Crystal Structure Defects and Imperfections

Alloying, heat treating, and work hardening are widely used to control material properties, and though they take different approaches, they all focus on imperfections of one type or other. This chapter provides readers with essential background on these material imperfections and their relevance in design and manufacturing. It begins with a review of compositional impurities, the physical arrangement of atoms in solid solution, and the factors that determine maximum solubility. It then describes different types of structural imperfections, including point, line, and planar defects, and how they respond to applied stresses and strains. The chapter makes extensive use of graphics to illustrate crystal lattice structures and related concepts such as vacancies and interstitial sites, ion migration, volume expansion, antisite defects, edge and screw dislocations, slip planes, twinning planes, and dislocation passage through precipitates. It also points out important structure-property correlations.

Structural Defects in TiNi-Based Alloys after Warm ECAP

The microstructure, martensitic transformations and crystal structure defects in the Ti50Ni47.3Fe2.7 (at%) alloy after equal-channel angular pressing (ECAP, angle 90°, route BC, 1–3 passes at T = 723 K) have been investigated. A homogeneous submicrocrystalline (SMC) structure (grains/subgrains about 300 nm) is observed after 3 ECAP passes. Crystal structure defects in the Ti49.4Ni50.6 (at%) alloy (8 ECAP passes, angle 120°, BC route, T = 723 K, grains/subgrains about 300 nm) and Ti50Ni47.3Fe2.7 (at%) alloy with SMC B2 structures after ECAP were studied by positron lifetime spectroscopy at the room temperature. The single component with the positron lifetime τ1 = 132 ps and τ1 = 140 ps were observed for positron lifetime spectra (PLS) obtained from ternary and binary, correspondingly, annealed alloys with coarse-grained structures. This τ1 values correspond to the lifetime of delocalized positrons in defect-free B2 phase. The two component PLS were found for all samples exposed by ECAP. The component with τ2 = 160 ps (annihilation of positrons trapped by dislocations) is observed for all samples after 1–8 ECAP passes. The component with τ3 = 305 ps (annihilation of positrons trapped by vacancy nanoclusters) was detected only after the first ECAP pass. The component with τ3 = 200 ps (annihilation of positrons trapped by vacancies in the Ti sublattice of B2 structure) is observed for all samples after 3–8 ECAP passes.

Recognition of dislocation structure of silicon carbide epitaxial layers by а neural network

Technological features of the growth of single crystal silicon carbide inevitably create condi-tions for the formation of crystal structure defects in them. A method is proposed for recognizing and analyzing a dislocation structure of single crystal silicon carbide based on the use of optical microscopy and a direct distribution neural network. The method was tested on homoepitaxial lay-ers of 4H-polytype silicon carbide. Software has been developed that allows building maps of the dislocation structure distribution over the surface of single crystal silicon carbide. The software was tested on digital images of the surface of silicon carbide epitaxial layers. The accuracy of recognition of dislocation structure was 95%. The dislocation mapping is used in the development of process technologies for reducing their density during the growth of single crystals.

NUMERICAL MODELING OF RADIATION EXPOSURE ON A QUARTZ CRYSTAL STRUCTURE DEFECTS AND SIGNAL ANALYSIS THERMOLUMINESCENT

Developing thermoluminescent (TL) dating methods for quartz-containing deposits has required a series of numerical experiments within the framework of the band model to study the rate of electron accumulation in quartz crystal structure defects under radioactive radiation. The crystal model contained two different types of electron traps and one emission center. A system of differential equations was solved numerically, relating the rates of change in the electron concentration in defects of the crystal structure and the electron concentration in the conduction band and holes in the valence band. The results have shown that the intense radiation exposure alters the dynamic equilibrium concentration of electrons in the traps, which significantly modifies the TL signal. In accordance with this, the sensitivity marker material to the radioactive dose, required for absolute dating and received by its intense radiation exposure, cannot be determined correctly. In addition, the numerical experiments have confirmed the possibility of dating the samples by the position of the TL signal’s maximum on the time axis, as well as the possibility of obtaining the dating by the TL signal’s amplitude.

Blackbody emission from CaF2 and ZrO2 nanosized dielectric particles doped with Er3+ ions

We have revealed that band gap of the material and the presence of crystal structure defects are key factors contributing to the appearance of thermal emission in dielectric nanoparticles doped with RE ions under intense laser excitation.

Получение тонких пленок графита на диэлектрической подложке с помощью гетероэпитаксиального синтеза

The results of testing the methods of preparation of thin graphite films on a dielectric substrate by the method of annealing the structure of (0001) Al2O3/(111) Ni/ta-C are present. The method is based on catalytic decomposition of hydrocarbons on the surface of the monocrystalline film of the metal-catalyst on the surface of the dielectric substrate and subsequent diffusion and crystallization of carbon between the metal film and the substrate. After chemical etching of the metal film, a thin graphite film with low density of crystal structure defects on a dielectric substrate is obtained.

Influence of crystal structure defects on the small-angle neutron scattering/diffraction patterns of clay-rich porous media

Analysing the structure and microstructure of compacted swelling clay minerals is important because of the applications of these minerals in engineering and environmental sciences. Given the typical sub-micrometre size of the particles and pores in clays, small-angle scattering techniques are well suited for such analysis. Interpretation of the intensity patterns, however, remains complex, especially in the intermediate region between the first Bragg peak and the small-angle range. In this study, theoretical small-angle neutron scattering and neutron diffraction patterns are calculated for three-dimensional virtual porous media representative of packed swelling clay particles (i.e. 0.1–0.2 µm size fraction of vermiculite). This packing represents the distribution of the size, shape and particle orientation of a bulk vermiculite sample, for which experimental scattering/diffraction patterns were also collected. It was found that a good fit between the experimental and calculated scattering/diffraction profiles can be obtained only if the presence of crystal-structure defects in the particles is considered. The existence of such defects was supported by transmission electron microscopy analysis. Their influence on power law exponents extracted from intensity profiles is assessed in detail. The analysis is further extended to the influence of mineral dehydration and particle orientation on the intensity profiles. This work shows that using virtual porous media as toy models makes it possible to evaluate the roles of different microstructural parameters in the extent of variation of power law exponents. Such knowledge can be used for better interpretation of small-angle scattering data of natural compacted swelling clay-rich media.