Исследуются магнитные свойства и сверхтонкие взаимодействия в карбиде Fe7C3, полученном путем механосинтеза (МС) -Fe в толуоле. Установлено, что граница температурной устойчивости МС-карбида не превышает 775 К. Температура Кюри синтезированной фазы изменяется от 509 до 525 К. Данные мессбауэровских экспериментов (57Fe) показывают, что элементарная ячейка Fe7C3 содержит 32 атома Fe, локализованные по пяти кристаллографическим позициям с мультиплетностью: 4 : 8 : 8 : 8 : 4. Для каждого неэквивалентного положения рассчитаны параметры сверхтонкого взаимодействия. Особенности распада при T 775 К позволяют предполжить, что наиболее вероятной последовательностью фазовых переходов в МС-карбидах Fe–C является последовательность: ? , где ? – модифицированный карбид Экстрёма–Одкокка Fe7C3.

Methods of thermomagnetic analysis and Mössbauer experiments (57Fe) were used to investigate the formation of Hägg carbide (χ-Fe5C2) under the conditions of mechanical milling of α-Fe in a medium of liquid hydrocarbons. It has been established that, with the employed parameters of milling, the synthesis of χ carbide begins after the completion of the stage of the formation of cementite (θ phase). The borderline of temperature stability of the monophase state of the χ carbide has been determined to be no more than 800 K. At T > 800 K, χ carbide decomposes into cementite and free carbon. The optimum temperature of heating of the synthesized Hägg carbide at which the population of the crystallographically nonequivalent positions of the Fe atoms is close to the ideal (0.2: 0.4: 0.4) is 775 K; the Curie temperature is T C = 520 K. The analysis of the Mössbauer data and of the results of a geometrical simulation of the configurations of Fe atoms in the the χ carbide unit cell made it possible to establish that the above relationship between the populations of positions is satisfied with the allowance for the anisotropic component h an of the field of hyperfine interaction. Under the effect of h an, the crystallographically equivalent atoms Fe(4e) become nonequivalent (Fe(e 1) and Fe(e 2)) in the magnetic sense. This specific feature manifests in the appearance in the presence of the distribution of hyperfine fields P(H) of two Mössbauer contributions, i.e., p(e 1) and p(e 2) with equal fractions of iron atoms in each of the contributions f Fe(e 1) = = f Fe(e 2) = 0.1 with the magnitudes of the fields H ≈ 11 and 16 T, respectively.

Methods of differential thermal analysis and Mössbauer spectroscopy ( 57 Fe) have been used to study the process of the formation of cementite in α-Fe upon the low-temperature mechanosynthesis ( T < 375 K) in the medium of liquid hydrocarbons. It has been established that this process occurs in the absence of austenite and corresponds to a two-stage model suggested previously for describing the mechanism of the decomposition of the quench martensite α″-(Fe)C with the precipitation of the θ phase in the process of aging or tempering. Upon dilatometric studies of the single-phase samples of cementite in the range of 750 ≤ T ≤ 925 K, a significant increase was revealed in the linear elongation Δ l / l 0 and linear thermal expansion coefficient (LTEC). It has been assumed that this increase is not connected with the localization of carbon atoms in the positions C(4 a ) and C(4 b ), but rather is determined by the anomalously high concentration of equilibrium carbon vacancies V C in the unit cell of cementite. The concentration of this type of vacancies can be sufficient for the growth of a graphite component of the carbon layer on the surface of the particles of the mechanosynthesized cementite (θ phase).

Методами термомагнитного анализа (ТМА) и мёссбауэровской спектрометрии (57Fe) исследовалась карбонизация -Fe в условиях механического измельчения в среде жидких углеводородов.Установлено, что при использованных условиях синтеза карбидов процесс карбонизации при Т сС( ) > сС( . Установлена граница температурной устойчивости цементита. Обнаружен эффект распада -фазы Fe3C при термическом циклировании в области 300 < Т < 1075 К. На основании полученных данных представлена схема последовательности фазовых превращений, протекающих в системе Fe–C в условиях низкотемпературного механосинтеза.

Methods of thermomagnetic analysis (TMA) and Mössbauer spectrometry (57Fe) have been used to study the processes of the carburizing of α-Fe under the conditions of mechanical milling in a medium of liquid hydrocarbons. It has been established that, under the chosen conditions of the mechanical synthesis of carbides, the process of carbonization at T < 375 K occurs through the decomposition of the deformation-induced martensite, i.e., the supersaturated bct solid solution α″-Fe(C) with the formation of transitional hcp ε and ε′ phases that precede the formation of cementite. The milling of the metallic iron in the toluene medium substantially enhances the catalytic capability of disperse powders of α-Fe in the process of conversion of cyclic structures of hydrocarbons into other chemical forms. The increase in the dispersity of the iron powder to a nanocrystalline state leads to an increase in the chemical activity of carbon and an increase in the rate of diffusion sufficient for the formation in the Fe-C mixture of both primary cementite (θ′) with an anomalously low Curie temperature T C(θ′)(first stage) and secondary cementite (θ″) at the second stage of mechanosynthesis. The parameters of hyperfine interactions have been calculated for a number of synthesized carbides. It has been shown that the change in the carbon concentration in iron carbides is determined by the following inequality: c C(θ′) > c C(ε) > c C(ε′). The boundary of the temperature stability of cementite has been established. The effect of the decomposition of the θ phase (Fe3C) upon thermal cycling θ ⇔ γ in the temperature range of 300 < T < 1075 K has been revealed. Based on the results obtained, a scheme of the sequence of phase transformations that occur in the Fe-C system under the conditions of low-temperature mechanosynthesis has been derived.

The potential of the extended local atomic displacement approximation (ELADA) is investigated in application to the description of the mechanism of the effect of the diverse atomic position occupancy (DAPO effect), which manifests itself in the “absorption and precipitation” of α-Fe in Fe 23 B 6 without a change in the temperature of the magnetic transformation T C of the phase in the process of compacting of powders and subsequent annealing of the compacts at T ≥ 325 K, respectively. It is shown that the nature of the observed effect is determined by the localization of Fe(4 a ) atoms in the crystallographic positions Fe(4 b ) of the fcc unit cell of the metastable Fe 23 B 6 phase (diverse atomic position occupancy (DAPO)).

Magnetic properties and hyperfine interactions in the carbide Fe 7 C 3 prepared by mechanical synthesis (MS) of α-Fe in toluene have been studied. It has been found that the boundary of temperature stability of the MS carbide does not exceed 775 K. The Curie temperature of the synthesized phase varies between 509 and 525 K. The Mössbauer experiments ( 57 Fe) have demonstrated that the unit cell of Fe 7 C 3 contains 32 iron atoms distributed over five types of crystallographic positions with a multiplicity of 4: 8: 8: 8: 4. The hyperfine interaction parameters have been calculated for each nonequivalent position. It may be conjectured from the specific features of the decomposition at T ≥ 775 K that the most probable sequence of the phase transitions in the MS carbides Fe-C is → θ, where -Adcock carbide Fe 7 C 3 .

Experimental results that indicate an unusual state detected in the fcc structure of the metastable Fe23B6 phase upon compacting and additional annealing are presented. This state manifests itself macroscopically in the form of the processes of α-Fe dissolution and precipitation without changing the magnetic-transition temperature T C of the mechanically synthesized alloy. The Mössbauer data of the mechanically synthesized Fe23B6 alloy suggest that the nature of this phenomenon is determined by changes in the parameters of the local surrounding of iron atoms in crystallographic position Fe(4a) in the fcc unit cell.

The possibility of application of the local atomic displacement approximation (LADA) for describing the low-field part ( H hf ≤ 24.0 T) of the hyperfine-field distribution P ( H ) in the metastable Fe 23 B 6 phase produced by mechanical alloying has been studied. Within the LADA, the appearance of Mössbauer contributions that are not typical of the fcc Cr 23 C 6 -type structure in the P ( H ) function of the mechanically alloyed Fe 23 B 6 phase is related to a decrease in the interatomic spacings by Δ r Fe-B / r Fe-B ≤ 0.10 for some local configurations of iron and boron atoms within the unit cell.

The possibility of application of the local atomic displacement approximation (LADA) for describing the low-field part (H-hf <= 24.0 T) of the hyperfine-field distribution P(H) in the metastable Fe23B6 phase produced by mechanical alloying has been studied. Within the LADA, the appearance of Mossbauer contributions that are not typical of the fcc Cr23C6-type structure in the P(H) function of the mechanically alloyed Fe23B6 phase is related to a decrease in the interatomic spacings by Delta r(Fe-B)/r(Fe-B)<= 0.10 for some local configurations of iron and boron atoms within the unit cell.

The possibility of application of a local atomic displacement (LAD) model for describing the mechanism of formation of a low-field part ( H hf ≤ 23.6 T) of a hyperfine-field distribution P ( H ) in a Fe 2 B alloy at early stages of grain refinement has been investigated. It has been found that the appearance of Mössbauer contributions, which are not typical of the tetragonal C 16 structure, in the P ( H ) function of the Fe 2 B alloy is induced by a decrease in the relative interatomic distances Δ r Fe-B / r Fe-B to ≤0.18 for some configurations of iron and boron atoms in the unit cell.

Mössbauer spectroscopy was used to study hyperfine interactions in the metastable phase Fe 23 B 6 that was for the first time obtained in macroscopic amounts in an almost single-phase state. Parameters of 57 Fe(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 Fe 23 B 6 compounds are determined by the modification of the parameters of local surroundings of Fe atoms in positions 48 h and 32 f of the unit cell.

The structure and magnetic propertiesof metastable fcc phases Fe23B6 and o-Fe3B produced by mechanical synthesis from a mixture of Fe and B powders have been studied. The compositions of the phases synthesizedare nonstoichiometric; the phases exist in a range of boron concentrations of 20 <= c(B) <= 23 AT %. In this composition range, the lattice parameter of the fcc Fe23B6 phase a varies from 1.06628(2) to 1.06373(2) nm. The lattice parameters of the o-Fe3B phase (with c(B) >= 22 at %) are a = 0.58612(4), b = 0.66761(3), and c = 0.44673(2) nm. The Curie temperature varies for the fcc Fe23B6 phase in the composition ranbge under study as T-C = 101 + 26.1 c(B); the magnetization sigma(H-i = 0) decreases from 190 to 153 A m(2)/Kg. The curie temperature of the synthesized orthorhombic phase Fe3B is close to 785 K; the magnetization is sigma congruent to 187 A m(2)/kg.

It has been established, using Mossbauer spectroscopy and magnetic measurements, that the compound Fe2B refers to a class of layered canted magnets with a canting angle of 7.4degrees and the magnetic moment at the Fe atom mu(Fe)(4.2 K) of about 2.00 mu(B.)

The properties of a metastable alpha modification of FeB obtained by mechanical synthesis of Fe and B powders were investigated by X-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. The existence of a structure of the B-f (CrB) type with the parameters of the orthorhombic cell of a = 0.248, b = 0.7579, and c = 0.2972 nm was confirmed for these compounds. The parameters of hyperfine interactions were refined to be H-hf = 10.19 T, IS = 0.312, and QS = 0.149 mm/s. The Curie temperature T-C for the low-temperature modification of FeB was found to be 550 +/- 5 K and the spontaneous magnetization sigma(0) (4.2 K) was found to be similar to89.4 A m(2)/kg. It is assumed that one of the principal factors that determine the stability of the B-f structure in the Fe-B system in a wide range of temperatures is the deviation of the composition of the alpha phase from the equiatomic one within the limits FeB1.04-1.10.

The distribution of iron cations in the crystal lattice of the Fe3−vO4 (v=0.153) cation-deficient spinel produced by mechanical dispersion of α-Fe2O3 hematite in water is investigated using x-ray diffraction and Mössbauer spectroscopy. Analysis of the Mössbauer data shows that the Fe2.847O4 magnetite prepared by mechanochemical synthesis is a chemically heterogeneous compound. The crystal structure of Fe2.847O4 is characterized by local environments of the (Fe2.5+)0 cations at v0≤0.1, v1≅0.12, v2≅0.18, and v3≅0.26, which are responsible for a broad distribution of magnetic hyperfine fields with the P(H) probability maxima near 37.0, 36.0, 34.0, and 30.0 MA m−1.