D. V. Matveev

Nanocrystallization of an amorphous Fe80B20 alloy during severe plastic deformation

The structural evolution of an amorphous Fe80B20 alloy subjected to severe plastic deformation at room temperature or at 200°C was studied. Deformation leads to the formation of α-Fe nanocrystals in an amorphous phase. After room-temperature deformation, nanocrystals are localized in shear bands. After deformation at 200°C, the nanocrystal distribution over the alloy is more uniform. Possible causes of the crystallization of the amorphous phase during severe plastic deformation are discussed.

Structural transformations in the Al85Ni6.1Co2Gd6Si0.9 amorphous alloy during multiple rolling

The effect of multiple rolling at room temperature on the structure and crystallization of the Al85Ni6.1Co2Gd6Si0.9 amorphous alloy has been studied using transmission electron microscopy, differential scanning calorimetry, and X-ray diffraction. The total plastic strain is 33%. It has been shown that the deformation results in the formation of aluminum nanocrystals with the average size that does not exceed 10–15 nm. The nanocrystals are formed in regions of localization of plastic deformation. The deformation decreases the thermal effect of nanocrystallization (∼15%) as compared to the heat release at the first stage of crystallization of the unstrained sample. The morphology, structure, and distribution of precipitates have been investigated. Possible mechanisms of the formation of nanocrystals during the deformation have been discussed.

Synthesis of α-SiC nanocrystals by carbothermal reduction of spherical nanoparticles of amorphous silicon dioxide

The method for carbothermal reduction of spherical particles of amorphous silicon dioxide is developed, and hexagonal α-SiC polytype nanocrystals are synthesized. The prepared samples are characterized by X-ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, and electron microscopy. The silicon carbide nanocrystals prepared have sizes in the range 5–50 nm depending on the diameter of initial silicon dioxide particles. A detailed analysis of the positions of the lines in the Raman spectra, their broadening, and shift makes it possible to reliably establish that the samples under investigation predominantly contain the 6H and 4H silicon carbide polytypes and insignificant amounts of the 2H and 3C phases. The 15R and 21R polytypes in the samples are absent. It is noted that the samples are characterized by a substantial size effect: the luminescence intensity of small silicon carbide nanocrystals is more than three times higher than that of large SiC nanocrystals.

Formation and structure of nanocrystals in bulk Zr50Ti16Cu15Ni19 metallic glass

The possible formation of a nanocrystalline structure in controlled crystallization of a bulk Zr50Ti16Cu15Ni19 amorphous alloy has been studied using differential scanning calorimetry, transmission and high-resolution electron microscopy, and x-ray diffraction. It was established that crystallization of the alloy at temperatures above the glass formation point occurs in two stages and brings about the formation of a nanocrystalline structure consisting of three phases. Local spectral x-ray analysis identified the composition and structure of the phases formed.

Structural and magnetic ordering of CrNb 3 S 6 single crystals grown by gas transport method

Paramagnetic layered semiconductor NbS2 doped with some transition metals can transform into ferromagnetic material. That is why such materials are promising candidates for spintronic devices. It is found that only at certain concentrations of a doping metal T crystallographic ordering is possible, which is essential for magnetic ordering of ternary compounds TNbS2. In particular, CrNb3S6 crystals are studied, which form almost completely ordered superstructure with intercalated Cr between NbS2 layers. The main difficulty in crystal growth is reaching stoichiometry of the compound. This problem is solved in the developed method of two-staged gas transport chemical reaction. This new approach provides growth of CrNb3S6 single crystals of several millimeters in diameter and 0.3–0.5 mm thickness. X-ray phase analysis (XRD) of powders is performed to identify all phases involved in synthesis and growth of the crystals. High frequency absorption in external periodic magnetic field as a function of temperature and intensity of magnetic field is used to estimate the temperature of ferromagnetic transition in CrNb3S6 single crystals. The Curie temperature is estimated as 115 K. Growth of CrNb3S6 crystals from vapor phase is studied in detail and full analysis of phase transitions during growth is given. It has been shown that using of high frequency absorption in the crystal provides reliable estimation of the point of ferromagnetic transition in this semiconductor. The authors are grateful to the Physical Science Department of Russian Academy of Sciences for financial support of the studies in the frameworks of the program “Physics of new materials and structures” (project no. 00-12-10).

Evolution of magnetic properties of amorphous Fe78Si13B9 alloy during deformation and subsequent heat treatment

The structure and properties of the amorphous Fe78Si13B9 alloy were studied after severe plastic deformation and subsequent heat treatment. The deformation was performed by torsion under high pressure at room temperature. The heat treatment was carried out at temperatures lower than the alloy crystallization temperature. The investigations were performed using X-ray diffraction, transmission electron microscopy, and vibrating-sample magnetometry. It was shown that the minimum coercive force (1.7 Oe) corresponds to the initial state and increases up to 2.6 Oe with an increase in the degree of deformation. It was found that the heat treatment of the deformed Fe78Si13B9 alloy improves its soft magnetic characteristics. The successive annealings for 1 h and, then, 2 h at a temperature of 200°C led to a decrease in the coercive force of the samples from 2.6 to 2.0 Oe.

Dependence of the amount of a nanocrystalline phase in the amorphous Fe80B20 alloy on the degree of severe plastic deformation

Conditions for the formation of nanocrystals in the amorphous Fe80B20 alloy under the effect of severe plastic deformation (SPD) have been investigated by electron microscopy and X-ray diffraction. The dependence of the size of nanocrystals and the fraction of a nanocrystalline phase that is formed during the crystallization of an amorphous alloy on the degree of SPD has been studied. It has been shown that the SPD induces nanocrystallization of amorphous samples. An average size of nanocrystals is about 6 nm. The formation of nanocrystals is observed only in the zones of localization of plastic deformation, i.e., shear zones. These zones have an elongated shape; their width is several microns; in these zones, regions of a low density of the material and, in some cases, even voids, are observed. The size of such discontinuities is from 5 to 50 nm. It has been found that an increase in a degree of deformation increases the fraction of the nanocrystalline phase. The estimations of the fraction of the phases performed by the method of resolution of the maxima present in the X-ray diffraction patterns into components showed that in the samples subjected to deformation by 15 rev of the anvils the fraction of nanocrystals is nine times greater than that in the samples subjected to deformation by 8 rev. At the same time, the average size of nanocrystals remains unchanged. Such behavior testifies to a step-by-step development of the deformation and its delocalization in the sample.

Magnetic structure and properties of a bulk Fe72Al5P10Ga2C6B4Si1 alloy in the amorphous and nanocrystalline states

The structure forming under controlled crystallization of a bulk Fe72Al5P10Ga2C6B4Si1 amorphous alloy has been studied using differential scanning calorimetry, transmission electron microscopy, and x-ray diffraction. Crystallization of the alloy was established to result in the formation of a nanocrystalline structure consisting of three phases. The domain structure and magnetic properties of amorphous and nanocrystalline samples were investigated using the magnetooptic indicating film technique (MOIF) and a vibrating-sample magnetometer. The coercive force and the saturation magnetization of the amorphous samples were found to be 1 Oe and 130 emu/g, respectively. It was shown that the formation of the nanocrystalline structure entails a dramatic decrease in domain size (down to 1–4 µm) as compared to an amorphous sample (∼1 mm). Simultaneously, a decrease in the saturation magnetization and a strong increase in the coercive force of the samples were observed.

Effect of thermal treatment on the microstructure and magnetic properties of a bulk amorphous Fe72Al5P10Ga2C6B4Si1 alloy

The structure and chemical composition of the phases that form in the controlled crystallization of a bulk amorphous Fe72Al5P10Ga2C6B4Si1 alloy are studied by differential scanning calorimetry, transmission electron microscopy, and x-ray diffraction. It was established that, when the alloy is annealed at a temperature above the glass transition point, a nanocrystalline structure consisting of three phases arises. The magnetic properties of amorphous and nanocrystalline samples were studied with a vibrating-sample magnetometer. The coercive force and saturation magnetization of amorphous samples were found to be 1 Oe and 130 emu/g, respectively. The structure and chemical composition of the forming phases and their correlation with the magnetic properties of the samples were determined.