Physics of Metals and Metallography (v.114, #11)

Effect of magnetism on the solution energy of 3p (Al, Si) and 4p (Ga, Ge) elements in iron by M. V. Petrik; O. I. Gorbatov; Yu. N. Gornostyrev (885-892).
The method based on the density-functional theory has been used to study the solubility of 3p (Al, Si) and 4p (Ga, Ge) elements in ferromagnetic and paramagnetic states of bcc iron. To simulate the paramagnetic state, two different approaches have been employed, which were implemented using the SIESTA and LSGF packages. It has been established that the solution energy of all these impurities decreases upon the transition into the paramagnetic state. The solution energies obtained by averaging over the ensemble of unpolarized magnetic configurations agree well with the values obtained in the coherentpotential approximation. At the same time, the allowance for the magnetic polarization in the vicinity of an impurity leads to a decrease in the solution energy, which is most clearly pronounced at temperatures close to T C. The temperature dependence of the solution energies of the impurities in the paramagnetic state is discussed.
Keywords: soft magnetic materials; iron alloys; paramagnetic state; first-principles calculations; solubility of impurities

Magnetic and magnetocaloric properties of (MnCo)1 − x Ge compounds by P. E. Markin; N. V. Mushnikov; E. G. Gerasimov; A. V. Proshkin; A. S. Volegov (893-903).
The crystal structure, magnetic properties, and heat capacity of the (MnCo)1 − x Ge compounds with x ≤ 0.05 have been studied. It was found that, as the deviation from the MnCoGe stoichiometric composition increases, the temperature of structural transition from the low-temperature phase with the orthorhombic TiNiSi-type structure to the high-temperature phase with the hexagonal Ni2In-type phase decreases rapidly, whereas the magnetic ordering temperature varies slightly. The temperature of structural transition for the composition with x = 0.02 approximately coincides with the Curie temperature of the hexagonal phase, and the transition is accompanied by a significant entropy change, namely, ΔS = 34 J/(kg K). The application of high magnetic field in the transition-temperature range causes an increase in the relative volume of the orthorhombic phase. An analysis of magnetocaloric properties of these compounds, which was performed with the formal application of the Maxwell’s relationship near the temperature of first-order structural phase transition, is shown to give overestimated values of the entropy change.
Keywords: intermetallic compounds; structural transition; X-ray diffraction; heat capacity; magnetocaloric effect

Optical properties of heusler alloys Co2FeSi, Co2FeAl, Co2CrAl, and Co2CrGa by E. I. Shreder; A. D. Svyazhin; K. A. Belozerova (904-909).
The results of an investigation of optical properties and the calculations of the electronic structure of Co2FeSi, Co2FeAl, Co2CrAl, and Co2CrGa Heusler alloys are presented. The main focus of our attention is the study of the spectral dependence of the real part (ɛ1) and imaginary part (ɛ2) of the dielectric constant in the range of wavelengths λ = 0.3–13 μm using the ellipsometric method. An anomalous behavior of the optical conductivity σ(ω) has been found in the infrared range in the Co2CrAl and Co2CrGa alloys, which differs substantially from that in the Co2FeSi and Co2FeAl alloys. The results obtained are discussed based on the calculations of the electronic structure.
Keywords: Heusler alloys; optical properties; electronic structure

The evolution of real and imaginary parts of the complex permittivity of hexagonal single crystals of RMnO3 manganites (R = Ho, Er, Tm, Yb) differing in the radius of rare-earth ion r R has been studied using spectroscopic ellipsometry in the range of 0.5–5.0 eV at room temperature. Spectra of dielectric functions show a strong polarization dependence. The optical-absorption edge for polarization Ec is determined by the intense narrow peak at 1.6 eV, whereas for polarization Ec, the peak is shifted toward high energies by 0.15–0.20 eV and its intensity is suppressed greatly. It has been shown that, when r R decreases, the energy position of the intense peak at 1.6 eV in the spectrum of the imaginary part of the dielectric function for polarization Ec shifts toward low energies by no more than 0.1 eV, which reflects changes in the local surroundings of the Mn3+ ion. For the both polarizations, a broad absorption band with the center at 2.4 eV has been revealed; the band was detected earlier in the antiferromagnetic phase in nonlinear spectra upon the optical generation of the second harmonic. Spectra of permittivity have been analyzed within available concepts on the electronic structure of hexagonal RMnO3 compounds and have been compared with corresponding spectra of previously studied orthorhombic RMnO3 compounds.
Keywords: hexagonal manganites; anisotropy of optical properties; ellipsometry

Effect of chemical composition of precursors and annealing temperature on structure and critical current of multifilament Bi,Pb-2223/Ag composites by D. N. Rakov; Yu. N. Belotelova; T. P. Krinitsina; E. I. Kuznetsova; Yu. V. Blinova; S. V. Sudareva; E. P. Romanov (918-927).
X-ray diffraction and transmission and scanning electron microscopy have been used to study ceramics of Bi,Pb-2223/Ag composites (with small deviations from stoichiometry) after standard treatment and annealings with some variations in temperatures compared to standard treatment (optimum annealings). The data obtained have been analyzed together with the results of measurements of the critical currents of the composites. A nonuniform distribution of the structure and components over the transverse section of ceramic filaments has been revealed after standard treatment. The content of carbon in the center of the filaments was higher by a factor of two or even greater than that in the subsurface layers adjacent to the Ag sheath. It is shown that each composition of the composite has its own optimum annealing temperature, which ensures a decrease in the carbon content in the center of the ceramic filaments and the creation of a structure that is more favorable for obtaining high critical currents.
Keywords: HTSC composites; structure inhomogeneities; carbon; critical currents

Structural and phase transformations that occur during mechanical alloying (MA) and subsequent annealing of nanocrystalline Cu-Cr-C alloys obtained from copper and chromium powders and graphite or xylene as the source of carbon have been studied. It is shown that, when using graphite, a supersaturated Cu(Cr) solid solution and an X-ray amorphous Cr-C phase are formed during MA. Heat treatment leads to their decomposition and the appearance of Cr3C2 in the nanocrystalline copper matrix. When xylene is used as the source of carbon, no strongly supersaturated Cu(Cr) solid solution and no X-ray amorphous Cr-C phase are formed, but the same volume fraction of chromium carbide, i.e., 20–24 vol %, appears. When graphite is used, the carbide is formed after shorter times of MA.
Keywords: mechanical alloying; nanocomposites; copper; chromium carbide

Structure of sputter-deposited films of β-tantalum-aluminum alloys by V. N. Volodin; Yu. Zh. Tuleushev; E. A. Zhakanbaev (935-939).
Ion-plasma sputtering and the codeposition of ultradisperse particles of Ta and Al have been used to prepare solid solutions in the entire range of concentrations of the binary system in the form of alloy coatings. The formation of the solid solution of these alloys directly in the process of codeposition confirms the theory of the thermofluctuation melting of small particles and coalescence of quasi-liquid clusters of subcritical size. Upon the formation of coatings via the deposition of nanolayers of tantalum of less than 0.8 nm for β-Ta and 1.1 nm for Al, the spontaneous mutual dissolution of the components occurs with the formation of solid solutions of one metal in the other. Beginning with a concentration of ∼85 at % Al in the alloy, Al atoms control the type of symmetry of the arising lattice. An increase in the characteristic dimensions (thickness of sublayers) of tantalum and aluminum leads to the appearance of solid solutions of these metals in these coatings in addition to the β tantalum and aluminum phases, as well as of amorphous regions and superlattices formed by nanoclusters of one metal in the matrix of the other. It has been established that the formation of these superlattices is controlled by the size factor.
Keywords: beta-tantalum; aluminum; structure; superlattice; films; size effect; solid solution

Effect of pulsed current on structure of Al-Mg-Si aluminum-based alloy during cold deformation by I. G. Brodova; I. G. Shirinkina; V. V. Astaf’ev; T. I. Yablonskikh; A. A. Potapova; V. V. Stolyarov (940-946).
The effect of the current density on the formation of the structure of the AD33 Al-Mg-Si alloy during deformation by rolling is studied. It is shown that, at a current density of 30 A/mm2, the electroplastic deformation by rolling (EPDR) of the AD33 alloy increases its ultimate strength. A decrease in the deformability of the material with increasing current density to 100 A/mm2 and higher is related to the melting of fusible eutectic, which leads to the appearance of microcracks at the grain and subgrain boundaries.
Keywords: aluminum alloy; electroplastic deformation by rolling; cold deformation; current density

Structure of boundaries in composite materials obtained using explosive loading by V. I. Lysak; S. V. Kuz’min; A. V. Krokhalev; B. A. Grinberg (947-952).
We have presented the results of studying the fine structure of interphase boundaries for a number of composite materials obtained by methods of explosive welding and explosive compacting of powder mixtures. Joints of different metals (titanium-low-carbon steel, copper-tantalum) and metals with refractory carbides (chromium carbide-titanium) have been investigated. Under welding, pairs differed from each other by the type of interaction. It has been found that, in these composites, interphase boundaries exhibit a final thickness on the order of 200 nm, throughout which the composition of the material changes gradually from a composition that corresponds to one of the components of the composite to a composition that corresponds to the second component. It has been shown that the structure of interphase boundaries is complex. With the limited solubility of components along boundaries, two fairly thick crystalline interlayers are detected, the total thickness of which is equal to the total thickness of the boundary; between the interlayers, there is a thin (to 5–7 nm in thickness) interlayer with a crystalline or amorphous structure.
Keywords: explosive welding; formation of joint; transition zone; interphase boundary

Initial stage of mechanical alloying in a binary system with composition Si70Fe30 by V. E. Porsev; D. A. Kolodkin; A. L. Ul’yanov; E. P. Elsukov (953-961).
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.
Keywords: silicon; iron; mechanical alloying; solid-state reactions; X-ray diffraction; Mössbauer spectroscopy

Studies have been performed on the effect of external compressive stress applied to samples of superplastic eutectic Bi-43 wt % Sn alloy during heating to a near-eutectic temperature on the specific heat of melting of the alloy. The alloy was prepared from chemically pure components by casting onto a massive copper substrate. After compression by ∼65% using a hydraulic press, the ingots were aged in air for approximately 7 months. The experiments were carried out using the method of differential thermal analysis. The samples were loaded using a specially designed device. A regular decrease in the specific heat of melting by ∼37% has been revealed while increasing the external pressure from 0 to 4.4 MPa. The experimental results are discussed using the available literature data on the heat of melting of the alloy and on the structure of the eutectic.
Keywords: specific heat of melting; eutectic alloy; superplasticity; heating; preliminary deformation; differential thermal analysis

Effect of spherically converging shock waves on phase and structural state of quenched Al-4 wt % Cu alloy by A. V. Dobromyslov; N. I. Taluts; A. N. Uksusnikov; E. A. Kozlov (968-976).
Optical metallography, scanning and transmission electron microscopy, and microhardness measurements have been used to perform a layer-by-layer study of the structure of a quenched Al-4 wt % Cu alloy subjected to loading by spherically converging shock waves. It has been established that, when using this mode of loading, the high-strain-rate plastic deformation of this alloy occurs via intragranular dislocation slip. No bands of localized deformation at the grain boundaries, twins, and adiabatic shear bands are formed. Highstrain-rate plastic deformation leads to the dissolution of Guinier-Preston zones.
Keywords: shock waves; high-strain-rate plastic deformation; Al-4% Cu alloy; structure