Research On Mechanical Properties Of Bimodal Nanocrystalline Materials

Nanocrystalline(NC) materials have a higher yield strength and a better wear resistance in comparison with conventional coarse-grained polycrystalline materials, however, these materials usually exhibit low ductility, which limit their applications in engineering. In order to overcome these defects of NC materials, many approaches have been proposed to enhance the ductility of NC materials. One efficient approach is processing NC materials by combining grains with bimodal distribution, in which NC grains can provide high strength, whereas coarse grains can enable strain hardening to enhance ductility, making fracture toughness of NC materials improved relatively under minor loss of strength, and we call it bimodal nanocrystalline(BNC) materials. Based on the dislocation mechanism and back stress theory, we proposed a theoretical model to study the constitutive behavior and mechanical properties of BNC materials. In this model, it is assumed that the intra-grain dislocation mediated interaction governs the primate deformation mechanism regardless of the contribution of other mechanical to the plastic deformation, such as grain boundary sliding, grain rotations, grain boundary diffusional creep. On the other hand, the contribution of back stress introduced by dislocation emission from nano-crack tip were used to describe the flow stress. The main research content and results are summarized as follows:(1) Using the model, we have calculated the overall stress-strain curves of BNC Cu-Ag materials, and compared it with the experimental results. The agreement between theoretical results and experimental data has proved that the proposed model can be used to accurately calculate the constitutive behavior of BNC materials.(2) In this model, it is assumed that nano-cracks nucleate in NC regions and the crack tip intersect at the grain boundary of the coarse grain. The dislocation is emitted from the crack tip along slip plane, and stopped at the grain boundaries. Dislocations stopped at the grain boundary cause back stress to impede the progress of similar dislocation, thereby induce kinematic strain hardening.(3)The dependence of mechanical properties of BNC Cu materials on grain size distribution was calculated. Analysis: 1) The strength of BNC materials decreases with increasing the NC matrix grain size while the strain hardening capability and the ductility increase with increasing the NC matrix grain size. 2) Effect of coarse grain size on the overall strength is complex phenomenon of isotropic strain hardening and kinematic strain hardening. The strength decreases with increasing coarse grain size without considering kinematic strain hardening. However, the increased coarse grain size give rise to the back stress. On the other hand, the strain hardening capability and the ductility are improved by increasing coarse grain size. 3) the strength decreases, while the strain hardening capability and the ductility increase, with the increased coarse grain volume fraction.(4) The dependence of mechanical properties of BNC Cu materials on external factor(strain rate and deformation temperature) was calculated. Analysis: the strength of BNC materials increases with increasing the strain rate while the ductility decrease with increasing the strain rate; the strength decreases, while the ductility increase, with the increased coarse grain volume fraction.