Changes in the vibrational energies and interatomic spacings upon the formation of vacancies in the volumes and in the cores of crystallite-conjugation regions of polycrystalline transition metals with cubic lattices

The changes in the vibrational energies and the signs of changes in the interatomic spacings upon the formation of vacancies in the bulk of metal and in the cores of the crystallite-conjugation regions (CCR) in polycrystalline transition metals with bcc and fcc lattices have been determined. The vibrational energy increases upon the formation of a vacancy in the bulk of metal because of a positive “relaxation” contribution to the change in the force constant of the atoms surrounding a vacancy. Positive “relaxation” contributions to the changes in the force constants and, correspondingly, an increase in the vibrational energy of the atoms surrounding a vacancy arise also upon the formation of “split” vacancies (S vacancies) in the cores of CCRs of polycrystalline transition metals with a face-centered cubic lattice. The positive “relaxation” contributions to the changes of the force constant of atoms in the region of localization of S vacancies are caused by a decrease in the interatomic spacings upon their formation, just as upon the formation of conventional vacancies in the bulk of metals. The vibrational energy of the nearest environment of the vacancies that are formed in the CCR cores in the polycrystalline d transition metals with a bcc lattice decreases because of a negative “relaxation” contribution to the change in the force constants. The cores of the high-angle CCRs in polycrystalline d transition metals with a bcc lattice are characterized by a negative internal pressure. Therefore, vacancies with positive relaxation volumes ν_BCC > 0 are formed in them, causing an increase in the interatomic distances in the nearest environment of such vacancies.     

Changes in the potential and vibrational energies upon the formation of vacancies in the oxygen-alloyed cores of crystallite-conjugation regions of polycrystalline Cr, Cu, and Ir

The changes in the potential and vibrational energies upon the formation of vacancies in the oxygen-alloyed (OA) cores of crystallite-conjugation regions (CCRs) in polycrystals of d transition metals Cr, Cu, and Ir have been determined. The potential energy increases upon the formation of vacancies (upon the formation of complexes of vacancies with oxygen atoms) in the OA cores of CCRs in the polycrystalline Cr, Cu, and Ir. However, the vibrational energy decreases upon the formation of vacancies in the OA cores of CCRs in these polycrystals, as in the OA cores of CCRs in polycrystals of the 4d and 5d transition bcc metals Mo, Ta, and W. The volume of the OA cores of CCRs in polycrystals of Cr, Cu, and Ir decreases upon the formation of vacancies. The changes in the interatomic interactions and dynamic properties in the regions of vacancy localization in the OA cores of CCRs coincide with analogous changes introduced into the lattices of metals by split interstitials.     

Changes in the potential and vibrational energies upon the formation of vacancies in oxygen-doped cores of crystallite-conjugation regions of polycrystalline 4d and 5d BCC transition metals

The changes in the potential (u _cmpl) and vibrational (?_cmpl) energies and the signs of changes in the interatomic spacings (Δa _cmpl) upon the formation of vacancies in oxygen-alloyed (OA) cores of crystallite-conjugation regions (CCR) in polycrystalline 4d and 5d transition metals Mo, Ta, and W have been determined. The potential energy upon the formation of vacancies (upon the formation of vacancy complexes with oxygen atoms—vacO complexes) in the OA cores of CCRs in polycrystalline Mo, Ta, and W increases (u _cmpl are positive), as upon the formation of vacancies in pure CCR cores of transition bcc d metals. The vibrational energy upon the formation of vacancies in OA CCR cores in polycrystalline Mo, Ta, and W decreases (?_vacO are negative). The negative sign of changes in the vibrational energy upon the formation of vacancies in the OA CCR cores in Mo, Ta, and W (?_vacO < 0) agrees with the independent determinations of the sign of changes in the vibrational energy in the OA CCR cores in polycrystalline tungsten (?_vacO)_W using Mössbauer measurements of the Debye temperature. The signs of changes in the interatomic spacings upon the formation of vacancies in the OA CCR cores in polycrystals of Mo, Ta and W are negative (Δa _VacO < 0), in contrast to positive (0 < Δa _BCC) changes in the interatomic distances in the nearest neighborhood of vacancies formed in pure CCR cores in Mo, Ta, and W.     

Crystallite-conjugation regions and adjacent lattice regions in polycrystalline iridium: III. Enthalpies of formation of vacancies and the energies of interaction of partners in vacancy complexes with oxygen formed in the cores of crystallite-conjugation regions in polycrystalline iridium

The interatomic spacings in the cores of crystallite-conjugation regions (CCRs) and adjacent lattice regions (ALRs) of polycrystals either decrease or increase upon alloying of nominally pure metals with oxygen and vacancy complexes with oxygen (mVacO, where m is the number of vacancies in the complex) that are formed during annealing. These changes in the interatomic spacings lead to an increase or decrease in the isomer shifts δ of the components of the Mössbauer spectra of atomic probes ⁵⁷Fe that are localized in the CCR cores and ALRs of polycrystals [1–6]. The enthalpies Q _mcmpl1, of formation of vacancy-oxygen complexes mVacO in the CCR cores have been measured for Ir and Cr polycrystals, and the enthalpies Q _Vac,1 of formation of vacancies in CCR cores have been determined for Ta, W, Ir, and Cr polycrystals. The enthalpies E _mcmpl of the interaction between the partners of the complexes increase with increasing number m of vacancies in the complexes.     

On the magnetic structure of the ground state of ordered Fe-Al alloys

Based on a comparison of the results of low-temperature magnetic and Mössbauer studies, a model of the magnetic structure of the ground state (T = 0) of ordered (D0₃ and B2 types) Fe-Al alloys is suggested. The model is based on the dependence of both the magnitude and sign of the local magnetic moment at an Fe atom on the number of Al atoms in its nearest neighborhood. It has been established that in terms of this model at Al concentrations exceeding 25 at. % there are formed magnetic inhomogeneities (regions with a magnetic moment oriented opposite to the total magnetization) on a nanometer scale (2–5 nm in size).     

Effect of the magnetism of impurities on their diffusion in metals: Bulk diffusion of iron,cobalt, and rhodium in iridium single crystals

The coefficients and parameters of the temperature dependences of the coefficients of bulk diffusion of Fe, Co, Rh, and Au atomic probes (APs) in iridium single crystals (mono-Ir) have been determined from the diffusion profiles obtained using secondary-ion mass spectrometry of the diffusion zones. The enthalpies of activation of diffusion of Fe, Co, and Rh APs are considerably lower than the enthalpy of activation of selfdiffusion in mono-Ir. This is caused by the negative contributions of the intraatomic exchange energy and energy of relaxation of the environment of the d transition APs to the enthalpy of interaction of magnetically active APs with the vacancies in the iridium lattice. The interaction energy of partners in such complexes and the relationships between the magnetic moments of d transition APs in complexes with vacancies have been estimated. The Rh APs in complexes with vacancies in iridium possess stable magnetic moments.     

Crystallite-conjugation regions and adjacent lattice regions in polycrystalline iridium: II. Composition and properties of cores of crystallite-conjugation regions in polycrystalline iridium during annealing in an ultrahigh vacuum

Composition and properties of cores of crystallite-conjugation regions (CCRs) formed during annealing of polycrystalline iridium (poly-Ir) in an ultrahigh vacuum (UHV) have been studied using the method of intercrystalline diffusion in combination with Mössbauer spectroscopy (ID+MS) that has been developed in our previous works. Upon annealing in a UHV, poly-Ir is doped with oxygen from the atmosphere of the vacuum chamber. Complexes containing two vacancies per oxygen atom are formed in the CCR cores of poly-Ir because of a rearrangement of the atomic structure of the CCR cores upon their doping with oxygen. Using the ID+MS method, we for the first time revealed a “compensated” state of CCR cores in poly-Ir samples annealed in a UHV and of CCR cores in poly-Cr annealed in technical vacuum. The cause of the appearance of “compensated” states in CCR cores is the mutual compensation of relaxation volumes with opposite signs characteristic of different point defects. The relaxation volume of an oxygen atom in the CCR core of poly-Ir is by an order of magnitude greater than that in poly-Cr.     

The influence of doping by transition metal elements on the vacancy formation energy in Fe-Al <Emphasis Type="Italic">B</Emphasis>2 phase

Based on ab initio calculations for Fe-Al B2 phase with and without the vacancies, the authors studied the influence of alloying by Ni, Cr, and V on the energy of vacancy formation. The authors predict that the presence of these alloying elements in the vicinity of the vacancy on the Fe site increases the energy of the vacancy formation. The present article discusses the changes of the electron charge redistribution in the vicinity of the vacancy in Fe-Al alloyed with these transition metals and link the preference of their site occupation with the peculiarities of the electron density redistribution. In addition, the authors report on the results of calculations of different types of antisite defects in Fe-Al and show that the formation of the antisite on the Al site is the energetically preferable configuration for this defect. The obtained results are in agreement with the experimental data.     

Evolution of grain–subgrain structure and carbide subsystem upon annealing of a low-carbon low-alloy steel subjected to high-pressure torsion

The effect of annealing on the evolution of an ultrafine-grain structure and carbides in a 06MBF steel (Fe–0.1Mo–0.6Mn–0.8Cr–0.2Ni–0.3Si–0.2Cu–0.1V–0.03Ti–0.06Nb–0.09C, wt %) has been studied. The grain–subgrain structure (d = 102 ± 55 nm) formed by high-pressure torsion and stabilized by dispersed (MC, M₃C, d = 3–4 nm) and relatively coarse carbides (M₃C, d = 15–20 nm) is stable up to a temperature of 500°C (1 h) (d = 112 ± 64 nm). Annealing at a temperature of 500°C is accompanied by the formation in regions with a subgrain structure of recrystallized grains, the size of which is close to the size of subgrains formed by high-pressure torsion. The average size and distribution of dispersed particles change weakly. The precipitation hardening and the increase in the fraction of high-angle boundaries in the structure cause an increase in the values of the microhardness to 6.4 ± 0.2 GPa after annealing at 500°C as compared to the deformed state (6.0 ± 0.1 GPa). After 1-h annealing at 600 and 700°C, the microcrystal size (d = 390 ± 270 nm and 1.7 ± 0.7 μm, respectively) increases; the coarse M₃C (≈ 50 nm) and dispersed carbides grow by 5 and 8 nm, respectively. The value of the activation energy for grain growth Q = 516 ± 31 kJ/mol upon annealing of the ultrafine-grained steel 06MBF produced by high-pressure torsion exceeds the values determined in the 06MBF steel with a submicrocrystalline structure formed by equal-channel angular pressing and in the nanocrystalline α iron.     

Crystallography of in-situ transformations of the <Emphasis Type="Italic">M</Emphasis><Subscript>7</Subscript>C<Subscript>3</Subscript> carbide in the cast Fe–Cr–Ni alloy

In the process of holding of the cast heat-resistant Fe–Cr–Ni (0.45C–25Cr–35Ni) alloy at 1150°C, the eutectic chromium carbide present in its structure undergoes a gradual transition M ₇C₃ → M ₂₃C₆. The gradual formation of domains of the M ₂₃C₆ carbide inside the particles of the M ₇C₃ carbide makes it possible to assume that the observed phase transition is the well-known carbide transformation of the in situ type. The mechanism of the in situ transformation of the crystal structure of the carbide from M ₇C₃ into M ₂₃C₆ with a change in the number of nearest metal neighbors of carbon atoms is explained within the previously developed combinatory model of polymorphic transitions in the metals.     

First-Principles Calculations of Defect Structures in B2 AlCo and GaCo

First-principles electronic structure calculations have been performed for defect structures in nonstoichiometric B2 AlCo and GaCo. To determine the type of constitutional defects, the compositional dependence of the energy of formation and lattice parameter was obtained by calculations employing supercells of various sizes (16 and 32 atoms) as well as special quasirandom structures (SQSs) developed for random pseudobinary A_1?x B _x C with compositions x = 0.25 and 0.5. According to the results, Co vacancies are the constitutional point defects in the Al-rich side of both B2 AlCo and B2 GaCo, while Co vacancies present the minimum energy for the Ga-rich side. For the Co-rich side of both B2 AlCo and B2 GaCo, the Co antisite is the most stable defect. To investigate the thermal defect concentrations at finite temperature, we adopted the Wagner–Schottky model using enthalpies of formation of point defects obtained from the SQS approach. The present results suggest that the predominant thermal defects in AlCo are of complex type whereas for GaCo they are of interbranche Co type. The results of these calculations show agreement with available theoretical and experimental data.     

Mechanical and magnetic properties of alloys near the concentration range of the existence of Co<Subscript>3</Subscript>(Al,W) intermetallic compound

An experimental study of the mechanical and magnetic properties of Co–8 at % Al–X at % W (where Х = 4.6; 6.8; 8.5; 10; 12) polycrystalline alloys has been carried out depending on the tungsten content. It has been shown that an increase in the tungsten content induces the following changes: Young’s modulus changes from 240 to 259 GPa and the microhardness HV changes from 290 to 413 MPa. The Curie temperature of the intermetallic γ' phase (L1₂) grows from 1025 to 1049 K, and the saturation magnetization of the alloys decreases. The value of the coercive force also increases, from 1 to 500 Oe. The calculations of the specific heat of the γ' (L1₂)–γ (dcc) phase transition has shown that the highest amount of heat (304 J/g) is absorbed upon the phase transition in the sample containing the largest amount of tungsten (12.6 at %). The results obtained indicate the diffusion character of the transition, the rate of which is controlled by the diffusion of the slowest component of the alloy (in this case, tungsten). It has been found that the solvus temperature of the γ' phase increases from 1009 to 1044°С with an increase in the tungsten content from 4.6 to 12.6 at %.     

The effects of pH and citric acid concentration on the characteristics of nanocrystalline NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> powder synthesized by a sol-gel autocombustion method

We have performed a qualitative analysis of the band structure of DyNi₂B₂C using the LDA + U method. The special structure of the antiferromagnetic order has a stabilizing effect on the superconductivity of the Ni 3d _xy band which is due to the canceling of four Dy moments nearest to each Ni site, which agrees with the phenomenological theory.     

ATOMISTIC SIMULATION OF POINT DEFECTS IN NiAl ALLOY

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Strain induced transformation in beta brass

The β-phase of Cu-Zn and Cu-Zn-Ga alloys partially transforms when cold worked at low temperatures. X-ray data are presented on the new structures formed (which depend upon composition), on densities of stacking faults in them, on M_d temperatures, and on the tendency to revert to body-centered-cubic structure.     

Mössbauer spectroscopy and the structure of interfaces on the atomic scale in metallic nanosystems

A microscopic model of the formation of an alloy on the interface has been constructed, which takes into account the exchange of atoms with the substrate atoms and the “floating up” of the latter into the upper layers in the process of epitaxial growth. The self-consistent calculations of atomic magnetic moments of spatially inhomogeneous structures obtained in this case are used for the interpretation of data of Mössbauer spectroscopy. The proposed scenario of mixing leads to the appearance of a preferred direction in the sample and the asymmetry of interfaces in the direction of epitaxial growth. In the multilayer M ₁/M ₂ (M _1,2 = Fe, Cr, V, Sn, or Ag) systems, this asymmetry makes it possible to understand the difference in the magnetic behavior of M ₁-on M ₂ and M ₂-on-M ₁ interfaces which has been observed experimentally. The correlation between the calculated distributions of magnetic moments and the measured distributions of hyperfine fields at iron atoms confirms the assumption about their proportionality for a broad class of metallic multilayer systems. However, a linear decrease of hyperfine fields at the ⁵⁷Fe nuclei with increasing number of impurity atoms among the nearest and next-nearest neighbors is not confirmed for Fe/Cr systems, although is correct in Fe/V superlattices. In the Fe/Cr multilayer systems, the experimentally measured value of magnetoresistance grows with increasing fraction of the “floated up” atoms of ⁵⁷Fe. Thus, it is the bulk scattering by impurity atoms that gives the basic contribution to the effect of giant magnetoresistance. The problem of the influence of mixing and adsorption of hydrogen in the vanadium layers on the state of the spin-density wave in V/Cr superlattices has been considered.     

Photoelectron spectroscopy and diffraction of NbO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Nb(110) surface

Methods of X-ray photoelectron spectroscopy and diffraction were used to investigate the oxygen-induced surface structures on the Nb(110) face which were formed as a result of oxygen segregation from the volume of the crystal upon thermal annealing at 2000 K in a vacuum and/or oxygen adsorption in situ at temperatures higher than 1100 K. The niobium atoms on the crystal surface form quasi-ordered structures in the form of monatomic Nb chains surrounded by oxygen atoms and from the viewpoint of the nearest surroundings and chemical bonding they are close to the states of the metal in NbO _x . The thickness of such NbO _x -like structure is estimated as 0.5 nm at the degree of coverage of the surface of 50%. Two chemically nonequivalent states of oxygen at Nb(110) have been distinguished, supposedly, (1) atomic chemisorbed oxygen on the surface of Nb metal and (2) oxygen in the composition of NbO _x -like clusters on the Nb(110) plane. The NbO _x /Nb(110) model of the surface allows for the distortion of the structure of NbO _x clusters, i.e., periodic atomic displacements of metal atoms in Nb chains by height and changes in the Nb-O bond angles.     

B2-RuAl点缺陷结构的第一原理计算

采用第一原理赝势平面波方法,计算了B2-RuAl金属间化合物的基本物性及其点缺陷结构的几何、能态与电子结构,通过对不同点缺陷结构形成热与形成能的计算与比较,分析和预测了RuAl金属间化合物中点缺陷结构的种类与存在形式.结果表明：RuAl金属间化合物的点缺陷主要是Ru空位和Al反位,在富Ru合金中主要为Ru反位,在富Al合金中则主要是Al反位.这些点缺陷主要以Ru-Ru双空位和Al-Al双反位的组态结构形式出现,并且双空位以Ru-Ru为第一近邻时其点缺陷结构最稳定,而双反位则是以Al-Al为第三近邻时稳定性最高.进一步通过对NiAl和RuAl不同点缺陷结构Cauchy压力的比较,发现点缺陷对RuAl塑性的降低程度比NiAl低,因而含有点缺陷的实际合金的室温塑性RuAl比NiAl好.     

Creep correlations of metals at elevated temperatures

Creep data for pure metals at temperatures above those at which rapid recovery occurs (above about 0.45 the melting temperature) are correctable by means of the equations ? = f (te^?ΔH/RT, σ) and σ = f (?&#x0307 _s e^ΔH/RT). These correlations were applied successfully to data for aluminum, iron, nickel, copper, zinc, platinum, gold, and lead as well as for simple alloys. For a given metal, ΔH is a constant about equal to the activation energy for self-diffusion.     

The research of powder fullerene and ultra-dispersed diamond composites with metal and oxide nanoparticles

Magnetic properties of new nanophase materials based on oxide and metal particles of d transition metals included in the powder fullerite C₆₀ and detonative ultradispersed diamonds (UDD) are investigated. The materials are obtained by an original method of catalytic combustion reaction.