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Methods of thermomagnetic analysis and Mossbauer experiments (57Fe) were used to investigate the formation of Hagg 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 Hagg 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 Mossbauer 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 Mossbauer 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 Mossbauer spectroscopy (57Fe) 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 Δl/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(4a) and C(4b), 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).

Methods of thermomagnetic analysis (TMA) and Mossbauer 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 cC(e) > cC(e′). 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 Fe23B6 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(4a) atoms in the crystallographic positions Fe(4b) of the fcc unit cell of the metastable Fe23B6 phase (diverse atomic position occupancy (DAPO)).

Magnetic properties and hyperfine interactions in the carbide Fe7C3 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 Mossbauer experiments (57Fe) have demonstrated that the unit cell of Fe7C3 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 Fe7C3.

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 TC of the mechanically synthesized alloy. The Mossbauer 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 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 Fe2B alloy at early stages of grain refinement has been investigated. It has been found that the appearance of Mossbauer contributions, which are not typical of the tetragonal C16 structure, in the P(H) function of the Fe2B alloy is induced by a decrease in the relative interatomic distances Δr Fe-B/r Fe-B to ≤0.18 for some configurations of iron and boron atoms in the unit cell.

Mossbauer 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 Mossbauer 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.