A hydrogenation-induced nonequilibrium oscillating structural transformation in Pd<Subscript>3</Subscript>Ni alloy

The results of X-ray research on the structural characteristics of the Pd₃Ni alloy after its electrolytic saturation by hydrogen during relaxation are presented. The synchronous changes in the period of a lattice and the value of elastic stresses, which started immidiately after sample hydrogenation and continued for 500 hours of relaxation, were found for the first time. The observed changes are caused by the cooperative processes of the displacements of vacancies and nickel atoms between defect complexes and an alloy matrix.     

Evolution of the Pd-Ta-H alloy structure in the Edwards thermodynamic representation

It is shown that the time evolution of x-ray diffraction patterns of a deformed Pd-Ta alloy after its saturation with hydrogen can be determined by the multiwell energy profile of the states of the system. Within the Lorenz synergetic approach, a phenomenological model is proposed in which the evolution of the alloy structure is represented as a random walk of the nonergodic system from one internal-energy minimum to another. In this case, the order parameter is the fraction of states with minimum energy occupied by the system, the conjugate field is associated with the Edwards entropy, and the control parameter is the internal energy. The evolution of the Pd-Ta-H alloy structure is interpreted as that of a complex nonergodic system in terms of thermodynamics.     

Hydrogen-induced phase transformations in Pd-8.3 at % Y alloy

X-ray diffraction analysis is used to study the effect of hydrogenation on the structural state of Pd-8.3 at % Y foil. The decomposition of the single-phase Pd-8.3 at % Y alloy into two phases that was found experimentally after its saturation with hydrogen and subsequent relaxation at room temperature for 840 h indicates a substantial increase in the coefficient of diffusion of yttrium in the alloy under study. Such an increase in the mobility of yttrium atoms is likely to be related to the high contents of hydrogen and vacancies introduced into the matrix of the alloy under study during the hydrogenation.     

Role of vacancies in the structural relaxation of Pd-Mo alloys saturated with hydrogen

The presence of an anomalously large number of vacancies in Pd-Mo alloys subjected to hydrogenation is revealed using precise x-ray diffractometry. These alloys are found to undergo nonmonotonic structural evolution during long-term relaxation. The evolution is characterized by aperiodic time variations in the number of coexisting phases, in the volume of each of them, and in the defect structure and by the cooperative motion of vacancies (as well as hydrogen in the early stage) between the matrix and defect regions. The key features of this evolution are an anomalously high concentration of not only hydrogen but also vacancies and a high concentration of defect regions, which causes thermodynamic instability of the system. The structural evolution has an oscillating character, because the maxima of thermodynamic instability of the matrix and of an ensemble of defect regions are separated in time.     

Behavior of hydrogen and defects in zirconium under irradiation

The accumulation of hydrogen and defects in the E-125 zirconium alloy (Zr-2.5% Nb) is investigated. The hydrogen concentration is maximum on the surface of zirconium alloy samples after electrolytic hydrogenation. The hydrogen concentration decreases at a depth of about 0.5 μm and then gradually grows with increasing depth. The surface of the zirconium alloy is strengthened and becomes more fragile after hydrogenation. A plastic deformation of the zirconium alloy gives rise to traps with different binding energies of hydrogen. The primary type of traps, the binding energy, and the amount of hydrogen captured by traps depend on the deformation magnitude and the sequence of deformation and hydrogenation processes. High mobility of hydrogen in plastically deformed samples is observed under bombardment of the surface of the zirconium alloy by a helium ion beam with an energy of 2.34 MeV. The variation of the hydrogen concentration in the near-surface region of zirconium under ion bombardment depends on the extent of deformation: upon bombardment by helium ions, the hydrogen concentration in the near-surface region of the metal increases for deformations from 1 to 3% and decreases for deformations of 4 and 5%.     

Elastoplastic properties of a low-modulus titanium-based β alloy

The elastoplastic properties (elastic modulus, amplitude-independent damping ratio, microplastic flow stress) of a Ti-26Nb-7Mo-12Zr titanium β alloy are determined using an acoustic resonance method. The effect of the strain during thermomechanical treatment on the structural features of the micro-crystalline alloy and, hence, its elastoplastic properties is analyzed.     

A general model for hydrogen trapping at the inclusion-matrix interface and its relation to crack initiation

Abstract

The role of non-metallic inclusions in hydrogen-induced failure of structural materials has been a controversial topic for many years. In this paper, hydrogen trapping and its relation to the crack initiation at the inclusion-matrix interfaces are studied by considering the interfacial structure and the interaction between the dissolved hydrogen atoms and the elastic strains produced by lattice matching and misfit dislocations. A model is proposed to analyse the change of interfacial structure with inclusion size and its relation to hydrogen trapping. Hydrogen accumulation at the interfaces is quantitatively analysed. The obtained results are in good agreement with the experimental observations. The multiple factors, such as interfacial structure, chemical composition, elastic properties of matrix and inclusions, crystallographic relationship between inclusions and matrix, inclusion morphology and size, simultaneously control hydrogen trapping. In addition, the mechanism of hydrogen-induced crack initiation at the interface is investigated. A criterion is proposed to determine critical conditions for crack initiation. For the first time, the inherent relationship between hydrogen trapping and hydrogen-induced cracking at the interface is clarified. This work paves a way for an in-depth understanding of the effects of inclusions on hydrogen-induced degradation of mechanical properties.     

Evolution of the structural and phase states of a Ti-6Al-4V alloy in forming submicrocrystalline structure with the use of temporary hydrogenation

The special features of the evolution of the structural and phase states of a Ti-6Al-4V alloy in forming submicrocrystalline structure with the use of temporary hydrogen treatment are studied by electron microscopy and x-ray diffraction analysis. Plastic deformation in the α + β two-phase region at 1023 K is found to initiate a complete β → α transformation in a Ti-6Al-4V-H alloy to form an α phase with lattice parameters different from those of the equilibrium α phase. Isothermal annealing at a dehydrogenation temperature of 873 K gives rise to α + β two-phase submicrocrystalline structure with a grain size of ∼ 0.3 μm. The use of nonequilibrium hydrogen release in the deformed Ti-6Al-4V-H alloy exposed to electron beams is shown to result in one-phase submicrocrystalline structure and grain refining. Possible reasons underlying the phase transformations in the alloy under study are discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 86–91, April, 2006.     

Tight-binding molecular dynamics study of the hydrogen-induced structural modifications in tetrahedral amorphous carbon

A tight-binding molecular dynamics study of the structural evolution in tetrahedral amorphous carbon networks under dynamic hydrogen saturation is presented. The incorporation of hydrogen results in higher degrees of network disorder in second-neighbour distances, and initiates orbital re-hybridization that relaxes network stress. Using the simulated structures, numerical tests are performed to verify the effectiveness of a new structural order parameter for tetrahedrally-bonded solids. It is found that the island of accessible information, within the order parameter field shows a linear dependence between the fluctuations in first- and second-nearest-neighbour distances at a preferred orientation of 36°. A comparison with similar studies on hydrogenated amorphous silicon suggests that the local network structure of tetrahedrally-bonded amorphous solids obey the same ordering rule irrespective of differences in chemical species.     

Magnetic and electrical properties of Co-rich cobalt-iron films

The crystalline structures and galvanomagnetic properties have been studied on Co-Fe films of 80–100% Co. It is found that in single- to multiphase transition region the structure of this alloy system is characterised by the minimum electron number per atom. Moreover, the variations in such magnetic parameters as the anomalous Hall constant and saturation induction do not follow the simple mixing rule, but depend on a crystal structure type of the alloy.     

Strain behavior of the hydrogenated submicrocrystalline Ti–6Al–4V alloy

Comparative studies of the influence of 0.002–0.12 mass % hydrogenation on the structure and phase composition of the submicrocrystalline and coarse-grained Ti–6Al–4 V alloys are performed. The evolution of the strain processes in the hydrogenated alloy is studied for both alloys upon tension at a temperature of 293 K depending on the hydrogen content and alloy state. It is established that the presence of hydrogen in the nanostructured Ti–6Al–4 V alloy in the solid solution leads to a decrease of its yield stress and an increase of its tensile strength and total strain before failure. The possible reasons for the increased duration of the uniform strain stage and the effect of strain hardening of the alloy in the presence of hydrogen in the solid solution are discussed.     

Features of the plasma saturation of nanocrystalline and coarse-crystalline titanium samples with hydrogen and deuterium

We investigate the laws governing the saturation of nano- and coarse-crystalline Ti-6Al-4V alloy samples with hydrogen from high-frequency and glow discharge plasmas with a view to finding materials suitable for making hydrogen accumulators. We conclude that nanocrystalline Ti-6Al-4V alloy holds promise for creating hydrogen accumulators, and hydrogen plasma of high-frequency discharge origin is a cleaner hydrogen saturation medium than that of non-self-sustained discharge origin.     

Crystalline structure of copper-gold alloys investigated by DEPES: Theoretical approach

We present results of calculations concerning the angle integrated electron emission from crystalline samples as a function of the incidence angle of the primary electron beam. Diffraction processes of primary electrons in directional elastic peak electron spectroscopy (DEPES) were described by the single scattering cluster (SSC) approximation applied to clusters composed of sublattices formed by different atoms. Theoretical polar profiles were calculated for Cu(0 0 1), Au(0 0 1), AuCu(0 0 1), and AuCu₃(0 0 1) in terms of phase shifts, inelastic mean free paths and scattering cross-sections. In calculations a real experimental geometry was considered concerning the acceptance angle of the analyser and polar rotation of the sample. Profiles calculated for particular incident electron beam energies depend on the structure and stoichiometry of crystalline samples. Application of above investigations is discussed in the context of superficial alloy formation and order-disorder transitions associated with the change of the concentration and order of alloy components in first few atomic layers.     

Al_xCrFeNiTi系高熵合金成分和弹性性质关系

为了探索Al_xCrFeNiTi系高熵合金组成成分和弹性性质的关系,结合固溶体特征参数和第一性原理计算,研究Al元素含量对Al_xCrFeNiTi (x=0, 0.5, 1, 2, 3, 4)合金结构和弹性性质的影响,并分析合金固溶体特征参数与弹性性质之间的关系.结果表明:Al_xCrFeNiTi系合金的价电子浓度随着Al含量的增加逐渐减小,合金在体心立方结构下的形成焓均低于面心立方结构,说明研究的Al_xCrFeNiTi系合金会形成单一的体心立方结构固溶体;合金的晶格常数和形成能力强弱随着Al含量的增加而增大,但合金的结构稳定性略有下降;当合金元素按照等原子比进行成分配比时,合金的原子尺寸差异最大; Al_xCrFeNiTi系合金中不同原子之间除了金属键结合外,还表现出一定的共价和离子键结合特征;对于Al_xCrFeNiTi系合金而言,随着热力学熵焓比的增大,合金体弹模量和韧性随之增大;随着合金混合焓的增加,合金在压缩方向的各向异性程度明显降低.热力学熵焓比和混合焓可作为Al_xCrFeNiTi系高熵合金成分设计的重要参数.     

InOOH压致氢键对称化及其弹性性质

利用第一性原理研究了InOOH在高压下的氢键对称化行为及其对InOOH弹性等性质的影响.结果表明约在18 GPa时InOOH中的氢键发生了对称化转变,导致轴比率b/c对压力的斜率由负值变为正值;压缩弹性常数、非对角弹性常数、体积模量和纵波波速出现异常增加,如体积模量增加了20%—40%.高压下InOOH弹性性质呈现出更加明显的各向异性.常压下InOOH呈现韧性,且伴随着氢键对称化韧性异常增加.对畸变金红石型MOOH(M=Al,In,Ga,Fe,Cr)化合物在高压下的弹性性质转变与氢键性质转变的耦合规律进行了初探.     

Self-organization processes in the cellular structure formation of Ni-W alloys

The self-organization processes, the growth steadiness, and the stability of the cellular structure morphology of Ni-W alloy single crystals with a tungsten concentration of 25–40% have been investigated. The temperature gradient in the crystallization front, the oriented crystallization rate, doping with aluminum, titanium, chromium, and molybdenum, and the tungsten concentration have been considered as nonequilibrium factors. The nonlinear dependence of the structure characteristics on the growth parameters, including the temperature gradient, the oriented crystallization rate, and the tungsten concentration, has been determined. Dendrite features of the cellular morphology of the crystallization front are the result of the self-organization processes and promote the steady growth of single crystals with a high degree of structural perfection. A mechanism of the morphological transition from hexagonal to square cells has been proposed.     

Statistical calculations of tracer and intrinsic diffusion coefficients in concentrated alloys and estimates of microscopic parameters of diffusion from experimental data

The earlier-developed statistical theory of diffusion in concentrated alloys based on the master equation approach is generalized to treat tracer diffusion in binary alloys. The theory developed is used to describe concentration dependencies of both tracer and intrinsic diffusion coefficients and to estimate microscopic parameters of diffusion in alloys CuNi, CuZn and AgCd for which necessary experimental data are available. We show that all main features of strong and peculiar concentration dependencies of diffusion coefficients observed in these alloys are naturally explained by the theory. Signs and scales of interatomic interactions important for diffusion in these alloys are found to strongly depend on the ratio of atomic sizes of alloy components, and types of these dependencies agree with simple physical considerations. We also discuss physical reasons for sharp concentration dependencies of diffusion coefficients characteristic of real alloys.     

Hydrogen adsorption and diffusion on the PdTa alloy surface

The hydrogen adsorption on the PdTa alloy surface is studied using a pseudopotential method with a generalized gradient approximation for an exchange-correlation functional. The most preferable hydrogen adsorption sites on two low-index surfaces ((001), (110)) are determined. It is shown that hydrogen adsorption at the bridge site is preferred on the PdTa(001) surface that terminates by one or two tantalum layers and on PdTa(110). The preference of hydrogen adsorption at tantalum-rich sites is caused by partial population of its d shell. During adsorption, the electronic structure of the states involved in interaction with hydrogen is shown to change most substantially, which is accompanied by the corresponding shifts of these states and the appearance of peaks in the densities of states of the metal in the region of the hydrogen valence band. The effect of hydrogen on the electron and structural characteristics of the surfaces is analyzed. The hydrogen diffusion barriers are calculated in the bulk of the alloy and from the surface into the bulk.     

Hydrogen accumulation and distribution during the saturation of a VT1-0 titanium alloy by an electrolytic method and from a gas atmosphere

The accumulation, distribution, and thermally stimulated release of hydrogen in a VT1-0 titanium alloy during electrolytic saturation and gas-phase saturation are studied. After electrolytic saturation, a 0.4-μm-thick surface layer consisting of δ hydrides with a binding energy of 108 kJ/mol forms in the alloy. The hydride dissociation after electrolytic saturation in heating occurs in the temperature range 320–370°C. After saturation from a gas atmosphere, δ hydrides with a binding energy of 102 kJ/mol form throughout the alloy volume. The dissociation of the hydrides formed during gas-phase saturation in heating occurs in the temperature range 520–530°C. A further increase in the temperature is accompanied by the transformation of titanium from the α into the β modification. At 690–720°C, the phase transformation is completed, and another hydrogen desorption peak appears in a thermally stimulated hydrogen desorption spectrum.