M. V. Karpets

Nanostructural inhomogeneities acting as pinning centers in bulk MgB2 with low and enhanced grain connectivity

Regularly distributed structural inhomogeneities in the MgB2 matrix, such as nano-areas with a high concentration of boron (MgBx) and impurity oxygen (Mg?B?O nano-layers or inclusions), are observed in all materials independently of the preparation method, pressure (0.1?MPa?2?GPa) and temperature (600?1100??C), and in materials with different connectivity (18?98%) and density (55?99%). Such inhomogeneities can act as pinning centers in MgB2 because the variation of their size and distribution are well correlated with variations of the critical current density, jc. The decrease in size of MgBx inclusions, the transformation of 15?20?nm thick Mg?B?O nano-layers into separated inclusions, and the localization of impurity oxygen are accompanied by an increase in critical current density in low and medium magnetic fields. The efficiency of these defects is evidenced by a shift from grain-boundary pinning to point pinning.

Mechanical properties of materials based on MAX phases of the Ti-Al-C system

By studies of materials based on the Ti3AlC2 MAX phase containing inclusions of titanium carbide it has been shown that as a titanium carbide content increases from 2 to 99%, the nanohardness and Young modulus of the material increase from 2.0 ± 0.4 to 23.6 ± 1.2 GPa and from 137 ± 21 to 447 ± 11 GPa, respectively. The exponent in the equation of creep for these samples has been found to vary from 104 to 140, which indicates that mechanical properties of the material and, hence, of the Ti3AlC2 MAX phase depend on the strain rate only slightly. The formation of broad hysteresis loops has been observed in the cyclic loading/unloading of the indenter for samples consisting mainly of the Ti3AlC2 MAX phase. This points to serious losses in elastic energy of the MAX phase in strain cycling and, hence, the prospects of the MAX phase application as a damping material. It has been found that the microhardness of a sample consisting of 98% Ti3AlC2 produced by sintering under a load of 4.9 N was 2.1 GPa and its fracture toughness was high (no cracks from the indent corners were observed even under a load of 149 N). Microhardness and fracture toughness of the material consisting of 71% Ti3AlC, 6% Ti2AlC, and 23% TiC were 3.0 ± 0.6 GPa and 4.3 ± 1.4 MPa·m1/2, respectively.

Effect of Electron Density on Phase Composition of High-Entropy Equiatomic Alloys

A series of high-entropy equiatomic alloys have been analyzed to determine the main factors that influence the formation of various solid solutions and chemical compounds. The key factor leading to the formation of phases in high-entropy equiatomic alloys is mean electron density (e/a). The necessary condition for the high-entropy σ-phase to emerge is the presence of elements forming it in two-component alloys in various ratios, the electron density of the alloy is to be between 6.7 and 7.3 e/a. The Laves phase shows up in the high-entropy equiatomic alloys at a mean electron density of 6–7 e/a in the presence of atoms differing by more than 12% in size and having mixing enthalpy lower than −30 kJ/mol. It is revealed that the lattice parameter in bcc high-entropy equiatomic alloys influences their elastic modulus and hardness.

Features of phase and structure formation in high-entropy alloys of the AlCrFeCoNiCux system (x = 0, 0.5, 1.0, 2.0, 3.0)

Alloys of the AlCrFeCoNiCux system (x = 0, 0.5, 1.0, 2.0, 3.0) were smelted by argon-arc smelting in pure argon. The phase composition and structure of fabricated alloys are investigated and their mechanical properties are determined. The results showed that an increase in the amount of copper in alloys leads to a change in the phase composition from single phase (bcc) to three phase (bcc + fcc1 + fcc2), which is accompanied by the structural change from coarse-grain polygonal structure to complex dendritic structure (primary dendrites (DR) + secondary dendrites (SDR) + interdendrite phase (ID)). The region of electron concentrations of alloys, in which bcc and fcc phases are present simultaneously, is determined. The limiting electron concentration of stability of the bcc lattice is found experimentally. Microhardness is measured and Young moduli of alloys over the entire range of varying the copper concentration are determined.

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Bulk MgB₂- and YBaCuO-based materials are competitive candidates for applications. The properties of both compounds can be significantly improved by high temperature-high pressure preparation methods. The transformation of grain boundary pinning to point pinning in MgB₂-based materials with increasing manufacturing temperature from 800 to 1050^°C under pressures from 0.1 MPa to 2 GPa correlates well with an increase in critical current density in low and intermediate magnetic fields and with the redistribution of boron and oxygen in the material structure. As the manufacturing temperature increases (to 2 GPa), the discontinuous oxygen-enriched layers transform into distinct Mg-B-O inclusions, and the size and amount of inclusions of higher borides MgB_X (X>;2) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC addition. The oxygenation of melt-textured YBa₂Cu₃O_{7 - δ} (MT-YBaCuO) under oxygen pressure (16 MPa) allows one to increase the oxygenation temperature from 440°C to 700-800°C, which leads to an increase of the twin density in the Y123 matrix and to a decrease of dislocations, stacking faults, and the density of microcracks, and as a result, to an increase of the critical current density, <i>J</i>_c, and the trapped magnetic field. In MT-YBaCuO, practically free form dislocations and stacking faults and with a twin density of 22-35 μm^-1, <i>J</i>_c of 100 kA/cm² (at 77 K, 0 T) has been achieved, and the importance of twins in Y123 for pinning was demonstrated experimentally.

\hbox{MgB}_{2}

Ternary carbides have been synthesized from a mixture of Ti, Al, and C powders taken in the stoichiometric relation 3/1.2/2 under quasihydrostatic pressures and temperatures. The amount of the MAX phase in samples and the material density has been increased by the two-stage synthesis at high pressures (1–2 GPa) and temperatures followed by homogenizing. The material, which had been produced at a pressure of 2 GPa, a temperature of 1200°C and annealed in an argon atmosphere, contained 94.3 Ti3AlC2-4.3 TiC-1.4 Al2O3 wt %. The material density and Vickers microhardness at the load of 4.9 N have been measured to be 4.27 g/cm3 and 3.3 GPa, respectively.

- and YBaCuO-Based Superconductors: Effect of Manufacturing Pressure and Temperature

The mechanical properties and temperature stability in air and hydrogen of the highly dense (ρ=4.27 g/cm3, porosity 1 %) material based on nanolaminated MAX phase Ti3AlC2 (89 % Ti3AlC2, 6 % TiC, 5 % Al2O3) manufactured by hot pressing (at 30 MPa) have been investigated. At room temperature the samplesexhibited microhardness HV = 4.6 GPa (at 5 N), hardness HV50 = 630 MPa (at 50 N ) and HRA=70 (at 600 N), Young modulus was 140 ± 29 GPa, fracture toughness K1C=10.2 MPa·m0.5compression strength 700 MPa and bending strength 500 MPa. After 1000 hours of exposition at 600 °C the oxide film (containing mainly Al2O3 and TiO2) formed on the surface and material demonstrated a higher oxidation resistance than chromium ferrite steels. Due to the surface oxidation the defects self-healing took place and the bending strength of the porous Ti3AlC2 (22% porosity) after exposition for 3 h at 600 oC in air slightly (for 3%) increased as compared to that at 20 oC. Besides, the porous Ti3AlC2 material resisted to high-temperature creep and after being kept in H2 at 600 °C for 3h its bending strength reduced by 5 %.

Synthesis of ternary compounds of the Ti-Al-C system at high pressures and temperatures

The effect of aluminum on the phase composition, microstructure, and mechanical properties of CrFeNiCuCoAlx multicomponent high-entropy alloys is examined. According to X-ray diffraction data, the phase composition of the alloys significantly varies depending on aluminum content: with higher aluminum content, the two-phase structure (mixture of FCC1 + FCC2 solid solutions) changes to a single-phase one (BCC solid solution). Scanning electron microscopy is employed to examine the alloy microstructure and determine the chemical composition of dendritic and interdendritic regions. The primary and secondary dendrites include all elements of the alloy. The interdendritic region has high content of copper as it possesses high pairwise positive enthalpy of mixing with most elements of the alloy. The microhardness increases from 3.1 to 8.4 GPa with greater aluminum content of the CrFeNiCuCoAlx system.

Thermal Stability and Mechanical Characteristics of Densified Ti3AlC2-Based Material

The DTA and TG study in air of Ti2Al (C1-xNx) and Ti3AlC2 synthesized under Ar 0.1 MPa pressure and densified in thermobaric conditions at 2 GPa, 1400 °C, for 1 h showed that the increase of the amount of TiC layers in Ti-Al-C MAX phases structures leads to the increase of their stability against oxidation: 321 MAX phase Ti3AlC2 are more stable than Ti2AlC and Ti2Al (C1-xNx) solid solutions both before and after thermobaric treatment. The oxide film formed on the surface of the highly dense (ρ=4.27 g/cm3, porosity 1 %) material based on nanolaminated MAX phase Ti3AlC2 (89 % Ti3AlC2, 6 % TiC, 5 % Al2O3) manufactured by hot pressing (at 30 MPa) made the material highly resistant in air at high temperatures: after 1000 hours of exposition at 600 °C it demonstrated a higher resistance to oxidation than chromium ferrite steels (Crofer GPU and JDA types). Due to the surface oxidation self-healing of defects took place. Besides, the Ti3AlC2 material demonstrated resistance against high-temperature creep and after being kept in H2 at 600 °C for 3h its bending strength reduced by 5 % only. At room temperature the Ti3AlC2 bulk exhibited microhardness Hμ = 4.6 GPa (at 5 N), hardness HV50 = 630 (at 50 N ) and HRA = 70 (at 600 N), Young modulus was 140 ± 29 GPa, bending strength =500 MPa, compression strength 700 MPa, and fracture toughness K1C=10.2 MPa·m0.5.

Mechanical Properties and Formation of Phases in High-Entropy CrFeNiCuCoAl x Alloys

The superconducting characteristics, such as the critical current density and the critical magnetic fields, of MgB2-based materials, which in fact belong to the Mg-B-O system because of the high concentration of admixed oxygen (up to 17 wt. %), depend on the inhomogeneity of the oxygen and boron distribution, which can be controlled by the synthesis temperature (600-1200 oC) and pressure (up to 2 GPa) as well as by SiC and Ti additions (10 wt%). With increasing manufacturing temperature grain boundary pinning transforms into point pinning, which is well correlated with the transformation of discontinuous oxygen enriched layers into separately located Mg-B-O inclusions in the MgB2 nanostructure and with a reduction of the size and amount of inclusions of higher magnesium borides MgBX (X>2). Ti or SiC additions can influence the oxygen and boron distribution as SEM and Auger structural studies showed.