Annotation of the results obtained in 2022
We studied comprehensively several structures of iron-based alloys. Using synchrotron radiation, our studies were carried out with the purpose to search for micro- or nano-ordered inhomogeneities with tetragonal lattice distortion (structures of the type D022, L60, etc.). It is shown that no signs of these structures are found in bulk samples using the best current intensities of synchrotron radiation beams. This means that in the volume of Fe-Ga or Fe-Al samples these structures are not formed in a significant volume, which can determine the macroscopic properties. The superstructure peaks typical for the m-D03 phase are absent in the diffraction patterns for both ND and XRD tests measured even at extremely high counting statistics. Thus, it is concluded that m-D03 does not exist in the quenched alloy of the Fe-27Ga type in any significant amount. Nanoscale inclusions with tetragonal symmetry may be present in the surface layer, but their possible volume fraction is too small to affect bulk physical properties such as magnetostriction. Based on a comprehensive study of the structures of a wide range of alloys and long-term annealing regimes, TTT-type (temperature-time-transition) diagrams were experimentally constructed for a set of Fe-Ga alloys. The constructed diagrams indicate that the fastest kinetics of the transition from the nonequilibrium A2 or D03 structure to the equilibrium L12 structure is observed in a temperature range of about 450–500°C and, it weakly depends on the gallium content in the alloy. As a result of a comprehensive study using long annealed binary alloys with a Ga content of 15 to 45 at.% (up to 75 days) at temperatures from 300 to 575°C the low temperature range of an equilibrium Fe-Ga phase diagram is revised and corrected. The resulting diagram largely refines the previously existing diagrams in the low-temperature (up to 600°C) region. Alloying of Fe-(25-28)Ga alloys with REM (REM = Tb, Er, Yb, Pr and Sm) has a clear suppressive effect on the phase transition from D03 to L12, which is confirmed by a decrease in the PTR peak in the TDIF curves, neutron diffraction and several helper methods. The effect of doping REM becomes pronounced from a concentration of 0.2% REM and increases with an increase in the content of REM up to 0.5-0.6%. The reason for this effect is the formation of enriched precipitates at the grain boundaries of the cast sample, which suppress the nucleation of the L12 phase during heating or annealing of the cast Fe-(24-28)Ga alloy and, therefore, stabilize the D03 structure. An increase in Δλ with an increase in the content of Tb from 0 to 0.1 at.% in Fe-20Al-xTb alloys, associated with an increase in the degree of texture (110), significantly enhances dλ///dH and, consequently, the effects of ΔE and ΔG. At a higher Tb content (x =0.15), texture (110) leads to a decrease in Δλ and dλ///dH and, accordingly, the effects K, ΔE, and ΔG. Optimized magnetomechanical properties achieved for Fe79.9Al20Tb0.1 alloy obtained by directional crystallization are Δλ 144ppm, dλ///dH  = 0.28ppm/Oe, K = 29.74%, ΔE-effect 9.1% and an ΔG effect of 5.4%.

 

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Annotation of the results obtained in 2019
Structure and phase transformation in the bulk samples of the Fe-Ga alloys with different gallium content have been investigated by neutron diffraction at High Resolution Fourier Diffractometer at the pulsed neutron source IBR-2 (Frank Laboratory of Neutron Physics, JINR, Dubna, http://flnph.jinr.ru/ru/facilities/ibr-2/instruments/hrfd). The structure and dependencies of lattice parameter for A2 and D03 on Ga content in Fe–Ga alloy after cooling at several rates have been obtained (Golovin et al., Intermetallics 114 (2019) 106610). The cluster-like distribution structure of the ordered phase in a less ordered or disorderd matrix has been established. The results obtained indicate the correlation between structure ordering of the alloys with their magnetostriction. The first and second critical cooling rates for Fe-27%Ga composition with respect to the beginning and end of formation of L12 phase have been determined and temperature-time-transition (TTT) diagram has been constructed. Two as-cast Fe-Ga alloys with structures close to the stoichiometric A3B composition (25.5 and 26.9 at.% Ga) were studied during heating and cooling with different rates, including long-term annealing (up to 300 h) by neutron diffraction, XRD, DSC, VSM, dilatometry, internal friction, HV, SEM and TEM (Golovin et al., Journal of Alloys and Compounds 811 (2019) 152030). Heating of these alloys and their subsequent cooling leads to a cascade of phase transitions which change their structural, mechanical and physical properties. Transition from metastable D03 to equilibrium L12 phase at heating occurs through the sequential formation of the disordered bcc A2 and fcc A1 structures according to a D03 → A2 → A1 → L12 scheme and leads to significant changes in macro and micro structure, hardness and magnetic properties. Structural features and kinetics of the transition between D03 and L12 phases of Fe–Ga alloys (27.2 and 28.0 at.% Ga) have been analyzed by in situ real-time neutron diffraction during isothermal annealing in the temperature range from 405 to 470 °С (Balagurov et al. Acta Cryst B75 (2019), 1024-1033). The transition proceeds according to a D03 → A2 → A1 → L12 scheme. Deformations of the crystal lattice arising due to these transitions are determined.The kinetics of L12 phase nucleation and growth were analyzed in the frame of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model; only the early stage of the D03 to L12 transition is well-described by the JMAK equation. The value of the Avrami exponent corresponds to the constant growth rate of the new L12 phase and decreasing nucleation rate in Fe-27.2Ga alloy and indicates the presence of pre-existing nucleation centers of L12 phase in Fe–28.0Ga alloy. Damping capacity of the Fe-18Cr alloy was measured to study the correlation between heat treatment, grain size, damping capacity and magnetostrictio (Mohamed et al., Journal of Magnetism and Magnetic Materials 494 (2020) 165777). The optimal properties for Fe-18Cr binary alloy were achieved after annealing of cold-rolled sheets at 840 °C. In contrast with damping capacity, the most significant changes in mechanical properties of the cold rolled alloy with the increase of annealing temperature took place at a lower temperature (600–650 °C). Damping capacity of the alloy decreases with the increase of the grain size, so annealing at a temperature higher than 900 °C decreases damping capacity and provides reduction of mechanical properties of the alloy, including the yield strength and the ultimate tensile strength. Slow cooling of the samples during high-temperature heat treatment causes a marked decrease of in the impact toughness, reduction of damping capacity and increase in the coercive force of the alloy. Annealing of cold rolled samples of Fe-18%Cr alloy at 600–950 °C leads to valuable increase in longitudinal magnetostriction (more than 2.5 times compared with the cold rolled state). The enhanced magnetostriction provides a necessary condition for the activation of magnetic domain walls motion (under the field of external alternating elastic stress) and hence for the formation of high damping state.

 

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Annotation of the results obtained in 2020
When Fe-Ga alloys are cooled after heating to the A2 region, the final structure is determined by the gallium content and the cooling rate. The temperature – time – transformation (TTT) diagram of the decomposition of the nonequilibrium high-temperature phases of Fe-27% Ga upon cooling was constructed, and the critical cooling rates were determined with respect to the transformation between metastable and equilibrium phases. The first critical rate is the cooling rate at which the formation of the equilibrium L12 phase begins (VCr1 = 30 K/min), the second critical rate VCr2 (5–8 K/min) is the cooling rate at which the formation of the equilibrium phase has time to pass almost completely on the sample surface. It was found that, depending on the cooling rate in the interval between the first and second critical rates in alloys with 27% Ga, two transformation schemes can be realized: A2 → B2 → A2 → А1 → А3 → D019 → L12 or A2 → B2 → A2 → А1 → А3 → L12. With slow cooling, during which all transformations have time to go through, the first scheme is first realized with the formation of the D019 phase, which passes into the L12 phase with decreasing temperature. With an increase in the cooling rate, the D019 phase does not have time to form, and the L12 phase is formed as a result of the bcc → fcc transformation. These results are published in [Materials Letters, 263 (2020) 127257]. New data have been obtained on the effect of an external magnetic field on the transformation from D03 to L12 phase. For this, the Fe-24% Ga and Fe-27% Ga samples were subjected to isothermal annealing for 40 or 60 minutes without and with the application of a magnetic field of 25 Tesla. For all samples, neutron diffraction spectra were measured, from which it followed that the volume fraction of the L12 phase in the samples exposed to the field increased several times. These results are published in [Journal of Magnetism and Magnetic Materials, 514 (2020) 167284]. Diffraction experiments with a synchrotron radiation beam (ESRF, Grenoble) were additionally performed on the compositions Fe-19Ga and Fe-27Ga, previously studied in detail using neutron diffraction. The high intensity and small size of the synchrotron beam made it possible to record diffraction from one single crystal grain with a minimum contribution from neighboring grains. The synchrotron stations BM01A (λ = 0.6867 Å) and ID28 (λ = 0.6968 Å), on which the experiments were carried out, made it possible to carry out an almost continuous scanning of the reciprocal space of single crystals. It was found that the average structure of the Fe-19Ga composition is very close to the disordered A2, but spatially it is modulated by waves of the ordering factor with a characteristic scale of ~ 5 nm. For the Fe-27Ga composition, it was found that some of the observed reflections correspond to the D03 structure, but, in addition, a large number of reflections corresponding to a triple cubic cell were recorded. Further analysis showed that additional reflexes can be indicated in a hexagonal lattice commensurate with the alpha-iron lattice. This hexagonal phase has not previously been observed. Its explicit presence in the studied single crystal in the form of inclusions in the D03 matrix may be of interest for theoretical calculations of the structure of these and alloys with similar compositions. From a methodological point of view, these experiments showed that the neutron and synchrotron diffraction data are mutually complementary and their joint analysis leads to new results. A similar conclusion was made based on the results of a joint analysis of neutron and X-ray diffraction data obtained on the Fe-27Ga composition when it is slowly heated to 850 ° C and then cooled. It is shown that the different penetrating ability of neutrons and X-rays and, accordingly, their sensitivity to the bulk and surface structures of the alloy, leads to noticeably different results. In particular, it was shown that an A2 phase deficient in gallium content with traces of D03 forms on the sample surface. Only it is identified by X-ray diffraction spectra. At the same time, neutron diffraction unambiguously indicates the formation of a mixed state (L12+D03) in the sample volume. These results were published by [Journal of Applied Crystallography, 53 (2020) 1343–1352].

 

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Annotation of the results obtained in 2021
One of the main goals of the project in 2021 was to increase the accuracy of equilibrium diagrams of the Fe-Ga compositions. For this, experimental studies of almost 30 different alloys with a Ga content of 15 to 45 at% were carried out. For studied alloys, structural phases composition, the kinetics of phase transformations during heating, and long isothermal holdings in the range of 300 to 600°C were clarified. It was found that annealing time of 300 h turned out to be insufficient to achieve the equilibrium state of the alloys; therefore, it was increased to 1800 h (up to 2.5 months) for a few compositions. Comparison of obtained data with the known equilibrium phase diagrams, final analysis, and publication is scheduled for the next stage of the project. The existing phase diagrams (equilibrium and metastable) of Fe-xGa alloys in the range of Ga concentrations up to 30 at.% contain structural phases A2, B2, D03, L12, and D019. With a further increase in x, more complex phases appear in Fe-xGa, the structure of which has remained controversial until recently. In addition, the literature reports three possible non-standard types of ordering, namely, D022, m-D03, and L60, which are not present in any of the published Fe-Ga phase diagrams. Hints of their existence in the form of locally ordered regions have been found by electron diffraction in a few experiments. The D022 structure belongs to the BCT structure and is considered as a precursor of the L12 phase during the D03 → L12 transition. The modified D03 (m-D03) structure can be converted to L60 by changing the coordinate system, and both belong to the MCP type. Interest in the non-standard structures D022, m-D03, and L60 is due to an increase of magnetostriction of Fe-Ga alloys. This increase is associated with the presence of nanostructured inclusions with tetragonal lattice symmetry, which were detected by the split of the main diffraction peaks, which was observed in several X-ray studies. Within the framework of the project, in 2021, a series of neutron diffraction experiments were carried out on Fe-xGa alloys with different gallium contents, aimed at searching for tetragonal phases, which, however, were not detected. It was assumed that the observed split of the peaks can be associated with the formation of two-phase states (D03 + B2) or (D03 + A2) in the surface layer, but not with inclusions of D022 or m-D03. Accordingly, a study of the crystal structure of a bulk sample of a quenched Fe-27Ga alloy by neutron and X-ray diffraction was carried out in order to establish the distribution of the phase composition over the volume of the material. The results obtained confirmed the presence of an ordered D03 phase in the bulk of the sample and revealed the formation of B2 and A2 phases in its surface layer, which led to the split of diffraction peaks. Thus, the split of the peaks detected in the surface layers of alloys or thin ribbons by X-ray or electron diffraction methods can be associated with the formation of incoherent two-phase states. These results are currently being verified and, if confirmed, will be published. The structural features of the Fe-xGa compositions in the range of the phase diagram up to x ≈ 30 at.% are widely presented in the literature. On the contrary, the region of high gallium concentrations (x> 30 at.%) has been insufficiently studied, and until now, it is still unclear which phases can be formed under certain conditions, and their atomic structure continues to be clarified. In the reporting year, we carried out studies of several compositions with x> 30 at.%, namely, Fe-31.1Ga, Fe-31.25Ga, Fe-32.9Ga, Fe-38.4Ga, Fe-45.0Ga. To determine their phase composition and atomic structure, neutron, and X-ray diffraction, scanning electron microscopy (SEM), and energy dispersive X-ray analysis were used. Some of the samples were further examined using transmission electron microscopy (TEM). Calculations using the density functional theory (DFT) were carried out to independently analyze the atomic structures of the compositions Fe-38.4Ga and Fe-45.0Ga. It was found that the main phase for Fe-38.4Ga is the Fe13Ga9 intermetallic compound. This structure belongs to the known type Ni13Ga9 and according to DFT calculations, it is stable (at least at low temperatures), although it was previously assumed to be metastable. For the Fe-45.0Ga composition, the main phase is the Fe13Ga13 intermetallic compound, which was previously designated in the phase diagrams as β-Fe6Ga5. DFT calculations have confirmed the experimental data. Cycles of slow (2 ° C / min) heating to 900°С and subsequent cooling to room temperature of a series of Fe-Ga alloys, during which neutron diffraction spectra were measured in situ, made it possible to reveal the structural phase transitions. A comprehensive study of anelastic effects in Fe-Ga alloys with 15 <Ga <45% allowed us to establish the regularities of the formation of thermally activated Snoek and Zener anelastic effects and to analyze transient effects associated with both phase transitions and relaxation of the structure of quenched samples during annealing. The obtained results will be examined and published in 2021. The study of the kinetics of the formation of the equilibrium structure (D03) of an alloy of the Fe3Al type upon heating and cooling was planned with the composition Fe-26Al, which was prepared in three states: in as-cast, after annealing at 450°C (for D03 ordering), and after water quenching. The Fe-26Al ingot was cooled in a graphite mold in an argon atmosphere inside the furnace, and the average ingot cooling rate was about 2 K/min (cooling time was 14 h), which led to the formation of an equilibrium state with the D03 structure. According to the plan on experiments using neutron diffraction, Mössbauer spectroscopy, VSM, and DSC, the following measurements have been carried out: (i) measurements of phase transformations (in situ neutron diffraction), anelasticity (DMA Q800), and magnetization, as well as magnetostriction, (ii) measurements during heating and cooling at different speeds. Due to some technical problems, the completion of these studies has been shifted to the next stage. The performed measurements are analyzed in order to prepare a corresponding publication, in which special attention will be paid to the cluster structure in Fe-Al alloys and its comparison with the structure of Fe-Ga alloys. According to the plan, the influence of microalloying of iron-gallium alloys with rare earth elements (Dy, Er, Tb, Yb, La, etc.) has been completed. Investigations of anelastic effects in Fe-(16-21)at.%Ga alloys without and with RE (RE = La, Tb, Dy, Er, Yb) at room and elevated temperatures were carried out. In most of the alloys, two thermally activated and two transient effects reflecting structural, or phase transformations were obtained. For interpretation of the obtained results on anelasticity, the structure and phase transitions in several binaries and ternary Fe-Ga alloys were investigated. D03 ordering of rapidly cooled alloys with the A2 structure start with heating to 300°C and disordering D03 happened when the sample cooled down to 500°C. For annealed samples were used three in situ methods: neutron diffraction, magnetometry, and internal friction, which were confirmed by experiments on the annihilation of positrons. In addition to thermally activated effects, two transition effects were found, caused by first-order phase transitions D03 → L12 and B2 / D03 → Fe13Ga9 (in alloys with Ga content> 29 at.%). According to the data of internal friction, REM elements reduce the rate of phase transitions. Measurements of the temperature dependences of internal friction on Fe-Al-RE alloys in the cast state have been carried out. Terbium was selected as RE based on the literature review. The addition of terbium to the Fe-12Al alloy did not have a noticeable effect on the TDIF. A thermally activated Zener peak is observed at TDIF for an alloy of the Fe-12Al-0.2Tb type upon cooling in the range 450-600°C and low frequencies of 0.1-1 Hz. There is no similar thermally activated effect on the TDIF during heating, which can be explained by the nonequilibrium state of the material in the cast state. A thermally activated Zener peak in the temperature range 400-600°C was found on the TDIF during heating and cooling for as-cast alloys of the composition Fe-21Al-0.2Tb and Fe-21Al-0.3Tb. The nature of this peak is evidenced by the activation parameters calculated using the Arrhenius dependences. The obtained results demonstrate a correlation between temperature-dependent internal friction, neutron diffraction, and magnetometry. The RE-effect during prolonged annealing (up to 1800 h) was investigated on several samples; after completion of all studies, the results will be analyzed and published in 2022. The results of the studies and their analysis were published in 2021 in 6 articles in the journals of the first quartile (Q1), and they were also presented in three reports at conferences, included in the special issue ICIFMS-19 of J. Alloys and Compounds (https: // www .sciencedirect.com / journal / journal-of-alloys-and-compounds / special-issue / 10MHG1DWWC1). The results of the research performed were also included in the materials of Mohamed A.K.'s thesis, successfully defended in 2021 "Formation of the structure of cast Fe-Ga alloys under controlled cooling and annealing" (on June 21, 2021), and were used in the review "Neutron scattering in studies of functional Fe-based alloys (Fe-Ga, Fe-Al)" (A.M. Balagurov, I. S. Golovin, Physics. Uspekhi 64 (7) 702-721 (2021) = Uspehi Fiz. Nauk 191 (7) (2021) 738-759). Based on the results obtained in 2021, new papers will also be presented for publication in 2022.

 

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