Molecular Dynamics Simulation Study On Mechanical Properties And Deformation Mechanism Of Nanocrystalline Metal Materials

Compared with conventional metals,nanocrystalline metals have unique mechanical properties.For example,the strength of nanocrystalline metals is generally much higher than that of the same material,so they have potential applications in many fields.However,due to the insufficiency of material preparation methods and measurement technology,there are still many problems to be solved in the research of nanocrystalline metals,for example,the inverse Hall-Petch relationship that appears as the size of the grain is reduced below 20 nm,It is quite controversial.Nanocrystalline metals and films inevitably introduce vacancy defects during the preparation.At the same time,researchers have recently prepared porous materials specifically to reduce the quality of the materials,and then use their nanoporous properties to apply to other fields,such as catalysis.These potential applications are introduced some new research topics.Among them,in mechanics,a basic problem is the relationship between vacancy defects and mechanical properties.In addition,the fracture limit of the nanocrystalline structure and the effect of annealing on the mechanical properties of nanocrystalline materials also need to be resolved urgently.With the improvement of computing technology,molecular dynamics simulation can explore the structural evolution and mechanical and thermal properties of more than one million atomic systems,especially the mechanical problems related to grain boundaries.This provides an important annlysis means for the deficiencies in experimental research.This article takes the important basic problems in the nanocrystalline metals mentioned above as the research motivation,and takes the nanocrystalline gold(including bulk and thin films)as the typical model,and studies the uniaxial tension by molecular dynamics simulation.Under the influence of its grain size,strain rate,film width,nanopores and annealing temperature on its mechanical properties and deformation mechanism.The main results are as follows:1.There is a mixed region between the Hall-Petch and inverse Hall-Petch regions.By simulating tensile deformation process of nanocrystalline gold with grain size from 2.65 nm to 18 nm.It is found that the grain size is in the range of 10 nm-18 nm,the flow stress does not change significantly,and the grain boundary atomic weight and stacking fault atomic weight are almost equal at strain 14%,dislocation activity and grain boundary sliding control the plastic deformation,this region can be called the mixed region.The grain size is between 5 nm and 10 nm.The flow stress decreases rapidly with the decrease of grain size.The atomic weight of grain boundaries is larger than that of stacking faults.Grain boundary sliding is the main plastic deformation mechanism.The region is a typical inverse Hall-Petch relationship.Therefore,between the Hall-Petch relationship and the inverse Hall-Petch relationship,there is a mixed region where the flow stress does not change significantly with the grain size.This can explain the contradiction between the existence of the inverse Hall-Petch relationship proposed by the early theoretical simulation and the fact that its existence was not observed in experiments.2.The failure strain of the nanocrystalline film with a width of 130 nm is the largest,and its failure strain can be as high as 96.1%.The mechanical behavior of nanocrystalline gold films with different widths through molecular dynamics simulation.By analyzing the relationship between film width and failure strain,it is found that when the width of the film is less than 80 nm,the failure strain decreases as the width decreases.The fracture is caused by necking caused by the grain boundaries sliding and stacking fault.When the width of the nanocrystalline film is greater than 130 nm,the failure strain decreases slowly as the width of the nanocrystalline film increases.The stress concentration and local thinning of the nanocrystalline film can promote the formation of shear bands.For nanocrystalline film with a width from 80 nm to 130 nm,they have good ductility and large fracture strain.And,we found that the failure strain of film with a width of 130 nm is the largest,which can exceed 96.1%.3.The mechanical laws and deformation mechanism of nanoporous materials are different on both sides of the density of 94%.Through simulation,it is studied that the mechanical properties of nanocrystalline closed-cell gold in tensile process,and the relationship between density and mechanical properties is calculated.It is found that the Young's modulus of closed-cell materials varies with relative mass density similar to that of open-cell materials,but the strength of closed-cell materials is greater than that of open-cell materials.For the structure with voids only near the grain boundary,the relative mass density of 0.94 is the turning point of the material's Young's modulus,maximum stress,flow stress and deformation mechanism.When the relative mass density exceeds 0.94,the grain boundary hinders the dislocation activity,which leads to the increase of the flow stress,and the voids reduces the dislocation interaction,which leads to the decrease of the flow stress.When the relative mass density is less than 0.94,the voids mainly control the deformation mechanism,which is consistent with the deformation mechanism of the open-cell material,and the mechanical properties of the closed-cell material conform to the modified Gibson-Ashby relationship.4.The melting point of nanocrystalline gold remains constant with small grain size.Using molecular dynamics simulation,we calculated the melting point of nanocrystalline gold with different grain sizes,and analyzed the law of melting point change with grain size.The research results show that when the grain size of nanocrystalline gold is greater than 7.7 nm,there is a linear relationship between its melting point and the reciprocal of the grain size.When the grain size is reduced to 7.7 nm,the relationship of the melting point and the grain size begins to deviate from the linear relationship.And when the grain size is less than 4 nm,the melting point of the sample is almost constant,about 1070 K.We analyzed the change process for the grain size and grain boundary atomic weight of nanocrystalline gold with increasing temperature,and found that the reason why the melting point of nanocrystalline gold with small grain size remained unchanged was the growth of the grains.