Research On Microstructural Evolution And Fracture In Nanocrystalline Materials

Nanocrystalline materials with special physical properties(superior high strength,strong hardness,excellent wear resistance,etc.)represent the subject of rapidly growing research efforts motivated by a wide range of technological applications,especially in micro/nano-technology.At the same time,the novel properties are at the expense of low tensile ductility and low fracture toughness,which severely restricts their practical utilization.Nevertheless,some recent examples of nanocrystalline materials showing considerable tensile ductility at room temperature or superplasticity at elevated temperature and improvement of fracture toughness have been detected and reported.They evoke high interests of many scholars and practitioners in exploring the fundamentals of special toughening mechanisms in nanocrystalline materials.Subsequently,many theoretical models such as GB sliding,GB migration,rotational deformation,twinning and triple junction diffusion creep have been extensively developed to account for the toughening mechanisms of nanocrystalline materials.Various defects such as dislocations,micro-cracks,nano-inclusion will be inevitably produced in the course of manufacturing and employing of nanocrystalline materials.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.However,the microscopic mechanism of this phenomenon,as well as the quantitative relationship between the microstructural evolution and the mechanical properties has not been revealed.Thus,research on the influence of microstructural evolutions near crack tips on the emission of dislocations from crack tips in nanocrystalline materials can provide a theoretical basis for micro structure design and fracture prevention of nanocrystalline materials.In this paper,based on the experimental observation and atomistic simulations,mechanical models are established.Utilizing the complex variable method of Muskhelishvili,superposition principle of elasticity and so on,we systemically investigate the evolution process of microstructures.The main achievements are summarized as follows:(1)A screw dislocation interacting with a circular nano-inhomogeneity near a bimaterial interface is investigated.The stress boundary condition at the interface between inhomogeneity and matrix is modified by incorporating surface/interface stress.The analytical solutions to the problem in explicit series are obtained by an efficient complex variable method associated with the conformal mapping function.The image force exerted on the screw dislocation is also derived by using the generalized Peach-Koehler formula.The results indicate that the elastic interference of the screw dislocation and the nano-inhomogeneity is strongly affected by a combination of material elastic dissimilarity,the radius of the inclusion,the distance from the inclusion'center to the bimaterial interface,and surface/interface stress between the inclusion and the matrix.Additionally,it is found that when the inclusion and the Material 3 are both harder than the matrix(?1>?2 and ?3>?2),a new stable equilibrium position for the screw dislocation in the matrix appears near the bimaterial interface,when the inclusion and the Material 3 are both softer than the matrix(?1<?2 and ?2<?3),a new unstable equilibrium position exists close to the bimaterial interface.(2)A theoretical model is established to describe the effect of cooperative grain boundary(GB)sliding and migration on dislocation emission from the tip of branched crack in deformed nanocrystalline solids.The explicit solutions of complex potentials are obtained by means of complex variable method and conformal mapping technique.The critical stress intensity factors(SIFs)for the first lattice dislocation emission from the tip of branched crack are calculated.The effects of the lengths of branched crack and main crack,and the angle between their planes on the critical SIFs for dislocation emission are evaluated in detail.The results indicate that the emission of lattice dislocations from the tip of branched crack is strongly influenced by cooperative GB sliding and migration.When main crack approaches the branched crack,dislocation emission from the tip of branched crack will be suppressed.The main crack tends to propagate while shorter branched crack is prone to be blunted by emitting lattice dislocations from its tip.(3)The problem of the special rotational deformation interacting with an internal crack is studied by developing a theoretical model in deformed nanocrystalline materials.Using complex variable method of Muskhelishvili and conformal mapping technique,the expressions of complex potentials and stress fields are obtained analytically.The SIFs near the crack tips and critical SIFs for the first lattice dislocation emission from the branched crack tip are calculated.The effects of important parameters such as grain size,the length of internal crack,and the angle between the main crack and its branched crack on the critical SIFs for dislocation emission are evaluated in detail.As a result,the special rotational deformation has great influence on the growth of internal crack and the emission of lattice dislocations from the branched crack tip.The disclination quadrupole produced by the special rotational deformation will shield the branched crack tip under certain condition.Moreover,when the main crack approaches its branched crack,it will stop the emission of lattice dislocations from the branched crack tip.