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Project Number19-72-20080

Project titleHeterogeneous structural states in Fe-based alloys with magneto-mechanical coupling: correlation between physical and engineering properties

Project LeadGolovin Igor

AffiliationNational University of Science and Technology "MISiS",

Implementation period2019 - 2022

Research area 02 - PHYSICS AND SPACE SCIENCES, 02-210 - Interaction of X-rays, synchrotron radiation and neutrons with condensed matter

KeywordsFe-based alloys, atomic ordering, structural decomposition, heterogeneous nanostructural states, neutron diffraction



Iron-based intermetallics and ordering alloys are functional materials, the physical and engineering properties of which strongly depend on their specific atomic structure, the volume content of various structural phases and the microstructural state. In particular, there is a strong dependence of strength properties, electrical conductivity, internal friction coefficient, and magnetostriction on the degree of atomic order in some iron-based alloys (Fe-Al, Fe-Co, Fe-Ga, Fe-Si, etc.), and on the atomic disordering of substitutional solid solution in the Fe-Cr composition. For example, in the Fe-xGa (Galfenol) system, the magnetostriction constant at x ≈ 19 is 40 times greater than in α-Fe. The doping of this alloy with a small amount (less than 1 at. %) of the rare-earth element further increases this value. The atomic ordering in Fe-Al compositions and atomic disordering in Fe-Cr leads to alloys hardening, embrittlement, and a significant decrease in the damping properties of these materials. Although the record values of magnetostriction in the Fe-Ga system and related internal friction values are already actively used in industry (manufacturing of sonars, sensors, actuators), the physical reasons for their formation remain unclear. Recently, the microstructural aspects of the formation of unusual properties of these materials are being given primary attention. According to the presently basic model, the giant magnetostriction appears due to the formation of nonequilibrium atomic structures with local inhomogeneities of atomic scales, including atoms of doping elements and, in particular, rare-earth metals. On the next size level, under certain conditions, the bulk microstructure of the alloy is formed from the regions (clusters) of nano- or mesoscopic dimensions with an ordered structure dispersely distributed in a structurally disordered matrix. Some models assume a formation of easy-moving nanoscale clusters with short-range order in the atomic arrangement. The change in external conditions, especially in temperature, drastically affects the atomic structure, the microstructure and, accordingly, the material-science properties of iron-based alloys. Thus, in the cast Fe-27Ga composition, in the temperature range from room value to 900°C, three phase transitions occur, during which the cubic crystal system changes to hexagonal one and vice versa. In addition, the cubic structure varies from body-centered to face-centered. In Fe-xCr alloys with x > 20%, as a result of the spinodal decomposition of substitutional solid solution, clusters of Guinier-Preston type are formed. The magnetic and damping properties of alloys also drastically change. Accordingly, the most complete information on the structural and microstructural properties of the alloy can be obtained from experiments conducted in “in situ” mode with a continuous change in the external action. The main methods for studying the structural states of alloys at the atomic and nanoscopic levels are diffraction and small-angle scattering of short-wave radiation (X-ray, synchrotron and neutron) and transmission electron microscopy (TEM). The main advantage of X-ray and especially synchrotron X-ray diffraction is the high luminosity of the method and the ability to form a small cross-section beam (~50 μm), which makes it possible to analyze the local structure and thereby reveal the scale of the inhomogeneity. Conversely, neutron diffraction is a purely bulk method and allows to determine the average characteristics not distorted by local fluctuations. Small-angle neutron scattering allows tracing the formation of structure inhomogeneities at the nanoscopic level that arise during spinodal decomposition of the alloy. Using TEM it is possible to obtain information on the shape and size of the ordered regions, their changes under external action and on the type and distribution of defects of the ordered structure. Information received by the above mentioned experimental methods is, as a rule, mutually complementary, and within the framework of the project these methods will be used together. Nevertheless, in this project, the neutron scattering is considered as the main method, since it allows to obtain quantitative information about the atomic and magnetic structures of the material, its phase composition and the features of its microstructural state. Moreover, neutron experiments will be organized in in situ and real-time modes. From in situ diffraction experiments one can quantitatively determine the temperature dependences and the features of the kinetics of changes for volume fraction and unit cell parameters of the structural phases present, occupancy factors for the crystallographic positions, microstresses in the crystallites, and characteristic dimensions of the coherently scattering domains. The experiments will be carried out with neutron spectrometers operating at the IBR-2 pulsed reactor at JINR (Dubna). The IBR-2 reactor and its complex of spectrometers are the only infrastructural facility in Russia enabling to conduct neutron scattering experiments on the world level. Two scientific groups – from NUST MISIS (Moscow, team leader is I.S. Golovin) and from FLNP JINR (Dubna, team leader is A.M. Balagurov), which are effectively cooperating over the past few years, will be working together on this project. Specialists from MISIS will determine the material science direction of the research; provide preparation of samples and testing of their physical and mechanical properties, including magnetic and damping characteristics. Researchers from JINR will be responsible for carrying out neutron experiments and preliminary data processing. Both scientific groups consist of qualified specialists with extensive work experience. Their joint work has already resulted in a whole series of new, fundamental results, which is confirmed by publications in the highest-ranked international scientific journals. The data obtained in this project will be of fundamental importance for the theory of iron-based intermetallic compounds and ordered alloys and for the analysis of the correlations of the microstructural state with the physical characteristics of these materials.

Expected results
The main expected result is the establishment on several spatial levels of an interrelation between physical and engineering properties of iron-based intermetallics and ordering alloys (functional materials) with the organization of their atomic structure and microstructural state. The connection of unusual functional properties of these materials, for example, giant magnetostriction, with their heterogeneous structural state is already considered as established, but the reasons for their formation have not yet received a generally accepted physically reasonable interpretation. The solution of this scientific problem has a fundamental importance for the theory of intermetallides and ordered alloys and for a purposeful search for new compositions and methods for their preparation. Within the frame of the general problem, many particular results relating to the organization of heterogeneous structural states in different compositions and under various external influences will be obtained. In addition, unique experimental data on the processes of transition of as cast alloys to the equilibrium state will be obtained. For this, experimental maps will be systematically constructed in the coordinates "phase state - temperature - time". Maps will be build in three different modes of exposure: - during continuous heating - cooling process with a constant speed (in situ), - with isothermal annealing at a given temperature (in situ), - after long exposures at a given temperature (ex situ). In addition to the fundamental meaning, the obtained results will be of extremely important practical importance, since materials that have either already been widely used or attracted interest in recent years will be studied. The principal experiments within the frame of the project will be carried out at a world-class infrastructure facility - the IBR-2 high-flux pulsed reactor (JINR, Dubna). The unique characteristics of the instruments on it and the applied advanced methods of data processing will ensure world-class level and originality of the results obtained.



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, 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.



1. Головин И.С., Палачева В.В., Мохамед А., Балагуров А.М., Бобриков И.А., Самойлова Н.Ю., Сумников С.В. Phase Transitions in Metastable Fe-Ga Alloys IARIA, The Tenth International Conference on Sensor Device Technologies and Applications, Nice, France, 27-30.10.2019, p. 13-16 (year - 2019).

2. Головин И.С., Балагуров А.М., Бобриков И.А., Сумников С.В., Мохамед А.K. Cooling rate as a tool of tailoring structure of Fe-(9–33%)Ga alloys Intermetallics, Volume 114, 2019, 106610 (year - 2019).

3. Балагуров А.М., Самойлова Н.Ю., Бобриков И.А., Сумников С.В., Головин И.С. The first- and second-order isothermal phase transitions in Fe3Ga-type compounds. Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials, B75, 1024-1033 (year - 2019).

4. Головин И.С., Мохамед А.К., Палачева В.В., Чеверикин В.В., Поздняков А.В., Коровушкин В.В., Балагуов А.М., Бобриков И.А., Фазел Н., Моуас М., Гассер Д.-Г., Гассер Ф., Табари П., Лан К., Ковас А., Остендорп С., Хубек Р., Дивинский С., Вилде Г. Comparative study of structure and phase transitions in Fe-(25–27)%Ga alloys Journal of Alloys and Compounds, Volume 811, 152030 (year - 2019).

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