The Effect Of Void And Helium Bubble On The Mechanical Properties Of Palladium Nanowire:a Molecular Dynamics Study

The mechanical properties of Pd in the nuclear irradiation enviroment, has attracted much attention, as its application in the tritium-storage technology and the excellent helium-retention properties. Helium atoms can diffuse, nucleate and release in Pd, inducing the production of some defects such as point defect, line defect, face defect, etc. in the materials. We employ the molecular dynamics (MD) method to study the void and helium bubble effects on the mechanical properties of palladium nanowire under uniaxial tensile strain at300K. In the simulation, the MAE AM potential is used to study the effect between Pd and Pd atoms, Morse potential and L-J potential to study the interaction effect of Pd-He and He-He, respectively. The mechanical properties of pure Pd nanowire are also investigated in our work to make a comparison.The existence of a void or helium bubble has obviously degraded the mechanical properties of the Pd nanowire. Under the uniaxial tensile strain, the void is firstly filled by the metallic atoms in the vicinity region during wire’s plastic deformation process for all the voids with different size. Then, it experiences the appearance of dislocation, necking, and elongation until its completed rupture. However, the deformation process and necking position is much different for different sized voids in the wire. Our results indicate that there exists a critical size of void,～1.6nm. In the nanowire, as the diameter of void is smaller than critical size, it is necking and ruptured at a random place, while at vicinity region of void as exceeding the critical value. It also reduces the ductility of the wire as increasing the size of void. It is shown that the rupture strain of nanowire decreases from44.2%to19.8%, as the diameter of void is from0to2.2nm. By the implement of the CNA method, it is demonstrated that increasing the void size can effectively inhabit the glide of dislocation. The glide planes can cross over the void area completely as the diameter of void increases until reaching the critical size.Under the uniaxial tensile strain, helium bubble has played an important role in the degradation of material’s mechanical properties. During the tensile process, helium bubbles reduce the ductility of Pd nanowires and facilitate their rupture. The local tensions will enhance around the bubble implanted and the nanowires will crack there due to its obvious inhibition for the relative motion of glide planes. It is also demonstrated that increasing the bubble size or decreasing the wire’s cross sectional width, it accelerates the wire’s rupture and enhances the impedation of dislocation glides. There also exists a critical value of effective cross section, exceeding which the glides planes cannot completely cross over the bubble and the wire’s configuration reconstruction are inhibited obviously. Here, the effective cross sectional width is defined as the ratio of the discrepancy between cross sectional width and helium bubble diameter to wire’s cross sectional width, k=(d-D)/d, and the effective cross sectional width is0.56in our work. Based on our simulation results, it is concluded that the existence of defects can degrade mechanical properties of materials by inhabiting the gliding of dislocation during the tensile process mainly.