NMR and Mössbauer study of peculiarities of the structure formation in Fe–Сo alloys

Fe–Co alloys with an Fe content of 20–95 at % have been investigated using NMR and Mössbauer spectroscopy. A comparison of the hyperfine fields measured at ⁵⁹Со and ⁵⁷Fe nuclei on the same samples allowed us to determine that the magnetic moment at an Fe atom in the disordered alloys with Fe concentrations of 20–80 at % varies from 2.8 to 2.4 μ_B, as well as to ascertain peculiarities of the formation of an ordered state. The data obtained indicate the existence of regions with a short-range order of the D0₃ type in alloys with an Fe content of 70–80 at %. In alloys with iron concentrations greater that 80 at %, the phase separation of the structure into regions of elemental iron and short-range-ordered Fe₈₉Co₁₁ regions is observed. The obtained results explain the contradictions in the values of the magnetic moment at the Fe atom and the anomalous dependence of the lattice parameter of the alloy with a Со content less than 20 at %, as well as prove the accuracy of the phase diagram containing regions of three bcc phases, rather than one, as is commonly accepted.     

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.     

Influence of supersaturation on structure of sodium aluminate solutions with medium concentration:a solution X-ray diffraction study

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Mössbauer and x-ray diffraction phase analysis of rusts from atmospheric test sites with different environments in Sweden

Rust samples from mild steel panels exposed at eight different atmospheric test sites in Sweden have been studied by transmission ⁵⁷Fe-Mössbauer spectroscopy and X-ray powder diffraction. All samples exhibited superparamagnetic effects in Mössbauer spectra recorded at room temperature, due to fine particles. The corrosion products were mainly identified in low temperature (∼10 K) Mössbauer spectra and with X-ray powder diffraction. In all samples, α-FeOOH and γ-FeOOH were the main constituents (>70%). Only in the marine samples were β-FeOOH and Fe_3-σO₄ found. The α-FeOOH/γ-FeOOH ratio obtained was found to increase with the SO₂ concentration at the test sites.     

Emission Mössbauer spectroscopy of grain boundaries in ultrafine-grained W and Mo produced by severe plastic deformation

Grain boundaries in ultrafine-grained W and Mo produced by severe plastic deformation (SPD) by high-pressure torsion method have been studied by emission Mössbauer spectroscopy on ⁵⁷Co (⁵⁷Fe) nuclei. The evolution of the state of the grain boundaries has been studied upon heating. It has been shown that, after SPD, the grain boundaries are in a nonequilibrium state, which is characterized by an excess free volume. Upon annealing, the state of the grain boundaries changes as it approaches the state typical of coarse-grained materials.     

Mössbauer study and thermodynamic modeling of Fe–C–N alloy

Mössbauer spectroscopy and Monte Carlo computer simulation have been combined to understand the reason for the solid-solution stability of Fe–0.93 C–0.91 N alloy (mass%). Interstitial concentrations in austenite and ferrite were determined on the basis of X-ray diffraction measurements of the lattice dilatation. The hyperfine structure of Mössbauer spectra was analyzed to identify different atomic configurations in solid solutions and determine their fractions. Thereafter Monte Carlo simulation of the interstitial distribution in ferritic and austenitic solid solutions was performed, and values of the interstitial–interstitial interaction energies were obtained for the first and second coordination spheres in austenite and the first to the fourth coordination spheres in ferrite. Simulation shows that in both austenitic and ferritic phases the interaction of interstitial atoms is characterized by a strong repulsion within the first two coordination spheres. Experimental data and simulated interstitial distributions are consistent and complementary. It is concluded that the absence of interstitial clusters prevents carbide and nitride precipitates and causes the higher thermodynamic stability of Fe–C–N solid solutions as compared with Fe–C and Fe–N ones.     

Mössbauer investigation of Sn diffusion and segregation in grain boundaries of polycrystalline Nb

The parameters of nuclear γ resonance (Mössbauer) spectra on ^119mSn nuclei inserted into grain boundaries of polycrystalline Nb have been determined at the annealing temperatures of 680 to 994 K [0.25–0.36 melting temperature (T _m)]. Two components are observed in the nuclear emission γ resonance spectra at all of the annealing temperatures considered. One of them (component 1) is suggested to result from the ^119mSn atomic probes localized in grain boundary cores, while the other one (component 2) corresponds to that located in the near-boundary areas. Diffusion annealing temperature dependences of both components parameters have been analyzed. The coefficients and enthalpy of Sn grain-boundary segregation are determined based on this analysis, as well as the ratio of the diffusion pumping zone extension to the grain boundary width and the enthalpy of the atomic probes “pumping” from grain boundaries into the bulk.     

Transition of solidification mode and the as-cast γ grain structure in hyperperitectic carbon steels

Formation processes of as-cast γ grain structures during casting of hyperperitectic carbon steels with 0.15–0.45 mass% carbon concentrations have been studied by means of a rapid unidirectional solidification technique. In steels with 0.15–0.41 mass% carbon concentrations, coarse columnar γ grains (CCGs) with a minor axis diameter of 1–3 mm developed along the direction of temperature gradient. In a steel with 0.38 mass% carbon, importantly, columnar γ grains (CGs) whose minor axis diameter is less than 500 μm form before the formation of CCGs and the grain structure changes discontinuously from CG to CCG. The fraction of the CG region increases with an increase in the carbon concentration. In the samples with a carbon concentration higher than 0.43 mass%, the as-cast structure consists of CGs over almost the entire ingots. Analyses of the relation between γ grain and dendrite structures and their crystallographic orientations indicate that the formation of CGs originates from the primary solidification of γ phase instead of δ phase. This is supported by numerical analysis of the dendrite growths.     

Kinetic paths for B2 order in nanocrystalline FeCo–Mo: a MÖssbauer spectroscopic study

The kinetic evolution of B2 order in nanocrystalline FeCo–Mo alloys was studied at different temperatures using Mössbauer spectroscopy. The kinetic paths in the space spanned by the two short-range order parameters α_Co/Fe and α_Mo/Fe were found to be independent of temperature within experimental errors. This behavior is to be compared with that observed in coarse-grained FeCo–Mo alloys, where the paths were not the same at different temperatures. This can be attributed to the presence of more grain boundary regions in the nanophase alloys, which provide short-circuited diffusion paths for atom movements.     

X-ray study of the short-range order structure in Cu–24.3 at.% Mn alloy

The diffuse X-ray scattering from a single crystal of Cu–24.3 at.% Mn aged within the fcc phase at 770 K was measured at room temperature. Diffuse peaks were found at 2n₁+0.2, 2n₂+0.2, 0 and equivalent positions (n₁ and n₂ are integers) in reciprocal space. The Warren–Cowley short-range order parameters were derived from the separated short-range order scattering and used to generate the corresponding local atomic arrangements on a computer. The generated arrangements indicated that Mn atoms form clusters. A set of effective pair interaction energies obtained using an inverse Monte Carlo method showed that the interaction between the first nearest neighbors mainly affects the clustering of like atoms.     

Formation,stability and crystal structure of the σ phase in Mo–Re–Si alloys

The formation, stability and crystal structure of the σ phase in Mo–Re–Si alloys were investigated. Guided by thermodynamic calculations, six critically selected alloys were arc melted and annealed at 1600 °C for 150 h. Their as-cast and annealed microstructures, including phase fractions and distributions, the compositions of the constituent phases and the crystal structure of the σ phase were analyzed by thermodynamic modeling coupled with experimental characterization by scanning electron microscopy, electron probe microanalysis, X-ray diffraction and transmission electron microscopy. Two key findings resulted from this work. One is the large homogeneity range of the σ phase region, extending from binary Mo–Re to ternary Mo–Re–Si. The other is the formation of a σ phase in Mo-rich alloys either through the peritectic reaction of liquid + Mo_ss → σ or primary solidification. These findings are important in understanding the effects of Re on the microstructure and providing guidance on the design of Mo–Re–Si alloys.     

Special aspects of diffusion mechanisms and dynamic behavior of Mössbauer iron atoms on a nanoclay surface

Using a technique that is developed using Mössbauer spectroscopy, parameters of the diffusion motion and data on geography of the exchangeable iron complexes on a silica–alumina surface of clays were presented. The analyzed natural causes of the temperature behavior of the dynamic Mössbauer parameters allowed the pattern of the atoms' motion to be determined on the basis of various models of diffusion motion. For iron atoms adsorbed in the interwrapper space of the mineral, a jumplike translational diffusion is typical, which allowed determination of the sizes of the fluctuation hollow (r ~ 0.16 nm) necessary for the diffusion of atoms. A nonisotropic character of the diffusion for the interlayer and surface iron atoms caused by structural anisotropy of a clay substrate and by the structure of the iron complexes themselves was found. The surface diffusion of iron dimers occurs in a bounded area (L ~ 0.25 nm) and is rotational.     

Simultaneous Acquisition of Mössbauer Spectra and Microbalance Data in Situ: A Possible New Technique for the Study of Iron Corrosion and Oxidation

Abstract

Mössbauer spectra were obtained from an iron foil suspended from the arm of a microbalance. The collection of simultaneous Mössbauer and microbalance information appears to provide a powerful in situ technique for the study of iron corrosion and oxidation.     

Crystal structure, 57Fe Mössbauer spectroscopy and magnetization of UxFe6Sn6 (0≤x≤0.6)

The U_xFe₆Sn₆ (0≤x≤0.6) intermetallics were prepared as polycrystalline samples by arc melting followed by annealing. They crystallize in the P6/mmm space group as a lacunary variant of the HfFe₆Ge₆-type, with an ordered insertion of U within the Sn₈ voids of the FeSn framework, and are isostructural with the R_xFe₆Sn₆ compounds (R=Sc, Zr, Tm, Yb, Lu). ⁵⁷Fe Mössbauer spectroscopy shows that the Fe magnetic moments, μ_Fe, are already ordered at room temperature. Anisotropic contributions to the total hyperfine field at the Fe site indicates that μ_Fe are perpendicular to the crystallographic c axis in contrast to the Sc, Zr and rare-earth (except Yb) containing compounds, where μ_Fe are directed along the c axis. The values deduced for the angles between the main axes of the electric field gradient and μ_Fe further show that, within each (001) plane, μ_Fe have a ferromagnetic arrangement similar to that in FeSn and Yb_0.6Fe₆Sn₆. In U_xFe₆Sn₆, the hyperfine fields increase with U content. Magnetization data in the 2–300 K show small ferromagnetic components for the U_0.4Fe₆Sn₆ and U_0.6Fe₆Sn₆ compositions.     

Mössbauer spectroscopy study of MgAl2O4-matrix nanocomposite powders containing carbon nanotubes and iron-based nanoparticles

Materials involved in the catalytic formation of carbon nanotubes are for the first time systematically studied by Mössbauer spectroscopy between 11 K and room temperature. Mg_1−xFe_xAl₂O₄ (x=0.1, 0.2, 0.3, 0.4) solid solutions are transformed into carbon nanotubes–Fe/Fe₃C–MgAl₂O₄ composite powders by reduction in a H₂–CH₄ gas mixture. The oxides are defective spinels of general formulae (Mg_1−x²⁺Fe_x−3α²⁺Fe_2α³⁺□_αAl₂³⁺)O₄²⁻. Ferromagnetic α-Fe, ferromagnetic Fe₃C and a γ-Fe form, the latter possibly corresponding to a γ-Fe–C alloy, are detected in the composite powders. An attempt is made to correlate these results with the microstructure of the powder. It seems that the nanoparticles, which catalyze the formation of the carbon nanotubes, are detected as Fe₃C in the post-reaction Mössbauer spectroscopy analysis.     

In situ synchrotron X-ray diffraction study of stress-induced martensitic transformation in a metastable β-type Ti-33Nb-4Sn alloy

In this study, the stress-induced martensitic (SIM) transformation of a recently developed metastable β-type Ti-33Nb-4Sn alloy consisting of a mixture of β and α″ phases are investigated by in situ synchrotron X-ray diffraction (SXRD). It is shown that though the SIM transformation covers a wide strain range, some remaining β phase is still observed after loading, indicating that the SIM transformation is incomplete. During SIM, the parameter of b_α″ increases with macroscopic strain within the strain range of 1.5–4.7%, while the parameters of a_α″ ([100]_α″) and c_α″ ([002]_α″) remain constant or decrease with increasing strain, respectively. This provides a plausible explanation for why the (020)_α″ peak intensifies, but the (002)_α″ peak decreases and even eliminates in the loading direction during loading. Additionally, the activation sequence of different deformation mechanisms is clarified unambiguously.     

Effect of carbon,chromium, and nickel on the α ⇄ γ transformation and properties of Cr - Ni - Mo - V steels

The aim of the present investigation is to determine some regular features of alloying Cr - Ni - Mo - V steels for critical parts with allowance for the structure, hardenability, level of properties, and semibrittleness temperature. For this purpose the concentration of carbon in Cr - Ni - Mo - V steels was varied from 0.03 to 0.47%, chromium from 0 to 4%, and nickel from 0 to 5% at a constant concentration of the other elements (0.5% Mo, 0.6% Mn, 0.25% Si, -0. 1% V, 0.015% S, 0.012% P), and the cooling rate from the austenitization temperature was varied from 25 to 1000°C/h.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 7 – 9, March, 1996.