Research On Deformation Mechanisms And Mechanical Properties Of Copper-based Nano Multilayered Composites

Nanolayered composites(nano multilayered films)have received extensive attentions due to the ability to customize their properties.Interfaces and modulation parameters are two of the most critical parameters that affect their deformation behaviors and mechanical properties.In this work,molecular dynamics(MD)simulations have been conducted to assess the role of interfaces in Cu-based nano-metal multilayered films,including Pd/Cu,Ni/Cu and graphene/Cu nano multilayers.Deformation behaviors,strengthening and weakening mechanisms under different thermo-mechanical loads have been explored to uncover the influence of interfaces,modulation periods,and loading methods on mechanical properties of Cu-based nano-metal multilayered films.The main progresses obtained are as follows(1)The MD simulation models of Pd/Cu nano multilayered films with cubic on cubic(COC)and twin(Twin)interfaces were established.After relaxation,the interface misfit dislocation nets show periodic triangular distributions.The COC interface is composed of coherent regions,triangular stacking fault regions,dislocation lines and their intersections(nodes),and the Twin interface is composed of Cu or Pd atomic twin interfaces,dislocation lines and nodes.When stretched along the <110> direction,the maximum stress of the Pd/Cu multilayered film increases,while the flow stress decreases with the increase of modulation period.The flow stress of COC is larger than that of Twin,which is different from the conventional understanding that twin interfaces usually lead to strengthening.MD simulations were further performed for the responses of in-plane single crystal and in-plane polycrystalline Pd/Cu multilayered films with COC and twin interfaces subjected to tension in x and y directions respectively.The results show that when stretched in <110> direction,the strengthening effect of the COC interface is more obvious;when stretched in <112> direction,the strengthening effect of the twin boundary is more obvious,showing that the effect of interface structures on the mechanical properties is anisotropic.In the in-plane honeycomb polycrystalline model,twin interface is found to induce significant strengthening.(2)MD simulations were also conducted for the responses of Ni/Cu multilayered films with coherent,semi-coherent,and twin interfaces subjected to tensile straining at different temperatures.It was found that dislocation movement is hindered by all three kinds of interfaces,which are restricted to propagate within Cu layers.While twin interfaces which show stronger resistances to dislocation movement are able to absorb inclined dislocations,reduce dislocation density and restrict the dislocations propagation into Ni layers,thus results in significant strengthening in plastic flow stage.Indentation MD simulations were conducted for Ni/Cu nano-twinned multilayered film using a spherical indenter,and it was found that both partial slip parallel with twin interface(PSPTB)and twinning partial slip(TPS)can reduce the hardness.A critical twin thickness was identified,at which the hardness reaches the maximum.When the twin thickness is smaller than the critical thickness,TPS formed due to the reaction between partial dislocations and the twin interfaces,accounting for softening.When the twin thickness is larger,the combination of the limited strengthening caused by the confined layer slip with the softening caused by TPS and PSPTB would lead to obvious softening and the decrease of hardness.(3)In the MD simulations for the uniaxial out-of-plane compression of nano multilayered graphene/Cu with different Cu layer thickness,it showed that the stress-strain curves of the multilayered films include three stages.The load-bearing capacity of graphene and the hindrance to dislocation movement by graphene layer are the primary strengthening mechanisms.The elastic modulus,yield strength,and average stress in plastic stage of graphene/Cu multilayered composites increase with the decrease of Cu layer thickness.The yield and fracture strains reach maximum at the critical thickness of 1.9 nm.The concept of graphene influence zone(GAZ)was proposed,GAZ integrated rule of mixtures and a confined layer slip model were then established,which can well describe the elastic and plastic strengthening of multilayered films.MD simulations were then performed for the tension and compression in the <112> direction and compression in the <110> and <111> directions of nano multilayered graphene/Cu samples,to study the effects of loading direction on its deformation mechanisms.It was found that the graphene/Cu multilayered films exhibit asymmetrical tensile and compressive responses under straining in the <112>direction.During tension,the deformation is dominated by dislocation slip forming stacking faults;during compression,the deformation is dominated by dislocation slip forming twins.The deformation behavior and twin structure of the graphene/Cu nano multilayered film under compression strongly depend on loading directions.Under the<112> compression,high-density and ordered nanotwins would form at the graphene/Cu interfaces which is consistent with experimental observations,deformation twin thickeness would increase due to the wrinkle of the grapheme layer,and two different twinning mechanisms were found.Under the compression in the <110>direction,multiple twins would form due to cross-slip of dislocations.While under the compression in the <111> direction,deformation would be dominated by dislocation slip induced stacking faults.