A polycrystalline mechanical model for bulk nanocrystalline materials using the energy approach

To systematically understand the grain size, strain rate and defect development dependent mechanical behavior of bulk nanocrystalline materials, a new constitutive model is proposed to describe the deformation mechanism, microstructure evolution and mechanical response of bulk nanocrystalline materials using the energy approach. In this model, the grain interior and grain boundary were not taken as two independent phases with different volume fractions, but as an integral object sustained dislocation and accommodated grain boundary sliding mechanisms. Meanwhile, defect creation and evolution and their effects on the overall stress-strain relation as well as the failure process of bulk nanocrystalline materials were considered in the model. For experimental verification, we have prepared nanocrystalline Ni powder by the DC arc plasma evaporation method. Bulk nanocrystalline Ni samples were then made by compaction and hot sintering. Experimental measurements on the mechanical response of bulk nanocrystalline Ni were performed under different strain rates and grain sizes. Comparison between experimental data and model predictions show that the method developed appears to be capable of describing the mechanical response of bulk nanocrystalline materials. The model applications to nanocrystalline Mg and Cu have shown that it can reflect the asymmetric defect development between tension and compression under quasi-static conditions; this results in its good capacity to describe the dynamic strain rate sensitivity and strain hardening behavior over a relatively large strain range under both compression and tension conditions.