Structural features of Σ = 19, [110] GaAs tilt grain boundaries

= 19, [110] tilt grain boundaries have been observed to facet parallel to particular planes; the facets lie along _A/ _B, ( )_A/ )_B and ( )_A/( )_B. The structural unit of the = 19 ( )_A/( )_B [110] boundaries consists of 5- and 7-member rings, which are similar to the core structure of a/2[110] edge dislocations. The polarities in each grain on either side of the boundaries has been confirmed by CBED methods; a lower number of anti-site type cross-boundary bonds occur along the boundaries compared to when the polarity of one grain is reversed. The presence of 7-member rings and anti-site cross-boundary bonds results in a more open atomic structure at the boundary, shortening the distance between the first and the second {331} atomic planes from the boundary plane by 40%.     

Prediction of Fracture Toughness of Reactor Pressure-Vessel Steels Using the “Master Curve” Concept and Probabilistic Model

Using a probabilistic model and the Master curve approach, the temperature dependence of the brittle fracture toughness of reactor pressure-vessel steel 15Kh2NMFA in the initial state and highly embrittled state is predicted from the results of fracture toughness testing of a cracked Charpy-type specimen at a given temperature. A comparative analysis has shown that for the steel in the initial state the curves calculated by the probabilistic model and by the Master-curve approach are in good agreement. By testing compact-tension specimens of embrittled 2T-CT steel in a wide temperature range the authors have obtained experimental fracture toughness values and compared them with the calculated curves. It is demonstrated that, in the case of the embrittled steel, the curve as calculated by the Master-curve approach fails to describe adequately the experimental results, while the curves plotted by the probabilistic model agree well with the experimental fracture toughness values.     

Prediction of Ductile Fracture Toughness for Neutron-Irradiated Reactor Pressure-Vessel Steels. Part 1

We discuss the theoretical background of modeling the influence of neutron irradiation on the upper-shelf level of the relation. The modeling involves a local criterion and a model for ductile fracture proposed by the authors earlier. A physical interpretation of the influence of irradiation on the mechanisms that control ductile fracture is presented. The parameters of the model that are sensitive to neutron irradiation are determined.     

Alternative Fracture Criterion in Terms of the Energy-Based Theory of Strength

Within the framework of the energy-based strength criterion, the transition from the stress space to the stress-intensity-factor space is performed in a such way so that the fracture criterion obtained contains all characteristics of the material crack resistance , as well as dimensions and of the local fracture zone at the crack-tip vicinity under different modes of the crack-face loading. We propose an alternative approximation of the fracture criterion, which is exact in terms of the model under consideration. The theoretical results are compared with some known experiments. An example of the calculation of a structural element with a macrocrack is given.     

Anisotropic flow and fracture in beryl [Be3Al2(SiO3)6]

Flow and fracture resulting from Vickers indentation testing on {0 0 0 1} and {10 0 } planar orientations have been examined. Flow characterized by indent shape differentiation was analysed to belong to the slip system with planes of the types { 10 0} and {11 0}. The ensuing fracture paths were resolved to propagate along {1 0 0} and {1 1 } cleavage planes whileK _c values obtained for them were 0.196 and 0.248 MPam^1/2, respectively.     

Effect of the addition of Y2O3 on the structure, microstructure and piezoelectric properties of PZT(53/47)

In this work, an attempt is made to study system with x=0.0125, 0.025, 0.050, 0.075 and 0.1. It was shown that there is limited solid solubility (0.625 mol %) of or 1.25 mol % of in PZT(53/47). For higher levels of dopant, mainly two other extra second phases were detected. The first was a Zr-rich phase in which some and small amounts of was dissolved. The second one was a Pb solid solution composed of mainly PbO, and which was initially also seen in calcined samples. The formation of Zr-rich phase is thought possibly to originate due to the sublimation of Pb from source during the sintering process. For higher x values, a structural shift towards Ti-rich region of PZT's phase diagram is seen. All piezoelectric parameters of the doped samples such as , are seen to decline sharply compared to that of undoped samples. Increasing the level of dopant gave rise to the increase of conductivity and dielectric loss of sintered samples. The formation of non-ferroelectric extra phases, and the Zr/Ti change of the main formed phase is believed to be responsible for this behavior.     

Computer experiment on deformation of nanocrystals of the chromium-niobium system. Part 1. Atomic-structural rearrangements

Molecular-dynamic simulation of the process of stretching of chromium and niobium nanocrystals and chromium-niobium bicrystals was performed to a pair-potential approximation. Atomic mechanism of deformation and fracture is illustrated, which depends on the crystal orientation with respect to the acting force . In the case of the initial orientation (100) , the reorientation mechanism (100) (110) is in action. In a bicrystal, it is niobium that deforms first and then the relay-race transfer of deformation from Nb to Cr takes place. With the initial configuration (110) , the fragmentation of the crystal and grain-boundary rearrangements are observed. Niobium crystallite in a bicrystal deforms only within the limits of elastic deformation whereas the deformation of chromium is of brittle-ductile mode. With the initial configuration (111) , the mechanism of pure brittle fracture is observed. In all cases, fracture of a bicrystal occurs along the interphase boundary. The causes of such atomic-structural transformations are explained.Translated from Problemy Prochnosti, No. 1, pp. 5–23, January–February, 2005.     

Fabrication and electrical properties of lead zirconate titanate thick films using a new sol-gel processing technique

Lead zirconate titanate (Pb ( , PZT) ferroelectric films 2–60 m in thickness have been successfully fabricated on Pt-coated oxidized Si substrates by a new sol-gel-based process. The films are a 0-3 ceramic–ceramic composite formed by dispersing ceramic powders in a sol-gel solution. The precursor solution for spin coating was prepared from lead acetate, tetrabutyl titanate, and zirconium nitrate. The microstructure and morphology of the prepared PZT thick films were investigated via X-ray diffractometry (XRD) and scanning electron microscopy techniques. XRD analysis shows that the thick films possess single-phase perovskite-type structure, no pyrochlore phase exists in thick films, and SEM micrographs suggest that the PZT thick films were uniform, dense, and crack free. A dielectric constant of 860, loss tangent of 0.03, remanent polarization of , and a coercive field of were measured on 50 m thick films.     

Electrical and magnetic properties of BaTiO3−(Ni0.3Zn0.7)Fe2.1O4 composites

Ceramic composites consisting of ferroelectric and ferrite ( were synthesized by the mixed oxide route. The phase assemblage, electrical and magnetic properties of the samples were investigated. The results indicates that the phase is compatible with ( phase, and dense diphasic composite ceramics were obtained. Electrical resistivity of composites varies with increasing amounts of ( phase, and shows a percolation-like drop. Magnetic hysteresis loops were observed in the composites.     

The effect of temperature on the electric conductivity property of Bi3Zn2Sb3O14 pyrochlore type phase

The electric conductivity characteristic of ceramics was investigated by impedance spectroscopy in the frequency range from 5 Hz to 13 MHz. Electric measurements were performed at temperatures in the range from 25 to 700 °C. The phase type pyrochlore was synthesized by the polimeric precursors method. Ceramic presenting a relative density of 98% of the theoretical density was prepared. The data were presented in Nyquist diagrams form, from which the electric resistivity was determined. The electric conductivity followed the Arrhenius law with an apparent activation energy of the conduction process equal to 1.37 eV. The electric conductivity at room temperature was determined by extrapolation being equal to . Between 400 and 700 °C, the conductivity values were and , respectively.     

The Smallest Correction of an Inconsistent System of Linear Inequalities

The problem of calculating a point x that satisfies a given system of linear inequalities, A x #x2265; b, arises in many applications. Yet often the system to be solved turns out to be inconsistent due to measurement errors in the data vector, b. In such a case it is useful to find the smallest perturbation of b that recovers feasibility. This motivates our interest in the least correction problem and its properties.Let A x #x2265; b be an inconsistent system of linear inequalities. Then it is always possible to find a correction vector y such that the modified system A x #x2265; b #x2212; y is solvable. The smallest correction vector of this type is obtained by solving the least correction problem Let denote the convex cone which consists of all the points for which the system A x #x2265; u is solvable. Let denote the polar cone of . It is shown that the least correction problem has a simple geometric interpretation which is based on the polar decomposition of into and . A further insight into the least correction concept is gained by exploring the duality relations that characterize such problems.     

A comparison of strengthening mechanisms in rolled and axisymmetrically deformed Ti-20Y composites

Two Ti-20%Y metal-metal composites were deformation processed: one axisymmetrically and the other by rolling. The microstructures, preferred crystallographic orientations, and tensile strengths of each were measured periodically as the deformation progressed. The axisymmetrically deformed Ti matrix developed a [10 0] fiber texture, and the rolled composite acquired a texture with the <0001> tilted 31° from the sheet normal toward the transverse direction with [10 0] parallel to the rolling direction. The orientations of the {10 0}<11 0> and (0002)<11 0> slip systems in Ti with these textures were used in conjunction with the maximum possible slip distances for dislocation travel in each specimen to demonstrate that the axisymmetrically deformed material should be stronger than the rolled material for composites of equal phase thickness and spacing. The strengths of the two composites measured in this study were compared at similar microstructural phase sizes and spacings, and the axisymmetrically deformed composite was indeed found to be somewhat stronger, although the difference in strengths was not large.     

Cu3Ge Schottky contacts on n-GaN

The electrical properties and thermal stability of (hereafter referred to as ) Schottky contacts to n-GaN as a function of the Ge concentration and annealing temperature are studied. Upon rapid thermal annealing the /n-GaN Schottky diode is formed at 300 °C and is stable up to 650 °C. At 700 °C the agglomeration occurs in the films. For the Cu-25 at % Ge films the ideality factor, n, barrier height, , and flat band barrier height, , of the /n-GaN diodes are in the ranges of 1.22–1.36, 0.53–0.72 eV, and 0.65–0.97 eV, respectively, being decreased with the annealing temperature. Higher Ge concentration in the Cu-35 at % Ge films results in larger n, , and of /n-GaN Schottky diodes. The film is expected to be a suitable candidate for stable Schottky contacts to n-GaN at elevated temperatures.     

Growth of aluminium nitride whiskers by sublimation-recrystallization method

Aluminium nitride (AlN) with various morphologies was fabricated through a sublimation-recrystallization method. The influences of type of reactor and temperature gradient were explored, as well as the orientations and growth mechanism of the obtained AlN whiskers. In the early stage of preparation, a vapour-liquid-solid (VLS) mechanism dominated, producing AlN pillars, whiskers and noncrystalline fibres. In the later stage, as the catalyst liquid was removed by volatilization, the pillars and noncrystalline fibres stopped growing, but the growth of AlN whiskers continued through a vapour-solid (VS) mechanism. By Laue method and rotating-crystal method of x-ray diffraction, together with electron diffraction, most of the AlN whiskers were discovered to grow on planes , or , where l=0, 1, 2, 3, along crystal axes , or , where w=0, 1, 2, 3. Oblique grown whiskers also appeared, with a growth direction at an angle of about 54° to the growth plane, .     

Transmission electron microscope study of a directionally solidified Cu-MgCu2 eutectic

The crystallography and the interface structure of a unidirectionally solidified Cu-MgCu₂ eutectic alloy have been examined by transmission electron microscopy. The microstructure of the eutectic was found to be lamellar and regularly interrupted by faults. The preference of the particular orientation relationship could not be explained by relative atomic densities of the planes comprising the interface. Based on the defect contrast observed and extinction distance calculations, it is suggested that the fine array of defects observed at the interface may be characterized as steps with step vectors parallel to or Dislocations were also observed at the interface but they were rarely regular.     

Hole Density in Charge Stripes and Effect of Next-Nearest-Neighbor Transfer

By applying a variational Monte Carlo method to the two-dimensional t-J model, a couple of properties of striped states are studied. In the charge domain walls vertical (or horizontal) to the lattice (VDW), the hole density is favorable in energy similarly for 0.5 , whereas in the diagonal domain walls (DDW) the stable hole density is limited to = 1. Negative next-nearest-neighbor transfer (t/t < 0) further stabilizes VDW with = 0.5 and DDW with = 1 against the -wave superconducting state.     

The Dual-Phase-Lag Heat Conduction Model in Thin Slabs Under a Fluctuating Volumetric Thermal Disturbance

In the present work, the thermal behavior of a thin slab, under the effect of a fluctuating volumetric thermal disturbance described by the dual-phase-lag heat conduction model is investigated. It is found that the use of the dual-phase-lag heat conduction model is essential at large frequencies of the volumetric disturbance. It is found that the hyperbolic wave model deviates from the diffusion model when and the dual-phase-lag model deviates from the diffusion model when . where is the angular velocity of the fluctuating wall temperature, is the phase-lag in the heat flux vector and is the phase-lag in the temperature gradient vector.     

Morphological characterization of selectively overgrown GaN via lateral epitaxy

Morphology as well as the dislocation networks in epitaxial GaN thin film, prepared via selectively lateral overgrowth has been characterized using TEM combined with focused ion beam (FIB) tool. The results showed that orientations of the sidewalls dependent on the orientations of mask strips. The sidewalls coincide with the planes that form V type voids when the mask strips aligning along directions and correspond to the planes that result in rectangular voids if the strips arranging along the directions. The dislocations were observed along the plan view direction. The dislocations in the lateral overgrown region mainly developed along the direction perpendicular to the strips. The genetic aspect of such morphologies of GaN films may have very close relation with the change of growing fronts of the epitaxial layer.     

Physico-Phenomenological Model of Metal Resistance to Plastic Deformation for the Calculation of the Processes of Metal Plastic Working. Part 1. Statement of the Problem and Derivation of the General Equation

We propose a model for metal resistance to plastic deformation in a wide temperature and strain-rate range, wherein dislocation sliding within grains is the main deformation mechanism. The model makes it possible to take into account the effect of the loading history, which is related to the strain-rate time dependence, by specifying the parameters of thermally activated micromechanisms, by which dislocations overcome barriers, and by rearrangement of the dislocation structure in dynamic polygonization and recrystallization processes.     

Evaluation of the fracture toughness of structural steels under high-speed deformation

We investigated the dynamic crack resistance (fracture toughness) of low-alloy structural steels within a broad range of change of temperature (173–323°K) and loading rate (10^–5–120 m/sec) on standard specimens tested under conditions of three-point bending by a drop weight. As velocity parameters determining the conditions of high-speed deformation of metal in the zone of prefailure we considered the speed of opening of the crack tip and the rate of local plastic deformation on the contour of its blunted tip. To calculate in dependence on the loading conditions, the corresponding expression is presented. We evaluated experimentally the mechanical properties of the investigated steels at different temperatures T and strain rates, and we also considered the possibility of determining theoretically the dynamic yield strength that is used in establishing the value of . We present the correlation between the transition temperature of the metal to ductile failure T_v and the velocity parameters .Translated from Problemy Prochnosti, No. 5, pp. 51–59, May, 1993.