A. L. Ul’yanov

Microwave absorbing properties of Fe powders milled in various media

Principal factors determining the microwave-absorption material parameters (shape, size, and chemical and phase compositions of the particles) and their dispersion relations in a range from 0.1 to 3 GHz were determined for composites containing milled Fe particles as the filling agent. The basic physical mechanisms of the effect of the aforementioned factors were assumed to be the domain-wall resonance and ferromagnetic resonance.

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.

The initial stage of mechanical alloying in Cr-Fe binary systems

The initial stage of mechanical alloying in Cr-Fe binary systems with atomic ratios of 80: 20 and 99: 1 (57Fe) has been studied by Auger spectroscopy, X-ray diffraction, and Mössbauer spectroscopy. Cr(Fe)xOy oxide clusters are formed at the sites of the contact between Cr and 57Fe particles at the earliest stages of mechanical treatment of the Cr(99)/57Fe(1) mixture. As the treatment duration is increased and a nanostructured state develops, oxide clusters are destroyed and O dissolves in the Cr matrix to increase the lattice parameter of Cr. As the nanostructure is formed in Cr, Fe atoms penetrate through the grain boundaries into the close-to-boundary distorted zones and then into the grain bulk. The process is characterized by a nonuniform concentration distribution of Fe atoms in Cr. The yield of reaction products and specific surface area of grain boundaries have been established to linearly depend on the mechanical energy dose at the initial stage of mechanical alloying.

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.

Initial stage of mechanical alloying in a binary system with composition Si70Fe30

Mössbauer spectroscopy and X-ray diffraction have been used to study the sequence of solidstate reactions that occur upon the mechanical alloying of mixtures of Si and Fe powders taken in an atomic ratio of 70: 30 in a planetary ball mill. In the course of the formation of a nanocrystalline state, the interpenetration of Si atoms into Fe particles and of Fe atoms into Si particles occurs. In the Si particles, clusters with a local neighborhood of Fe atoms that is characteristic of the deformed α-FeSi2 phase are formed. In the Fe particles, clusters of the ɛ-FeSi and the β-FeSi2 type arise. With increasing time of mechanical treatment, second phases of α-FeSi2 in Si particles and of ɛ-FeSi and β-FeSi2 in Fe particle are formed. In the latter case, a reaction ɛ-FeSi + Si → β-FeSi2 occurs up to the complete disappearance of the ɛ-FeSi phase if the mixture under study is not contaminated by the material of the vessel (Fe) and balls.

On the problem of the cementite structure

Evolution of the crystalline structure of cementite that was formed in contact with α-Fe upon heat treatments of a mechanically alloyed iron-amorphous Fe-C phase nanocomposite has been studied using X-ray diffraction and Mössbauer spectroscopy.

The mechanism and kinetics of initial stage of mechanical alloying in Si(Al, Mg, Cr)9957Fe1 binary systems

X-ray diffraction and Mössbauer probe spectroscopy data on mechanically alloyed binary systems of the Si(Al, Mg, Cr)9957Fe1 composition have been analyzed within the framework of the energetic approach. It has been established that the initial stage of mechanical alloying is preceded by a “preparatory” period, during which grains of a main component (element) reach critical size Lcr, followed by the alloying process per se. It has been shown that, the maximum of derivative dN/d(logD) (N is the yield of reaction products, and D is the mechanical energy dose), with the maximum corresponding to grain size Lmax = 17 nm irrespective of the type of the main component, is a universal characteristic when considering the kinetics of the initial stage of alloying. The chemical interaction of a main component with Fe has been found to play the decisive role in the kinetics of mechanical alloying, with the significance of this role increasing in a series Mg, Al, Si, and Cr. A phenomenological explanation has been given to the linear N(D) dependence at the linear dependence of the specific surface area of grain boundaries on the mechanical energy dose.

Structural and phase transformations during heat treatment of the Fe(71.4)Si(14.3)C(14.3) amorphous alloy prepared by mechanical alloying

Differential scanning calorimetry, X-ray diffraction, Mössbauer spectroscopy of 57Fe nuclei, magnetic measurements, and different heat treatments have been used to study the sequence and mechanisms of solid-state reactions in the Fe-Si-C amorphous alloy in the course of the structure transition to equilibrium. Three stages of structural and phase transformations have been found; these are the structural relaxation, formation of an Fe5SiC silicocarbide, and its decomposition. It has been shown that the second and third stages occur during isochronous annealing within sufficiently narrow temperature ranges of 380–405 and 530–555°C, respectively. The kinetics of the decomposition of the metastable Fe5SiC silicocarbide and the formation of the ordered Fe-Si alloy during isothermal annealing has been studied.

Deformation-induced dissolution of the intermetallic compound FeSn in nanocrystalline α-Fe

The structure and phase transformations during the mechanical treatment of an α-Fe98Sn2/FeSn mixture with a total Sn content of 14 at % in a ball planetary mill are studied by X-ray diffraction and 57Fe and 119Sn Mössbauer spectroscopy. Three stages of the dissolution of the intermetallic compound FeSn are found. The key role of a nanostructure with a grain size of ≤10 nm in this dissolution is shown, and a mechanism for its realization is proposed.

Mössbauer study of the local atomic structure of cementite

Mössbauer measurements of nanocrystalline (mechanically alloyed) and annealed at 525 and 775 K cementite have been performed at temperatures of 77 and 300 K. On the basis of the X-ray diffraction data, results of magnetic measurements, and calculations, it is concluded that carbon atoms are located in the octahedral and prismatic positions of nanocrystalline cementite and only in the prismatic positions of the cementite annealed at 775 K.