Vol 119, No 3 (2018)

A three-dimensional micromagnetic computer simulation of transition structures that separate regions of C- and S-shaped vortex asymmetric domain walls in films with easy magnetization axes parallel to the surface has been performed. Films with uniaxial and triaxial magnetic anisotropy (with the surface parallel to crystallographic plane (100)) have been examined. New types of transition structures (including those containing Bloch points) have been obtained.

A short review of the results of a computer simulation of ferromagnets of limited sizes (nanoplatelets, thin films, and rods of rectangular section) using the method of minimizing the functional of the free energy of a magnetic system has been carried out. The implicit dependence of the functional on the magnetization distribution has been taken into account via the potential of the intrinsic field, which made it possible to exclude the points of the labile equilibrium upon minimizing the functional. The efficiency and adequacy of this method for studying micromagnetic properties of nanoobjects have been established. The specific features of the formation of domain structures of nanoobjects, as well as new possibilities of employing them in the magnetic recording of information have been shown.

The composition and the structure of ceramic EuBa₂Cu₃O_{6 + δ} (Eu-123) oxide samples annealed in steps with varying processing conditions (in air or oxygen and argon atmosphere at a temperature of 940–960°С for 1–70 h with or without homogenization) were studied by the X-ray phase and chemical analysis, electron diffraction pattern analysis, elemental analysis, and high-resolution transmission electron microscopy. Regardless of the processing conditions, Eu-123 nanostructured oxide with a tetragonal or orthorhombic structure and domains 1–20 nm in size was obtained as a result of annealing. Nanostructuring of the samples, which was revealed by high-resolution electron microscopy, is attributed to their chemical nature: the presence of identical structural elements in members of the homologous Eu_nBa_mCu_{m + n}O_y series of oxides allows them to intergrow coherently and create an illusion of a single crystal. Just like any other member of the Eu_nBa_mCu_{m + n}O_y series, oxide Eu-123 is disproportionate depending on the annealing conditions to form other members of this series located on either side of the dominant oxide. Temperature T_c of the superconducting transition of each member of the series depends on the average oxidation state of copper \(\overline {Cu} \). At \(\overline {Cu} \) < 2, all members of the series have a tetragonal structure and do not exhibit superconducting properties. At \(\overline {Cu} \) = 2.28, five members of the Eu_nBa_mCu_{m + n}O_y series with matrices (Ba : Cu) 5 : 8, 3 : 5, 2 : 3, 5 : 7, and 3 : 4 exhibit superconducting properties with T_c = 82–90 K.

Results of an analysis of the mechanical properties and macro- and microstructures and the fractographic analysis of the fractures of samples of the Al–6 wt % Ni and Al–7.6 wt % Ca aluminum–matrix composite alloys of eutectic type after thermomechanical treatment, including radial–shear rolling (RSR),—have been presented. The hot deformation of preliminarily annealed ingots of studied alloys with a circular section 60 mm in diameter using RSR method at 400–450°C with total reduction μ = 9.0 can lead to the formation of the gradient microstructure with external more deformed layer characterized by high microhardness and thickness of about 1.5–2.5 mm. The microhardness decreases smoothly from the periphery to the center of samples. Uniaxial tensile tests revealed that the strength of alloys after RSR increases by 2.0–2.5 times compared to the as-cast or the annealed state, the plasticity is the same as in the annealed state or increases by several times as in the case of the Al–7.6 wt % Ca alloy. The latter fact is clearly illustrated by the results of the analysis of the fractures of samples, for which the transition from the brittle or mixed type of the fracture before deformation treatment to the pronounced ductile dimpled type after RSR has been observed.

Using X-ray, Mössbauer, and magnetic measurements, the formation of phases has been investigated upon mechanosynthesis in a ball planetary mill and upon the subsequent annealing of samples of the cementite composition (Fe_0.95–уСr_0.05Ni_y)₇₅C₂₅, where у = 0–0.20, which contains two alloying elements (chromium and nickel). It has been shown that, in the mechanosynthesis process, cementite alloyed with chromium and a small amount of nickel and an amorphous phase alloyed with chromium and nickel have been formed. Upon heating above 300°С, the amorphous phase is crystallized into nickel-enriched cementite. In the process of annealing at higher temperatures, the most nickel-rich cementite decomposes with the formation of austenite. As a consequence, after annealing at medium temperatures, the composition of the alloys contains cementite alloyed mainly with chromium and some amount of alloyed austenite, which can be found in ferromagnetic or paramagnetic states depending on the Ni content. Annealing at 800°С bring about the complete or partial decomposition of cementite contained in the alloys. The intensity of the decomposition has been determined by the nickel content in the samples.

Quenched titanium–ruthenium alloys containing 0.25–4 at % ruthenium have been studied using X-ray diffraction analysis, optical metallography, transmission electron microscopy, and microhardness measurements. It has been found that, during the quenching of the alloys containing 0.25, 0.5, and 1 at % ruthenium, a polymorphic β → α transformation occurs with the formation of a two-phase (α + β) structure. In Ti–1.5 at % Ru and Ti–2 at % Ru alloys, a martensitic β → α″ transformation occurs. The quenched Ti–3 at % Ru alloy has a β + ω structure. The complete stabilization of the β phase takes place in the alloy with 4 at % ruthenium. In the electron-diffraction patterns of alloy containing 4 at % ruthenium, diffuse scattering that indicates the formation of ω-phase-related displacements in the locations of atoms has been observed.

Combined studies of the properties of the 50Ti–40Pd–10Ni alloy with a high-temperature shape memory effect have been carried out. The elemental and phase compositions and its mechanical and thermomechanical characteristics have been determined. Samples cut from the strip with a thickness of 2.04 mm have been studied. The phase transformations are in the range of M_f = 371.3°С to A_f = 436.9°С. These are acceptable values. The maximum values of the shape memory effect ε_SME and of the degree of shape recovery η_SME are 3.9 and 49%, respectively. These values are insufficient to create workable safety devices. These characteristics can be improved via optimal changes in the elemental composition of the shape memory alloy and by an adequate choice of the regimes of heat treatment and of the regimes of providing strain in the investigated objects. The results of these studies are necessary in order to develop different devices used in nuclear power plants.