ABSTRACT
An overview of key experimental data and theoretical representations on fracture processes in nanoceramics is presented. The focuses are placed on crack growth in nanoceramics and their toughening micromechanics. Conventional toughening micromechanisms are discussed which effectively operate in both microcrystalline-matrix ceramics containing nanoinclusions and nanocrystalline-matrix ceramics. Particular attention is devoted to description of special (new) toughening micromechanisms related to nanoscale deformation occurring near crack tips in nanocrystalline-matrix ceramics. In addition, a new strategy for pronounced improvement of fracture toughness of ceramic materials through fabrication of ceramic-graphene nanocomposites is considered. Toughening micromechanisms are discussed which operate in such nanocomposites containing graphene platelets and/or few-layer sheets.
ABSTRACT
A new physical mechanism of plastic flow in solids is suggested and theoretically described. The mechanism represents stress-driven rotations of grain boundaries (GBs) in subsurface areas of solids. The stress and energy characteristics of the GB rotations are calculated. In the case of nickel, we find that such rotations are energetically favorable processes in a wide range of GB parameters. Our theory is consistent with the experimental observation [D. Jang and J. R. Greer, Scr. Mater. 64, 77 (2011).] of GB rotations in deformed nanocrystalline nickel nanopillars.
ABSTRACT
A new physical mechanism or mode of plastic deformation in nanocrystalline metals and ceramics is suggested and theoretically described. The mode represents the cooperative grain boundary (GB) sliding and stress-driven GB migration process. It is theoretically revealed that the new deformation mode is more energetically favorable than "pure" GB sliding and enhances the ductility of nanocrystalline solids in wide ranges of their structural parameters.
ABSTRACT
A new physical mechanism of plastic deformation in nanowires is suggested and theoretically described. This mechanism represents formation of near-surface nanodisturbances-nanoscopic areas of plastic shear with tiny shear vectors-in deformed nanowires. We calculated the energy characteristics for nanodisturbance formation and compared them with those for conventional dislocation generation. It is shown that the nanodisturbance deformation mode tends to dominate in Au nanowires deformed at high stresses and zero temperature.
ABSTRACT
The new mechanism for relaxation of misfit stresses in nanoislands (quantum dots) is suggested and theoretically examined which is the formation of partial misfit dislocations. The parameters of nanoislands are estimated at which the generation of partial misfit dislocations is energetically favorable, with emphasis on the case of Ge/Si nanoislands. Different dislocation structures are shown to be energetically preferred in different regions of the interface.
ABSTRACT
A theoretical model is suggested that describes the behavioral features and energetic characteristics of dipoles of grain boundary dislocations in nanocrystalline films. Such dislocation dipoles in nanocrystalline films are shown to play the role of misfit defect configurations that compensate, in part, for misfit stresses that occur due to a mismatch between crystal lattice parameters of films and substrates. Ranges of parameters (misfit parameter, grain size, etc.) are revealed at which the formation of dislocation dipoles is energetically favorable in nanocrystalline films. It is demonstrated that dislocation dipoles are typical structural elements of nanocrystalline films fabricated at highly nonequilibrium conditions.
Subject(s)
Crystallization/methods , Crystallography/methods , Models, Chemical , Models, Molecular , Nanotechnology/methods , Computer Simulation , Energy Transfer , Membranes, Artificial , Molecular Conformation , Motion , Stress, MechanicalABSTRACT
In the context of the concept of determinate chaos, introduced in psychology in the nineties, a modified stimulus-response formula is suggested which deals with the probabilistic predictions of responses of animate systems (brainless animate systems, animals, human persons) on stimuli. The suggested formula is used here in elaboration of a model describing the relationship among thoughts, feelings, and actions. In the framework of the model, a Darwin-like selection of behavioral and mental responses on stimuli is realized which is influenced by feelings. Transformations of human persons into mentally unhealthy states are discussed with the help of representations of the elaborated model. The model invites empirical test.
