13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of cooperative grain boundary sliding and migration on emission of dislocations from a crack tip in nanocrystalline materials H. Feng 1, Q.H. Fang 1, 2,*, Y.W. Liu 1 1 State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body (Hunan University), Changsha 410082, China 2 School of Mechanical and Manufacturing Engineering, The University of New South Wales, NSW 2052, Australia * Corresponding author: fangqh1327@tom.com ; Qi-hong.fang@unsw.edu.au Abstract Interaction of the cooperative grain boundary sliding and migration with a crack in deformed nanocrystalline materials is investigated using the complex variable method. Effects of the two disclination dipoles produced by the cooperative deformation on the emission of lattice dislocations from the crack tip are theoretically described. The complex form expressions of the stress field and the force field are divided. The critical stress intensity factors for the first dislocation emission are calculated. Influences of disclination strength, grain size, locations of the two disclination dipoles as well as crack length on the critical stress intensity factors are discussed in detail. Results show that, the cooperative deformation has great influence on dislocation emission from the crack tip. In general, the cooperative deformation can promote the lattice dislocation emission from the crack tip, thus improve the toughness of the nanocrystalline materials. Keywords nanocrystalline materials; crack; grain boundary sliding; grain boundary migration; dislocation emission; 1. Introduction Nanocrystalline metals and ceramics show outstanding mechanical and physical properties, which represent the subject of rapidly growing research efforts motivated by a wide range of their applications [1-8]. However, in most cases, nanocrystalline materials have superior strength, hardness and good wear resistance but at the expenses of both low tensile ductility and low fracture toughness, which considerably limit their practical utility [3-5, 9-11]. At the same time, there are several examples of nanocrystalline materials showing considerable tensile ductility at room temperatures [4, 12-14], or superplasticity at elevated temperatures [14, 15], and significant fracture toughness that can be often higher than that of their polycrystalline or singlecrystalline counterparts [4, 16-19]. The nature of the outstanding combination of good ductility and superior strength is not quite clear, which creates high interest in understanding the toughening mechanisms that specific for nanocrystalline materials. Recently, many models have been developed to explain this phenomenon. In most of them, intergrain sliding, grain boundary migration, triple junction diffusional creep, Coble creep, rotational deformation and nanoscale deformation twinning have been theoretically described as specific deformation modes in nanocrystalline materials. And the specific toughening mechanisms are attributed to specific deformation modes in nanocrystalline materials [20-25]. Recently, rapidly growing attention has been focused on a new physical mechanism or mode of plastic deformation in nanocrystalline metals and ceramics. The new specific deformation mode represents the cooperative grain boundary sliding and stress-driven grain boundary migration process near the tips of growing cracks [26]. Grain boundary sliding is an important deformation mechanism in nanocrystalline materials [3, 4, 10]. It is also one of the specific deformation modes showing superplasticity in nanocrystalline materials [4, 15]. Non-accommodated grain boundary sliding in nanocrystalline solids creates defects (dislocations and disclination dipoles) at triple junctions of grain boundaries. These defects can initiating the formation of cracks, in which case a nanocrystalline solid tends to exhibit brittle behaviour [27, 28]. In contrast, if grain boundary
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