Quasicontinuum Study The Influence Of Crystal Defects On The Deformation Behavior Of Materials

Advanced plastic forming technology plays an increasingly important role in many aspects, such as promoting economic development, promoting the national defense modernization and improving competitiveness. At present, the research on defect-free thin film material nanoindentation process has a certain depth. Initial plastic deformation of the material plays an important role on the follow-up of plastic deformation, However, the materials used in the engineering are not always perfect but more or less have defects, therefore, investigating the impact of nano-cavity and grain boundaries on initial plastic deformation is vital. In this study, in order to elucidate the impact of nano-cavity and grain boundaries on initial plastic deformation, the model of tip and substrate with nano-cavity and grain boundaries are modeled respectively.Multiscale simulations using the quasi-continuum (QC) method are performed in two situations (defect matrix and defect-free matrix), so as to understand the dislocation nucleation in the early stages of plastic deformation in defect matrix under the action of indenter. the results of defect matrix show that the presence of nanocavity in the matrix delays the emission of dislocation. The effects of shape, size and position of nano-cavity are investigated. The results showed that the shape of nano-cavity has no effect on the delay of dislocation emission. There is a critical distance for the impact for a particular size of nano-cavity for the delay effect of dislocation emission, when the distance between the nano-cavity and the indenter is larger than this critical distance, no delay effect exist. Critical distance increases with increasing nano-cavity size.In order to reveal the nano-cavity defects of the material elastic modulus and yield strength, we use quasi-continuum (QC) method to study the nanoindentation process with nano-cavity just below indenter. The effects of shape, size and position of nano-cavity are investigated. The results showed that when the distance between the nano-cavity and the indenter farther, nano-cavity has little effect on its elastic modulus and yield strength, When the distance between the nano-cavity and the indenter nearer, nano-cavity horizontal dimension significantly reduced elastic modulus and yield strength. By varying the ratio of the elliptical nano-cavity semi-major axis, the relationships between semi-major axis ratio and elastic modulus and yield strength are investigated.Finally, a model with grain boundary based on quasi-continuum (QC) method is applied to explores the influence of crystal orientation and the thickness of upper film on the first dislocation emission. The results show that the film thickness of the upper layer significantly affects the initial plastic deformation of the bi-crystal, with the increase of the thickness of upper layer film the yield strength of the material is increased, however, the mechanism for different orientations of the upper films are different.