S. F. Lomayeva

Structural and phase transformations, thermal stability, and magnetic and corrosive properties of nanocrystalline iron-based alloys obtained by mechanoactivation in organic media

A review of works on the investigations of structural and phase transformations that occur in iron during its mechanical disintegration in the presence of liquid organic media. It is shown that the mechanoactivation of metals in the presence of organic media leads to the formation of a nanocrystalline structure of complex phase composition which is determined by the chemical nature of the milling medium and can include (in different proportions) α-Fe, amorphous phases, carbides, oxides, etc. By varying the composition of the medium, the duration of milling, and the temperature of a subsequent heat treatment, it is possible to obtain nanocrystalline systems with different structural and phase composition, dispersity, and thermal, magnetic and corrosive properties. It has been shown that for the investigation of processes that take place in nanocrystalline materials, it is necessary to use, apart from methods of analysis of the composition and structure of substances at the mesoscopic level (electron microscopy, X-ray diffraction, etc.), also methods which give information at the level of local atomic environment. In this work, Mössbauer spectroscopy was used for the systems on the basis of iron.

Electrochemical behavior of Co3C carbide

The electrochemical properties of the Co3C carbide have been studied. In contrast to metallic cobalt, its carbide is characterized by higher electrocatalytic activity in the hydrogen evolution reaction and low rate of anodic oxidation in acidic sulfate solutions. The oxidation in borate solutions (pH = 6.3–9.0) results in the formation of oxide films different in phase composition. In the case of metallic cobalt, a passive film consisting of CoO and Co(OH)2 forms, while the cobalt carbide oxidation results in the formation of the Co3O4 film. The oxidation of cobalt carbide is an irreversible process accompanied by the accumulation of carbon on the surface.

The electrochemical properties of Ni3C carbide

The electrochemical properties of Ni3C was studied. In acidic sulfate solutions, the carbide is characterized by high overpotential of its oxidation as compared with nickel. In the case of carbide oxidation, the anodic reaction orders with respect to anions are low, indicating a weak dependence of the rate of the anodic process on the solution composition. Significant differences in the kinetics of the anodic processes indicate different mechanisms of the oxidation of nickel and its carbide. The rate and kinetic parameters of the hydrogen evolution reaction are comparable on Ni and Ni3C. In neutral and alkaline solutions, the metal and carbide samples were similar in their electrochemical characteristics. The anodically grown oxide film is thinner on nickel carbide than on nickel metal, and the oxide formed on the carbide is more readily reduced under cathodic polarization. This film is also more resistant to the pitting attack than the oxide film on nickel metal.

Microwave properties of powders produced by high-energy milling of iron with paraffin

The structural and phase composition and magnetostatic and microwave properties of powders produced by simultaneous milling of carbonyl iron and paraffin taken in volume ratios ranging from 10 : 90 to 80 : 20 have been studied. It has been shown that the shape of the powder particles depends substantially on the amount of Fe in the mixture. At an Fe content of 30 vol %, the particles acquire a platelet shape; at an Fe content of more than 40 vol %, the agglomeration of the particles is observed. At an Fe content above 30 vol %, the Fe3C phase is formed in the particles, which leads to an increase in the coercive force and a decrease in the specific saturation magnetization of the powder. The frequency dependences of the microwave magnetic and dielectric permeabilities of the composite materials containing the prepared powders have been measured, and the frequency dependences of the intrinsic magnetic permeability of these powders have been determined. It has been shown that the static magnitude of the intrinsic magnetic permeability reaches a maximum in powders containing 30 vol % Fe in the initial mixture.

Cementite formation in copper matrix under mechanical activation using carbon media

We studied the structure and phase transformations of nanocrystalline alloys prepared with mechanical alloying (MA) and annealing by milling copper and iron powders with graphite and xylene. At an early stage of MA, a supersaturated solid solution of iron is formed in copper, regardless of the carbon-precursor type used. In the case of graphite, the formation of iron carbides occurs at a later stage of milling. MA in xylene results in an insignificant amount of carbon phases. Heat treatment leads to the formation of nanocrystalline copper composites with 30 vol.% Fe3C in the two cases of using graphite and xylene. The grain size (30 nm) of the annealed (800 °C) Cu + Fe3C composite produced by MA with xylene is five times less than that of the annealed sample produced with graphite.

Formation of interfacial layer in Fe-polystyrene system during mechanochemical synthesis in the presence of alkyl surfactants

The structure of surface organic layers formed during the mechanical milling of Fe with polystyrene and alkyl surfactants is studied using NEXAFS and FT-IR spectroscopies. It is shown that the main constituents of the layers formed are condensed aromatic structures (products of the dehydrogenation of an organic medium) that contain covalently bound polystyrene fragments.

Corrosion-electrochemical properties of nanocomposites of α-Fe+Fe3C+VC in acidic and alkaline sulfate solutions

This work studies corrosion-electrochemical properties of three-phase composites of α-Fe+ Fe3C + VC in acidic and alkaline sulfate solutions. Nanosize carbide inclusions are characterized by a high level of activity in the reaction of cathodic hydrogen evolution and result in a high rate of composite dissolution in acidic solutions. The presence of carbides and carbon has a negative effect on passivation of the iron matrix. The resistance of vanadium carbide inclusions to anodic oxidation increases upon a decrease in the amount of carbon in the carbide.

Microwave Properties of Mechanosynthesized Fe-Paraffin Nanocomposites

The structure and microwave properties of composites consisting of paraffin as a dielectric host matrix and Fe particles as inclusions have been studied. The composites were prepared by the mechanochemical synthesis and contain 7.7% vol. concentration of the inclusions. The microwave properties were studied in 0.1 to 6 GHz frequency range. The microwave properties of the composites are shown to depend mostly on the size and shape of the inclusions, with the phase composition of the inclusions exerting only a slight effect. With decreasing the inclusions size, the low-frequency peak (300−400 MHz) attributed to the domain wall motion diminishes.

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.

Structural and phase transformations during copper and iron mechanical alloying in liquid medium studied by Mössbauer spectroscopy

Mössbauer spectroscopy and X-ray diffraction have been used to study the kinetics of structural and phase transformations in Cu + 2 at% 57Fe during mechanical activation in liquid media (heptane, distilled water) and subsequent heat treatment (600 and 700 °С). The initial stages of mechanical alloying are associated with the transition of components to the nanostructural state. Iron atom groups form near the grain boundaries, and isolated iron atoms penetrate from the boundaries into the grains. Oxidation of groups of iron atoms that form highly dispersed phases of ternary oxide and magnetite occur in the initial stages of mechanical alloying of Cu + 2 at% 57Fe in water. The formation of the solid solution in the form of isolated iron atoms in the lattice of copper proceeds, regardless of the milling media used. Samples prepared in heptane contain carbon and oxygen, and upon heat treatment, carbide and oxide phases are formed.