D. A. Shishkin

Magnetic properties and structure of nanocrystalline FINEMET alloys with various iron contents

The effect of the composition and annealing temperature on the structure and magnetic properties of soft magnetic nanocrystalline Fe-Cu-Nb-Mo-Si-B alloys has been studied. An increase in the iron content compared to that in the traditional FINEMET alloy is shown to allow one to increase the magnetic induction by 18% at a coercive force of no less than 6 A/m. It has been found that, along with the amorphous phase, rapidly quenched ribbons of alloys enriched in Fe contain crystalline α-Fe-based phase precipitates, the (100) crystallographic directions of which are perpendicular to the ribbon plane. Thermomagnetic analysis and differential scanning calorimetry were used to determine the temperatures of structural and magnetic phase transformations of the alloys with different iron contents. It was found that the separation of amorphous phase into areas of different compositions precedes the precipitation of nano-sized soft magnetic Fe-Si phase grains in the rapidly quenched iron-enriched ribbons.

Magnetic state and magnetocaloric properties of rapidly quenched Gd75M25 alloys (M = Co, Ni)

Effect of rapid quenching on the magnetic state and magnetocaloric properties of the Gd3Co and Gd3Ni intermetallic compounds has been studied. The rapidly quenched Gd75Co25 and Gd75Ni25 alloys were found to exhibit a ferromagnetic behavior below the Curie temperature TC = 172 and 117 K, respectively, whereas their crystalline counterparts Gd3Co and Gd3Ni are characterized by an antiferromagnetic order and Néel temperatures TN = 130 and 99 K, respectively. Data have been obtained which indicate that the amorphization of Gd3Co leads to the appearance of a magnetic moment at cobalt atoms. The rapid quenching is shown to lead to a significant increase in the low-field magnetocaloric effect.

Magnetic properties, thermal stability, and structure of the nanocrystalline soft magnetic (Fe0.7Co0.3)88Hf2W2Mo2Zr1B4Cu1 alloy with induced magnetic anisotropy

The effect of magnitude of tensile stresses (σ) applied to the (Fe0.7Co0.3)88Hf2W2Mo2Zr1B4Cu1 alloy with refractory-metal additions during its nanocrystallization at 620°C for 20 min on the magnetic properties, structure, and thermal stability of the alloy is studied. It has been found that, during the nanocrystallization of the alloy under the effect of tensile stresses of 6–250 MPa, longitudinal magnetic anisotropy with an easy magnetization axis parallel to the long size of ribbon is induced in the alloy. The thermal stability of magnetic properties of the alloy under study has been shown to be determined by the thermal stability of induced magnetic anisotropy and to depend on the magnitude of tensile stresses applied during nanocrystallizing annealing (NA). The better thermal stability of magnetic properties has been observed for the alloy subjected to NA at σ = 170 MPa. After annealing at 570°C for 25 h, the magnetic properties of the alloy are unchanged.

Study of magnetic properties and structural and phase transformations in the Co-19 at % Al-6 at % W alloy

The Co-19 at % Al-6 at % W alloy prepared by two methods of melting in an inert atmosphere, namely, by arc melting followed by casting into a water cold copper mold and induction melting followed by casting in a ceramic (Al2O3) mold, has been studied. It was found that the phase composition of the alloy and its magnetic properties depend on the cooling rate of ingot after melting. Samples are ferromagnetic up to 800°C; the specific magnetization equal to σ= 10 emu/g is retained up to ∼700°C. The Curie temperatures of all phases found in the alloy have been determined. It was found that the formation of the Co7W6 phase in the alloy increases the coercive force of the alloy, whereas the saturation magnetization demonstrates a 1.5-fold decrease.

Structure and magnetic properties of a Ni3(Al, Fe, Cr) single crystal subjected to high-temperature deformation

The structure and magnetic properties of the Ni3(Al, Fe, Cr) single crystal subjected to high-temperature tensile deformation to failure at 850–900°C have been studied. No recrystallized grains and metastable phases were found. The rupture zone of the alloy subjected to deformation (at 900°C) to the highest degree demonstrates the fragmentation accompanied by rotation of atomic layers and changes of the chemical composition in the nickel and aluminum sublattices. Magnetic studies of the alloy have shown the existence of two Curie temperatures for samples cut from the rupture zone. Samples cut away from the rupture zone exhibit no additional magnetic transitions; twines and planar stacking faults in the alloy structure. The alloy deformed to the lower degree of deformation (at 850°C) also demonstrates twins; no ferromagnetic state was found to form.

Study of the martensitic transformation in the Co–9 at % Al alloy

Phase transformations in the Co–9 at % Al have been investigated after slow furnace cooling. It has been shown that the structure and phase composition of the alloy after slow cooling do not correspond to the equilibrium phase diagram of the alloy of this chemical composition. It has been established that the α → ε martensitic transformation does not require overcooling and occurs even during a slow cooling of the alloy. It has been found that the formation of 4H modulated martensite is a specific feature of the binary alloys of cobalt and is not connected with the rate of their cooling. The Curie temperatures for the B2, α, and ε phases have been determined.

Magnetic properties, thermal stability, and structure of soft magnetic (Fe0.7Co0.3)88Hf2W2Mo2Zr1B4Cu1 alloy that underwent high-temperature nanocrystallization

The effect of the temperature of nanocrystallizing annealing (NA) at temperatures of 670–750°C on the magnetic properties, structure, and thermal stability of the (Fe0.7Co0.3)88Hf2W2Mo2Zr1B4Cu1 alloy has been studied. The NA was performed both in the presence of tensile stresses applied to samples and in the absence of stresses. It was found that the nanocrystallization of the alloy under applied tensile stresses induces the longitudinal magnetic anisotropy with an easy magnetization axis parallel to the long side of the ribbon. The optimum conditions for nanocrystallization of the alloy, which stabilize its magnetic properties at a fairly high temperature of 570°C, have been determined.

Layer-preferential substitutions and magnetic properties of pyrrhotite-type Fe7-yMyX8 chalcogenides (X = S, Se; M = Ti, Co).

A comparative study of four series of pyrrhotite-type chalcogenide compounds Fe(7-y)M(y)X(8) (X = S, Se) with substitution of Ti or Co for iron has been performed by means of x-ray and neutron powder diffraction, and by magnetization measurements. In Fe(7-y)M(y)X(8) compounds having a ferrimagnetic order at y = 0, the substitution of either Ti or Co for iron is observed to result in a monotonous decrease of the magnetic ordering temperature, while the resultant magnetization shows a non-monotonous behavior with a minimum around y = 1.0-1.5 in all the Fe(7-y)M(y)X(8) families except Fe(7-y)Co(y)Se(8). Suppression of a magnetically ordered state with substitutions in Fe(7-y)M(y)X(8) is ascribed to nearly zero values of Ti and Co magnetic moments, while the non-monotonous changes of the resultant magnetization are explained by the compensation of the sublattice magnetizations due to the non-random substitutions in alternating metallic layers. The difference in the cation partitioning observed in Fe(7-y)Ti(y)X(8) and Fe(7-y)Co(y)X(8) is attributed to the difference in the spatial extension of Ti and Co 3d orbitals. High coercive field values (20-24 kOe) observed at low temperatures in the Ti-containing compounds Fe(7-y)Ti(y)X(8) with y ⩾ 3 are suggested to result from the enhancement of Fe orbital moment due to the Ti for Fe substitution.

Analysis of the Deformation Process in Ni3(Al, Fe) Intermetallic under Longitudinal Bending

The structure and magnetic properties of an Ni3(Al, Fe) single crystal after high-temperature rolling deformation have been studied. It has been shown that high-temperature rolling deformation induces longitudinal bending in Ni3(Al, Fe), which is accompanied by the nonuniform distribution of stresses along the length of the sample. It has been found that longitudinal bending leads to change in either the mechanical or physical properties of the metal. Dynamic recrystallization has been found to occur at high degrees of deformation (starting at 40%). Local change in the distance between nickel and aluminum atoms, as well as the chemical composition, that takes place in the bending zones (distortions) of a crystal lattice of Ni3Al intermetallic compound is accompanied by change in the saturation magnetization and Curie temperature.

Heat-resistant alloys based on intermetallic Co3(Al, W)

The structures and phase compositions of a series of Co–9 at % Al–x at % W alloys (where x = 4.6, 6.8, 8.5, 10.0, 12.6) have been studied. All of them are dominated by the structure γ + γ′, with the content of other phases (the μ phase and the phase D019 based on Co3W) being insignificant. With increasing tungsten content of the alloy (to 10 at % W), the volume fraction of the γ′ phase increases, and so does its degree of long-range order. The alloy that is optimal in structure and mechanical properties has the composition Co–8.2 at % Al–8.5 at % W. The dissolution range of the γ′ phase was determined as 920–1054°C. Young’s moduli of cobalt alloys exceed E of commercial nickel-based superalloys. The samples of the studied alloys are in the ferromagnetic state with a Curie temperature of 870°C. With increasing tungsten content of the alloy, its saturation magnetization decreases and the coercive force increases. The coercive force is minimal in an as-cast sample and increases as it is subjected to deformation or heat treatment.