Investigation Of Size Effects And Deformation Mechanisms Of Cu/Ta Multilayers Under Uniaxial Tension With Molecular Dynamics Simulation

Nanoscale metallic multilayer is a new film developed on the basis of monofilm and composite membrane.It has the special performance that the bulk materials and single materials are difficult to achieve.It has good development and application prospects in the aerospace,machining and other fields.Hence,the nanoscale metallic multilayers have attracted the attention of many scholars and become a hot spot in the study of materials.In recent years,a large number of scholars have discovered the excellent mechanical properties of multilayers through experiment,such as high strength,high hardness and high toughness,etc.But its internal strengthening and toughening mechanism are not yet entirely clear.At present the deformation and strengthening and toughening mechanisms of fcc/fcc multilayers has been explained and understood by molecular dynamics method.However,there are relatively few potential mechanisms and theoretical studies for multilayers with two phase interface structures(fcc/bcc,fcc/hcp).Therefore,this paper is intended to study the size effect and deformation mechanism of Cu/Ta multilayers under uniaxial tension by means of molecular dynamics(MD)method.In this paper,the models of Cu,Ta films and Cu/Ta multilayers are established.We have analyzed the influence of modulation periods(λ),interface structures,temperatures,and the modulation ratio(r)on the mechanical properties under uniaxial loading.The deformation mechanisms of the Cu/Ta multilayers are revealed from atomic scale combined with the σ-ε curves.The main research contents and conclusions in this article are as follows:The Cu and Ta films with two lattice orientations are simulated and analyzed,respectively.The results show that the nucleations of <112> partial dislocations lead the K-S Cu film to yield,but for N-W Cu film which is ascribed to the generation of <112> and <110> dislocations;the yield strength of K-S Cu film is relatively smaller but the mean flow strength is larger.For the K-S Ta film,the nucleations of <100> dislocations make it yield,however the phase transition of bcc Ta leads the N-W Ta film to yield.Comparing Ta films with two lattice orientations the N-W Ta film yields firstly.The influence of λ on the deformation mechanism of K-S Cu/Ta multilayers has been analyzed.It is found that the Cu(111)/Ta(110)interfaces can serve as the sources for dislocation nucleations and the barriers impeding the motion of dislocations.The <112> partial dislocation nucleations make the Cu/Ta multilayers yield,and the yield strength basically remains unchanged under different λ.The smaller the λ the stronger the interface strengthening ability,and the dislocations nucleate difficultly from the interface in Ta layer,so the maximum strength increases with the decrease of λ.There is a critical λ(about 11.18 nm)in plastic flow stage,in which the flow strength of Cu/Ta NMMs is maximized.In this stage,the dislocation movements in Cu layers and Ta layers dominate the plastic deformation in the material,jointly.The changes of the σ-ε curves of Cu/Ta multilayers with two interfacial configurations(K-S,N-W)have been analyzed from atomic scale.The kinds of dislocations and the positions of dislocation nucleation in Cu layers are different under different interface configuration,leading to the yield strength of Cu/Ta multilayers is different and which of N-W Cu/Ta multilayers is bigger.The maximum strength of K-S Cu/Ta multilayer increases with the decrease of the λ,but that of the N-W Cu/Ta multilayer increases with the increase of λ,which is due to the different deformation mechanism under two kinds of interface structures.After entering the plastic flow stage,the mean flow strength of K-S Cu/Ta multilayer increases with the increase of λ,but that of N-W Cu/Ta multilayer obtains the maximum at λ=8.94 nm,and the mean flow strength of K-S Cu/Ta multilayers is larger.We have also analyzed the effects of different temperatures(10K,300K)on the micro-deformation of K-S Cu/Ta multilayers from atomic scale.It is found that the yield strength of Cu/Ta multilayer is basically the same at both different temperatures,indicating that the temperature has a little effect on the yield strength of Cu/Ta multilayers under certain temperature range.We also find that the maximum strength of K-S Cu/Ta multilayer all decreases with the increase of λ under two kinds of temperatures,but the higher the temperature the motions of <112> partial dislocations in Cu layer the acuter,so as to make the dislocations nucleate from the interface in Ta layer easily,namely the higher the temperature the maximum strength of Cu/Ta multilayer the smaller.Besides,the mean flow strength of K-S Cu/Ta multilayers all increases with the increase of λ under two kinds of temperatures,but that is basically the same when the λ at the same.Combining with the σ-ε curves we have analyzed the influence of r on the micro-deformation of Cu/Ta multilayers.It can be found that the volume fraction of Cu in Cu/Ta multilayers is increasing with the increase of r,which makes the yield strength decrease gradually.We also find that the maximum strength of Cu/Ta multilayer also decreases with the increase of r,and the maximum strength of Cu/Ta multilayer decreases quickly when the r < 1.It is indicated that the maximum strength of Cu/Ta multilayer is not only related to the volume fraction of Ta but also the multilayer characteristic structures and the interaction between the dislocations and interfaces.The mean flow strength basically remains unchanged.