Structure,Magnetic and Magnetocaloric Properties of Nonstoichiometric TbCo<Subscript>2</Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Compounds

TbCo₂Mn_x (x ≤ 1) alloys were synthesized and their crystal structure, heat capacity, magnetic and magnetocaloric properties were studied. Single-phase compounds with the MgCu₂-type structure were formed at х < 0.4. In alloys with х > 0.4, additional phases with the PuNi₃- and Th₆Mn₂₃-type structures form. It was shown that there is a substantial increase in the Curie temperature and magnetic moment of 3d?metal sublattice of the nonstoichiometric compounds when compared to those of the TbCo₂ binary compound. The magnetocaloric effect of single- and multiphase alloys were estimated based on magnetic and heat capacity measurements.     

Hydrogen in the Ti<Subscript>3</Subscript>Al intermetallic compound: Study by the NMR method

The times of nuclear spin–lattice relaxation have been measured and the spectra of NMR for ¹H and ²⁷Al in Ti₃AlH _x hydrogenated intermetallic compounds with hydrogen concentrations of х = 0, 0.31, 0.51, 1.0, 2.0, and 4.32 in a wide range of temperatures (10–500 K) and resonance frequencies (8.2–90 MHz) have been recorded. The analysis of low-temperature relaxation data has shown that the addition of hydrogen with a concentration of х ≥ 2 leads to a significant change in the density of electron states at the Fermi level. It has been found that the hydrogen mobility in Ti₃AlH _x strongly depends on its concentration. For solid solutions of hydrogen in Ti₃Al with х < 1, the mobility of hydrogen atoms on the scale of frequencies of NMR in the investigated temperature range has not been detected. In Ti₃AlH _x compounds with х ≥ 2, the fast diffusion of hydrogen is observed, which can be related to changes in the structure of these compounds.     

Magnetoresistive properties of Ni-doped La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganites

La_0.7Sr_0.3Mn_1?x Ni _x O₃ (x = 0, 0.025, 0.050 and 0.075) ceramics were prepared by the conventional solid-state reaction method. The partial substitution of Mn by Ni²⁺ leads to a decrease in cell volume as well as a structural transition from the rhombohedral to the orthorhombic structure. Ni²⁺ doping increases the electrical resistivity, decreases the semiconductor–metal transition temperature (T _ms) and relatively enhances the room temperature magnetoresistance (MR), especially in x = 0.025 and around T _ms. With respect to conduction mechanism, the small polaron hopping (SPH) and the variable range hopping (VRH) models were used to examine conduction in the semiconducting region.     

Effect of silicon on the phase formation in mechanically activated systems based on Fe<Subscript>75</Subscript>C<Subscript>25</Subscript>: Mechanosynthesis of composite states

The methods of X-ray diffraction analysis, Mögsbauer spectroscopy, and measurement of dynamic magnetic susceptibility have been used to study stationary phase states that develop at the later stages of mechanical alloying in a planetary ball mill. In the Fe(100 ? x)C(x), Fe(75)C(25 ? x)Si(x), and Fe(75 ? x)C(25)Si(x) (x ≤ 25) systems, the processes of phase formation are determined by the dynamic equilibrium between the crystalline and amorphous phases. Depending on the composition of the alloys, the conditions of this equilibrium are changed, which is reflected in the sets of the crystalline phases that are formed.     

Properties of the Ti<Subscript>40</Subscript>Zr<Subscript>10</Subscript>Cu<Subscript>36</Subscript>Pd<Subscript>14</Subscript> BMG Modified by Sn and Nb Additions

The results of investigation of the influence of additions of 2 and 3 at.% of Sn and simultaneously of Sn and 3 at.% Nb on microstructure and properties of the bulk metallic glasses of composition (Ti₄₀Cu_36?x Zr₁₀Pd₁₄Sn _x )_100?y Nb _y are reported. It was found that the additions of Sn increased the temperatures of glass transition (T _g), primary crystallization (T _x ), melting, and liquidus as well as supercooled liquid range (ΔT) and glass forming ability (GFA). The nanohardness and elastic modulus decreased in alloys with 2 and 3 at.% Sn additions, revealing similar values. The 3 at.% Nb addition to the Sn-containing amorphous phase decreased as well all the T _g, T _x , T _L, and T _m temperatures as ΔT and GFA; however, relatively larger values of this parameters in alloys containing larger Sn content were preserved. In difference to the previously published results, in the case of the amorphous alloys containing small Nb and Sn additions, a noticeable amount of the quenched-in crystalline phases was not confirmed, at least of the micrometric sizes. In the case of the alloys containing Sn or both Sn and Nb, two slightly different amorphous phase compositions were detected, suggesting separation in the liquid phase. Phase composition of the alloys determined after amorphous phase crystallization was similar for all compositions. The phases Cu₈Zr_3, CuTiZr, and Pd₃Zr were mainly identified in the proportions dependent on the alloy compositions.     

Effect of Titanium Additions on the Thermophysical Properties of Glass-Forming Cu<Subscript>50</Subscript>Zr<Subscript>50</Subscript> Alloy

Differential scanning calorimetry, laser flash method, and dilatometry were used to study the thermophysical properties of quenched Cu₅₀Zr_50–xTi_x (x = 0, 2, 4, 6, 8) alloys in the temperature range from room temperature to 1100 K. Data obtained on the heat capacity, thermal diffusivity, and density have been used to calculate the coefficient of thermal conductivity. Temperatures corresponding to the stability of martensite CuZr phase, its eutectoid decomposition, and formation in Cu₅₀Zr_50–xTi_x alloys with different Ti contents upon heating have been determined. It has been found that the thermal diffusivity and thermal conductivity of the studied alloys are low and a typical of metallic systems. As the titanium content increases, the coefficients of thermal conductivity and thermal diffusivity vary slightly. It has been shown that the low values of thermophysical characteristics correspond to the better capability of amorphization and can be a criterion for the glass-forming ability of Cu–Zr-based alloys.     

Synthesis and Structural Characterization of Sb-Doped TiFe<Subscript>2</Subscript>Sn Heusler Compounds

Ternary Heusler compounds form a numerous class of intermetallics, which include two families with general compositions ABC and AB₂C, usually referred to as half- and full-Heusler compounds, respectively. Given their tunable electronic properties, made possible by adjusting the chemical composition, these materials are currently considered for the possible use in sustainable technologies such as solar energy and thermoelectric conversion. According to theoretical predictions, Sb substitution in the TiFe₂Sn full-Heusler compound is thought to yield band structure modifications that should enhance the thermoelectric power factor. In this work, we tested the phase stability and the structural and microstructural properties of such heavily doped compounds. We synthesized polycrystalline TiFe₂Sn_1?xSb_x samples, with x?=?0, 0.1, 0.2 and 1.0 by arc melting, followed by an annealing treatment. The structural characterization, performed by x-ray powder diffraction and microscopy analyses, confirmed the formation of the pseudo-ternary Heusler structure (cF16, Fm-3m, prototype: MnCu₂Al) in all samples, with only few percent amounts of secondary phases and only slight deviations from nominal stoichiometry. With increasing Sb substitution, we found a steady decrease in the lattice parameter, confirming that the replacement takes place at the Sn site. Quite unusually, the as-cast samples exhibited a higher lattice contraction than the annealed ones. The fully substituted x?=?1.0 compound, again adopting the MnCu₂Al structure, does not form as stoichiometric phase and turned out to be strongly Fe deficient. The physical behavior at room temperature indicated that annealing with increasing temperature is beneficial for electrical and thermoelectrical transport. Moreover, we measured a slight improvement in electrical and thermoelectrical properties in the x?=?0.1 sample and a suppression in the x?=?0.2 sample, as compared to the undoped x?=?0 sample.     

Changes in the temperatures of phase transformations of the Ni<Subscript>2</Subscript>MnGa alloy upon substitution of nickel for manganese

Temperatures of phase transformations of a number of Ni₂MnGa-based alloys (four compositions), which are characterized by substitution of nickel for manganese at an unchanged gallium content (25 at %), have been determined. As the nickel concentration increases with respect to the stoichiometric composition (or the electron concentration e/a increases), the liquidus (T _liq), solidus (T _sol), and martensitic transformation (T _mart) temperatures increase, whereas the magnetic transformation temperature (T _C) decreases slightly.     

Structure and phase transformations in copper-alloyed rapidly melt-quenched Ni<Subscript>50</Subscript>Ti<Subscript>32</Subscript>Hf<Subscript>18</Subscript>-based alloys with high-temperature shape memory effect

Methods of transmission and scanning electron microscopy, chemical microanalysis, electron diffraction, and X-ray diffraction have been used to carry out the comparative study of the structure and chemical and phase composition of thin ribbons of four quasi-binary alloys (Ni₅₀Ti₃₂Hf₁₈, Ni₄₅Ti₃₂Hf₁₈Cu₅, Ni₃₅Ti₃₂Hf₁₈Cu₁₅, and Ni25Ti32Hf18Cu25) obtained in the amorphous state by rapid quenching from the melt by jet spinning. The critical temperatures of the devitrification and B2 ? B19′ martensitic transformation of the alloys have been determined based on the data of temperature dependences of the electrical resistivity. The specific features of the formation of the ultrafine-grained structure upon the devitrification and of the phase transformations have been studied depending on the heat-treatment regimes and chemical composition of the alloys (concentration of copper atoms).     

Superior Mechanical Properties of AlCoCrFeNiTi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High-Entropy Alloys upon Dynamic Loading

High-entropy alloys with composition of AlCoCrFeNiTi _x (x: molar ratio; x = 0, 0.2, 0.4) under quasi-static and dynamic compression exhibit excellent mechanical properties. A positive strain-rate sensitivity of yield strength and the strong work-hardening behavior during plastic flows dominate upon dynamic loading in the present alloy system. The constitutive relationships are extracted to model flow behaviors by employing the Johnson-Cook constitutive model. Upon dynamic loading, the ultimate strength and fracture strain of AlCoCrFeNiTi _x alloys are superior to most of bulk metallic glasses and in situ metallic glass matrix composites.     

Short-range atomic order in Fe<Subscript>2</Subscript>B powders

The possibility of application of a local atomic displacement (LAD) model for describing the mechanism of formation of a low-field part (H _hf ≤ 23.6 T) of a hyperfine-field distribution P(H) in a Fe₂B alloy at early stages of grain refinement has been investigated. It has been found that the appearance of Mössbauer contributions, which are not typical of the tetragonal C16 structure, in the P(H) function of the Fe₂B alloy is induced by a decrease in the relative interatomic distances Δr _Fe-B/r _Fe-B to ≤0.18 for some configurations of iron and boron atoms in the unit cell.     

Microstructure and mechanical properties of multi-principal component AlCoCrFeNiCu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloy

By introducing Cu, AlCoCrFeNiCu _x (x values in molar ratio, x = 0, 0.1, 0.5, 1.0, 1.5, 2.0, and 2.5) alloys were designed and prepared. The effects of Cu on microstructure and properties of AlCoCrFeNi alloy were investigated. The introduction of Cu results in the formation of Cu-rich FCC solid solution phase when Cu content is low. There are two FCC solid solution phases, i.e., Cu-rich FCC solid solution phase and phase transformation-induced FCC solid solution phase, when the Cu content is more than 1.0. Both the yield stress and plastic strain of alloy show a turning point when the Cu content is 0.5. Among the seven alloys, Cu_0.5 alloy exhibits the largest yield stress of 1187 MPa and the lowest plastic strain of 16.01 %.     

Microstructure and Room-Temperature Mechanical Properties of FeCrMoVTi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High-Entropy Alloys

FeCrMoVTi _x (x values represent the molar ratio, where x = 0, 0.5, 1.0, 1.5, and 2.0) high-entropy alloys were prepared by a vacuum arc melting method. The effects of Ti element on the microstructure and room-temperature mechanical properties of the as-cast FeCrMoVTi _x alloys were investigated. The results show that the prepared alloys exhibited typical dendritic microstructure and the size of the microstructure became fine with increasing Ti content. The FeCrMoV alloy exhibited a single body-centered cubic structure (BCC1) and the alloys prepared with Ti element exhibited BCC1 + BCC2 mixed structure. The new BCC2 phase is considered as (Fe, Ti)-rich phase and was distributed in the dendrite region. With the increase of Ti content, the volume fraction of the BCC2 phase increased and its shape changed from a long strip to a network. For the FeCrMoV alloy, the fracture strength, plastic strain, and hardness reached as high as 2231 MPa, 28.2%, and 720 HV, respectively. The maximum hardness of 887 HV was obtained in the FeCrMoVTi alloy. However, the fracture strength, yield stress, and plastic strain of the alloys decreased continuously as Ti content increased. In the room-temperature compressive test, the alloys showed typical brittle fracture characteristics.     

Magnetic properties of <Emphasis Type="Italic">R</Emphasis>Co<Subscript>2</Subscript> compounds in the exchange-striction model of ferrimagnets

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.     

Low-temperature properties of the orderable Cu<Subscript>3</Subscript>Pd alloy

The effect of atomic ordering on the transport properties of the Cu₃Pd alloy in the low-temperature range (T < 70K) in strong magnetic fields (H ≤ 150 kOe) has been studied. The temperature dependence of the resistivity during alloy disordering has been found to have a minimum at T ～ 10 K. The Hall effect has a negative sign. The magnetoresistance, on the contrary, is positive; the longitudinal effect equals ～50% of the transverse one.     

Effect of Hydrogen Intercalation on the Structure of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> with a Low Oxygen Content

We used Raman spectroscopy to study the interaction of YBa₂Cu₃O_y (123) (y ≤ 6.5) with hydrogen at temperatures of 150–200°С. In contrast to the YBa₂Cu₃O_6.96 compound, compositions with low oxygen contents show a lower tendency to form stacking faults. We found that the hydrogenation of these compositions does not lead to the transformation of the 123 phase to the pseudo-124 phase. Absorbed hydrogen does not react with oxygen in 123 and does not form hydroxyl groups. The H_xYBa₂Cu₃O_y oxyhydride is the hydration-reaction product.     

Effect of partial substitution of calcium for yttrium on the structure and properties of the Y<Subscript>0.9</Subscript>Ca<Subscript>0.1</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.8</Subscript> superconductor

The crystal structure of the high-temperature Y_1–xCa_xBa₂Cu₃O_6.8 superconductor has been studied in a temperature range of 80–300 K using low-temperature X-ray diffraction analysis; its microstructure has been studied by scanning and transmission electron microscopy. Changes of the bond length in the structure of principal phase and precipitation topology of impurity phases and their compositions have been analyzed. An addition of calcium was shown to increase the environmental tolerance of the principal Y123 phase and its microhardness and ensures the low unchanged coefficient of thermal expansion. All of the facts indicate that the material can be used to manufacture composite superconducting articles.     

Superconducting cellular MgB<Subscript>2</Subscript>

A new composite superconducting material in the form of a cellular structure consisting of large Mg grains surrounded by thin MgB₂ layers has been prepared. The superconducting properties of such a cellular structure were found to depend on the thickness of the superconducting layer d _s. As d _s decreases from ～30 to 1 μm, the critical temperature decreases by more than 10 K. The derivative of the upper critical field with respect to the temperature (dH _c2/dT) near T _c increases from ～0.2 T/K at high d _s to ～0.35 T/K at d _s ～6 μm. The critical current density of the cellular samples calculated for the area of the superconductor section is above 10⁵ A/cm² (T = 4.2 K and H = 1 T).     

Electrical resistivity and thermal electromotive force of Ni<Subscript>75</Subscript>V<Subscript>25</Subscript>, Ni<Subscript>72</Subscript>V<Subscript>28</Subscript>, and Ni<Subscript>67</Subscript>V<Subscript>33</Subscript> alloys at high temperatures

The temperature dependences of the electrical resistivity and thermal electromotive force (thermal e.m.f.) of the Ni–25 at % V, Ni–28 at % V, and Ni–33 at % V alloys in a temperature range of 300–1600 K have been reported; the dependences have been measured during slow heating and cooling of quenched and annealed samples. It has been shown that, near the order–disorder phase-transformation temperature, the temperature dependences of the electrical resistivity of the Ni₇₅V₂₅ and Ni₆₇V₃₃ alloys demonstrate a kink (second-order phase transition) and a jump (first-order phase transition), respectively. The behavior of the experimental dependences is discussed in terms of the band Mott s–d scattering model.     

Effect of water intercalation on the structure and electrophysical properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.9</Subscript>

The influence of water vapors and plastic deformation on the structure and electrophysical properties of YBa₂Cu₃O_6.9 (123) has been studied. It has been established that, at T = 200°C, the introduction of water into the structure of YBa₂Cu₃O_6.9 leads to its transition into a defect tetragonal phase of the 124 type as a result of the formation of planar stacking faults. After annealing at T = 930°C, these defects are partially retained and are efficient centers of pinning in the magnetic fields applied perpendicularly to the c axis, which makes it possible to increase (by an order of magnitude) the critical current density in the high-textured ceramics at 77 K in the external magnetic field of 5–10 T. The plastic deformation of the hydrated ceramics favors the reverse transition of the arising 124 phase to the 123 phase at T = 930°C and is accompanied by a recrystallization of the material, which leads to the appearance of a texture and an increase the critical current density.