Molecular Dynamics Study on the Simulation Model Building and Fracture Mechansims of Metallic Nano-materials

Author:Li Peng Tao

Supervisor:yang yan qing


Degree Year:2018





With the rapid development of nanotechnology,more and more researches clearly realize that when the size of material structure is reduced to the nanometer level,the mechanical behavior at nano-scale shows significant different from macro scales,and presents many new mechanics phenomenon.It is of great significance to study the mechanical properties and deformation mechanism of the nanomaterial,which could be applied during the design processes of new nanometer material.Due to the constraints of experimental conditions and technology,the deformation mechanism and mechanical properties of nanomaterial are not fully understood by experimental methods.Recently,using molecular dynamics simulation to carry out the related research of nanomaterial has become a powerful method for theoretical exploration and experimental research,which has already become an important tool for nanomaterial study.Using the molecular dynamics,not only can get the optimal nanostructures through energy optimization process,but also can observe the microstructure evolution process,which has ability to analyze the mechanical behavior of different deformation stages,and reveal the inner material deformation mechanism.Therefore,in our work,the optimal structure and the atomic deformation mechanism for the three-dimensional TiAl nano polycrystalline,two-dimensional nano pure aluminum plate and one-dimensional Cu-Ag core-shell nanowires are studied based on molecular dynamics method.In order to obtain the nano polycrystalline TiAl alloy,the rapid quenching method was used.When the quenching rate is greater than 0.02 K/ps,the finally solidification product has amorphous structure with feature of medium-range order and long-range disorder.When the quenching rate≤0.02 K/ps,the finally products are TiAl polycrystalline alloy with nano structures,and the component analysis results show that the main phases of nano TiAl alloy consist ofγ-TiAl andα2-Ti3Al.During the formation process of nano TiAl alloy structures,the first solid phase formed in the undercooling melt is the atomic cluster with BCC configuration,which plays a role in relaxation regime during transformation from icosahedral(ICO)configuration to initial crystal nucleus.Once the metastable BCC phase is formed,subsequently,HCP configuration appears,followed by the atoms cluster with FCC configuration,while the BCC clusters disappear and the solidification process of nanocrystals begin.During the process of the deformation of the nano TiAl alloy,grain boundary sliding and lamellar domain increasing are the two main deformation mechanisms at the strain below9.0%.With further increasing strain,voids nucleate and grow along the grain boundaries,and the coalescence of voids by forming a crack results in subsequent failure.The research results of the deformation mechanism of two-dimension nano aluminum crystals are described as follow.Firstly,the amorphization process at crack tip during the crack propagation in FCC Al is observed by high-resolution transmission electron microscopy(HRTEM).The results rationalize the amorphization assumption that the amorphous zone is likely at crack tip in FCC structures at high stress intensity.Secondly,the simulation results show that there exists a transition point KI=0.205 eV?-2.5 between twinning partial dislocation and Lomer one,with Lomer dislocation is preferable one at high stress intensity.A systematic study on the optimal structure and possible deformation mechanism for Cu-Ag core-shell nanowires(NWs)is carried out.After energy minimization process,the actual structure of the Cu-Ag core-shell nanowires at 300 K are observed,the results show that when the shell thicknesses are smaller than 1.0 nm,the atoms configuration in shell Ag are reconstructed,when the shell thickness increases from 1.0 nm to 1.5 nm,the shape of NWs change into ellipse.And then the plastic deformation mechanism of Cu-Ag core-shell NWs by applying uniaxial tension is systematically investigated.When with high shell thickness(≥1.0 nm)and at low temperature(<500K),a new deformation mechanism,i.e,dislocation emission from misfit dislocation tube is found,which controls the plastic deformation of core-shell NW and makes the yield stress abruptly decrease.If we decrease the shell thickness to be smaller than 1.0 nm while maintain low temperature(<500K),the plastic deformation mechanism will transform from the misfit dislocation emission mechanism to the surface dislocation nucleation mechanism.On the other hand,if raising the temperature to a certain level(≥500K),the plastic deformation mechanism then transforms into the high temperature influence dislocation nucleation mechanism.