Study On Structure And Mechanical Properties Of Metallic Glass Nanowires By Molecular Dynamics Simulation

Metallic glass nanowires have promising potential in catalysis, data storage media, microelectronicdevices and optical devicesowingto their unique structure and nano-size effect. Due to the limitation of experimental conditions, the knowledge of the microstructure and mechanical properties of the metallic glass nanowires is still limited.Therefore, doing the research on this topic has great significance. In this thesis, based on the Cu64Zr36 amorphous nanowires, the microstructure and mechanical properties of metallic glass nanowires fabricated by different methods are studied systematically through extensive molecular dynamics(MD) simulations, and also the size effect of metallic glass nanowires is discussed deeply.Firstly, FIB nanowire, hot imprinting(HI) nanowireand cast nanowire are prepared by MD simulations via the processes mimicking focused ion beam(FIB) milling, hot imprinting and cast in experiment. It is found thatthe microstructure of the nanowires are very different. The internal structure of FIB nanowire is quite similar to the structure of the bulk metallic glass. While that of the cast nanowire deviates far from the structure of the bulk metallic glass. Chemical segregation in the surface is observed in all the nanowires. The energy gradient in the surface is a major cause for the chemical segregation.Then,both tensile and compressive tests are conducted on the nanowires by MD simulations. It is found that metallic glass nanowires made by different processing methods have different mechanical properties.The HI nanowirewhich contains the most icosahedral clusters exhibit the highest strength but the poorest ductility, whereas the cast nanowire which contains the fewest iscosahedral clusters shows excellent ductility but reduced strength.The strain localization occurs in FIB nanowire and HI nanowire, whilethe cast nanowire deforms homogeneously. The different responses are closely related to the atomic structure and packing density.At last, size effect of metallic glass nanowires is investigated based on FIB nanowires and cast nanowires. The apparent mechanical properties of metallic glass nanowires are determined by the combination of surface and internal stresses. For tension, the surface of the metallic glass nanowires acts as a defect, weakening the nanowires, while pre-existing residual internal compressive stressesreinforce the nanowires. Thecombined effects of the two factors cause high resistance to degradation of tensile strength when the size reduces.For compression,pre-existing compressive internal residual stresses weaken the wires and the surface has no effect on the stability of the internal part of the wires. The difference of the surface state leads to different size effect between FIB nanowires and cast nanowires.The tensile strength of the FIB nanowire slowly decreases when the size reduces while that of the cast nanowire slowly increases. The change of the compressive strength of the FIB nanowire has the same trend as the residual internal stress when the size decreases while that of the cast nanowire does not follow the trend.