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A new theoretical model is proposed to describe the mechanical properties of bimodal nanocrystalline (BNC) materials. This composite model is comprised of coarse grains evenly distributed in the nanocrystalline (NC) matrix. In this study, we have studied the effect of grain size distribution on the constitutive behavior of BNC materials. During the plastic deformation, effects of nanocracks and dislocation emission from crack tips on the constitutive behavior of BNC materials are also analyzed. Numerical calculations have been carried out according to the model, and it is found that the nanocracks make a positive effect on the strain hardening, and the results show that this model can describe the enhanced strength and strain hardening of BNC materials successfully. The prediction of the bimodal Cu–Ag material is in good agreement with the experimental results.

I. INTRODUCTION

Contributing Editor: Susan B. Sinnott a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.134

It was found that the plastic deformation of the UFG Al–Mg alloys with a bimodal microstructure was highly localized. The fracture of the alloys was attributed to shear localization under the compressive tests and to a combination of shear localization, cavitation, and necking under the tensile tests. However, the grain size of their UFG matrix did not reach the nanoscale. Han et al.16 investigated the strain rate sensitivity and the strain rate effect on the ductility of bimodal 5083 Al alloys. The mechanical responses of several bimodal 5083 Al alloys with different fractions of coarse grains were studied at different strain rates. It was found that the failure strain increases with the decreasing of strain rate for plastic deformation when the strain rate is less than 10
1 s
1. Magee et al.17 studied the tensile test parameter effects on the mechanical properties of a bimodal Al–Mg alloy. It was found that decreasing the specimen thickness and strain rate served to increase both the strength and ductility of the material, and increasing the coarse grain (CG) ratio would lead to increase ductility and slightly reduce strength. Long et al.18 investigated the ductility in the bimodal UFG Ti–6Al–4V alloy fabricated by high energy ball milling and spark plasma sintering. It was found that the high strength primarily results from the contribution of ultrafine grains, while the enhanced ductility may be attributed to the improved strain hardening capability by the presence of coarse grains and occurrence of crack blunting and deflecting. Although there have been numerous experimental studies that have provided insight into the constitutive behavior of BNC materials, it scarcely builds explicit and universally theoretical models to take quantitative analysis. For instance, Ovid’ko and Sheinerman19 built a theoretical

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Ó Materials Research Society 2015

Nanocrystalline (NC) materials have attracted great interest in recent years owing to their excellent mechanical and special physical properties.1,2 Ge