Research On Fracture Toughness 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 fracture toughness, which limit their applications. In order to overcome these defects of NC materials, many approaches have been proposed to enhance the ductility of NC materials. One of the several strategies that have been suggested for enhancing fracture toughness in NC materials is to develop a bimodal grain size distribution, in which NC grains can provide high strength, whereas coarse grains can enable strain hardening to enhance fracture toughness, making fracture toughness of NC materials improved relatively under minor loss of strength,and we call it bimodal nanocrystalline(BNC) materials. In order to explain the mechanism of BNC materials having a higher fracture toughness than the corresponding NC materials. On the basis of fully understanding the deformation mechanisms of BNC materials, a dislocation cohesive model has been developed to study fracture toughness K BB IC BB of BNC materials by considering a typical case where crack lies at the interface of two neighboring NC grains and the crack tip intersect at the grain boundary of the coarse grain, the cohesive zone size is assumed to be equal to the grain size d of NC materials.Blunting and propagating processes of the crack is controlled by a combined effect of cohesive and dislocations. Edge dislocations emit from the cohesive crack tip and make a shielding effect on the crack to increase fracture toughness of materials. This work not only possesses very important scientific value, but also provides a solid theoretical foundation for designing ideal NC materials both with high strength and with good fracture toughness which impulse the development of mechanical theory and engineering application of NC materials. The main research content and results are summarized as follows:(1) In the dislocation cohesive model of BNC materials, the maximum number of dislocations, emitted from a crack tip, increases with increasing the coarse grain size D.N is relatively small and the increase rate tends to be reduced. It is because that the driving force being exerted on the dislocations will be getting smaller as the dislocations move far away from the crack tip, but the dislocations are arrested at grain boundaries which make the sliding of later emitted dislocations become more and more difficult.(2) In the model, we take copper as an example, and have analyzed the relationship between fracture toughness of BNC materials and two kinds of grain size. Analysis:when the grain size d of NC materials is unchanged, fracture toughness of BNC materials will increase as coarse grain size D increases, and this change is obvious; when the coarse grain size D is unchanged, fracture toughness of BNC materials will increase asgrain size d of NC materials increases, but the increase is not obvious. According to the relationship between fracture toughness and two kinds of grain size, we can know that BNC materials with higher fracture toughness than the corresponding NC materials.