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The study of phase transformations in iron-based alloys is one of the key areas of materials science. Particular attention is devoted to Fe-based systems alloyed with elements such as Ga and Ge. These additions can significantly affect magnetostrictive properties, making such alloys promising for practical applications [1]. It is well established that the initial chemical composition of an alloy plays a decisive role in the formation of phases, their stability, and the sequence of transformations with changing temperature [2]. While extensive data have been accumulated on structural transformations and functional properties in the binary Fe–Ga and Fe–Ge systems, ternary Fe–Ga–Ge alloys remain insufficiently studied [3]. Understanding the influence of the initial phase composition of Fe–Ga–Ge alloys on the sequence of phase transformations is essential not only for clarifying the fundamental principles of structure formation, but also for tailoring their functional and operational properties.
In the present work, the evolution of the phase composition was investigated in alloys of closely related elemental compositions, Fe74.4Ga13.1Ge12.5 and Fe75Ga12.5Ge12.5, which differed in their initial phase states. In the as-cast condition, the Fe74.4Ga13.1Ge12.5 alloy contained only Fe3(Ga,Ge) with D03-type ordering, whereas Fe75Ga12.5Ge12.5 exhibited two polymorphic modifications of Fe3(Ga,Ge) with D03- and D019-type ordering. The alloys were subjected to continuous heating up to 850 °C (Fe74.4Ga13.1Ge12.5) and 1000 °C (Fe75Ga12.5Ge12.5), followed by cooling at a rate of 2 °C/min. Measurements of Fe75Ga12.5Ge12.5 were performed using neutron diffraction on the General Purpose Powder Diffractometer (GPPD) operating at the spallation source CSNS (China), while the Fe74.4Ga13.1Ge12.5 alloy was studied on the high-resolution Fourier diffractometer (IBR-2, JINR).
It is known that the phase composition of Fe75Ge25 in the as-cast state consists of a mixture of Fe3Ge (D019) and B82 [4], whereas Fe75Ga25 exists in a single-phase D03 state [5]. Thus, the phase composition of Fe75Ga12.5Ge12.5 and Fe74.4Ga13.1Ge12.5 naturally represents an intermediate variant between the limiting binary alloys Fe75Ge25 and Fe75Ga25.
Upon heating the Fe74.4Ga13.1Ge12.5 alloy to 700 °C, the D019 phase appears. The two-phase mixture D03 + D019 persists up to 850 °C as well as during subsequent cooling down to room temperature.
In Fe75Ga12.5Ge12.5, an increase in the integral intensities of the D019 peaks is observed at ~680 °C, indicating an increase in the volume fraction of this polymorphic modification. At 950 °C, a D03 + D019 → B2 transition occurs. It is likely that this transformation also takes place in Fe74.4Ga13.1Ge12.5, but due to the experimental temperature limitation it was not observed. During cooling of Fe75Ga12.5Ge12.5, the D03 phase reappears at an unusually high temperature (T ≈ 900 °C), corresponding to the reverse B2 → D03 transformation. The D019 phase emerges at T ≈ 880 °C.
From the variation of the integral intensities of the (111) D03 and (101) D019 reflections, the Curie temperatures of both phases were determined to be ~500 °C and ~550 °C, respectively.
Thus, regardless of the initial phase composition, both studied alloys demonstrate a similar sequence of phase transformations and comparable Curie temperatures for the D03 phase.
