Simulation Studies On The Mechanical Behaviors Of Nano-metallic Materials Regulated By Grain Boundaries/Hetero Interfaces/Twin Boundaries

As a structural material with a long history,metallic materials have been playing an irreplaceable role in various fields.With the rapid development of science and technology,higher requirements are put forward for the mechanical properties of metallic materials.Nanostructured metallic materials have attracted the attention of the public because of their excellent properties different from those of conventional coarse-grained metals.However,with the development of the research,it is found that for nanostructured metallic materials,although the strength still increases with the decrease of grain size within a certain range of characteristic size,the ductility of the material is sacrificed.Besides,high-density grain boundaries(GBs)store a large amount of energy,which often leads to the decrease of the thermal and mechanical stability of nano-metallic materials.The inverted dilemma of strength and ductility and the stability of nano-metallic materials greatly limit the processing and technical application of this new material.Recently,research has found that the comprehensive properties of nano-metallic materials can be significantly improved by adjusting the distribution of some low-energy interfaces,providing a new possibility for the development of nano materials with high strength and ductility.Exploring the modulatory role of interface structures during deformation through the atomic-scale simulation can help people understand the influence of interface structures on mechanical properties and strengthening mechanisms at micro scale more deeply.This is of great significance for guiding the development and design of nanostructured metallic materials with excellent comprehensive properties.In the present dissertation,the effects of interface structures,including GB,hetero interface and twin boundary(TB),on the mechanical properties and deformation mechanisms of nano-metallic material are studied.For GB,the influences of segregation of the second-phase elements at GB on GB stability,plastic and creep deformation are mainly investigated.For hetero interface and TB,the present dissertation focuses on the coupling effects of different interfaces on the mechanical properties and deformation mechanisms of nano-metallic multilayered materials.The main work includes the following four parts:Firstly,the tensile mechanical behavior of nanocrystalline Ni before and after the segregation of Mo atoms at GBs are simulated.Polycrystalline models of Ni with Mo atoms randomly distributed in grains and segregated at GBs are established.The effects of segregation at GBs on the mechanical properties and deformation mechanisms of nanocrystalline materials are investigated by molecular dynamics method.The results show that segregation of Mo atoms at GBs can effectively avoid softening behavior of nanocrystalline Ni below critical grain size.In addition,in order to explore the underlying mechanisms for the significant strengthening effect after the segregation of Mo atoms at GBs.some simple but representative bicrystal models are constructed to study the influences of segregation on GB energy and migration ability.It is found that the segregation of Mo atoms at GBs can drastically reduce the GB energy and the energy reduces almost linearly with the increase of segregation degree in a certain range.Besides,Mo atoms segregation at GBs can also effectively inhibit the grain growth driven by curvature.which further proves that segregation can reduce the migration ability of GBs.By checking the equilibrium configurations of nanocrystalline Ni before and after Mo atoms segregation at GBs,it is also found that the segregation of Mo atoms at GBs disrupts the regular dislocation network structures at GB,which are one of the main sources of dislocation nucleation during deformation.Thus,the ability of GBs to emit dislocations is suppressed dramatically,which is also one of the microscopic mechanisms that strengthen the material.Secondly,the creep behavior of nanocrystalline Ni before and after the segregation of Mo atoms at GBs are simulated.Three-dimensional polycrystalline models of pure Ni and Ni with Mo atoms segregated at GBs are established.The effects of grain size,applied stress.temperature and concentration of Mo atoms at GBs on the creep properties of nanocrystalline Ni at high temperature are systematically studied using molecular dynamics method.The results show that the creep strain rate of nanocrystalline Ni decreases significantly after the segregation of Mo atoms at GBs.By comparing the activation energy calculated based on the simulation results,it is found that higher energy is required to activate the creep mechanism for segregated samples than that for pure Ni samples,which means that the creep deformation in segregated samples is more difficult to occur,thus improving the creep resistance of the material.According to the analysis of stress exponent and grain size exponent,it is concluded that the corresponding creep mechanisms are diffusion,GB slip and dislocation activities under-low,medium and stress states.This indicates that the influence of applied stress and grain size on creep strain rate of nanocrystalline Ni can still be described by the classical Bird-DornMukherjee equation when Mo atoms are segregated at GBs.Through the statistical analyses of the influences of segregation on creep rate of the material under different applied stress,it is found that segregation has little effect on the creep process dominated by lattice diffusion,while can effectively reduce the creep strain rate of the creep deformation dominated by GB behavior and dislocation activities,and the higher the concentration of Mo atoms at GBs,the lower the creep rate.Thirdly,the tensile behaviors of monocrystalline Cu/Ag multilayered alloy are simulated.The effects of different hetero interface structures,individual layer thickness and twin thickness on mechanical properties and deformation mechanisms are systematically investigated by molecular dynamics method.The results show that the existence of hetero interfaces can improve the strength of materials,and the strength of increases with the increase of the density of hetero interfaces.Because of the structural differences between H etero-Twin(HT)interface and Cube-On-Cube(COC)interface,HT interface has a stronger strengthening effect on strength.In addition.the secondary strengthening can be achieved by the introduction of TBs into Cu/Ag multilayers,and the strengthening effect of TBs on multilayered materials still approximately follows Hall-Petch relationship.It is worth noting that the first dislocation nucleation is not from the hetero interface.Furthermore,the calculation results according to the adaptive NEB calculation method proves that the energy barriers for the dislocation nucleation in the bulk between TBs that far away from the hetero interface needs to overcome is lower.Fourthly,the tensile behaviors of polycrystalline Cu/Ag multilayered alloy are simulated.The model of columnar polycrystalline Cu/Ag multilayers is established to study the coupling effects of GBs,hetero interfaces and TBs on mechanical properties and deformation mechanisms using molecular dynamics method.The simulations show that there is a competitive mechanism between grain size and layer thickness in the process of regulating material properties.For multilayers with COC interface,the strength of material mainly depends on the grain size and increases with the decrease of grain size.For the multilayers with HT interface,the strength of material is mainly affected by the structure with smaller characteristic size.When the ratio of layer thickness to grain size is small enough,it is found that the deformation mechanism of polycrystalline multilayered material will change from dislocation activities to relative slip of hetero interface,and this new plastic deformation mechanism will cause a significant decrease of material strength.For polycrystalline multilayered materials containing TBs.the simulation results show that the flow stress increases with the decrease of twin thickness.However,due to the competitive relationship between GBs and TBs in the process of regulating plastic deformation,the dependence of the flow stress on the twin thickness gradually weakens as the grain size decreases.