Investigation On Forming And Bonding CuZr Metallic Glasses Via Molecular Dynamic Simulation

With excellent mechanical,electromagnetical,chemical properties,metallic glass has great prospect in application.However,those properties also contribute to its poor processing performance.As it is hard and wearable,the traditional cold working process can hardly be applied to metallic glass.The hot working process is also limited facing the risk of crystallization.To solve the problems and make a better application of these properties,one should make a further understanding of its processability and combine forming and bonding,so as to machine metallic glass parts with complex structure,high precision and good stability.In this work,forming and bonding metallic glass are investigated via molecular dynamic simulation.The evolution of inner structure,atomic behavior and energy transfer are observed in detail to reveal the mechanisms in the machining process based on the thermodynamic and dynamics principles.The results provide a theoretical support for machining metallic glass parts.The main contributions are as follows.The Cu46Zr54 metallic glass was prepared by rapid-quenching from its melt using molecular dynamic simulation,laying a foundation for further research on the processability.The atomic packing in the solid state metallic glasses was investigated,including short-range ordering structure and middle range ordering structure,which showed that the obtained sample was in steady amorphous state.We also revealed the structural evolutions during the rapid-quenching process and analyzed the effect of the cooling rate on the glass transition temperature and the properties of material.A numerical model for thermoplastic forming metallic glass was constructed via molecular dynamic simulation.The flowing deformation and filling process were depicted in the atomic level.The influences of adhesive attraction between metallic glass and silicon stamp,atomic rheological mechanism,capillary force and inner structure on the thermoplastic forming were discussed.Based on the analysis,a theoretical model of thermoplastic forming,which combined plane Poiseuille flow and capillary force,was established for optimization of technological parameter.We also showed the structural evolution of the short-range and middle-range orders in the metallic glass during the mold filling process.The results suggest that the thermoplastic deformation will change the atomic arrangement and contribute to the better plastic performance of the processed metallic glass,which is a viable method to improve the plasticity of metallic glass.Diffusion bonding metallic glass with crystalline Al was investigated via molecular dynamic simulation.The atomic behavior in the diffusion bonding was observed and the concentrations of all elements and the interface migration velocities were extracted to analyze the growth of diffusion zone.We found a new phase appeared in the middle of the diffusion zone and the interdiffusion was highly asymmetric.Due to the heterogeneous and asymmetric interdiffusion in the diffusion couple,the traditional calculation method was no longer applicable.We modified the widely used MSD method and Boltzmann-Matano analysis and obtained the intrinsic diffusion coefficients and the interdiffusion coefficients.The elastic modulus and the glass transition temperature of the alloy in the diffusion zone were measured,which explained the underlying cause of the anomalous new phase.The inner atomic structure though the diffusion zone was analyzed further,including amorphous icosahedra cluster and crystalline fcc structure.The results suggest that the interdiffusion drove the structural transformation of material from crystal to amorphous state and then to liquid state in the middle of the diffusion zone,contributing to the unique interdiffusion phenomenon in the diffusion couple.The amorphization of crystalline Al was investigated in the diffusion bonding.The migration of the amorphous-crystalline interface and the crystallinity were measured,showing that the amorphization and the interface migration were closely linked with one another.The amorphization was highly anisotropic.We found the essential of the anisotropy was the anisotropy of shear modulus,which was proved to be a key parameter affecting the migration velocity and bonding quality.To further understand the atomic migration and exchanging behavior,we proposed an atomic exchange principle on the base of thermodynamic and dynamics analysis,showing the individual roles and the cooperative work of Zr atoms and Cu atoms.According to the principle and combining the lattice plane evolution,we gave a complete picture for the amorphization of crystalline Al in the atomic level,divided the amorphization process into three subprocedures,including the atomic permeating,jumping and the collapse of lattice plane.All these results show a microcosmic perspective research on forming and bonding metallic glass with a deep analysis based on the thermodynamic and dynamics principles,and provide usefull guide for improving its processing property.