I. A. Ovid'ko

Triple junction diffusion and plastic flow in fine-grained materials

Abstract A theoretical model is suggested which describes the yield stress dependence on grain size in fine-grained materials, based upon competition between conventional dislocation slip, grain boundary diffusional creep (Coble creep) and triple junction diffusional creep. In the framework of the model, the contribution of diffusional creep mechanisms to plastic deformation increases with reduction of grain size, causing the abnormal Hall–Petch dependence in the range of small grains. A grain size distribution is incorporated into the consideration to account for a distribution of grain sizes occurring in real specimens. The results of the model are compared with experimental data from Cu and shown to be in good agreement.

Misfit dislocations in wire composite solids

A theoretical model is suggested, which describes generation of misfit dislocations in film/substrate composites of wire form. In the framework of the model, the ranges of the geometric parameters (wire radius, film thickness, misfit parameter) of a wire composite are calculated at which the generation of misfit dislocations is energetically favourable. The specific features of generation of misfit dislocations in wire composites are discussed and compared with those in conventional platelike composites.

Disclinations, amorphization and microcrack generation at grain boundary junctions in polycrystalline solids

Abstract Splitting of disclinations (decay of disclinations into small-power disclinations) at grain boundary junctions in polycrystalline solids is examined theoretically, and related to local amorphization. Energetic characteristics of the splitting process are revealed. It is shown that such a process competes effectively with microcrack generation at triple junctions of grain boundaries, causing a plastification of deformed polycrystalline materials.

Misfit dislocation loops in composite nanowires

A new mechanism for relaxation of misfit stresses in composite nanowires (quantum wires) is suggested and theoretically examined, namely the formation of misfit dislocation loops. The stress field of a prismatic dislocation loop in a cylinder (nanowire) is calculated. The parameters of two-phase composite nanowires at which the formation of misfit dislocation loops is energetically favourable are estimated. The effect of stress fields of dislocation loops on the formation of compositionally modulated nanowires is discussed.

Transformations of grain boundaries due to disclination motion and emission of dislocation pairs

A theoretical model is suggested which describes changes of grain boundary misorientation parameters in plastically deformed polycrystalline and nanocrystalline materials. In the framework of the model, the changes occur via grain boundary disclination motion associated with emission of dislocation pairs from grain boundaries into adjacent grain interiors. Energetic characteristics of the disclination motion in question are calculated.

Misfit dislocations in nanocomposites with quantum dots, nanowires and their ensembles

We review theoretical concepts and experimental results on the physics of misfit dislocations in nanocomposite solids with quantum dots (QDs) and nanowires (quantum wires). Special attention is paid to thermodynamic theoretical models of formation of misfit dislocations in QDs and nanowires, including composite core–shell nanowires. The effects of misfit dislocations on the film growth mode during heteroepitaxy and phase transitions in QD systems are analysed. Experimental results and theoretical models of the ordered spatial arrangement of QDs growing on composite substrates with misfit dislocation networks are discussed. The influence of subsurface dislocations in composite substrates on the nucleation of QDs and nanowires on the substrate surface is considered. Models of misfit strain relaxation and dislocation formation in nanofilms on compliant substrates are also reviewed.

Yield stress of nanocrystalline materials: role of grain- boundary dislocations, triple junctions and Coble creep

A theoretical model is suggested which describes the strengthening of nanocrystalline materials due to the effects of triple junctions of grain boundaries as obstacles for grain-boundary sliding. In the framework of the model, a dependence of the yield stress characterizing grain-boundary sliding on grain size and triple-junction angles is revealed. With this dependence we have found that, in as-fabricated nanocrystalline materials, the yield stress depends upon a competition between conventional dislocation slip and grain-boundary sliding. On the other hand, yield stress dependence on grain size in heat-treated nanocrystalline materials is described as that caused by a competition between conventional dislocation slip and Coble creep. Grain-size and triple-junction angle distributions are incorporated into the consideration to account for distributions of grain size and triple-junction angles, occurring in real specimens. The results of the model are compared with experimental data from as-fabricated and heat-treated nanocrystalline materials and shown to be in good agreement.

Emission of partial dislocations from triple junctions of grain boundaries in nanocrystalline materials

A theoretical model is suggested that describes emission of partial Shockley dislocations from triple junctions of grain boundaries (GBs) in deformed nanocrystalline materials. In the framework of the model, triple junctions accumulate dislocations due to GB sliding along adjacent GBs. The dislocation accumulation at triple junctions causes partial Shockley dislocations to be emitted from the dislocated triple junctions and thus accommodates GB sliding. Ranges of parameters (applied stress, grain size, etc) are calculated in which the emission events are energetically favourable in nanocrystalline Al, Cu and Ni. The model accounts for the corresponding experimental data reported in the literature.

Disclinations and rotational deformation in fine-grained materials

A theoretical model is presented which describes a new mechanism of plastic deformation in fine-grained materials. In the framework of the model, rotational deformation occurs via motion of dipoles of grain-boundary disclinations and is associated with the emission of lattice dislocations from grain boundaries into adjacent grain interiors. Ranges of defect system parameters are identified in which the disclination motion is energetically favourable. It is shown that the mechanism can contribute to plastic flow in fine-grained materials prepared by highly non-equilibrium methods such as ball milling, severe deformation and high-pressure compaction.

Grain-boundary dislocations and enhanced diffusion in nanocrystalline bulk materials and films

A theoretical model is suggested which describes the transformations of grain-boundary dislocation walls and their influence on diffusion processes in nanocrystalline materials fabricated under highly non-equilibrium conditions. It is shown that the decay of boundary dislocation walls of finite extent, occurring via the climb of boundary dislocations and the corresponding emission of vacancies, is capable of highly enhancing the grain-boundary diffusion in nanocrystalline materials. The enhanced diffusion, in turn, strongly affects the deformation behaviour of nanocrystalline materials. In the case of nanocrystalline films deposited on to substrates, the effects of misfit stresses on the transformations of boundary dislocation walls and the diffusion are analysed. It is demonstrated that the mean diffusion coefficient in a nanocrystalline film may increase by approximately several orders of magnitude owing to misfit stresses.