Investigation On Microstructural Evolution And Fracture Toughness In Nanocrystalline Materials

Nanocrystalline materials have caused widespread concern due to their outstanding mechanical and physical properties. In most cases, nanocrystalline materials have superior strength, strong hardness, and good wear resistance but at expense of low tensile ductility and fracture toughness. Nevertheless, certain nanocrystalline materials with good tensile ductility at room temperature or superplasticity at elevated temperatures, and significant fracture toughness have still been studied and reported. And the specific toughening mechanisms are attributed to specific deformation modes in nanocrystalline materials, including intergrain sliding, rotational deformation, cooperative grain boundary sliding and migration, as well as nanoscale deformation twinning. Meanwhile, the microscopic mechanism of this phenomenon, as well as the quantitative relationship between the microstructural evolution and the mechanical properties has not been revealed. Various defects will be inevitably produced in the course of manufacturing and employing of nanocrystalline materials, such as dislocations, inhomogeneity, micro-cracks and so on. The interaction between these defects and specific deformation modes is the foundation to understand the fracture of nanocrystalline materials. And the emission of dislocations from crack tips is the key transformation between brittle and ductile. Thus, research on the influence of microstructural evolutions near crack tips on the emission of dislocations from crack tips in nanocrystalline materials, can not only help to understand the evolution process of microstructures caused by movement of grain boundary dislocaitons, as well as its relationship with the fracture toughness of materials; but also can provide a theoretical basis for micro structure design and fracture prevention of nanocrystalline materials.In this paper, based on the experimental observation, and taking nanocrystalline materials as the object of study, mechanical models are established. Utilizing the elastic complex potential method in investigation of complex multiply connected domain and energy method, we systemically investigate the evolution process of microstructures (including coated cylindrical inhomogeneity, rotational deformation, cooperative grain boundary sliding and migration and nanoscale deformation twinning), the interactions between the microstructures and dislocaitons or cracks, as well as their influence on the fracture toughness of materials. The main achievements are summarized as follows:(1) A model is established to investigate the interaction between the screw dislocation and the nano-sized coated inhomogeneity. The analytical expressions of stress and displacement fields, as well as the image force acting on the screw dislocation are derived. The image force and equilibrium property of the dislocation are presented by numerical calculations and discussed in detail. The results show that, within the nanoscale range, the screw dislocation can be attracted by a hard matrix and hard coating layer, and be repelled by a soft matrix and soft coating layer. Additionally, the interface stresses can change not only the equilibrium position of the screw dislocation in the nano-sized coated homogeneity, the property of the dislocation stability in inhomogeneity, but also the value of the critical radius of inhomogeneity.(2) The special rotational deformation is described using the wedge disclination quadrupole mode. Theoretical models are established to study the interaction between the striate crack and the disclination quadrupole produced by special rotational deformation, as well as the interaction among the striate crack, special rotational deformation, intergrain sliding and dislocation emission from the crack tip. The critical stress intensity factor for the dislocation emission from the crack tip is derived. The combined effects of special rotational deformation, intergrain sliding and dislocation emission from crack tip on the fracture toughness of nanocrystalline materials are discussed. And the results show that, the special rotational deformation near the crack tip can release part of local stress, thus suppress the dislocation emission from the crack tip. The special rotational deformation is stronger; the edge dislocation emissions are more difficult. However, the combination of special rotational deformation, intergrain sliding and dislocation emissions near the crack tip can lead to an increase of effective fracture toughness by several times in nanocrystalline materials at fines grain size. It is also found that the fracture toughness of nanocrystalline materials is highly sensitive to grain size and there is an ideal grain size corresponding to the best toughening effects.(3) A mechanical model describing the interaction between the edge dislocation, the elliptical blunt crack and the fast nanoscale rotational deformation is established. The critical stress intensity factor for the dislocation emission from the elliptical blunt crack tip is derived. The influence of the deformation strength, grain size, and orientation of deformation, radius of curvature, dislocation emission angle, as well as crack length on the critical stress intensity factors for dislocation emission is evaluated. The results indicate that nanoscale rotational deformation releases the high stresses near the crack tip region and thereby enhances the critical stress intensity factor for dislocation emission. The nanoscale rotational deformation has great influence on the most probable angle for dislocation emission. And the critical stress intensity factor will increase with the increment of the grain size.(4) A mechanical model describing the interaction between the edge dislocation, the striate crack and the disclination structure produced by the cooperative grain boundary sliding and migration is established. The critical stress intensity factor for the dislocation emission from the crack tip is derived. The influence of the deformation strength (the strength of disclination), grain size, and the angle between grain boundaries, the distance of grain boundary sliding and migration, dislocation emission angle, as well as crack length on the critical stress intensity factors for dislocation emission is evaluated. Results show that, the cooperative grain boundary sliding and migration can promote the lattice dislocation emission from the crack tip, thus improve the toughness of the nanocrystalline materials.(5) A mechanical model is suggested to describe the nucleation and growth mechanisms of nanoscale deformation twins in hexagonal-close-packed metal magnesium. The<a> slip dislocaitons on the basal plane can dissociate into twinning dislocations and stair-rods under the applied stress. Then the twinning dislocaitons slip on every slip plane within a nanoscale region where a nanoscale deformation twin is thereby nucleated and grew. The energy and stress conditions of the nanoscale deformation twin nucleation and growth are calculated theoretically and discussed in detail. The results indicate that, the {1012} twin nucleation stress is about61.16MPa-92.05MPa. And the twin is harder to grow at its initial stage, and then it becomes much easier when the twin thickness reaches a certain value.