压痕位置对多晶铜纳米压痕变形机理的影响

为研究在纳米压痕过程中微观结构多晶铜力学特性及变形机理的影响机制,采用Poisson-Voronoi和Monte Carlo方法建立大尺度多晶铜分子动力学模型,针对多晶铜不同微观结构分别建立初始压痕位置位于晶粒、晶界面、三叉晶界、顶点团4类微观结构的纳米压痕模型,采用分子动力学方法模拟计算金刚石探针压入模型的纳米压痕过程,计算4种模型的纳米压痕力及原子的静应力与第三应力.采用中心对称参数法分析多晶铜表面及亚表面位错演化对纳米压痕过程的影响.结果表明:探针对4种微观结构的纳米压痕过程存在显著的规律性,纳米压痕力的增长速率、纳米压痕过程位错向邻近晶粒扩散的难度、纳米压痕后多晶铜亚表面位错分布范围、纳米压痕过程中低维数微观结构累积原子势能的难度等均满足降序关系:晶粒、晶界面、三叉晶界、顶点团;压痕位于高维数微观结构时,其相邻的微观结构呈拉应力;而压痕位于低维数微观结构时,其相邻的微观结构呈压应力.在多晶铜的纳米压痕过程中,为减少缺陷结构数量及其能量累积,降低材料的残余应力,应针对多晶铜的晶粒进行机械加工并避开顶点团、三叉晶界等微观结构.

Influence of indentation position on the nanoindentation deformation mechanism of polycrystalline copper

To study the effect of microstructural components on the mechanical properties and deformation mechanism of polycrystalline copper during the nanoindentation process, a large-scale molecular dynamics simulation model of polycrystalline copper is structured by Poisson-Voronoi method and Monte Carlo method. Based on the microstructural components of the nanocrystalline copper, the polycrystalline copper nanoindentation simulation models with initial nanoindentation position at different microstructural components that contain grain cell, grain boundary, triple junction and vertex points are established, respectively. The nanoindentation process with the four different initial nanoindentation positions are simulated by molecular dynamics method, and the nanoindentation force and internal stress of the microstructural components are calculated. Centrally symmetric parameter method is used to analyze the dislocation nucleation and propagation process in the surface and subsurface of the polycrystalline copper with different initial nanoindentation positions. The results show that there is obvious regularity of the microstructural components during the nanoindentation process: the nanoindentation force rate, the difficulty of dislocation propagate to adjacent grains, the size expansion of dislocation distribution range on the polycrystalline surfaces, as well as the ability of the microstructural component low dimension accumulating atomic potential energy satisfy the descending relationship: grain cell, grain boundary, triple junction and vertex points. In addition, when the indentation position is at the high-dimensional microstructural component, the adjacent microstructural component exhibits tensile stress, while the indentation position is at the low-dimensional microstructural component, the adjacent microstructural component exhibits compressive stress. Therefore, during the nanoindentation process of polycrystalline copper, it is suggested to machine the microstructural components like grain cells of the polycrystalline material and to avoid the microstructural components like vertex points and triple junctions to reduce the number and energy accumulation of dislocations and the residual stress in the workpiece.