Study on the strengthening mechanisms of Cu/CNT nano-composites

Recent experimental studies by Chen et al. [1], [3] showed that copper (Cu) matrix reinforced by a small amount of carbon nanotubes (CNT, about 4% volume fraction) will increase material strength by about 300% while sacrificing some material ductility. The strengthening mechanisms of Cu/CNT nano-composites were firstly studied numerically using 2D axial symmetric unit cell finite element (FE) models in Ls-Dyna, which consist of both copper matrix and CNTs. The simulation results were verified by existing experimental data. A round of parametric studies was performed to investigate the effects of several modeling parameters in the FE simulations. These parameters include the volume fraction of CNTs, aspect ratio of CNTs, size of hardening zone, and the “equivalent” hardened plastic strain in the hardened zone. Two main strengthening mechanisms are found that affect CNTs reinforcement prediction. The first one is the load-bearing effect resulting from boundary condition imposed in the models. The CNTs significantly affect the plastic flow of copper around CNTs during plastic deformation, which is one important reinforcement mechanism because of high aspect ratio (H/D) of CNTs. The second strengthening mechanism is found to be the hardened zone of Cu matrix around CNTs, which is introduced by manufacturing processes and/or the Orowan effect. The Orowan effect plays a key role in reinforcement especially in the nano-scale, which results in a very small inter-particle spacing. The Orowan effect was also studied using analytical methods. Both analytical solution and unit cell FE modeling well correlated with the experimental results for various Cu/CNT composites with different CNT outside diameters.