Molecular Dynamics Study On The Mechanical Behavior Of High-Entropy Alloy/Graphene Composites

FeNiCrCoCu high-entropy alloys(HEAs),with a single face-centered cubic(FCC)structure,are expected as potential candidates for next-generation structural applications in light of outstanding properties such as high hardness,excellent wear resistance and corrosion resistance,and good thermal stability.Therefore,it is of great significance to conduct research on high-entropy alloys with this component in order to further improve their performance.An effective strategy is to combine graphene with high-entropy alloys to form high-entropy alloy/graphene nanolayered composites.However,it is difficult for existing experimental techniques to detect and capture microstructural evolution in real-time with atomic resolution,both economically and technically.Fortunately,the leap forward in atomic simulation algorithms and computational capabilities has offered another avenue to us for studying the deformation behavior of metal/alloy systems at the atomic level.In particular,molecular dynamics,as a three-dimensional visualization approach,could real-timely detect and capture the dynamical evolution of nanoscale plasticity,revealing deformation behavior and corresponding mechanisms.Meanwhile,that in turn can also guide us to design materials with remarkable properties.For this end,the FeNiCrCoCu high-entropy alloy/graphene composite was identified as the main research object,and the mechanical behavior of the FeNiCrCoCu high-entropy alloy/graphene composite was studied under nanoindentation,nano-scratching,uniaxial compression,and shock compression loadings with the help of molecular dynamics simulation method.The main contents of this paper are as follows:(1)The nanoindentation process of FeNiCrCoCu high-entropy alloy/graphene composite material was studied using molecular dynamics simulation method.The nanoindentation response of the high-entropy alloy/graphene composite material was analyzed,the influence of graphene layers on hardness and Young’s modulus was explored,and the hardening mechanism of the high-entropy alloy/graphene composite material was revealed.The indentation hardness of the composite material was confirmed to be effectively improved after covering the surface of high-entropy alloys with the graphene layer.Due to the high in-plane stiffness,Gr coatings were demonstrated to effectively decrease the contact stress and inhibit the dislocation nucleation,which facilitated the decreased subsurface damage.Meanwhile,highdensity Shockley and Stair-rod dislocations induced by an increase of the actual loading area were verified to synergistically promote a strong strain hardening effect in the composites.The substantial hardening effect of the high-entropy alloy/graphene composite material was a result of the combination of the high in-plane stiffness of graphene and its induced strong strain hardening effect.(2)The nano-scratching process of FeNiCrCoCu high-entropy alloy/graphene composite material was studied by molecular dynamics simulation method.The nanoscratching response of the high-entropy alloy/graphene composite material was analyzed,the influence of graphene layers on friction force,the friction coefficient,and the number of wear atoms was investigated,the anti-friction and wear-resisting mechanisms of the high-entropy alloy/graphene composite material were revealed,and the related nano-scratching experiment was carried out to verify the results of simulation.The advent of graphene was demonstrated to significantly reduce the friction on the surface of high-entropy alloy substrates.By comparing the distribution of atomic forces at the tip,the significant anti-friction effect was confirmed to be caused by a weakened contribution to friction from local pinning points while an enhanced contribution from local pushing points.Meanwhile,covering graphene layers can also suppress the wear damage,which was verified to be the result of the reduction of subsurface deformation of the high entropy alloy and the inhibition of surface pile-up behavior after introducing a graphene layer.More importantly,the friction and wear damage can be further improved with an increase in the number of graphene layers,attributing to the interlayer repair effect.In addition,the FeNiCrCoCu high-entropy alloy/graphene layered composite was prepared.It turned out that compared with pure high-entropy alloy materials,the scratching depth of the composite material was reduced and the surface pile-up behavior was suppressed,which was consistent with the simulation results.(3)The compression process of FeNiCrCoCu high-entropy alloy/graphene composite material was investigated by means of molecular dynamics simulation method.The interactions between dislocations and graphene in the high-entropy alloy/graphene composite material after introducing graphene were analyzed,the graphene size-dependent strengthening mechanism in the high-entropy alloy/graphene composite material was revealed.The hindrance of graphene sheet to mobile dislocations was demonstrated to promote the onset of massive immobile dislocations,especially the Stair-rod dislocation which gave rise to the remarkable strengthening effect.The other immobile dislocations,including 1/3<100> Hirth,1/3<110> and1/6<301> dislocations,exerted the subsidiary effect on strengthening the composite pillars.Meanwhile,the diminished dislocation length induced by dislocation reactions and the absorption effect of the interface to mobile dislocations was verified to endow a more appreciable dislocation starvation for composite pillars which was conducive to forming more Shockley and Stair-rod dislocations.In addition,the results also confirmed that the reduction in graphene diameters would directly weaken the pillars attributing to the fact that the graphene edges acted as a dislocation source.(4)The shock compression process of FeNiCrCoCu high-entropy alloy/graphene composite material was investigated through the molecular dynamics simulation method.The shock wave characteristics of the high-entropy alloy/graphene composite material after the introduction of the graphene layer were analyzed,the dynamic deformation mechanism of the high-entropy alloy/graphene composite material was revealed,and the influence of the shock velocity on the deformation mechanism of the high-entropy alloy/graphene composite material was explored.The resistance to dislocation propagation imparted by graphene was corroborated to encourage the elevated local stress level by increasing the likelihood of dislocation interplays,which facilitated the onset of twins and hexagonal close-packed(HCP)phases.It was additional formation of these twins and HCP phases that led to energy penalty in the shock wave,which contributed to the improvement of the shock resistance of the composite material.The advent of Gr was demonstrated to endow the high-entropy alloy with an additional twinning pathway,i.e.,a structural conversion from HCP to parent FCC within the HCP phases.By virtue of an increase in flow stress,the transformation-induced plasticity(TRIP)effect was validated to be additionally evoked as the predominant strain accommodation mechanism of the composite material at higher strains,which was be conducive to extending the strain-hardening capacity.More than that,the dynamic deformation mechanism of the composite material after the introduction of the graphene layer shifted from dislocation slip at low strains to the HCP martensitic transformation at higher strains,followed by the body-centered cubic(BCC)phase transition at high strains.This varied obviously from those in the highentropy alloy materials,where the deformation mode gradually changed from prevalent dislocation slip at low strains to BCC phase transition at high strains.