The Effects Of Vacancy-type Defects On The Plastic Behaviors Of Metallic Materials And The Corresponding Mechanism

Due to the collision cascades induced by high-energy particles,the materials of the pressure vessel and fuel cladding in nuclear reactors suffered from high concentration of Frenkel pairs,i.e.vacancy and interstitial.Vacancies gather under multi-field coupling(including temperature,stress and concentration fields)and evolved into different kinds of vacancy-type defects.Lots of experimental evidences showed that the irradiated-induced hardening,brittleness or creep and closely related to the interaction between vacancy-type defects and the plastic carriers of different materials.Since the vacancy-type defects could be in micro-,nano-or even subnano-scale,understanding the irradiation-induced plastic responds in metals could be difficult by experimental method solely.Therefore,it is of importance to capturing the interaction details between vacancy-type defects and the plastic carriers in atomic scale.Basing on these,we can extract the key physical mechanisms and thermodynamic parameter which is useful for upper-scale methods.On the one hand,such atomic researches promote the theory of plastic mechanics to develop,by revealing the irradiation effects on the plastic behaviors of metal.On the other hand,they could guide the safety design of the radiation resistant material in future reactors,and therefore have important engineering application value.Based on the above understanding,the thesis focus on the effects different kinds of nano-scale vacancy-type defects on the plastic behaviors of ?-iron and Cu Zr type metallic glasses,which representing crystal and amorphous metals respectively,by molecular dynamic(MD)method.Four particular researches on these aspects are performed as follow:(1)By atomic scale simulations,we studied the interaction between ellipsoidal nanovoid and 1/2<111>{110} edge dislocation(ED)in ?-iron,and discuss the influence of nanovoids shapes on their obstacle strength to the ED.Under the quasi-static condition for T=0K,the axis length along the dislocation line is the dominant factor controlling the obstacle strength of nanovoids.While the axis length along the other two directions also affect non-negligibly by changing the dislocation configurations during interaction.Given that,this dissertation modified the classic theoretical prediction model for spherical nanovoids to include the effects of shapes.In addition,the MD results showed that,the temperature basically do not changed the trend of the shape effects.(2)A combination of Monte Caro and molecular dynamics method is used to probe the helium segregation on 1/2[1(?)1] DLs in ?-iron,and the interaction between 1/2<111>{110} ED and the decorated DLs.The simulation results show that,the helium atoms always tend to segregate as bubbles at six energetic favorable sites on the edge of DLs.The helium decoration suppresses the absorption of DLs by moving EDs effectively,and increases the obstacle of DLs synchronously.Three kinds of new residual products are observed when considering the helium-decoration,while the corresponding interaction mechanisms are discussed.(3)By simulating the effects of embedding nanovoids on the uniaxial tensile behaviors of Cu Zr type MGs,the ratio of void-surface area to MG volume ?,is revealed to be a critical factor for the plastic behavior of MGs.With increasing ?,the deformation mode of MGs with embedding nanovoids changed from localized shear banding to homogeneous flow.The intrinsic mechanism for such brittle-to-ductile transition may be the transition of energetic void-surface atoms into low-energy internal atoms.This process creates uniformly distributed nucleation sites for shear transformation zones,and thus provides another way to release the stored strain energy,rather than by the formation of a single dominant SB.(4)This dissertation also simulated the effects of sequential collision cascades on the microstructure and the mechanical property of Cu Zr type MG under uniaxial tensile test.It is showed that,the fraction of full icosahedron cluster and the relative density of MGs decrease and reach saturation synchronously with increasing dose.Under the uniaxial tensile test,the localization degree and the ultimate tensile strengths of MG sample decrease also with the increasing dose,while their deformation modes varied from single shear banding,to necking controlled by multiple shear bands,and finally to homogeneous flow.By examining the strain characteristics in irradiated MG sample,it is revealed that such irradiation-induced softening and toughening are rooted in the destroy of the full icosahedron cluster.