Molecular Dynamics Studies Of Plastic Deformation Of Nanotwinned Copper With Embedded Cracks

Nanotwinned (NT) materials have attracted much attention for their many superior mechanical properties as compared to traditional materials and conventional nanocrvstalline materials. It is believed that these properties are closely related to their microstructures and deformation mechanisms. Thus, it is of great importance to fully understand the mechanical behaviors of NT materials. In this work, the tensile and shear mechanical properties of NT copper with embedded cracks have been investigated by using the molecular dynamics simulations. Furthermore, the effects of crack length, temperature and loading rate on the mechanical properties have also been discussed.First, computer software has been developed for the analysis and visualization of the Burgers vector and density of dislocations. Then, the plastic deformation of bulk NT copper with embedded cracks under tension has been explored. It has been found that the stress concentration at the crack tip dominates the dislocation nucleation at the onset of the plastic deformation and occasionally as sinks at later stage. The joint action of twin boundaries (TBs) and dislocations controls the yield stress at the earlier stage of plastic deformation. The dislocation pile-up, accumulation and transformation at TBs control the plastic hardening and softening deformations. The dislocation/TB-dislocation interactions determine the plastic flow stage. The width of the TB dislocation pile-up zone was estimated to be5.6-8nm, which agrees well with previous experimental and simulation results. Furthermore, it has been found that the flow stress vs. dislocation density at the hardening stage follows the Taylor-type relationship.In addition, the influences of crack length, temperature and loading rate on the plastic deformation under tensile and shear loadings have been investigated. Simulation results show that under the tensile and shear loadings, with the increase of the crack length or the tempeiture, the strength of the sample decreases, while the influence of the loading rate shows an opposite tendency. Moreover, the relationship between the yield stress and the crack length is nonlinear, which is similar to that between the yield stress and the loading rate, but is different from the influence of temperature.