The Elasticity And Plasticity Of Abnormal One Dimensional Metal Nanostructures Studied By Molecular Dynamics Simulations

The present development and application of one-dimentional metal nano-materials are briefly reviewed in this thesis. Mechanical properties of metal nanowires can be manipulated both by adding internal micro structures and subtracting materials from the inside to form hollow nanotubes. In this thesis, two types of representative one-dimentional metal nano-materials, Ag nanowires with special five-fold twin boundaries and a surface groove and Au nanowires with hollow interior (or metal nanotubes), are studied to investigate the synergistic effects of sample size and microstructures on the mechanical behaviors of metal nanowires at different temperature. In particular, uniaxial compression, uniaxial tension and nano-indentation of the two types of metal nanowires are modeled and simulated using molecular dynamics method with embedded atom potential. By analyzing the respective deformation mechanism at atomic scale, it is found that the mechanical properties of metal nanowires are dramatically influenced by internal microstructures.The methodology of molecular dynamics method is elucidated in detail, including equations of motion, the algorithm, embedded atom potential, periodic boundary conditions, etc. Key techniqes used in the thesis, such as visualization, initial and periodic boundary conditions, and the determination of virial stress, are emphasized.The deformation behavior of five-fold twinned silver nanowires with surface groove are simulated under uniaxial tension, compression and nano-indentation. In particular, the synergistic effects from both internal and external structural defects, including twin boundaries, surface facets, and a special surface groove, are studied. It is found that the yielding modes vary among Ag nanowires when different microstructures are present. While a five-fold twin together with a surface groove generally lead to strengthening in Ag nanowires under tension, abnormal weakening effects are observed under compression at low temperatures when the nanowire diameter is increased. The peculiar size and temperature effects can be attributed to the toggling of dislocation nucleation between various preferential sites under different loading conditions. Furthermore, while fivefold twin boundaries are found to cause significant strain hardening under nano-indentation, they can either decrease or increase the initial yield stress of Ag nanowires under uniaxial deformation. The surface groove, in addition, will result in damping behavior of the Ag nanowire that leads to peculiar oscillating load-displacement responses under nanoindentation.The deformation behavior of gold nanotubes are simulated under uniaxial tension. It is found that gold nanotubes exhibit a combination of ultrahigh strength and extraordinary plastic flow during uniaxial tensile deformation through atomistic simulations. A remarkable transition from sharp to smooth yielding is observed in Au nanotubes with decreasing wall thickness. The orientation-dependent uniaxial tension deformation is characterized in this thesis. Particularly, the yield strength in [1 1 1] Au nanotubes is found to be up to 60% higher than the corresponding solid Au nanowire, which approaches the theoretical ideal strength in Au. The ultrahigh tensile strength in [1 1 1] Au nahotube might originate from the repulsive image force exerted by the interior surface against dislocation nucleation from the outer surface.It is revealed that the [110] Au nanotubes can maintain a ultrahigh plastic flow stress of more than 2 GPa up to～60% tensile strain while the corresponding solid nanowires normally experience sharp yielding at less than 5% tensile strain at which the stress drops immediately to less than 1 GPa. Meanwhile, it is found that the yield strength and ductility in [110] Au nanotubes can be up to～40% and ～72% higher than the corresponding solid nanowires respectively by tuning the wall thickness.The work in this thesis provides theoretical support for the design of materials at nano scale and inspires new ideas of manipulating the structure of metal nanowires, e.g., by subtracting materials from the nanowire interior to form hollow nanotubes with improved elastic and plastic properties.