I. A. Ovid’ko

Crossover from grain boundary sliding to rotational deformation in nanocrystalline materials

A theoretical model is suggested which describes cooperative action of grain boundary (GB) sliding and rotational deformation in mechanically loaded nanocrystalline materials. Focuses are placed on the crossover from GB sliding to rotational deformation occurring at triple junctions of GBs. In the framework of the model, gliding GB dislocations at triple junctions of GBs split into dislocations that climb along the adjacent boundaries. The splitting processes repeatedly occurring at triple junctions give rise to climb of GB dislocation walls that carry rotational deformation accompanied by crystal lattice rotation in grains of nanocrystalline materials. The role of GB sliding, rotational deformation and conventional dislocation slip in high-strain-rate superplastic flow in nanocrystalline materials is discussed.

Transformations of grain boundary dislocation pile-ups in nano- and polycrystalline materials

A theoretical model is suggested which describes several types of transformations of grain boundary dislocation pile-ups at triple junctions of grain boundaries in (super) plastically deformed nanocrystalline and polycrystalline materials. Ranges of parameters of defect configurations are revealed at which the transformations considered are energetically favourable. The role of transformations of grain boundary dislocation pile-ups at triple junctions of grain boundaries in plastic deformation processes in nanocrystalline and polycrystalline materials is discussed with special attention being paid to the influence of such transformations on competition between different deformation mechanisms in nanocrystalline materials.

Grain boundary migration as rotational deformation mode in nanocrystalline materials

Stress-induced grain boundary migration is theoretically described as a new mode of rotational plastic deformation in nanocrystalline materials. We have calculated the strain energy change due to migration of a grain boundary that carries rotational plastic flow. It is shown that, depending on the stress level, the grain boundary can either be immobile or mobile, and in the latter case it can migrate in either a stable or unstable regime. The critical stress values, which correspond to the transitions between these migration regimes, are estimated and discussed.

Special strain hardening mechanism and nanocrack generation in nanocrystalline materials

A special mechanism of strain hardening in deformed nanocrystalline materials (NCMs) is suggested and theoretically described. The mechanism represents generation of disclination dipoles at grain boundaries (GBs) due to GB sliding. It is shown that special strain hardening can effectively suppress plastic flow instability in metallic and ceramic NCMs and thus enhance their ductility. At the same time, the disclination dipoles formed due to GB sliding serve as dangerous stress sources that can induce nucleation of nanocracks, decreasing ductility of NCMs.

Misfit dislocations in composites with nanowires

A theoretical model is suggested which describes the generation and evolution of misfit dislocations in composite solids containing nanowires with rectangular cross-section. In the framework of the model, the ranges of the geometric parameters (nanowire sizes, misfit parameter, interspacing between the nanowire and the free surface of the composite) are calculated at which the generation of various misfit dislocation configurations (loops, semi-loops and dipoles) is energetically favourable. Transformations of these dislocation configurations and their specific features are discussed.

Crack-stimulated generation of deformation twins in nanocrystalline metals and ceramics

A theoretical model is proposed that describes the generation of deformation twins near brittle cracks of mixed I and II modes in nanocrystalline metals and ceramics. In the framework of the model, a deformation twin nucleates through stress-driven emission of twinning dislocations from a grain boundary distant from the crack tip. The emission is driven by both the external stress concentrated by the pre-existent crack and the stress field of a neighbouring extrinsic grain boundary dislocation. The ranges of the key parameters, the external shear stress, τ, and the crack length, L, are calculated within which the deformation-twin formation near pre-existent cracks is energetically favourable in a typical nanocrystalline metal (Al) and ceramic (3C-SiC). The results of the proposed model account for experimental data on observation of deformation twins in nanocrystalline materials reported in the literature. The deformation-twin formation is treated as a toughening mechanism effectively operating in nanocrystalline metals and ceramics.

Special mechanism for dislocation nucleation in nanomaterials

A special mechanism of dislocation nucleation in deformed nanocrystalline materials (NCMs) is suggested and theoretically described. The mechanism represents nonlocal homogeneous nucleation of a nanoscale loop of “noncrystallographic” partial dislocation whose Burgers vector magnitude continuously grows during the nucleation process. It is shown that the special mechanism can effectively produce nanoscale loops of lattice and grain boundary dislocations in NCMs deformed at high mechanical stresses.

Grain-boundary dislocation climb and diffusion in nanocrystalline solids

The effect of grain-boundary dislocation transformations on diffusion in nanocrystalline solids is discussed. A theoretical model describing the enhancement of diffusion processes associated with the climb of grain-boundary dislocations in nanocrystalline solids is developed.

Mechanism of Deformation-Twin Formation in Nanocrystalline Metals

A theoretical model is proposed to describe the nucleation of deformation twins at grain boundaries in nanocrystalline materials under the action of an applied stress and the stress field of a dipole of junction or grain-boundary wedge disclinations. The model is used to consider pure nanocrystalline aluminum and copper with an average grain size of about 30 nm. The conditions of barrier-free twinning-dislocation nucleation are studied. These conditions are shown to be realistic for the metals under study. As the twin-plate thickness increases, one observes two stages of local hardening and an intermediate stage of local flow of a nanocrystalline metal on the scale of one nanograin. In all stages, the critical stress increases with decreasing disclination-dipole strength. The equilibrium thickness and shape of the twin plate are analyzed and found to agree well with the well-known results of experimental observations.

Nanograin nucleation initiated by intergrain sliding and/or lattice slip in nanomaterials

Stress-induced nucleation of nanoscale grains (nanograins) in deformed nanocrystalline metals and ceramics is theoretically described as a process initiated by intergrain sliding and/or lattice slip. The nanograin nucleation occurs through splitting and migration of grain boundaries containing disclination dipoles produced by intergrain sliding and/or lattice slip. It is shown that the nanograin nucleation is energetically favorable in mechanically loaded nanocrystalline Al and α-Al2O3 in certain ranges of their parameters and the external stress level.