A. V. Protasov

Solid-state reactions upon mechanical alloying of an Fe32Al68 binary mixture

The sequence of solid-state reactions that occur upon mechanical alloying of powder mixtures of Al and Fe taken in an atomic ratio of 68: 32 has been studied by the methods of X-ray diffraction analysis, Mössbauer spectrometry, and Auger spectrometry. Upon the formation of a nanocrystalline state (<10 nm), there takes place a mutual penetration of Al atoms into Fe and Fe atoms into Al particles. The rate of consumption of the fcc Al is substantially higher than that of the bcc Fe. The process of the mechanical alloying (MA) was found to be two-stage. At the first stage, up to 2 at % Fe is dissolved in the fcc Al, and an amorphous Fe25Al75 phase is formed in the interfaces, whose amount reaches 70 at % at the finish of the initial stage. In the interfaces of the α-Fe phase, a disordered bcc phase of composition Fe66Al34 is formed, which contains up to 12 at % Al segregates. At the second stage, the amorphous phase crystallizes into an orthorhombic intermetallic compound Fe2Al5. The residual α-Fe, bcc Fe66Al34, and segregated Al form a bcc phase of composition Fe35Al65.

Magnetic properties and structure of nanocrystalline FINEMET alloys with various iron contents

The effect of the composition and annealing temperature on the structure and magnetic properties of soft magnetic nanocrystalline Fe-Cu-Nb-Mo-Si-B alloys has been studied. An increase in the iron content compared to that in the traditional FINEMET alloy is shown to allow one to increase the magnetic induction by 18% at a coercive force of no less than 6 A/m. It has been found that, along with the amorphous phase, rapidly quenched ribbons of alloys enriched in Fe contain crystalline α-Fe-based phase precipitates, the (100) crystallographic directions of which are perpendicular to the ribbon plane. Thermomagnetic analysis and differential scanning calorimetry were used to determine the temperatures of structural and magnetic phase transformations of the alloys with different iron contents. It was found that the separation of amorphous phase into areas of different compositions precedes the precipitation of nano-sized soft magnetic Fe-Si phase grains in the rapidly quenched iron-enriched ribbons.

Deformation-induced structural transformations in Si and the initial stage of mechanical alloying of Si and Fe

The methods of X-ray diffraction, Mössbauer spectroscopy, IR spectroscopy, and laser diffractometry are employed to study the changes in the structure and phase transformations that accompany mechanical alloying of Si and 57Fe used in an atomic ratio of 99: 1. It is established that the process comprises the development of a nanocrystalline state of Si with crystallite sizes smaller than 10 nm; the formation of an amorphous phase of Si at particle surfaces and in near-boundary distorted zones of interfaces in Si nanostructure; the penetration of 57Fe atoms along grain boundaries; and the formation of Si-Fe clusters, the local environment of Fe atoms in which is typical of a deformed α-FeSi2 phase, with these clusters being located in the interfaces.

Structural state and magnetic properties of cementite alloyed with manganese

Using X-ray diffraction analysis, Mössbauer spectroscopy, and magnetic measurements, the structure, parameters of hyperfine interactions, localization of Mn atoms in the lattice, coercive force, and specific saturation magnetization have been investigated in the mechanically alloyed and annealed cementite (alloyed with manganese) of compositions (Fe1 − xMnx)3C (x = 0–0.12). It has been shown that strongly deformed cementite resides in the low-coercivity state and, after annealing in the vicinity of 500°C, in the high-coercivity state. Alloying with manganese reduces the coercive force, the specific saturation magnetization, and the Curie temperature of cementite. Inhomogeneities of the distribution of manganese atoms indicate the temperature dependence of the coercive force of mechanically alloyed and annealed cementite samples.

Mössbauer probe spectroscopy studies of initial stage of Al-Fe mechanical alloying

The initial stage of mechanical alloying (MA) in a binary mixture of powdered Al and 57Fe in an atomic ratio of 99: 1 has been studied using X-ray diffraction and 57Fe Mössbauer probe spectroscopy. The proposed microscopic model of MA includes the formation of a nanostructured state (∼15 nm) in FCC Al, the penetration of Fe atoms across Al grain boundaries, and the formation of isolated Fe atoms and Fe-Al clusters in boundary distorted zones of Al matrix interface similar to the local atomic environment in deformed Al6Fe and Al9Fe2 phases. Based on the published data, it is assumed that with increasing Fe concentration in the initial mixture, and, correspondingly, increasing amount of clusters, distorted FCC interface regions are transformed into amorphous phase.

Effect of silicon on the phase formation in mechanically activated systems based on Fe75C25: Mechanosynthesis of composite states

The methods of X-ray diffraction analysis, Mögsbauer spectroscopy, and measurement of dynamic magnetic susceptibility have been used to study stationary phase states that develop at the later stages of mechanical alloying in a planetary ball mill. In the Fe(100 − x)C(x), Fe(75)C(25 − x)Si(x), and Fe(75 − x)C(25)Si(x) (x ≤ 25) systems, the processes of phase formation are determined by the dynamic equilibrium between the crystalline and amorphous phases. Depending on the composition of the alloys, the conditions of this equilibrium are changed, which is reflected in the sets of the crystalline phases that are formed.

Effect of the nanocrystalline state and electrical resistance of Fe and Fe75Si25 powders produced by the method of high-energy ball milling on the frequency dispersion of microwave material parameters

The influence of the nanocrystalline state of Fe and Fe75Si25 particles and their electrical resistance on the microwave properties of composite materials that contain these particles has been investigated experimentally. The main factors that determine changes in the frequency dispersion of the permeability are the skin effect and the decrease in the internal field of anisotropy of the particles. In the case of Fe particles, the role of skin effect of prevails.

Studying mechanosynthesized Hägg carbide (χ-Fe5C2)

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 TC = 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 han of the field of hyperfine interaction. Under the effect of han, the crystallographically equivalent atoms Fe(4e) become nonequivalent (Fe(e1) and Fe(e2)) 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(e1) and p(e2) with equal fractions of iron atoms in each of the contributions fFe(e1) = = fFe(e2) = 0.1 with the magnitudes of the fields H ≈ 11 and 16 T, respectively.

Dynamic equilibria of phases in the processes of the mechanosynthesis of an alloy with composition Fe72.6C24.5O1.1N1.8

X-ray diffraction, Mössbauer spectroscopy, and measurements of the dynamic magnetic susceptibility have been used to investigate phase states of the Fe72.6C24.5O1.1N1.8 alloy at different stages of the mechanosynthesis (MS) in a planetary ball mill. The introduction of impurities of O and N into an Fe75C25-based alloy changes the sequence of the formation of phases during MS: instead of Fe3C, the Fe7C3 carbide is first to be formed. The processes of phase formation in the alloy preliminarily subjected to MS have unidirectional nature upon the continuation of the MS and upon annealings and are determined by the interaction of the alloy components with one another under the effect of the accumulated excess energy. The phase compositions of the MS alloys depend on the conditions of the dynamic equilibrium between the crystalline and amorphous phases.

Probe Mössbauer spectroscopy of the evolution of mechanically alloyed Mo92O8(57Fe) system upon heat treatment

X-ray diffraction and Mögsbauer spectroscopy have been used to study the multistage character of the process of recovery to equilibrium in the course of isochronous (1 h) annealings (300–1300°C) of a mechanically alloyed nanocrystalline Mo92O8 system with 1 at % Mögsbauer isotope 57Fe. Three stages of the recovery to the equilibrium state have been established: at 300–700°C, the stage of structural relaxation; at 700–1100°C, the stage of normal grain growth and formation of a dislocation structure; and at 1100–1300°C, the stage of the formation of a composite Mo9957Fe1/MoO2.