Interaction between Intermetallic Compounds <Emphasis Type="Italic">R</Emphasis>Ni<Subscript>3</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Gd,Dy) and Hydrogen at Low Temperatures

The hydride phases of the intermetallic compounds GdNi₃ and DyNi₃ are synthesized at room temperature and at 273 K under a hydrogen pressure of 1–30 bar. The phase composition of the obtained samples is established using the X-ray diffraction method and the crystal-lattice parameters of the hydride phase are determined. The crystal phases are synthesized at room temperature under a hydrogen pressure of about 1 bar. At 273 K and under a pressure of 30 bar, amorphous samples are formed. The desorption of hydrogen from amorphous hydrides at 573 K leads to the formation of well-crystallized samples of the initial intermetallides. The amorphous samples are formed due to the ordering of hydrogen atoms in the metallic matrix of the hydride at low temperatures.     

Section images of dislocations normal to the surface of a 6H-SiC single crystal

Section topographs of edge and screw dislocations with an axis along [0001] in 6H-SiC are taken and interpreted, and the image formation is explained for this case. The contrast induced by various arrangements of dislocations within the Borrmann triangle is experimentally studied. The sign of the Burgers vector of an edge or screw dislocation normal to the crystal surface is shown to be unambiguously determined from the section-topograph image of this dislocation. The sign of the Burgers vector of a screw dislocation can also be determined from its image taken with Lang projection topography. The contribution of a long-range strain field to the section images of edge and screw dislocations normal to the crystal surface is revealed. The experimental contrasts recorded using section topography and Borrmann-effect-based topography are compared.     

Ti-Mo合金氢化物微观缺陷的小角X射线散射研究

采用小角X射线散射(SAXS)方法对Ti-Mo合金氢化物的微观缺陷进行了研究.结果表明,氢化物样品中Mo含量为5at％时,所测定的SAXS强度在高散射角区明显低于其他样品的SAXS强度.氢化物中的晶粒及其内部的亚结构是引起SAXS现象的散射体,而这些亚结构是由合金氢化时引入的大量位错所产生.Mo 含量为5at％合金主要为hcp结构而其他合金为bcc结构,但两种合金氢化后都成为fcc结构的氢化物.据此,认为hcp结构与bcc结构Ti-Mo合金在氢化时氢化物结构相同但引入的位错缺陷特征不同. 关键词： Ti-Mo合金氢化物 小角X射线散射 微观缺陷     

Self-consistent inhomogeneity of quantum wells in II–VI semiconductors

An X-ray diffraction method that uses a slightly diverging (3′) beam and maximally attainable diffraction angles ? _B (as large as 77°) was developed to study quantum wells (QWs) with widths of 5–8 nm separated by wide (100–220 nm) barrier layers. The advantage of this method compared to the use of a parallel beam is an increase by two orders of magnitude in the intensity of the beam incident on the sample and an increase in the probability of diffraction for all QWs as a unified single crystal. It is found that the growth on GaAs substrates misoriented by 10° from the (001) plane in the [111]_II direction brings about monoclinization of crystal lattices of the QW layers and barrier layers in opposite directions. Inhomogeneity of composition over the thickness of each well is observed. In the case of growth of a ZnSe/ZnMgSSe structure in which the layers have a crystal-lattice period close to the lattice period of the GaAs substrate, the QWs are inhomogeneously doped with elements from the composition of the barrier layers. The inhomogeneity of QW composition observed in the growth of mismatched layers in ZnCdSe/ZnSSe and ZnCdS/ZnSSe structures is caused by the fact that mismatch between the lattice parameters of QWs and barriers stimulates the growth of self-consistent compositions; this occurs due to a decrease in the Cd concentration in the Zn_1?x Cd _x Se QW in the initial stages of growth compared to the Cd concentration in the flow of gases and an increase in the Zn concentration in the Cd_1?x Zn _x S QW at small values of x up to the concentration matching GaAs (x = 0.4). The mismatch stresses are partially relaxed via dislocations with the (111)_II glide planes, as a result of which is observed the combination of rotation of the crystal planes of the layers and QW around the [1\(\overline 1 \)0] axis and almost cylindrical bending of the entire sample around the perpendicular [110] axis. Mismatch between lattice parameters of the ZnMgSSe barrier layers and the substrate brings about decomposition of these layers into two phases; this decomposition is caused by thermodynamic instability of the alloy.     

Ab initio study of H and He migrations in β-phase Sc, Y, and Er hydrides

Ab initio calculations based on the density functional theory have been performed to investigate the migrations of hydrogen(H) and helium(He) atoms in β-phase scandium(Sc),yttrium(Y),and erbium(Er) hydrides with three different ratios of H to metal.The results show that the migration mechanisms of H and He atoms mainly depend on the crystal structures of hydrides,but their energy barriers are affected by the host-lattice in metal hydrides.The formation energies of octahedral-occupancy H(H oct) and tetrahedral vacancy(V tet) pairs are almost the same(about 1.2 eV).It is of interest to note that the migration barriers of H increase with increasing host-lattice atomic number.In addition,the results show that the favorable migration mechanism of He depends slightly on the V tet in the Sc hydride,but strongly on that in the Y and Er hydrides,which may account for different behaviours of initial He release from ScT2 and ErT2.     

Interaction of shockley partial dislocations with reacting forest dislocations

Dislocation combinations formed as a result of interaction between a glissileBσ,d Shockley partial dislocation with reacting undissociated forest dislocations are considered. The value of the parameters characterizing the strength <k> of the dislocation combinations, the probability β_r of their formation, and the interaction intensity α_r of the reacting dislocations are determined for an orientation of the [100] deformation axis of an FCC single crystal for all components of the dislocation loop. Tomsk State Architectural-Building Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–8, May, 1997.     

Three-dimensional size and orientation of the precipitates in AZ91 magnesium alloys measured by TEM techniques

Knowledge of the microscopic structure, including three-dimensional (3-D) size and orientation of the precipitates, is essential to fully understand the mechanical properties of the magnesium alloys and designing the alloys with better performance. Analytical TEM with high spatial resolution offers the simultaneous measurements of 3-D size, structure, orientation, composition of the precipitates from one typical sample along an established crystallographic axis. Besides popular Burgers orientation relationship (OR), other ORs such as Pitsch--Schrader OR, Crawley OR, Potter OR and a new OR with the form of [0001]_α 1.0° from [311]_γ and (11\bar 20)_α 2.0° from (03\bar 3)_γ between the magnesium matrix and the precipitate γ -Mg₁₇Al₁₂ are identified by TEM imaging and diffraction techniques. As a case study, the thicknesses of the individual precipitates with Burgers OR are further measured to be 100--200~nm through both electron energy-loss spectroscopy and x-ray energy dispersive spectroscopy combining differential x-ray absorption and extrapolation, which are in agreement with the overall 3-D size statistic distribution results obtained through analysing various samples along various directions. Furthermore, the fabricated wedge-shape structure provides a platform on which to study the dependence of the interfacial strain on the variation of the thickness.     

Proton magnetic resonance in CoHgCl4· 4H2O

Proton magnetic resonance in a new paramagnetic crystalline hydrate, cobalt mercuric chloride tetrahydrate, was observed using a CW wide line NMR spectrometer. The resonance spectra showed appreciable asymmetry caused by the paramagnetic influence of Co²⁺ ions at the proton sites. The angular dependence of the resonance splittings about each of the three crystallographic axes was analysed to determine the p-p vectors in the unit cell of the crystal, and the angular variation of paramagnetic shift about the [001] axis of rotation was studied.     

Ni51Zr49非晶合金晶化过程的电子显微镜研究

用透射电子显微镜对Ni₅₁Zr₄₉非晶合金的晶化过程进行了研究,发现晶化初期首先从非晶基体上析出的是具有C心单斜点阵的NiZr＇相,a=0.3268nm,b=0.4101nm,c=0.5224nm,β=71.8°。与稳定相NiZr有很明确的取向关系:[100]_NiZr＇∥[100]_NiZr [001]_NiZr＇∥[110]_NiZr。它可以看成是NiZr相的亚稳态或超结构。随温度升高,逐渐转变成NiZr相。温度再高时,在非晶基体上还会析出少量Ni₁₀Zr₇相。晶化初期,无论是NiZr还是NiZr＇相,都是以多次孪晶的形式析出。孪晶有这两种基本形式,一种是以NiZr或NiZr＇的[021]方向为轴的180°孪晶,一种是以NiZr的[001]或NiZr＇的[010]方向为轴的36°旋转孪晶,其它孪晶为这两种孪晶方式的重复与组合。随着温度的升高,孪晶逐渐减少。 关键词：     

违反标度定律的一种相变模型

本文准确地求得了一种三维八顶点模型(能量参数满足条件ε₁+ε₂=ε₃+ε₄)的全部临界指数:α=α′=0,β=1/4,δ=7,γ=1/2,γ′=3/2,ν=ν′=1,η=3/2。它们不满足标度定律,其原因是在相变时极化关联的范围发生了收缩(从晶面(1,1,1)和(1,1,1)收缩成晶轴[1,0,1]。     

Generation of dislocations introduced by bending stress in a Si wafer

Distribution and morphology for dislocations introduced in (001) Si wafers subjected to bending stress at 800°, 900°, and 1100°C were investigated. For wafers bent around a [110] axis at 900° and 1100°C, straight dislocations appeared along the [110] direction only near the neutral plane, and were absent at the surfaces where bending stress is greatest. However, for wafers bent at 800 °C, such straight dislocations were not formed. Dependence of the dislocation distribution and morphology on heat treatment temperature is explained on the basis of interaction between bending stress and SiO₂ precipitates introduced in bulk. Also, it was found that the straight [110] dislocations remained still near the neutral plane, even when additional reverse bending stress was applied around an axis parallel to the dislocations, but were transfered toward the tensile surface by bending around an axis normal to the dislocation direction.     

Magnetostriction of a single cubic laves phase compound Tb0.27Dy0.73Fe2

The magnetostriction of a single crystal cubic Laves phase compound Tb_0.27Dy_0.73Fe₂ is measured with the applied field (0–25 kOe) in the [111] direction in the temperature range between 80 and 300 K. Evidence is seen of a possible transition to the[uv0] easy axis phase during the spin re-orientation form [100] easy axis direction to the [110] easy axis direction.     

Formation of titanium and niobium aluminides induced by hydrogen in a hydride cycle

The results of our study of formation of aluminides by the hydride cycle (HC) method in Ti–Al and Ti–Al–Nb systems and of their hydrides under the conditions of self-propagating high-temperature synthesis are presented. The HC method was developed at the High-Temperature Synthesis Laboratory of the Nalbandyan Institute of Chemical Physics, Armenian Academy of Sciences. The effect of various parameters: the ratio of titanium and niobium hydrides and aluminum powders in the reaction mixture, hydride powder grain size (micro and nano size), compacting pressure during compaction of hydrides, and conditions of dehydrogenation and sintering (heating temperature and rate) on the characteristics of the obtained aluminides: crystal structure, density, etc. was studied. As a result, single-phase aluminides α₂-Ti₃Al, Ti₂Al, γ-TiAl, TiAl₃, Ti_0.33Al_0.34Nb_0.33, Ti_0.5Al_0.23Nb_0.27, Ti_0.52Al_0.15Nb_0.33, TiAl₆Nb, etc., and hydrides were synthesized. The HC method has significant advantages over conventional procedures: low temperature (1000–1100°C) and processing time (30–60 min), one-stage formation of single-phase aluminides, etc.     

Dependence of the specific energy of the β/α interface in the VT6 titanium alloy on the heating temperature in the interval 600–975°C

The specific energy of interphase boundaries is an important characteristic of multiphase alloys, because it determines in many respects their microstructural stability and properties during processing and exploitation. We analyze variation of the specific energy of the β/α interface in the VT6 titanium alloy at temperatures from 600 to 975°C. Analysis is based on the model of a ledge interphase boundary and the method for computation of its energy developed by van der Merwe and Shiflet [33, 34]. Calculations use the available results of measurements of the lattice parameters of phases in the indicated temperature interval and their chemical composition. In addition, we take into account the experimental data and the results of simulation of the effect of temperature and phase composition on the elastic moduli of the α and β phases in titanium alloys. It is shown that when the temperature decreases from 975 to 600°C, the specific energy of the β/α interface increases from 0.15 to 0.24 J/m². The main contribution to the interfacial energy (about 85%) comes from edge dislocations accommodating the misfit in direction [0001]_α || [110]_β. The energy associated with the accommodation of the misfit in directions \({\left[ {\bar 2110} \right]_\alpha }\left\| {{{\left[ {1\bar 11} \right]}_\beta }} \right.\) and \({\left[ {0\bar 110} \right]_\alpha }\left\| {{{\left[ {\bar 112} \right]}_\beta }} \right.\) due to the formation of “ledges” and tilt misfit dislocations is low and increases slightly upon cooling.     

Effects of Al concentration and resulting long-period superstructures on the plastic properties at room temperature of Al-rich TiAl single crystals

The role of Al₅Ti₃ and h-Al₂Ti long-period superstructures on the plastic properties of TiAl at room temperature is investigated on five single crystals with aluminium content comprised between 54.7 at.%, and 62.5 at.%. After annealing at 1200°C for 1?h, the Al₅Ti₃ superstructure develops in the L1₀ (γ) matrix upon increasing Al concentration except for Ti–62.5 at.%Al where h-Al₂Ti substitutes for Al₅Ti₃. The CRSS for <110]{111} first increases abruptly with the development of the Al₅Ti₃-type ordering. Then, the CRSS reaches a plateau at which dislocations assemble in groups of four to prevent extra anti-phase boundary (APB) from being engendered during glide throughout the Al₅Ti₃ phase. In Ti–62.5 at.%Al, the CRSS for ordinary slip further increases upon the precipitation of h-Al₂Ti in the L1₀ phase, whereas it decreases when the crystal is fully transformed into single-phased Al₅Ti₃. <101] superlattice dislocations are primarily activated under both the [210] and [1?1?8.6] load orientations irrespective of the Al concentration, but the dislocation microstructure strongly depends on orientation as well as on the degree of Al₅Ti₃ ordering. In the [210] orientation, the frequency of the decomposition of <101] dislocations into 1/2<110] and 1/2<112] dislocations decreases abruptly with the development of Al₅Ti₃. This is interpreted in terms of the increased difficulty to move ordinary dislocations. Under the [1?1?8.6] orientation, the density of faulted dipoles diminishes remarkably with the development of Al₅Ti₃. This is consistent with the transformation of the low energy extrinsic stacking fault of the L1₀ phase into a higher energy complex extrinsic stacking fault.     

Crystal structure prediction of uranium hydrides at high pressure: A new hydrogen-rich phase

Uranium hydride is a novel hydrogen-rich system which contains 5f electrons. Uranium hydride can not only be used in the nuclear fuel industry, but also be a candidate of high superconducting-temperature materials. In this paper, we have searched the stable uranium hydride structures by using particle swarm optimization method and first-principles calculations. Besides UH₈ and UH₉, we find that UH₁₇, which contains larger hydrogen content than most hydride materials reported before, is also stable at high pressure. The atomic structures, electronic structures and phase diagram of uranium hydrides are provided, and we find that all of the discovered uranium hydrides are metals with negligible magnetic moments.     

α—SiC晶体中的位错

用腐蚀法和X射线形貌术研究了α-SiC晶体中的位错。所用的腐蚀剂为熔融氢氧化钾。证实了尖底蚀坑与位错的一一对应关系。由于[0001]方向的螺型位错的Burgers矢量比刃型位错的Burgers矢量大得多,故可从蚀坑的深浅来判别螺型位错和刃型位错。给出了蚀坑形状和多型体晶体结构的对应关系。研究了表面生长蜷线的形态与SiC晶体中的位错及位错运动的关系。X射线形貌图显示了α-SiC晶体中相当数量的位错处于基面C面上。生长位错从晶体“根部”成核并随着晶体生长前沿的向前推进而延伸,因而位错线的方向常常沿[101O]和[1120]方向。将腐蚀法和X射线形貌术结合起来才能全面显示α-SiC晶体中的位错。 关键词：     

NMR Line Shape for a Rectangular Configuration of Nuclei

We present an approach to describe nuclear magnetic resonance absorption of dipolar-coupled rectangular configuration of nuclear spins associated with reorientation of ethylene bridge (–CH₂–CH₂–) in the crystal. The approach was applied to investigate reorientational mobility of dabco-linkers (1,4-diazobycyclo[2,2,2]octane) in metal–organic framework [Zn₂(bdc)₂(dabco)] (bdc—benzenedicarboxylate). Dabco rotation around the N–N axis is shown to remain down to 80 K and the activation energy of the molecule reorientation does not exceed 12 kJ/mol.     

Formation of transition metal hydrides at high pressures

Silane (SiH₄) is found to (partially) decompose at pressures above 50 GPa at room temperature into pure Si and H₂. The released hydrogen reacts with surrounding metals in the diamond anvil cell to form metal hydrides. A formation of rhenium hydride is observed after the decomposition of silane and reaction of hydrogen with Re gasket. From the data of a previous experimental report [M.I. Eremets, I.A. Trojan, S.A. Medvedev, J.S. Tse, Y. Yao, Science 319 (2008) 1506], the claimed high-pressure metallic and superconducting phase of silane is identified as platinum hydride, that forms after the decomposition of silane. These observations show the importance of taking into account possible chemical reactions that are often neglected in high-pressure experiments.     

The orientation of formate and carbonate on ZnO(101&#x0304;0)

Polarisation dependent CK-edge NEXAFS spectra of carbonate and formate species on ZnO(101&#x0304;0) indicate that both species are aligned by the substrate. The carbonate species, formed by adsorption of CO₂, is oriented with its molecular plane close to the [0001] azimuth. This suggests a bond geometry in which one O atom of CO²⁻₃ is in the substrate, and a second interacts with a surface cation, consistent with the results of previous cluster calculations. In contrast, the formate species, formed by adsorption of formic acid, is not azimuthally ordered, although the molecular rotation axis is aligned close to the surface normal, consistent with bidentate bonding to a single cation.