Molecualr Dynamics Studies On The Mechanical Behavior And Properties Of Monocrystalline Metals And The Length Scale Effects

A Molecular dynamics simulation model suitable for mechanical behavior and properties of nanoscale metals was developed, and the mechanical properties, deformation mechanism and failure process of nanowire, nanofilm and nanobulk with face centered or body centered crystals were studied by this model. The relationship between mechanical properties of nano metals and the length size, surface ratio and loading rate was analyzed, and some prediction formula verified by numerical results were proposed to describe the relationship. Moreover, crack propagation in nano plane was studied by both two dimensional MD simulation and continuum-atomistic coupled theoretical method. Study shows that the method and MD model proposed here was proved to be suitable for simulation of metals.Simulation results show that free surfaces take large effect on the mechanical behavior and properties of nano-materials. Initial stress and high energy are found in nanowire and nanofilm without external load, and deformation begins from free surfaces. There are dislocation and lattice steps in nanowire and only lattice slide in nanobulk during their failure process.The fracture strength of monocrystalline metal is close to the theoretical strength of the lattice and agrees well with Griffith's theory about the fracture of ideal lattices.The modulus of elasticity of nano BCC metal is sharply lower than that of the identical macro metal, and it will arise after the yield period in BCC nanowire and nanofilm. In comparison, the modulus of elasticity of nano FCC metal is a little lower than that of corresponding macro metal.The plasticity of nano BCC metals is stronger than that of FCC metals, but yielding strength and modulus of elasticity are lower than that of FCC metals. Under the same loading condition, BCC metals will enter the yield period earlier and last longer than FCC Metals.There is a short yield period during the tensile failure process of FCC nanowires, which is decided by the metal and independent on external conditions such as loading rate and model size.The yielding strength and modulus of elasticity of an FCC nanowire are linear to the logarithm of both its cross-section size and external strain rate, and the fracture strength is linear to the logarithm of both external strain rate and the inverse exponential of its cross-section size. What's more, the yielding strength and modulus of elasticity of an FCC nanowire can be divided into three parts: the corresponding property of internal lattices, surface lattices and corner ones, and the weighting linear plus of the three parts may obtain that of the nanowire.The atomic energy and stress in nanowire are lower than in nanofilm, and the ductility higher. The modulus of elasticity of nanowire is about a half of that of nanofilm with the same size, and the tensile strength is about two thirds of that of nanofilm.With Embedded Atom Method, the plane MD model can study the crack propagation in nano metal slab, and the simulation results agree remarkably well with macro fracture mechanics method in which the elastic constants were obtained from the interatomic potential functions and the two-dimensional triangular lattices.