A Mössbauer spectroscopy study of Fe based cemented carbides

Mössbauer spectroscopy has been used as a novel characterization technique to investigate Fe charge states, Fe complexes and hyperfine interaction parameters of different phases in WC-10Fe and WC-10(FeNi) materials sintered at three different temperatures (1350, 1430 and 1510 °C). The materials were characterized using standard cemented carbide quality control, and spectroscopic techniques to evaluate the structural changes and magnetic effects of the binder. The WC-10Fe grade had the highest Vickers hardness ranging from 1282 to 1320 HV₃₀, for the different sintering temperatures. X-ray diffraction data showed the presence of WC and the metal binder phase, α-Fe and γ-FeNi. Transmission Mӧssbauer spectroscopy spectra obtained for the milled powders revealed only the α-Fe phase with a hyperfine magnetic field, B_hf ~33 T. Conversion electron Mössbauer spectroscopy on the sintered compacts revealed the presence of multiple fields, suggesting the possibility of minor phases present in the binder which were not detected using X-ray diffraction. The corresponding spectra for the sintered WC-Fe grades exhibited two magnetic fields with hyperfine parameters of ~33 T and ~17 T, respectively. These fields were assigned to α-Fe with some W atoms in solution and a W-rich Fe phase, respectively. The Mӧssbauer spectrum for the FeNi binder sample at the lowest sintering temperature of 1340 °C showed a paramagnetic doublet with an isomer shift, δ = −0.08 mm/s and electric quadrupole splitting, ∆E_Q = 0.00 mm/s and a weak hyperfine magnetic field, B_hf = 15.7 T. The doublet has been assigned to γ-FeNi and the magnetic component to a W-rich γ-FeNi phase. For the higher sintering temperature, a distribution of magnetic fields (~33 T, 25 T and 9 T) was evident in the Mössbauer spectrum. These magnetic fields were tentatively assigned to multiple W-rich γ-FeNi phases.     

Hyperfine structure and electrochemical behavior of Fe-Cr-Ni alloys

With the use of nuclear γ-resonance Mössbauer spectroscopy, the hyperfine structure of quenched (1100° C, 1 h, water) Fe-～20% Cr-(9.6, 40.8, 70.7)% Ni alloys is studied. With the Normos program package, the isomer (chemical) shift Γ (mm/s) of the singlet and doublet signals, the half-width Γ (mm/s) of the singlet and doublet spectral components, the area part S (%) of the components in the general spectra, and the probability distribution function of hyperfine magnetic fields P(H_eff) are calculated. The effect of nickel on the electron density distribution around ⁵⁷Fe nuclei is discovered. Novel data on the presence of ferromagnetic phases, their nuclei and clusters in Fe-19.6% Cr-9.6% Ni are obtained. The data of Mössbauer spectroscopy are compared to the anodic polarization curves recorded in a 1 N H ₂SO₄ solution with the aim to clarify the relation between the electronic structure and electrochemical behavior of alloys     

Mössbauer investigations of the metastable Fe23B6 phase

Mössbauer spectroscopy was used to study hyperfine interactions in the metastable phase Fe₂₃B₆ that was for the first time obtained in macroscopic amounts in an almost single-phase state. Parameters of ⁵⁷Fe(300 K) spectra, as well as local and average magnetic moments at iron atoms have been calculated. It is indicated that the specific features of Mössbauer spectra and hyperfine-field distributions P(H) in mechanically synthesized Fe₂₃B₆ compounds are determined by the modification of the parameters of local surroundings of Fe atoms in positions 48h and 32f of the unit cell.     

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

In this work, we have studied the characteristics of regions in which atomic probes (APs) ⁵⁷Co(⁵⁷Fe) were diffusionally localized in polycrystalline iridium (poly-Ir) using a previously developed method based on Mössbauer spectroscopy. Poly-Ir becomes alloyed with oxygen during annealing even in an ultrahigh vacuum already at a temperature of 0.18T _m (T _m is the melting point of the matrix). After the annealing temperature reaches a certain value, there arises a “compensated” state of lattice regions adjacent to crystallite-conjugation regions (“adjacent zones,” or AZs) in poly-Ir. Such a state of AZs arises due to the mutual compensation of positive relaxation volumes of oxygen atoms and negative relaxation volumes of oxygen-vacancy complexes that are formed during each annealing. Therefore, in the “compensated” state of AZs the isomer shifts δ₂ of components 2 of Mössbauer spectra of ⁵⁷Fe APs become equal to “intrinsic” isomer shifts δ_{intrs, 2} of the Mössbauer spectra of ⁵⁷Fe APs located in the AZs of impurity-free metals. The “intrinsic” isomer shifts depend parabolically on the charges Z of the matrix-atom nuclei.     

Mössbauer spectroscopy of the nanocrystalline materials

X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements were used to study the magnetic properties and the parameters of hyperfine interactions in the nanocrystalline (with a grain size less than 10 nm) and microcrystalline alloys Fe₉₀Ge₁₀, Fe₇₇Al₂₃ and pure α-Fe. It has been established that the nanocrystalline state does not affect the formation of the saturation magnetization, Curie temperature, isomeric shift, and hyperfine magnetic field. No additional sextets in the Mössbauer spectra and no additional features in the temperature dependences of dynamic magnetic susceptibility of the alloys investigated have been revealed. In the Mössbauer spectrum of pure nanocrystalline iron, a slight line broadening (～20%) is observed.     

The formation of structure and phase composition and magnetic properties of Fe(Fe<Subscript>3</Subscript>C,Fe<Subscript>5</Subscript>SiC)-SiO<Subscript>2</Subscript> nanocomposites upon mechanical alloying

Structural and phase transformations that occur in Fe(Fe₃C, Fe₃SiC)-SiO₂ (amorphous quartz) systems during mechanical alloying in an Ar atmosphere and in air have been studied by X-ray diffraction, Mössbauer spectroscopy, IR spectroscopy, electron microscopy, and magnetometry. It has been shown that the mechanoactivation treatment leads to the formation of isolated particles 2–20 nm in size with a complex phase composition (Fe, FeSi alloy, oxides, silicates, and carbides), which depends on the milling atmosphere. It has been found out that the magnetic properties of such systems strongly depend on the oxygen and carbon compounds existing in the system, which cannot be detected by X-ray diffraction, but their presence is testified by the data of Mössbauer spectroscopy.     

Are ferric chloride intercalated graphite compounds air stable?

Mössbauer spectroscopy has been used as a tool to analyze the effect of long term exposure to air on ferric chloride intercalated graphite samples. After periods of about a year three phenomena occur in sequence: there is a migration of some FeCl₃ from the sample, some of the FeCl₃ is converted into (FeCl₄)^? and finally the sample becomes unstaged.     

X-ray and intrinsic magnetic properties of nanocrystalline Sm2(Fe,M)17 (M = Si,Ga, Co,Cr, Zr or Mo)

The structure and magnetic properties of ball-milled and subsequently annealed Sm₂Fe_17–xM_x compounds (M = Si, Ga, Co, Cr, Zr, Mo, for x = 2) were investigated by X-ray diffraction coupled with magnetic measurements and Mössbauer spectroscopy. Rietveld analysis have shown that all these samples crystallize in the Th₂Zn₁₇ type rhombohedral structure. The substituting elements Si, Ga, Co occupy the 18h atom site, while Cr, Zr, Mo prefer the 6c site. Mössbauer studies corroborate these results. The X-ray analysis cannot solve the possible M atom distribution over Fe atom sites when atomic factors of Fe and M are close. Mössbauer spectroscopy appears as the efficient tool to raise this ambiguity. The Curie temperature increases upon Zr and Cr substitution reaching a maximum for x = 1, then decreases because of magnetic dilution.     

N–N interaction and nitrogen activity in the iron base austenite

The Monte Carlo computer simulation of the f.c.c. Fe–N alloy is performed using the data of abundances of different iron sites in the austenite lattice as obtained by means of Mössbauer spectroscopy. The validity of the interpretations of available Mössbauer spectra was tested based on the data of calculation of N–N interaction energies in the two first coordination spheres on the interstitial sublattice, which could satisfy the experimental data of nitrogen distribution in austenite derived from Mössbauer spectra. It is shown that no values of N–N interaction energies exist that could be consistent with the data of conversion electron Mössbauer spectroscopy (CEMS), where a thin surface layer only contributes to the Mössbauer spectra. A strong N–N repulsion (>0.14 eV) in the first coordination sphere and a soft N–N repulsion (<0.07 eV) in the second one was found to be consistent with the studies performed by means of transmission Mössbauer spectroscopy (TMS), according to which single nitrogen atoms and 180° N–N pairs exist in the nitrogen austenite. A possibility of the long-range order-like Fe₄N is shown at the energy values determined for two coordination spheres, provided a small N–N repulsion in the third coordination sphere occurs. It is also observed that the available data of nitrogen activity in Fe–N austenite are not consistent with the values of N–N interaction energies determined from the analysis of Mössbauer spectra, i.e. the available activity data do not correspond to the nitrogen distribution in the iron austenite as it follows from Mössbauer data. The concentration dependence of nitrogen activity and an effect of the N₂ gas pressure on the nitrogen solubility in Fe–N austenite are calculated using the values of N–N interaction energies in two coordination spheres.     

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.     

Anwendung der Mößbauer-Spektroskopie auf die Untersuchung von Deckschichten in Naturumlaufkesseln

Application of Mößbauer spectroscopy to the study of surface layers in natural convection boilers Qualitative and quantitative analysis of oxide layers on tubes in steam generators have been performed by ⁵⁷Fe Mössbauer Spectroscopy in transmission and backscattering geometry. The tubes were exposed to the following conditions for ca. 127000 hours: 120–174 atm., 320–353 °C and 9.5–10.5 pH; the water contained < 10 and < 4 mg/l of P₂O₅ and SiO₂, respectively (The results of the chemical analysis of the tube materials were: 15 Mo 3 : 0.12% C, 0.15% Si, 0.5% Mn, 0.04% P, 0.04% S, ? 0.3% Cr, 0.25% Mo, rest Fe; St. 45.8. III: < 0.22% C, 0.1% Si, ? 0.45% Mn, 0.05% P, 0.05% S, ? 0.3% Cr, rest Fe). The Mössbauer investigations showed that the protective oxide layers contained mainly non-stoichiometric magnetite (Fe_3?xO₄; x ? 0.03), partly in microcristalline form (<500 Å). In addition to magnetite, hematite (α-Fe₂ O₃) was also detected in one of the samples. The composition of the top surface layers (～5000 Å) was studied by Conversion Electron Mössbauer Spectroscopy (CEMS). The high phosphate content in two of the four investigated samples can be probably attributed to apatite or hydroxylapatite.     

Mössbauer spectroscopy of interphase boundaries of Co/CoO bilayers

Co films and Co/CoO bilayers that were deposited by the method of magnetron sputtering on the MgO(100) and Al₂O₃(110) single-crystal substrates have been studied using electron microscopy and Mössbauer spectroscopy. The surface, bulk, and interphase Mössbauer spectra of the layers and bilayers under examination have been investigated. It is shown that the ⁵⁷Co(⁵⁷Fe) atoms located in the region of a Co/CoO interphase boundary can be in different magnetic states and have different valences and chemical surroundings.     

On the problem of chromium effect on the passivability of Fe-Cr alloys

The hyperfine magnetic and electronic structure of annealed iron alloys containing 9.0, 11.8, and 12.7% chromium is studied using of Mössbauer spectroscopy. Probability distribution functions of hyperfine magnetic fields P(H_eff), mean-square intensities of the effective magnetic field H _{eff, m-sq}, average isomer (chemical) shifts δ, and other spectral parameters are calculated with the use of computer modeling. Data from Mössbauer spectroscopy are compared to the results of polarization measurements in 1 N H₂SO₄ solution at room temperature in order to find the correlation between the electronic structure and the passivability of the alloys studied.     

Fe/V multilayers studied by CEMS

Fe/V multilayers prepared by sputtering has been studied by conversion electron Mössbauer spectroscopy. An extended series of samples of the Fe(x)/V(y) system has been studied both at room temperature and at 10 K. The series consisted of samples with x=3 ML (monolayers), 5 ML, 7 ML, 10 ML and 20 ML and y=5 ML, 10 ML and 14 ML. The obtained hyperfine field distributions point to quite diffuse interfaces, where multilayers with Fe layer thickness less than 5 ML are mainly non-magnetic. The influence of the spacer layer thickness on the Fe hyperfine field has also been investigated. The results indicate that interlayer coupling may influence the hyperfine field distribution positively.     

Evolution of structural changes in nanocrystalline alloys with temperature

Structural features of the NANOPERM-type alloys Fe_{91 ? x} Mo₈Cu₁B _x with x = 12, 15, and 17 have been investigated by Mössbauer spectroscopy. The room-temperature Mössbauer spectra of the as-quenched alloys are characteristic of disordered structural arrangement, but traces of bcc-Fe(Mo) as well as a FeMo₂B₂ phase have been revealed by X-ray diffraction in all the samples. These results have been confirmed by conversion-electron Mössbauer spectroscopy. The differences between the opposite sides of the ribbon-shaped samples have been shown to stem from structural distinctions. From the point of view of hyperfine interactions, the x = 12 sample exhibits paramagnetic behavior. With increasing x, a contribution from ferromagnetic regions appears gradually, thus leading to an increase in the magnetic ordering temperature in the as-quenched state. Partially crystallized samples have been prepared by controlled annealing of the original precursors for one hour at temperatures ranging from 330 to 650°C in a vacuum. The temperature of the onset of crystallization has been determined to be of 430, 450, and 470°C for x = 12, 15, and 17, respectively. During the first step of crystallization, bcc-Fe(Mo) nanosized grains are formed. Surface features of the samples investigated have also been characterized by using atomic force microscopy.     

The effect of alloying molybdenum additives on the hyperfine structure and corrosion-electrochemical behavior of Fe-Cr alloys

With the use of Mössbauer spectroscopy, the effect of alloying molybdenum additives on the hyperfine magnetic and electronic structure of iron alloys containing ～21 and 28% chromium is studied. Probability distribution functions of hyperfine magnetic fields P(H _eff), intensities of the effective magnetic field H _eff, isomer (chemical) shifts δ, second-to-third spectral line area ratios (W ₂₃), and other spectral parameters are calculated with the use of the Normos program package. The data of Mössbauer spectroscopy are compared to the corrosion-electrochemical behavior of the alloys.     

The oxygen defect perovskites Ba3La3Cu6O14+y: a progressive transition from semi-conductive to semi-metallic properties. II. Electron transport properties

Magnetic and electric transport properties of the oxygen defect perovskites Ba₃La₃Cu₆O_14+y (0.05?y?0.43) have been investigated in the temperature range 100 – 450 K. They showed a large increase in conductivity with the inserted oxygen concentration y. The positive sign of the Seebeck coefficient in the whole temperature range and the variations of the magnetic susceptibility and electric conductivity with temperature have been interpreted in the Mott model of quasi localized holes trapped at the top of a filled 3d_x²?y² band.     

Mössbauer study of the process of the room-temperature aging of the alloy Cu<Subscript>79</Subscript>Ni<Subscript>14</Subscript>Fe<Subscript>7</Subscript>

Mössbauer spectroscopy was used to investigate the initial stage of the phase separation in the quasi-binary system Cu₇₉Ni₁₄Fe₇ and the subsequent transformation of the alloy structures as a result of prolonged aging at room temperature. For describing the Mössbauer spectra of ferromagnetic particles, which appear upon the spinodal decomposition in a paramagnetic matrix, a model was proposed and approved, which uses particle-size distribution in the approximation of the generalized Lifshitz-Slezov-Wagner (LSW) model and of the linear decrease of the hyperfine field at the ⁵⁷Fe nuclei in the near-surface layers of spherical particles.     

Atmospheric corrosion of Ni-advanced weathering steels in marine atmospheres of moderate salinity

One conventional and three Ni-advanced weathering steels have been exposed for one year in two marine atmospheres of moderate aggressivity (30 and 75 mg Cl⁻/m² d). The rusts generated have been analysed by polarised light optical microscopy, SEM, XRD, Raman spectroscopy and Mössbauer spectroscopy.The presence of high nickel (1–3% weight) contents in the steel composition leads to higher corrosion resistance in moderate marine atmospheres. The presence of nickel in the weathering steel also raises the proportion of nanophasic (superparamagnetic) goethite in the inner rust layer.     

Mössbauer studies of the thermal stability of layered metallic systems

Methods of Mössbauer spectroscopy at ⁵⁷Fe nuclei and X-ray diffraction analysis have been used to investigate layered systems Fe (5 μm)-Zr (2 μm) and Fe_0.966Ti_0.034(13 μm)-Ti (1 μ)-⁵⁷Fe(0.8 μm) obtained by the method of ion-plasma deposition and subjected to sequential isothermal annealings at T = 900 and 650°C, respectively. The relative content of phases that are formed in the systems at each stage of annealing has been established and the sequence of transformations and their kinetics have been determined. For both systems, thermal stabilization of the structural and phase state, which is inhomogeneous in depth, has been observed.