晶粒度对多晶铜纳米压痕表面变形机理影响

Influence of grain size on the nanoindentation deformation mechanism of polycrystalline copper

为研究晶粒度在多晶材料纳米压痕过程中对其塑性变形机制及位错演生过程影响.采用Poisson-Voronoi和MonteCarlo方法建立大规模多晶铜分子动力学模型,针对多晶铜Hall-Petch效应曲线建立具有不同晶粒度的多晶铜模型,并与单晶铜纳米压痕模型对比,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种模型的缺陷结构的配位数、内应力、原子势能等参数.采用中心对称参数法研究压痕过程中位错等缺陷结构的演化机制.结果表明:具有不同晶粒度的多晶铜纳米压痕过程存在显著的规律性,单晶铜压痕力高于多晶铜,多晶铜压痕力随着晶粒度降低而下降;多晶铜的晶界结构能够限制压痕缺陷、内应力与原子势能向材料内部传递,而单晶铜难以限制此传递过程;压痕过程中,具有较小晶粒度的多晶铜具有更高的静水压力、范式等效应力与原子势能,单晶铜内应力与原子势能低于多晶铜.表层及亚表层为较低晶粒度而材料内部为较大晶粒度的梯度晶粒度材料具有极大的研究价值.

To study the effect of grain size 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 Hall-Petch relationship of the nanocrystalline copper, the single-crystalline and polycrystalline copper nanoindentation simulation models with different grain size are established. The nanoindentation process with different grain size are simulated by molecular dynamics method, and the nanoindentation force, internal stress and atomic potential energy of the atoms 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. The results show that the indentation force of single-crystalline copper is higher than that of polycrystalline copper, with the decrease of grain size, that of polycrystalline copper continuously decreases due to softening phenomenon. The high internal stress and atomic potential energy under the indenter leads to the defect evolution region under the indenter. The range of horizontal distribution of defects is larger than that of the vertical distribution, and such defects are limited in the grains around the indenter due to grain boundary network. The internal stress and atomic potential energy in polycrystalline copper with smaller grain size is larger than that with higher grain size, and the stress and potential energy in single-crystalline copper are lowest. Hence, during the nanoindentation process of polycrystalline copper, to improve the mechanical properties and deformation mechanism of nanocrystalline materials, it is suggests to adopt the nanocrystalline materials with grain size gradient.