Investigations On Strengthening And Toughening Mechanisms Of Supra-nanometre-sized Dual-phase CoCrFeNiAl High-entropy Alloys

Supra-nanometre-sized dual-phase glass-crystal high-entropy alloy(SNDP-GC HEA)is a new nano-structured alloy material made by combining two toughening methods,i.e.alloying and structuring.The dual-phase microstructure is generated by embedding HEA grains with a grain size magnituide of 10 nm into HEA metallic glass(MG)shell with a thickness of few nanometers.The multi-principal-elements properties and the dual-phase microstructure composed of MG shell and crystalline grains result in higher strength,hardness,and toughness,having significant potential applications in micro-and nanoelectromechanical devices and surface engineering.Since SNDP-GC HEA is an emerging material,understanding its plastic deformation mechanism and under various loadings can help optimize the design of strength and toughness,and promote its practical engineering applications.In this study,the molecular dynamics simulation program LAMMPS is used to simulate the deformation of the SNDP-GC HEA under loadings such as low fatigue cycle and tensile or compression accompanied by high-frequency vibration(HFV).The evolution tendencies of toughness and micro structural stability are analyzed,and the corresponding plastic deformation mechanism is also revealed.An attempt is made to introduce the gradient distribution in MG shell thicknesses,and the effect on the hardness and damage tolerance of SNDP-GC high-entropy alloy is investigated.In addition,the source of the well-known MD-continuum mismatch with experiments on deformation rate is also explored by examining the typical spontaneous deformation of snap-through behavior of nano-strip,and it confirmed the possibility of quantitative comparison of molecular dynamics results with those of the continuous theoretical model on time scales.The details of the study are as follows:(1)The plastic deformation mechanisms of SNDP-GC HEA under low fatigue cyclic loadings are examined.Two SNDP-GC HEAs with different MG shell thicknesses are constructed,and their unique strength and microstructure evolution tendencies are revealed by comparison with the corresponding polycrystalline HEAs.For the polycrystalline sample,dislocation motion,and grain boundary(GB)-mediated deformation control the plastic deformation process,and the sample loses its original structure after a few cycles of the set amplitude due to the GB migration.However,for the two SNDP-GC samples,as the grain boundary is replaced by the MG shell,the dominant plastic deformation mechanism has changed to the concentrating shear transformation in the MG shell.Since the deformation is concentrated in the MG shell,the density of defects such as dislocations,twins,and stacking faults within the grains is much lower than that of polycrystalline grains.It makes these defects have stronger recoverability,ensuring a good microstructural stability of SNDP-GC HEAs.The results also show that structural stability is highly dependent on MG shell thickness.For the sample with a thinner MG shell,MG cannot accommodate sufficient deformation which induces the generation of voids.However,for the sample with a thicker MG shell,MG can store adequate deformations,thus dislocation initiations in crystalline grains are reduced and void generation is prevented in the MG shell.(2)The influence of high-frequency vibration(HFC)on the mechanical properties of SNDP-GC HEA is investigated,providing theoretical references for its processing and forming.(1)As a comparison,the tensile deformation of single-crystal aluminum(Al)accompanied by high-frequency vibration(HFV)is firstly studied.It is found that HFV induces softening,i.e.,reduction in yield stress and strain by causing dislocations earlier onset and the degree of softening increasing with the improve of vibration amplitude.It is also found that both lattice orientation and strain rate play important roles in yield stress and plasticity.Under a lower strain rate,high-frequency vibration has enough time to induce dislocations annihilate,resulting in strong ups and downs in the strain–stress curves,indicating that the strain-hardening was significantly suppressed.Under a higher strain rate,newly appearing dislocations caused by tension interact with previous ones before the latter annihilate,densifying the dislocation network.The effect of HFV is thus weakened,resulting in a rebound of strength.(2)Molecular dynamics simulations of tensile&compression deformation of SNDP-GC HEA accompanied by HFVs are carried out.The results show that the effect of HFV is weak on the strength of the SNDP-GC HEA,but strong on the plastic deformation mechanism during tension.During tensile deformation,the HFVs induced the formation of voids in the MG shell.Fortunately,the similarity in size between the grains and the MG shell can prevent the SNDP-GC HEA from softening.In compressive deformation,the effect of high-frequency vibration is significantly stronger.The thin MG shell of SNDP-GC1 has a lower deformation accommodation capacity,and the distribution of STZ in the MG shell is more concentrated,allowing more deformation to be transferred into the grains and leading to a reduction in material strength.The thicker MG shell of SNDP-GC2 prevents localization of the STZ,thus ensuring a relatively high strength of the material.In addition,the strain rate sensitivity of SNDPGC is less affected by high-frequency vibrations than that of single-crystal aluminum.(3)The plastic deformation mechanisms of gradient SNDP-GC(GGC)HEA in nanoindentation tests have been examined.The MG shell thickness in the alloy is adjusted to follow a specified gradient distribution along the indentation direction,resulting in the construction of a GGC high-entropy alloy.It is found that the MG shell with thickness gradient-distribution effectively reduce the degree of strain localization within the region around the indenter,which results in a considerable hardening effect.A proper gradient distribution led to both dislocation motion in the grain phase and plastic flow in the MG shell activated simultaneously,inducing a more significant strengthening effect.Comparing the results with the literature data,it is found that the hardness of the GGC HEA in this study is close to that of the corresponding singlecrystal HEA.The MG shell with a thickness gradient distribution hinders the occurrence of the FCC→HCP phase transition in the grains,thus increasing the damage tolerance of the GGC HEA.With the reduction of aluminum content in the MG shell,a more excellent strain propagation capability is exhibited within the material under the action of the indenter,which is an effective means of regulating the deformation pattern of the dual-phase microstructure.In addition,the magnitude of the sensitivity of the aluminum content is strongly correlated with the thickness distribution of the MG shell.With increasing aluminum content,the decrease in hardness is greater for GGC1,while the decrease in hardness is relatively smaller for GGC2.(4)For the molecular dynamics simulation method,the fundamental issue of the mismatch between experimental and simulated deformation rates is initially explored by investigating the snap-through behaviour of nano-strip.The elastic snap-through behavior of single-crystal copper strips is examined by numerically and theoretically to investigate factors that influence the characteristic snap-through time scale.The strip is simply supported on both ends,and the snap-through is launched by suddenly removing the concentrated forces that have already been statically applied to produce an initial bending configuration.On the one hand,the process is implemented in the MD method.On the other hand,a theoretical formulation is provided with the consideration of surface tension.Increasing surface tension is found to increase the snap-through time.The results show that the snap-through behavior is further closely related to the magnitude of the initially stored deformation and the strip thickness.Finally,snapthrough times provided by the numerical and theoretical analyses are on the same order of magnitude.This is an interesting agreement,especially considering that the huge gap in time scales between MD simulations and experiments has been a well-known fundamental issue.It is believed that the study about spontaneous processes such as snap-through has cast some light on the fundamental issue that deformation in MD simulations generally happens much faster than in physical experiments.