Quasi-continuum Method At The Microscopic Crystal Deformation Multi-scale Simulation Applications

As the development of Micro-electronics and Microsystems, researchers pay much attention on the micro-mechanical properties of materials. Because micro-components always show different characteristics from components in macro-scale (such as higher intensity, hardness, abrasion resistance, ductibility and low temperature super plasticity, etc), the microcosmic mechanism of deformation is important to study the properties of material in micro-scale. However, traditional methods based on continuum theories do not work in investigating the mechanism of micro-deformation, and microcosmic methods such as Molecular Dynamics and Monte Carlo method can not be used to simulate the deformation of a large region in common personal computers for its large computation memory caused by large degrees of freedom. In this situation, the multi-scale methods coupling atomistic scale and continuum scale are widely applied in computer simulations.The Quasi-continuum Method is developed as a typical multi-scale method. The key idea is that of selective representation of atomic degrees of freedom. Instead of treating all atoms making up the system, a small relevant subset of atoms is selected to represent, by appropriate weighting, the energetics of the system as a whole. Based on their kinematic environment, the energies of individual "representative atoms" are computed either in nonlocal fashion in correspondence with straightforward atomistic methodology or within a local approximation as befitting a continuum model. The representation is of varying density with more atoms sampled in highly deformed regions (such as near defect cores) and correspondingly fewer in the less deformed regions further away, and is adaptively updated as the deformation evolves.In this thesis, the effect of size, crystal direction, and grain boundary in micro-deformation are investigated using Quasi-continuum Method. The mechanisms of the dislocation nucleation and the deformation twinning are also have been analyzed. Based on the Quasi-continuum Method, the works of this thesis are as follows:1) The processes of nano-indention of four FCC metals (Al, Ag, Ni, and Pd) under four different indenters with various widths are simulated. Load-displacement curves, strain energy vs displacement curves, as well as nano hardness are obtained. The values of the critical load are in good agreement with theoretical values. The relation between rigidity and indenter width is discussed. The widths of extended dislocation are in agreement with theoretical values. To study the deformation mechanism, the dislocation pictures are plotted and the relation between load-displacement curves and dislocations are discussed.2) The nano-indention of Al film in three crystal orientations(x[111], y[-110], z[11-2]; x[-1-12], y[111], z[-110]; x[1-10], y[001], z[-1-10]) are simulated. Load-displacement curves and atomistic arrangements pictures are obtained. The results are in good agreement with experiments. Dislocations and deformation twining are observed in the simulation. The differences between different crystal orientations are analyzed by dislocation theory.3) The nano-indentation on an Al film with a grain boundary is simulated. The load-displacement curve and micro-structures under the indenter are compared with the situation with no grain boundary. The interaction between dislocations and grain boundary is analyzed.4) The deformation of crack tips in FCC Cu under modeâ… andâ…¡load in three crystal orientations are simulated. Dislocations and deformation twining are observed in the simulation. The deformation mechanisms in three crystal orientations arediscussed. From the results of the simulation, we can find that the {111}<112>deformation twinning and dislocation nucleation present itself when the slippage of the dislocation dried up.