E. P. Yelsukov

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.

Phase composition, structural state, and magnetic properties of nanocomposites of composition (Fe,Cr)75C25: Mechanosynthesis and isochronous annealings

The effect of the alloying element Cr on the structure, parameters of hyperfine interactions, and magnetic properties of the nanocomposites of compositions (Fe1 − xCrx)75C25, where x = 0.01–0.10, which was obtained by mechanical synthesis with subsequent annealing, has been investigated. It has been established that alloying with chromium reduces the coercive force, saturation magnetization, and Curie temperature of cementite. At the same time, alloying with chromium increases the thermal stability of cementite.

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 chromium-alloyed cementite in the process of mechanosynthesis and subsequent annealings

Mössbauer spectroscopy and magnetic measurements have been used to investigate the formation of phases in the process of mechanosynthesis of a nanocomposite of composition (Fe0.93Cr0.07)75C25 in a planetary ball mill with the subsequent annealings at various temperatures. It has been shown that, at the initial stage of the mechanosynthesis, an amorphous Fe-C phase is formed. Later, an amorphous Fe-Cr-C phase is formed and, in the process of the subsequent mechanosynthesis, an alloyed cementite is formed from it. It has been found that, due to the products of the pickup from the milling balls made of ShKh15 steel, the changes in the initial composition (Fe0.93Cr0.07)75C25 of the powders that reach 5 wt % have almost no effect on the phase composition, specific saturation magnetization, and the coercive force of samples annealed at 800°C for 1 h.

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 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.

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.

Mössbauer study of the mechanical alloying of highly concentrated Fe–Cr alloys

Mössbauer spectroscopy and X-ray diffraction are used to investigate the kinetics of the mechanical alloying (MA) of Fe and Cr powdered mixtures with Cr contents of 20 to 48 at % in the initial mixtures. Variations during mechanical alloying in specimens with Cr contents of ≤30% and >30% in the initial mixtures are observed for the first time. After MA, specimens are characterized by heterogeneous concentration distributions of Cr and Fe atoms in particles, especially at Cr concentrations of >30% in the initial mixture.