The interaction between irradiated helium bubble and the semi-coherent interface in Cu/Nb multilayers,as well as the effect of interfacial helium bubble on interface shear properties and tensile properties are investigated.It is found that: 1)The matrix energy decreases as the helium bubble approaches to the interface,which implies that the semi-coherent interface in Cu/Nb multilayer is a sink to helium bubbles.2)The shear strength of semi-coherent interface in Cu/Nb multilayer is anisotropic,over-pressured interfacial He bubbles will reduce the shear strength of interface.3)The Cu/Nb multilayer has high tensile strength with two consequent peaks in the stress-strain curve.The two peaks are respectively ascribed to dislocation nucleation in Cu layer and Nb layer.Over-pressured helium bubble can lead to the decrease of the first peak,but has little effect on the second peak(ultimate stress),which indicating the great irradiation property of Cu/Nb multilayer.The quantitative results in this paper can provide basic parameters and guidance for the research and design of semi-coherent multilayers.The adopted method can be extended to investigate other irradiated materials.