Physics of Metals and Metallography最新文献

Abstract

A distinctive feature of oxide dispersion-strengthened alloys and steels, which provides a significant increase in heat resistance in comparison with traditional materials, is a significant number of homogeneously distributed nanoscale inclusions (oxides and clusters). For detailed characterization of such materials, a set of techniques is used, such as transmission electron microscopy, atom probe tomography, as well as small-angle scattering of X-rays and neutrons. The latter techniques make it possible to analyze the largest volume of material, while maintaining the ability to detect various nanoscale features. Since ferritic-martensitic oxide dispersion-strengthened steels are ferromagnetic materials, magnetic scattering has to be taken into account in the processing of small-angle neutron scattering data. The nanostructure of ferritic-martensitic oxide dispersion-strengthened steels with different alloying systems (different content of Cr, V, W, Al, and Zr) is investigated by small-angle neutron scattering. A comparison of the results of the study of the nanostructure of steels (oxide particles and clusters) in the ferromagnetic state with and without magnetic scattering is carried out. It is shown that oxide particles have a significantly higher magnetic contrast in comparison with nanoscale clusters. At the same time, the most accurate hardness values can be obtained by taking into consideration of both oxide inclusions and clusters.

Abstract

We study the dependence of magnetic properties: Curie temperature, mean and local magnetic moments—on the type of crystal lattice and average number of d electrons per atom. The problem is considered in two approximations: with spin fluctuations not taken into account, in the Stoner mean field theory, and with spin fluctuations taken into account, in the dynamic spin fluctuation theory (DSFT). In the DSFT, we obtain an analogue of the Slater–Pauling curve for the mean magnetic moment at finite temperatures. Numerical results in the DSFT are in qualitative agreement with experiment: with magnetic phase diagram and dependence of magnetic moment on concentration in ferromagnetic alloys.

Abstract—The atomic structure of single-crystal samples of soft-magnetic Fe–9 at % Ga alloys (the A2-phase region) subjected to thermomagnetic treatment (TMT) is studied by X-ray diffraction. In the course of TMT, the samples in the ferromagnetic state are annealed at 450°C and slowly cooled to room temperature in an external saturating magnetic field. In the alloys subjected to TMT and in the alloys subjected to heat treatment (HT) in zero magnetic field, B2 clusters are found, which are pairs of bcc cells centered with Ga atoms. The TMT and HT are shown to have virtually no effect on the size and volume fraction of B2 clusters in the alloy under study. The formation and stability mechanisms of B2 clusters and their role in the formation of induced magnetic anisotropy in the Fe–Ga alloys are discussed.

Abstract

The influence of a large spin-polarized current on the coupled dynamics of vortices in spin-transfer nanooscillators with a diameter of 400 nm is investigated. New stationary modes of coupled oscillations of vortices have been discovered, both for the same and opposite polarities of their cores. The dependence of the frequency of stationary coupled oscillations of magnetic vortices on the magnitude of the spin-polarized current has been studied. The found effect can be used to increase the operating frequencies of spin-transfer nano-oscillators.

Abstract

Currently, the development of new high performance magnetoelectric engines and generators that require high strength materials with specific magnetic characteristics is a relevant problem. The possibility of using maraging steels—in particular, the steel developed under brand 04Kh13N8MTYu–VI by Lasmet Co.—as rotor poles for high speed electric machines with permanent magnets due to the favorable combination of their mechanical properties, improved corrosion resistance, and impact toughness has been considered. The mechanical and magnetic properties of steel 04Kh13N8MTYu–VI have been investigated. Despite the fact that the maximum value of magnetic permeability in steel 04Kh13N8MTYu–VI is substantially lower than that in steels 3 and 30KhGSA, it is shown that the difference in the magnitude of the electromagnetic torque between engines in which different steels 30KhGSA and 04Kh13N8MTYu–VI are used is less than 0.2%. Considering the higher strength characteristics of steel 04Kh13N8MTYu–VI compared to 30KhGSA, there is a potential opportunity to increase the rotor rotation frequency by 25% with the use of a rotor made of steel 04Kh13N8MTYu–VI.

Abstract

X-ray diffraction and electron microscopy were used to examine the crystal structure features of ultrathin (3–50 nm) yttrium orthoferrite films, obtained by magnetron sputtering of stoichiometric composition target on α-Al₂O₃ substrates with orientation. The morphology and crystal structure of the films differ significantly depending on their thickness. In the thinnest films, several phases are formed, including yttrium orthoferrite with an orthorhombic crystal lattice (o-YFeO₃), yttrium hexaferrite with a hexagonal crystal lattice (h-YFeO₃), iron-yttrium garnet (Y₃Fe₅O₁₂), and iron oxides such as hematite and maghemite. The study examines the local composition and determines the orientation ratios between the crystallized phases and the substrate. The films with a thickness greater than 10 nm predominantly exhibit a highly textured phase of o-YFeO₃ with a small admixture of iron-yttrium garnet.

Abstract

The paper presents three-dimensional micromagnetic simulation of the magnetization reversal process of permalloy film with additional relief elements of step-like shape made of the same material. It is shown that in the course of magnetization reversal in a constant magnetic field, the initial magnetization distribution containing a C-shaped domain wall is transformed into a magnetization reversal zone filled with vortex structures. In this case, the magnetization reversal dynamics becomes irregular. The peculiarities of the dynamics of the magnetization reversal zone (change in magnetization reversal rate, temporary or final cessation of motion) are revealed for different types of surface relief elements (strips, linear or two-dimensional arrays of rectangular parallelepipeds of different sizes). Visualization methods of vortex structures are described.

Abstract

This study focuses on investigating the effect of Mg and Si content in different ratios on the microstructure evolution and superplasticity after processing by multidirectional isothermal forging of the Al–Mg–Si–Cu based alloys with additions of Fe and Ni forming large particles of phases of crystallization origin and dispersoid-forming elements Sc and Zr. The study examined alloys with the following Mg/Si content: 1.2/0.4 (3), 1.2/0.7 (1.7), and 2.0/0.7 (2.8) (wt %). The alloys underwent six cycles of isothermal multidirectional forging (MDF) at a temperature of 325°C with cumulative deformation of up to ∑ε = 14.4. During the MDF process, fragmentation occurred in particles of Mg₂Si and Al₉FeNi eutectic phases, resulting in the formation of particles with sizes of 0.6–0.7 and 1.2–1.5 µm, respectively. Due to particle stimulated nucleation mechanism and suppressed grain growth by nanoscale dispersoids the finegrained structure with a mean grain size of 2 μm was formed in alloys. An increase in Si concentration at constant Mg content, as well as an increase in Mg and Si concentration at a close Mg/Si ratio leads to an increase in fraction of recrystallized grains and a decrease in grain size after MDF. The alloy with the lowest Mg and Si content has shown the highest elongation under superplastic deformation, which is explained by the reduced fraction of Mg₂Si phase particles of crystallization origin.

Abstract

The distribution of Co²⁺ ions over sublattices and structurally nonequivalent positions in the unit cell of the crystal lattice of a single crystal of lithium gallium spinel Li_0.5Ga_2.5O₄ is shown. This distribution determines the properties of both mono- and nanocrystalline substances. The distribution is obtained by a special technology and is manifested in the electron paramagnetic resonance (EPR) spectra. The distribution of Co²⁺ ions depends on the structural and magnetic nonequivalence. The structural and magnetic nonequivalence causes a multiminimum behavior of the crystal field potential in the unit cells of single crystals at the locations of Co²⁺ ions. The Co²⁺ ions are found in complexes with tetrahedral and octahedral oxygen ions. Three types of EPR spectra of Co²⁺ ions have been found and investigated. The ({text{Co}}_{{{text{tetr}}}}^{{2 + }}) spectrum is attributed to the Co²⁺ ion, which replaces the Ga³⁺ ion located in a tetrahedral oxygen environment. The spectrum of the ({text{Co}}_{{{text{oct}}}}^{{2 + }}) ion located in the crystal field with axial symmetry belongs to the Co²⁺ ion replacing the Li⁺ ion located in an octahedral oxygen environment. The spectrum of the ({text{Co}}_{{{text{oct}}}}^{{2 + }}) ion located in a low symmetry crystal field belongs to the Co²⁺ ion replacing the Ga³⁺ ion located in an octahedral oxygen environment. The nearest cationic environment of the ion creates rhombic distortions due to the different valence numbers of Li⁺ and Ga³⁺. The results of studying the angular dependences of the spectra show the presence of four and twelve magnetically nonequivalent positions in the unit cells.

Abstract

Cerium(IV) oxide nanoparticles are promising agents for use in radiotherapy. The morphology of nanoparticles largely determines their effectiveness. This article presents the results of a study of the conditions for the deposition of cerium(IV) oxide nanoparticles, a detailed variation of the parameters of the syntheses, and assessment of their effectiveness from the point of view of the morphology of the resulting structures. The conditions for obtaining nanoparticles with optimal physicochemical properties, high stability, and reproducibility of synthesis have been selected.