Molecular Dynamics Simulations On Mechanical Behaviors And Deformation Mechanisms Of CoCrNi/CoCrFeMnNi Nano-multilayered Composites

Metallic nano-multilayered materials（i.e.,a kind of periodic multilayer composites）exhibit excellent mechanical properties,e.g.,high hardness,high strength and high toughness,by which it has a good application prospect in aviation,aerospace,national defense and other fields.Due to the rich phase interface and periodic multilayer structure,the metallic nano-multilayered materials present a strong size effect and with the unique mechanical behavior different from traditional rule of mixture of composites.However,most existing studies are generally aimed at the multilayered composites composed of different pure metals（such as Cu/Nb and Cu/Ni）,which provides relatively perfect understanding of the deformation behavior and strengthening mechanism for the multilayered composites.Different from pure metals and traditional alloys,high entropy alloy（HEA）is a single-phase solid-solution alloy composed of five or more principal elements in equal or near-equal atomic ratio,and has excellent properties（e.g.,high strength,high plasticity and radiation resistance）.At present,the relative research of HEA nano-multilayered materials is inadequate,and the deformation mode and intrinsic physical mechanism need to be further explored.In view of this,the mechanical response and deformation mechanism of high entropy alloy Co Cr Ni/Co Cr Fe Mn Ni（FCC/FCC）nano-multilayered composites under tensile and cyclic loads are systematically studied by molecular dynamics simulation.The research contents and conclusions are listed as follows.（1）Based on molecular dynamics simulation,the tensile mechanical response and micro-deformation mechanism of medium entropy alloy（MEA）Co Cr Ni and HEA Co Cr Fe Mn Ni at different temperatures（77 K,300 K）were studied.The results show that both Co Cr Ni and Co Cr Fe Mn Ni have higher yield strength and plastic flow strength at low temperature.At the same temperature,Co Cr Ni has higher yield strength and flow strength than Co Cr Fe Mn Ni.The plastic deformation mechanism of Co Cr Ni and Co Cr Fe Mn Ni is dominated by dislocation slip,and Lomer-Cottell lock is simultaneously formed by the interaction of dislocations,which effectively hinders the dislocation slip in the process of plastic deformation and is conducive to enhancing the strain hardening ability.With the decrease of temperature,the Shockley partial dislocation density of the two materials increases significantly,and low temperature is more conducive to the nucleation and movement of partial dislocation.In addition,twin nucleation and detwinning can be observed in the plastic deformation stage of Co Cr Ni and Co Cr Fe Mn Ni,which is a beneficial supplement to the plastic deformation behavior.（2）Based on the above studies,the mechanical response and micro plastic deformation mechanism of Co Cr Ni/Co Cr Fe Mn Ni nano-multilayered composites under tensile load are further analyzed,and the effects of layer thickness（h）and modulation ratio（R）(i.e.,the ratio of h _{Co Cr Ni} to h _{Co Cr Fe Mn Ni})are discussed.The results show that the coherent interface with mismatch dislocation network in multilayered materials can be used as the nucleation source of dislocation as well as a barrier to the movement of dislocation.In the initial stage of plastic deformation,dislocation nucleation and slip in Co Cr Fe Mn Ni layer are dominant.With the increase of applied load,slip dislocations accumulate at the interface and propagate and slip through the interface in the Co Cr Ni layer,which promotes the transfer of plastic deformation from the Co Cr Fe Mn Ni layer to the Co Cr Ni layer.At the initial stage of plastic deformation,stacking faults are formed by partial dislocation slip nucleation,and deformation twins areg radually generated.The flow strength of multilayered composites gradually increases with the decrease of layer thickness,showing a typical Hall-Petch phenomenon.With the decrease of the layer thickness,the dislocation density in the multilayer composites gradually increases,resulting in stronger Taylor hardening and higher flow strength.In addition,the flow strength of multilayered composites increases with the increase of modulation ratio（volume fraction of Co Cr Ni）.The flow strength of Co Cr Ni/Co Cr Fe Mn Ni multilayered composites with different layer thickness and modulation ratios are higher than the calculation of the rule of mixture of composites,and the flow strength of multilayered composites with partial layer thickness and modulation ratios is even higher than that of Co Cr Ni.（3）The mechanical response of CoCrNi,CoCrFeMnNi and CoCrNi/CoCrFeMnNi nano-multilayered composites under tensile and compressive cyclic load is compared,and the effect of layer thickness on the micro-cyclic deformation behavior of multilayered composites is analyzed.The results show that the tension-compression asymmetry occurs in Co Cr Ni,Co Cr Fe Mn Ni and Co Cr Ni/Co Cr Fe Mn Ni multilayered composites under monotonic load.With the increase of the number of cycles,the flow strength of the pure materials quickly reaches saturation and remains stable,while the phenomenon of cyclic softening first and then cyclic strengthening appears in the multilayered composites,showing a cyclic plastic deformation mode of a gradual accumulation of non FCC structure.Compared with pure materials,high entropy alloy multilayered composites generate more lattice disordered structures during cyclic deformation,which hinders the reverse slip of dislocations and weakens its Bauschinger effect.The lattice disorder degree of multilayered composites gradually increases with the decrease of layer thickness,and the Bauschinger effect also shows a weakening trend.