Physics of Metals and Metallography (v.118, #1)

The anticommutative Green’s functions and the energy spectrum of fullerene C60 have been calculated using the Hubbard model in the mean-field approximation. Based on the obtained energy spectrum, the interpretation of the experimentally observed bands of optical absorption has been proposed and the parameters of this fullerene, by which it is characterized in the Hubbard model have been calculated. The results agree fairly well with the experimental data.
Keywords: Hubbard model; Green’s functions; energy spectrum; nanosystems; C60 fullerene

Effect of anisotropy of elastic energy on the phonon propagation in single-crystal nanowires made of Fe, Cu, MgO, InSb, and GaAs materials that are used to fabricate spintronics devices in the regime of the Knudsen flow of phonon gas has been studied. A new method of analyzing the focusing of quasi-transverse modes has been suggested, which made it possible to determine the average values of the densities of phonon states in the regions of focusing and defocusing slow and fast quasi-transverse modes. The effect of phonon focusing on the anisotropy of heat conductivity and lengths of the phonon free paths has been analyzed for all acoustic modes that exist in spintronics nanostructures. It has been shown that for all the nanowires investigated the angular dependences of the free paths of fast and slow transverse modes in the {100} and {110} planes correlate with the angular dependences of the densities of phonon states for these modes. Directions of the heat flux that ensure the maximum and minimum phonon heat conductivity in the nanowires have been determined.
Keywords: spintronics; nanowires; heat conductivity; boundary scattering of phonons

The original version of the exchange-striction model of a ferrimagnet has been employed for calculating a number of magnetic properties of RCo2 ferrimagnets, where R = Er, Ho, Dy, Tb, and Gd are rareearth ions. The following magnetic properties are calculated: pressure dependence of the Curie temperature (Т С), temperature dependences of magnetization in sublattices of cobalt and rare-earth atoms, and isotherms of magnetization of these lattices at Т > Т С. For an ErСо2 sample, the Н–Т phase diagram has been constructed and the magnetization in the magnetic fields Н = 0–70 Т has been calculated. The calculated and experimental results have been compared. Based on the exchange-striction model, the qualitative explanation of the difference in the type of the magnetic phase transformation in the intermetallic compounds with R = Tb and Gd and R = Er, Ho, and Dy is given.
Keywords: intermetallic compounds; magnetic properties; exchange-striction model; rare-earth metals

Simulation of hydrogen diffusion in TiH x structures by A. S. Rokhmanenkov; A. Yu. Kuksin; A. V. Yanilkin (28-38).
Quantum-mechanical calculations of the energies of the formation of Frenkel pairs and barriers for hydrogen migration via different mechanisms in the titanium hydride δ-TiH x and in the α phase of titanium have been carried out. Using the potential of interaction developed for the molecular-dynamic simulation, diffusion coefficients of hydrogen in fcc and hcp lattices of TiH x were calculated depending on the temperature. The opportunity to approximate the coefficients of hydrogen self-diffusion has been analyzed in terms of the model of non-interacting point defects. For δ-TiH x , the range of concentrations and temperatures was separated where the self-diffusion of hydrogen becomes liquid-like (ceases be dependent on the hydrogen concentration), upon the transition into which there takes place a sharp increase in the isochoric heat capacity. The effect of defects in the Ti sublattice on the coefficient of self-diffusion of H has been investigated.
Keywords: titanium hydride TiH x ; hydrogen diffusion; model of point defects; ADP potential

The effect of the composition and cooling rate of the melt on the microhardness, phase composition, and fine-structure parameters of as-cast and splat-quenched (SQ) high-entropy (HE) Al–Cu–Fe–Ni–Si alloys was studied. The quenching was performed by conventional splat-cooling technique. The cooling rate was estimated to be ~106 K/s. Components of the studied HE alloys were selected taking into account both criteria for designing and estimating their phase composition, which are available in the literature and based on the calculations of the entropy and enthalpy of mixing, and the difference between atomic radii of components as well. According to X-ray diffraction data, the majority of studied Al–Cu–Fe–Ni–Si compositions are two-phase HE alloys, the structure of which consists of disordered solid solutions with bcc and fcc structures. At the same time, the Al0.5CuFeNi alloy is single-phase in terms of X-ray diffraction and has an fcc structure. The studied alloys in the as-cast state have a dendritic structure, whereas, after splat quenching, the uniform small-grained structure is formed. It was found that, as the volume fraction of bcc solid solution in the studied HE alloys increases, the microhardness increases; the as-cast HE Al–Cu–Fe–Ni–Si alloys are characterized by higher microhardness compared to that of splat-quenched alloys. This is likely due to the more equilibrium multiphase state of as-cast alloys.
Keywords: high-entropy alloy; splat quenching; phase composition; structure; microhardness

Microstructure of the β-phase in the Sn11Sb5.5Cu babbit by A. Kh. Valeeva; I. Sh. Valeev; R. F. Fazlyakhmetov (48-51).
It has been shown that radial-shear rolling leads to the formation of a recrystallized structure in the β-phase crystals in the Sn11Sb5.5Cu babbit. Moreover, small grains form inside a β-phase crystal when the degree of deformation (reduction) increases.
Keywords: microstructure; babbit; EBSD analysis

Structural and phase transformations and micromechanical properties of the high-nitrogen austenitic steel deformed by shear under pressure by A. V. Makarov; S. N. Luchko; V. A. Shabashov; E. G. Volkova; A. L. Osintseva; A. E. Zamatovskii; A. V. Litvinov; V. V. Sagaradze (52-64).
Using transmission electron microscopy, X-ray diffraction analysis, Mössbauer spectroscopy, microdurometry, and microindentation, the effect of large plastic deformations (through shear under pressure in Bridgman anvils) on the structure, phase composition, and micromechanical properties of high-nitrogen (1.24 wt % N) 08Kh22GA1.24 steel has been investigated. The steel was obtained by the casting method with counterpressure of nitrogen and was subjected to different heat treatments (quenching from1180°С, aging at 450 and 550°С) that form an austenitic (FCC) structure of the metallic matrix with chromium nitrides. It has been established that deformation by shear under pressure at room temperature results in the dispersion and deformation-induced partial dissolution of primary nitrides Cr2N in quenched and aged steel and in the complete (after aging at 450°С) and partial (after aging at 550°С) dissolution of secondary nitrides CrN. It has been noted that, for aged steel that contains finely dispersed secondary chromium nitrides upon shear deformation, as compared to the quenched state, the dispersion of the austenitic structure (down to nano- and submicrocrystalline states) is more intense and the enhancement in the microhardness and resistance to elastic–plastic deformations upon contact loading is more effective.
Keywords: austenitic steel; nitrogen; shear under pressure; nanostructurizing; dissolution of nitrides; micromechanical properties

Deformation of nanocrystalline binary aluminum alloys with segregation of Mg, Co and Ti at grain boundaries by A. V. Zinovev; M. G. Bapanina; R. I. Babicheva; N. A. Enikeev; S. V. Dmitriev; K. Zhou (65-74).
The influence of the temperature and sort of alloying element on the deformation of the nanocrystalline (NC) binary Al alloys with segregation of 10.2 at % Ti, Co, or Mg over grain boundaries has been studied using the molecular dynamics. The deformation behavior of the materials has been studied in detail by the simulation of the shear deformation of various Al bicrystals with the grain-boundary segregation of impurity atoms, namely, Ti, Co, or Mg. The deformation of bicrystals with different grain orientation has been studied. It has been found that Co introduction into grain boundaries of NC Al has a strengthening effect due to the deceleration of the grain-boundary migration (GBM) and difficulty in the grain-boundary sliding (GBS). The Mg segregation at the boundaries greatly impedes the GBM, but stimulates the development of the GBS. In the NC alloy of Al–Ti, the GBM occurs actively, and the flow-stress values are close to the values characteristic of pure Al.
Keywords: molecular dynamics; aluminum alloys; segregation; grain-boundary sliding; grain-boundary migration

Formation of the structure and properties of β-type titanium alloy upon thermomechanical treatment by O. P. Shaboldo; Ya. M. Vitorskii; V. V. Sagaradze; N. L. Pecherkina; M. A. Skotnikova (75-80).
The rapid quenching of β-type titanium alloy from 800°C and cold deformation by drawing (ε = 24%) leads to the formation of a cellular-banded structure with a cell size of 200 × 400 nm and high density of dislocations (~5 × 1014 m–2). During subsequent aging at 450°C, the decomposition of the β-phase occurs with a heterogeneous precipitation (at dislocations) of plates of the α phase with a thickness of 10–30 nm and length of 50–100 nm. The small size and high density of α crystals (5 × 1021 m–3) provide a substantial increase in the strength characteristics of the alloy.
Keywords: β-type titanium alloy; cold deformation; aging; α phase; electron microscopy; mechanical properties; strengthening

Based on the results of phase physicochemical analysis of high-carbon chromium–vanadium steel, the predominant type of carbide that provides high wear resistance has been established, and its amount and amount of carbon in martensite have been determined. Data on the composition and the amount of carbide phase and on the chemical composition of the martensite of high-carbon steel have been obtained, which allows determination of the alloying-element concentration limits. The mechanical testing of heats of a chosen chemical composition has been carried out after quenching and low-temperature tempering. The tests have demonstrated benefits of new steel in wear resistance and bending strength with the fatigue strength being retained, compared to steels subjected to cementation. The mechanism of secondary strengthening of the steel upon high-temperature tempering has been revealed. High-temperature tempering can be applied to articles that are required to possess both high wear resistance and heat resistance.
Keywords: phase physicochemical analysis; chemical composition; high-carbon steel; wear resistance; martensite; strengthening; heat resistance

This work is devoted to the effect of processes initiated by the combined action of aging (A) and accumulative roll bonding (ARB) on the evolution of the microstructure and the mechanical characteristics of an Al–0.2 wt % Zr alloy. Upon solution treatment (ST), followed by aging at temperatures of 350 and 450°C, the specimens were subjected to deformation to a degree of deformation of 80% using ARB. The evolution of the microstructure was examined using atomic force microscopy and the mechanical characteristics of the specimens were determined using tensile tests and Vickers microhardness measurements. The results have shown that, upon ten ARB cycles, the grain size decreased to 0.3, 0.4, and 0.32 μm in the specimens subjected to ST followed by ARB (ST–ARB), ST followed by A at a temperature of 350°C and ARB (350°C–A–ARB), and ST followed by A at a temperature of 450°C and ARB (450°C–A–ARB), respectively. This study has also shown that the combined use of preliminary A and subsequent ARB holds promise in enhancing the mechanical characteristics of the alloy due to precipitates that appear in the course of annealing. Fracture surfaces of the rolled specimens subjected to the tensile tests were examined using scanning electron microscopy. The results of these examinations have shown that in the specimens subjected to ST followed by ARB brittle fracture has been observed at the stage of the final ARB cycles, while in the A–ARB specimens cleavage facets (sites of fracture over the cleavage plane) and river lines have appeared on the fracture surfaces.
Keywords: aging; accumulative roll bonding; microstructure; mechanical characteristics; fracture

Formation of microstructure and the superplasticity of Al–Mg-based alloys by A. A. Kishchik; A. V. Mikhaylovskaya; V. S. Levchenko; V. K. Portnoy (96-103).
The influence of a 3–10% content of magnesium in Al–Mg–Mn(Cr) alloys on the characteristics of the microstructure of sheet blanks and their superplasticity has been examined. It has been shown that the minimum size of grains and the best superplasticity are characteristic of the alloy that contains about 7% magnesium and is additionally alloyed simultaneously with manganese and chromium. An increase in the content of magnesium leads to the formation of conglomerates of particles of a chromium–manganese phase and, as a result, to a coarsening of the grain structure and a deterioration of superplasticity.
Keywords: aluminum alloy; magnaliums; grain size; superplasticity