Molecular Dynamics Study Of Nanoindentation Properties Of Crystal Tungsten

With the development of nanotechnology,more and more attention has been paid to the mechanical properties and deformation of materials at nanometer scale.In recent years,researchers have made a lot of achievements in many fields around its application.Single crystal is a kind of crystal with only one orientation in the crystal,and has a unique three-dimensional structure.With the in-depth study of metals,the grain boundaries of metals have attracted scientists’ attention.Tungsten is an important strategic metal.As an important part of new materials in modern science and technology,tungsten has many excellent properties,especially the characteristics of high melting point and high strength of tungsten are used in extreme working environment such as high temperature and high pressure.Molecular dynamics simulation has the characteristics of high strain rate,so it is suitable to be used as a research tool in this paper.In this paper,the effect of nano indentation on the mechanical properties of crystal tungsten is studied by means of molecular dynamics,and the initiation and evolution of dislocations in the indentation process of single crystal tungsten and bicrystal tungsten are studied,respectively,and the effect of nano indentation on the mechanical properties,the influence of dislocation generation and its interaction with grain boundary are studied.The research in this paper is divided into two parts:Firstly,the numerical model of single crystal tungsten is established by means of molecular dynamics,and the nanoindentation simulation of single crystal tungsten is carried out.The deformation mechanism,dislocation formation mechanism and dislocation propagation law of single crystal tungsten under nanoindentation are discussed.The effects of different indenter radius and loading speed were studied.The same situation under the control variable is analyzed by indentation simulation.Theresults show that:(1)When the radius of the indenter is different,as the radius of the indenter increases,the ratio of the depth of the deformed region of the tungsten atom around the indenter to the radius of the indenter gradually decreases;When the indenter is unloaded and moved to the initial position at the same speed,the larger the indenter radius,the smaller the slope of the pit;In the case where the indenter is pressed into the same depth,the load increases as the radius of the indenter increases,and contrary to the load-displacement curve,the hardness of the smaller indenter is higher than that of the larger indenter.(2)When the loading speed of the indenter is different,the load difference in the pre-loading period is not large,and the gap starts to increase gradually as the indentation depth increases;and as the indentation rate increases,the hardness increases.Secondly,a numerical model of bicrystal tungsten was established by molecular dynamics method to study the mechanical properties of load-displacement curve,hardness,dislocation initiation and plastic deformation during the indentation of bicrystal tungsten.The effects of the radius of the indenter,the indente speed,the position of the indenter distance from the grain boundary and the maximum indentation depth on the nanoindentation of the bicrystal tungsten were discussed in depth.It is found that the residual dislocation after the unloading of the indentation is the root cause of the plastic deformation of the material.(1)as the indenter distance decreases from the grain boundary distance,that is,the closer the indenter is to the grain boundary,the higher the hardness of the material.(2)when the indenter is pressed into the same depth,the load increases with the increase of the radius of the indenter.When the indenter radius increases from 1.0 nm to 5.0 nm,the nanohardness decreases by about 60%.(3)the greater the maximum indentation depth,the greater the corresponding load value,and the corresponding hardness is gradually increased.(4)as the indentation rate increases,the corresponding load peaks are not much different but the hardness increases.In the all nanoindentation process of bicrystal tungsten,a similar conclusion can be drawn: during the whole loading process,dislocations are generated and gradually spread to the periphery,and the dislocations extend to the grain boundaries,but they are not worn through the grain boundary to the right substrate,the ring slip continues in the grain boundary region,which also indicates that the grain boundary hasthe effect of blocking dislocations to a certain extent.