Investigation On The Interaction Mechanism Of Generalized Dislocations With Micro-structures In The Nanocomposites

In recent decade, as developing rapidly nanotechnology and promising wide applications for future devices in micro-electro-mechanical systems and nano-intelligent devices, nano-structured composites have become the focus of intensive research in mechanics, material science and solid state physics. Research results have increasingly shown that the mechanical properties of nano-structured materials and the micro structure evolution are intensely dependent on the dimension and size of the nanostructures. This is due to the high ratio of the surface/interface area to the volume of the nano-structured materials, which leads to the prominent surface/interface effect. Thus, the studies of the influence of surface/interface effect on the interaction of various dislocations and disclinations with nanoinhomogeneities, nanocrack and nanovoid in nano-structured composites are motivated by the need to provide rational explanation concerning a number of the size-dependent physical and mechanical properties of nanosized materials, such as certain strengthening and hardening mechanism of the generalized dislocations in nano-structured composites. They also demonstrate how these are meaningful for the applications in nano-structured composites science and engineering.In this paper, taking nano-structured composites as the research object, we systemically study the interaction of edge dislocations and disclination with nanoinhomogeneities, nanocrack and nanovoid in nano-structured composites, their relativity with the strengthening and hardening mechanism as well as fracture failure of nano-structured composites. Based on the practical engineering application, all kinds of complex mechanical models are presented. The efficient analytical methods for a complex multiply connected region and size-dependent surface/interface plane problem are further developed. And then they are used to solve and analyze the macro, meso and micro physics quantities and phenomena of a series of complex nano-structured problems, such as the stress fields, displacement field, image force on the dislocation and disclination, their equilibrium position and stability, and shielding effects on blunt crack, the stress intensity factors at the crack tip, the critical conditions for the lattice dislocation emission. The main work is summarized as follows:1. The elastic interaction force of the edge dislocations and the critical conditions of their generation inside nano-structured composites (containing nanoscale coating, nanopores, nanowire, etc.) are obtained. By using the complex variable function method, the closed form solutions of the problem are derived. The influences of the material elastic dissimilarity, the thickness of the coating or nanoscale film, the surface/interface effects and the relative location of the dislocation on the generation of edge misfit dislocation as well as the equilibrium location and the stability of the dislocation are revealed in detail. The results show that, the thinner the film becomes, the more impact of the inhomogeneity and matrix has on the normalized glide force characterized by the surface/interface intrinsic lengths and the residual surface/interface tensions. The surface/interface stress evidently impacts the critical film thickness, which may be increased, decreased, even disappears. The thicker film or the smaller misfit strain leads to the greater influence of the surface/interface stress.2. The elastic behavior of a wedge disclination dipole with nano-structured composites, such as a nanoscale coated cylindrical inhomogeneity, core-shell nanowires, nanotube, is presented with or without surface/interface effects. By using the complex variable method, the analytical solutions of the force acting on the wedge disclination dipole center are derived. The influences of the feature of disclination dipole, the nanoscale coating or film thickness, the material elastic dissimilarity and surface/interface stress on the equilibrium location and the stability of the dipole are evaluated. The results indicate that the material properties of the coating or film strongly influence the equilibrium location and the stability of the disclination dipole in nano-structured composites. The effect of the nanowire on the force acting on the disclination dipole is shielded by coating layer, when coating layer is thick enough. There exist two critical coating thicknesses to alter the equilibrium of the disclination dipole. The negative interface stress decreases this range of the two critical coating thicknesses, even makes the equilibrium position disappear. The effect of surface/interface stresses on the force becomes more distinct with the increment of the dipole arm and the disclination strength.3. Based on the surface elastic theory and the basic knowledge of fracture mechanics, the elastic interaction between a wedge disclination dipole and the elliptical blunted crack is studied. By using the complex variable method, the complex form expression of the stress intensity factors at the crack tip and then critical stress intensity factors for the first lattice dislocation emission from crack surface are derived under mode I and mode II loading conditions, respectively. The influences of the morphology and blunting of the elliptical blunt crack, the features of the disclination dipole as well as the deformation of cooperative nanograin boundary sliding and stress-driven nanograin boundary migration (CNGBSM) process characterized by disclination on the SIFs at the elliptical blunt crack tip and critical SIFs for dislocation emission are examined in detail. The results show that disclinations, the cooperative CNGBSM deformation and crack blunting have great influence on the SIFs at the elliptical blunt crack tip and dislocation emission from blunt nanocrack tip. The shielding effect to SIF increases with increasing the crack length, the relative dipole arm and the rotation strength of disclination dipole, and the influence of the shielding effect to SIF is distinctly stronger with the increment of them. Influence of disclination strength on critical normalized SIFs strongly depends on nanocrack blunting. When nanocrack blunting is relatively small, critical normalized SIFs sharp decrease with rising disclination strength. Within CNGBSM deformation, there exists a critical nanocrack length to make dislocation most prone to emit. Effect of nanograin size on critical normalized SIFs is highly sensitive to the angle between nanograin boundaries. Blunt nanocrack surface characterized by positive surface stress changes the influence of nanocrack blunting on critical normalized SIFs. The larger nanocrack blunting contributes to the influences of surface stress on critical normalized SIFs and emission angle.4. A generalized self-consistent model is suggested to describe nanovoid growth by the dislocation emission from nanovoid surface accounting for the effect of neighboring nanovoids and surface stresses in ductile porous materials. By means of the complex variable method, the explicit solution of the critical stress for dislocation emission from nanovoid is derived. The effects of various factors on critical stress and the corresponding most favorable slip plane for dislocation emission are evaluated quantitatively. The results indicate that an increasing nanovoid size rapidly reduces critical stress for the dislocation emission from nanovoid surface and promotes nanovoid growth. When nanovoid size is fixed, the larger nanovoid volume fraction in the nanoporous materials makes the dislocation emission take place more easily, and the relative most probable critical emission angle more insignificantly depart from the direction45°. Under the condition of constant void volume fraction, the larger the neighboring number of voids is, the higher the critical stress becomes. The dislocation emission from nanovoid is more difficult in nanoporous materials, which is characterized by a dilatational eigenstrain. NGB sliding deformation releases, in part, the high stresses near the nanovoid, thereby nanovoid growth is slowed down or even arrested with the stronger NGB sliding deformation in nanocrystalline solids. Surface stresses characterized by positive (negative) surface elasticity increase (decrease) the critical stress. Surface stresses characterized by positive (negative) surface residual stress decrease (increase) the critical stress and the relative most probable emission angle. When NGB sliding deformation appears, there exists a range of the nanovoid size to make the critical stress keep almost unchanged. There is also a critical nanograin size making dislocation emission and nanovoid growth occur most difficultly.5. Based on creep mechanism of the lattice self-diffusion controlled dislocation climb process, the creep model of NiAl nanoprecipitate strengthened ferritic Fe-Cr-Ni-Al alloy is built in the temperature range873-923K. The configurational force driving dislocation climb is derived. The applied stress, temperature, nanoprecipitate size, glide plane height and interface effect dependence of the creep rate of nanoprecipitation strengthened materials is performed. The results show that creep resistance visibly falls with increasing the temperature, applied stress and glide plane height, which makes nanoprecipitates more readily overcome by dislocation climbing and contributes to creep deformation. Creep resistance significantly strengthens as nanoprecipitates grow. This gives a poorer creep resistance with notable positive interface effect, while the negative interface effect has significantly greater contribution to the creep resistance, even though the temperature and applied stress remain the same. A great effect of interface stresses on creep resistance is more visible at small nanoparticle size.