Scientists of the Laboratory of Neutron Physics JINR have conducted a series of experiments on the study of phase transitions in iron-gallium alloys – galfenols. In cooperation with researchers at the University of Science and Technology MISIS and the Institute of Materials Science of Freiberg University of Mining and Technology, they have revealed a Fe₁₃Ga₉ phase, deciphered its structure, studied the area of its existence, i.e. conditions under which it appears and disappears, for the first time. The study focused on the determination of conditions, under which such a phase can exist in an alloy when it stops having such a property as giant magnetostriction, a special reaction to the magnetic field. This will help obtain materials with a specified structure and determine how this structure affects their properties in the future.

Specialists use materials with giant magnetostriction for the production of devices for transmitting sound, including ultrasound, and magnetic field sensors. They are also used in microdisplacement mechanisms and pressure devices, audio and electrical signal delay lines, linear and rotary motors, magnetomechanical relays, and other radio engineering and telecommunication devices.

Magnetostriction is a phenomenon of rapid metal stretch and compression under the influence of the magnetic field. If the magnetic field pulsates, the metal item pulsates with it. There are metals with magnetostriction orders of magnitude higher than others. This phenomenon is called giant magnetostriction (GM). For example, rare earth metals, but not only, have such a property. Gallium is not one of them. However, in the early 2000s, specialists found out that the addition of gallium to iron could enhance the magnetostrictive effect of iron by about ten times. To date, the most widely used material with the largest magnetostriction coefficient is terphenol-D. It consists of iron, terbium, and dysprosium. Its disadvantage is low mechanical properties, as it is very fragile and difficult to machine. Iron-gallium alloys have a slight advantage in this regard: they are not as stretchable as iron, but they are much better processed than terphenol-D.

It is also important that they have giant magnetostriction at normal, room temperature. “The first alloys in which this effect was observed obtained GM at significantly negative temperatures. Their scope of application was very limited. Fe-Ga alloys have a significant advantage in this regard, and they are very promising for their practical application,” a FLNP JINR senior researcher and a co-author of the study Tatiana Vershinina said. She explained that scientists had not studied the very nature of giant magnetostriction in iron-gallium alloys. “Scientists are trying to reveal this secret. If we can control magnetostriction, we will manage to create different devices with specified characteristics This is the main task of everyone involved in the study of these alloys,” she said. The research by FLNP was focused on the part of the task that determined the existence of one particular phase with a Fe₁₃Ga₉ chemical formula in alloys. Scientists also studied the way it reacted to different temperature ranges.

A phase is a part of an alloy with uniform chemical composition, crystal structure, physical properties. It is separated from other phases by a definite boundary, crossing which all these characteristics abruptly change.

Tatiana Vershinina explained that scientific groups dealing with galphenols have focused their attention for a long time on alloys with a relatively low gallium concentration, < 30 atomic percent (at. %), since it was known that the Fe-Ga system was characterized by the presence of two peak magnetostriction values corresponding to 19-20 and 27-29 Ga. In the current study by JINR, scientists used samples with 31-35 at. % of gallium. "It is necessary to study them, since the Fe₁₃Ga₉ metastable phase can be obtained at high concentrations of gallium. Researchers can observe this phase in these alloys, according to theoretical estimates, at the decreased concentration of Ga down to 25 atomic percent,” the scientist said. The presence of this phase in an alloy can lead to a rapid decrease of magnetistriction at a concentration of Ga > 27 at. %.

FLNP JINR scientists have conducted a series of diffraction experiments at the IBR-2 Reactor aimed at the determination of concentration ranges and conditions in general under which the Fe₁₃Ga₉ can appear. To carry out research using extracted beams at the IBR-2 Reactor, they used a high resolution Fourier diffractometer (HRFD). “Neutron diffraction allows us to study large volumes of material. For comparison, for example, X-ray diffraction analysis allows scientists to study only a thin surface layer of the sample,” the scientist added. Researchers were making diffractograms in situ – continuously while heating.

At the diffractometer, they performed a comparative analysis of evolution in phase composition in as-cast Fe-(31-35)Ga alloys during continuous heating up to 850 °C and subsequent cooling. The as-cast state means that specialists cooled down the heated sample very fast.

Scientists have discovered that phase transitions in these metastable alloys occur similarly. Moreover, they have several stages characterised with a definite set of features.

Fig. 1 Evolution in phase composition of Fe–31Ga alloys during heating and subsequent cooling

Alloys with a high Ga concentration of 32 – 35 at. % had the Fe₁₃Ga₉ phase before the research. It was in samples until scientists heated them up to ~ 570 °C, which led to the Fe₁₃Ga₉ – α-Fe₆Ga₅ phase transition (alpha phases of iron that can exist at low temperatures). “It is interesting to note that it is necessary to reduce the concentration of iron in Fe₁₃Ga₉ beforehand to make the phase transition possible. That is why specialists can obtain this phase transition after the formation of L12 phase rich in iron,” Tatiana Vershinina commented. At the same time, researchers have discovered that the Fe₁₃Ga₉ phase does not occur again during the cooling of alloys with 32-35 at. % Ga.

Samples of alloys with 31 at. % Ga did not have the Fe₁₃Ga₉ phase initially. However, it occurred during the heating up to ~ 425 °C and existed, like in other alloys, until temperature became 570 °C. Unlike alloys with higher concentration, the Fe₁₃Ga₉ intermetallic is formed in the Fe-31Ga alloy during cooling at 570 °C, inhibiting the formation of α-Fe₆Ga₅.

According to theoretical estimates, the Fe₁₃Ga₉ phase can also exist at those concentrations of gallium and temperatures that lead to giant magnetostriction. “We believe that the formation of the Fe₁₃Ga₉ intermetallic and its increased concentration in an alloy has a negative effect on alloy properties. That is why it is desirable to control its presence in materials,” the researcher highlighted. We have studied conditions under which it occurs. Specialists can use the obtained results, among other things, to select those conditions of alloy processing that specialists must fulfil to prevent the formation of this phase in alloys. We have shown that its existence when heating at a rate of 2 ˚/min is limited by 570 °C. If we heat it up to even higher temperatures, we will definitely get rid of it.”

In 2023, the Journal of Alloys and Compounds published an article about the research “Comparative study of structures and phase transitions in Fe-(31-35)Ga alloys by in situ neutron diffraction” authored by T. N. Vershinina, N. Yu. Samoylova, S. V. Sumnikov, A. M. Balagurov, V. V. Palacheva, I. S. Golovin. FLNP JINR scientists will continue studying iron-gallium alloys with and without various rare earth elements when specialists switch the IBR-2 Reactor from the shut-down mode.