Research On Nanoindentation Response Of Metallic Glasses Via Molecular Dynamics Simulations

Amorphous alloys,also referred to as metallic glasses(MGs),is one of emerging metal materials with rapid developing of science and technology.The basic atomic structure of MGs long-range disorder arrangements.With this structure character,MGs have both extraordinary mechanical properties of metal materials and high corrosion resistance of glass materials,which makes it a promising structural and functional material in the fields of microelectronics,corrosion-resistant coatings and photocatalysis,etc.From the view of industrial application,MG materials inevitably face a complex mechanical environment in practical engineering applications,making the study of deformation mechanism and mechanical properties a focus and a hotspot in the scientific research of amorphous materials.The correlation between the deformation mechanism and the microstructure of the MGs is the key to cracking the mechanical nature of the amorphous material.In the experimental researches,traditional mechanical testing methods can only provide ex-situ observations for local structure transition,far from satisfying the research on microstructure transition under stress.The development of advanced observation technology can realize nano-scale in-situ mechanical testing,strongly promoted the study of the microscopic mechanical properties of MGs.However,such testing and observing methods have great dependence on the advanced level of the instrument,high testing costs,and high testing environment requirements.Furthermore,the urgent need for research on the diverse parameters of MG materials(such as deformation rate,temperature,and shear band interaction)cannot be satisfied.As mentioned above,the research on the mechanical properties and deformation behavior of the MG materials at the micro-nano level is in urgent needs,while proper in-situ testing technology is highly costly and harsh in contradiction.In this thesis,molecular dynamics(MD)simulations are conducted on Cu-Zr binary amorphous alloy to solve the contradictions.The following research work has been carried out around the response of the amorphous alloy indentation test.(1)Firstly,a Cu-Zr binary MG model was established with different compositions and cooling rates.The 2D cylindrical and the 3D spherical indentation simulations were performed on the MG sample.Information on the yield strength,elastic moduli,and hardness of the two simulations was obtained by Hertz contact analysis and other methods.The results showed the mechanical parameters obtained are basically the same,which means that the 2D simulation has certain practical significance in the calculation of the MG material mechanical parameters.By investigating the effects of the preparation parameters and the indentation test parameters on indentation processes and deformation morphologies of MG,it was found that the MG preparation parameters,such as cooling rate and composition,could affect the mechanical parameters by changing the main SRO structure.The indentation test parameters mainly affect the measured mechanical parameters by changing the atomic rheological properties.(2)Nanoindentation simulations were performed on pre-compressed and pre-stretched MG samples via MD methods,to investigate the effects of pre-stresses on the mechanical behavior,shear bands spreading and deformation mode transitions.The results showed that pre-tensile stress has a more intuitive impact on the mechanical performance of the MG.With increasing of pre-tensile stress,the yield stress and hardness of MG decreased.While,for pre-compressive cases,with increasing of compressive strain,the pre-stress has a “harden” to “weaken” effects transition.Besides,the shear bands morphologies under indentations are also found to be influenced by the sign and magnitude of the pre-stress.If the pre-stress is large enough,the shear banding would propagate spontaneously and rapidly,resulting in brittle deforming mode happened during the indentation process.Furthermore,By using the energy analysis method,it was found that if the stored potential energy exceeded the essential forming energy of shear bands,the fracture would happen once local area is triggered by an external stress.(3)The cyclic indentation simulations and the rolling-indentation simulations were performed to investigate the influence of plastic deformation on the indentation mechanical behavior and rheological path of the MG materials.For the cyclic indentations,a large-depth indentation after cycling is conducted,and the indentation curves show that hardening behavior occurs with cyclic indentation amplitudes exceeding elastic range.The atomic Von Mises shear strain distributions during the large-depth indentation are monitored,and the pre-existing plastic deformation induced by cyclic indentation is found to be the main contributor to the hardening behavior.In addition,it was also found higher temperature would induce more severe hardening.While with lower loading rate,the hardening phenomenon would be suppressed.For the rolling-indentation simulations research,the rolling treatment could impact the deformation of the surface and sub-surface layer to a certain extent,thereby affecting the mechanical properties of the material surface.(4)To improve the toughness of MG materials,amorphous-crystalline nanolaminates(ACNLs)composed of Cu-Zr metallic glass layers and single crystal copper layers were constructed.The deformation behaviors of ACNLs were investigated by conducting shear loadings in parallel deforming and serial deforming directions,the results showed the yielding mechanism,the deforming mechanism and the roles of ACIs in the two modes were different.In parallel deforming mode,the yielding of the material was mainly triggered by generation of dislocations and slips in crystalline layer,and both crystalline and amorphous phases participated in the subsequent plastic deforming.In serial deforming mode,the yielding and plastic deforming of the material was induced by shear localization of amorphous layers only.Furthermore,the increasing of crystalline layer thickness would promote the material shear modulus and shear strength in both deformation modes.The promotions and incentives varied and had dependence on the coupled deforming mechanisms.In addition,through the indentation test of ACNL,it was found that deformation such as dislocation slip in the crystal layer could easily affect the formation of shear bands in the amorphous layer through ACI,which can achieve plastic subdivision of the amorphous layer material.In summary,the MD simulation method was used to study the indentation response of MG materials in different test environments,as well as internal variables such as pre-stresses and pre-deformation.The micro-nano-scale mechanical response and deformation mechanism under complex factors was investigated,and provided a research basis for the preparation and research,testing and engineering applications of MG materials.