随着材料尺度的减小，传统塑性力学对位错密度的连续化假设不再适用，无法解释纳米材料超高强度（达到几个GPa）的实验现象。纳米尺度材料中的缺陷非常不稳定，很容易被自由表面吸收形成缺陷很少接近完美的晶体。完美晶体的塑性屈服取决于位错的形核，形核需要的应力要远大于位错开动所需的应力，这是导致纳米材料具有高强度的主要原因之一。 本论文发展了基于原子势能的金属Cu的非线性本构关系，并将材料稳定性理论应用于FCC晶体铜的位错形核判断，结合分子动力学的结果证实了该理论的适用性。研究了FCC铜晶体的理论强度，该值体现了很强的各向异性。同时，研究了FCC铜单晶体拉伸压缩强度的不对称性，并将基于位错形核屈服强度和传统塑性理论做比较，结果表明材料呈现与传统塑性屈服面不同的奇异屈服面，并且以最大临界切应力作为位错形核或者位错开动的准则已经不再适用。应用能量极小原理，对非均匀原子系统的位错形核进行预测，计算结果表明由于表面原子的键位缺失，薄膜结构易在表面形核。同时，结合位错形核理论对纳米压痕的尺度效应做出了解释：圆形压头预测的材料模量随着压头半径的减小而增大；方形压头下方的位错形核所需要的临界平均压力随着压头的减小而增加。 用分子动力学方法研究了在不同扭转角度下的Cu（100）失配晶界强度。结果表明：在小角度扭转晶界上形成失配位错网，失配位错密度随着晶粒之间的失配扭转角度的增加而增加。在拉伸变形过程中，随着位错网格密度的增加，位错之间的塞积作用增强，界面的屈服强度得到提高。大角度扭转晶界将形成面缺陷，面缺陷位错形核强度趋于定值。在剪切变形过程中，高角度晶界以相互滑移为主而非以位错形核扩展为主，因此它的剪切强度随着晶界能的增加而减小。对纳米摩擦过程压头下方微观结构演化的分子动力学模拟表明：位错环的开动与相互反应影响纳米系统的摩擦学性能，穿晶层错和棱形位错环是金属材料中传递塑性变形两种不同的载体。纳米薄膜晶界能够阻滞两者向基体扩展，将塑性变形集中于薄膜中或者晶界处。
As material scales decrease, the continuum assumption on dislocation density from the classical plastic mechanics is no longer applicable, which can not be used to explain the experiment phenomenal of ultrahigh strength, reach up to a few GPa, for nano-materials. Defects are unstable in nano-material, which are absorbed by the free surface during deformation process. The material will be formed less defects or close to a perfect crystal. Yield of perfect crystal was decided by dislocation nucleation, and the strength required by dislocation nucleation is much higher than that by dislocation initiation. This is the main reason for the nano-material with ultrahigh strength.The nonlinear constitutive of copper crystal is developed in this thesis based on atomistic potentials. The crystal stability theory is used as dislocation nucleation criterion for FCC copper single crystal and its applicability is validated by comparing with the results obtained from classical molecular dynamics simulations(MD). The ideal strength of FCC copper is also studied and the value of it appears significant anisotropy on the crystallographic orientations. Meanwhile, the asymmetry strengths of tension-compression are investigated for FCC copper. Compared with the classical plastic theory, the singular yield surface is appeared in the material, which is different from that of the classic yield surface. The maximum critical shear stress is no longer applicable for dislocation nucleation or dislocation initiation criterion.Based on the minimum energy principle, the dislocation nucleation criterion is predicted for inhomogeneous atomic system. The simulation results are illustrated that the thin film is easily nucleated on the surface because the atomic bonds lose. The size effect of nano-indentation is explained by dislocation nucleation theory: the values of Young's module predicted by round tip indenter increases with the decrease of tip radius; the value of critical average pressure required for dislocation nucleation, predicted by flat rectangular indenter, increases with the decrease of tip size.Used MD, Cu (100) strength of misfit grain-boundary (GB) is investigated at different twist-angles. The results obtained show that there is misfit dislocation network formed at a small twist-angle GB, and the density of misfit dislocations increase as the increase of twist-angle between crystals. During tensile deformation, as the dislocation density increases, the interaction between dislocations increases, and the yield strength increases at the GB. The surface defect is formed at a large twist-angle GB, and the dislocation nucleation strength is kept almost the same value. During shear deformation, GB slip dominates the deformation rather than the dislocation nucleation at large-angle GB, so the shear strength decreases as the GB energy increases. In the nano-scratch process, micro-structure evolution under the indenter is simulated by using MD method. The results indicate that nucleation and reaction of dislocation loops can affect the tribology performance of nano-system. There are two carriers by stacking faults and prismatic dislocation loops resulted in plastic deformation in metal material. The GB of nano-film can prevented them from propagation into the matrix, which concentrates the plastic deformation in the film or on the GB.
