electron microscopy. It was found that irradiation at temperatures below the threshold of grain-boundary relaxation causes a local expansion of the boundaries. Obtained data show that as a result of intergrain adsorption, the point defects not only annihilate each other, but also induce substantial changes in the structure of the grain boundaries. Comparison with the results of a study of the evolution of the structure of the grain boundaries as a result of intergrain adsorption of helium and interstitial atoms suggests that the more mobile interstitial atoms wind up at the free surface during intergrain adsorption, resulting in an enrichment of the grain boundaries by vacancies. Computer simulation using molecular dynamics shows that intergrain adsorption of vacancies can lead to the formation of three-dimensional grain-boundary structures. The three-dimensional grain-boundary structures were revealed also in NdFeB alloy.
The atomic structure of the interfaces in amorphous-crystalline tungsten obtained in situ by ultrafast quenching to low temperatures from liquid phase are investigated by the methods of FIM. A high degree of localization of imperfections and interfacial incoherence was established. Transition regions with intermediate positions of atoms were observed only in the vicinity of boundary regions between closely packed crystallographic planes and paraplanes in amorphous phase. It was shown that microscopic topography of interfaces was characterized by alternation of extended plane segments oriented predominantly along the planes with low Miller indices and microscopically rough regions containing nanosteps. Extended plane regions were perfectly atomically smooth. Atomic displacements in the boundary region of the crystalline phase do not disturb the crystallogeometrical regularity of atomic packing. As a rule, conjugation effects are not observed in the boundary regions of the amorphous phase: irregular displacements are typical even for atoms closest to the interface. The structural width of the interface determined from the width of the region with a peculiar morphology in atomic packing in the interfacial core can be assumed to be equal to zero. The effect of conjugation of crystallographic planes and paraplanes was observed in separate regions of the interface, and a high extent of their orientational correspondence was indicated. It is shown that the width of interfacial region in which peculiarities of deformation field were observed with the help of ion microscopy is considerably larger that the structure width of the interface.
Oscillatory atomic relaxation in symmetrical grain and twin boundaries in metals is investigated by molecular-dynamics methods. It is found that density of atoms varies nonmonotonically in various metastable grain boundaries and that the amplitude, period, and character of damping of the oscillations are virtually the same in these boundaries. Analysis of the feature of the stress field in the neighborhood of the twin and grain boundaries allows one to replace the complex pattern of interaction of adjacent crystals by periodic rows of parallel line forces for describing the strain field. Linearly distributed forces are localized along the crystallographic directions with low Miller indexes, which have the maximum packing density at the boundary and are characterized by an increased stress level. The distance between the lines of application of the forces along the grain boundary is equal to the lattice period in the direction normal to the axis of misorientation. The adjacent crystallites are treated as elastic continua bounded by the plane of the surface. The forces, uniformly distributed along the axis of misorientation, are directed normally to the boundary. A proposed continuum model for surface layer deformation produced by linear distributed forces adequately describes oscillatory grain-boundary relaxation. This model can give a satisfactory quantitative description of the oscillatory character of the atomic relaxation at the grain and twin boundaries.
To describe the configuration of barrier controlling the field evaporation a semiempirical universal potential was used. An analytical expression for the electric field of evaporation was obtained. The experimental data confirmed the results of the mathematical modeling of field evaporation of refractory metals. Intentional nanoscaled modification of metal surfaces is realized using the field-controlled technology. In this way, the possibility of surface smoothing on an atomic scale was obtained. Developed high-field technologies were used for the surface forming of the field emitters, microprobes and microsurgical instruments.
Dissertation which consists of the introduction, five chapters, conclusions and a list of the references (135 tittles) is presented on 162 pages (including 46 figures, 3 tables and cited literature).
Key words: nanocrystalline metal, grain boundary, triple junction, nanostructured tungsten, field ion microscopy, field evaporation, computer modeling, field emitter, microsurgical instrument.