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Iron-based alloys have many useful functional properties that have been widely used in various technologies for a long time. One of these properties is "giant" magnetostriction, which was discovered in the early 2000s in Fe-Ga alloys. These alloys, whose magnetostriction shows two peaks depending on Ga content, have been actively studied for the past twenty years. [1.]. An interesting research topic is studying how adding rare-earth (RE) elements to Fe-Ga alloys affects their structure. Adding small amounts of rare-earth elements improves the magnetostrictive properties of these functional alloys. The physical and technical properties of such alloys largely depend on their specific atomic structure, the amount of different structural phases, and their microstructural state. Despite many studies, the mechanism of formation of increased magnetostriction in Fe-Ga alloys and its connection to atomic ordering is still under discussions.
Studies were carried out over a wide temperature range to observe changes in the phase composition and microstructure of as-cast magnetostrictive Fe$_{100-(x+y)}$Ga$_{x}$RE$_{y}$ alloys (where $x\approx$ 19 at.% and 27 at.%). These alloys were doped with small amounts ($y\approx$ 0.1 – 0.5 at.%) of rare-earth elements (Dy, Er, Pr, Sm, Tb, Yb), totaling 14 different compositions. The results were obtained from neutron diffraction experiments performed at the HRFD facility (JINR, IBR-2, Dubna) in two modes: high resolution in interplanar distance and high intensity with continuous temperature scanning. The samples were heated to $\sim$900$^\circ$C and then cooled at a rate of $\pm$2$^\circ$C/min [2.]. Information about the microstructural state of the alloys was obtained using the Williamson-Hall and Pelashek methods. These methods help to estimate the typical size and size distribution of coherent scattering domains by analyzing diffraction peak profiles.
Heating and subsequent cooling of Fe$_{81}$Ga$_{19}$RE alloys leads to the formation of $D0_3$ phase clusters with sizes ranging from 200 to 300 Å within a disordered $A2$ phase matrix [3.]. The structural changes in these alloys generally occur in the same way as in the original Fe$_{81}$Ga$_{19}$ alloy. In Fe$_{73}$Ga$_{27}$RE alloys with RE content from 0.1 to 0.5 at.%, both the sequence of structural phases that appear and disappear during heating and cooling and the final state of the alloy depend on the type and amount of the rare-earth element [4,5.]. In these alloys, a suppression effect was observed: adding RE elements inhibits the formation of phases with structures based on FCC ($L1_2/A1$) and HCP ($D0_{19}/A3$) unit cells and stabilizes phases with structures based on BCC ($D0_3/A2$) unit cells. The behavior of the Fe$_{73}$Ga$_{27}$Yb$_{0.2}$ alloy is different. In its initial state, a significant presence of the $L1_2$ phase (17%) was detected (Fig. 1). During heating, phases with structures based on the FCC unit cell ($A1/L1_2$) remain present up to 900$^\circ$C, which is not observed in any other composition. During cooling of the Yb-containing alloy, the equilibrium $L1_2$ phase with a lattice parameter of $a$ $\approx$ 3.687 Å becomes the main phase at T < 500$^\circ$C (Fig. 2), similar to other compositions. However, an additional phase (about 20%) also forms. It has the same peak system as $L1_2$ but with a larger lattice parameter of $a$ $\approx$ 3.708 Å.
The search for a structural order in the bulk of the alloy corresponding to the tetragonal $L6_0$ phase, which has been found in several electron diffraction studies of Fe-Ga alloys, did not give a positive result.
Literature:
[1.] E.M. Summers, T.A. Lograsso, M. Wun-Fogle, J. Materials Science, 42. 9582–9594 (2007). DOI: https://doi.org/10.1007/s10853-007-2096-6.
[2.] Balagurov A.M. and et al, Nuclear Inst. and Methods in Physics Research B, 436, 263–271 (2018). DOI: https://doi.org/10.1016/j.nimb.2018.09.045.
[3.] Balagurov A.M. and et al, Phys. Met. Metallogr. 125, p. 185–195 (2024). DOI: https://doi.org/10.1134/S0031918X2360286X.
[4.] Balagurov A.M. and et al, Phys. Met. Metallogr. 125, p. 525–534 (2024). DOI: https://doi.org/10.1134/S0031918X24600131.
[5.] Yerzhanov B., and et al, Appl. Phys. A 131, 356, (2025). DOI: https://doi.org/10.1007/s00339-025-08432-y.
