Phase equilibria among η-Fe2Al5 and its higher-ordered phases.

Phase equilibria among the η-Fe2Al5 phase and its higher-ordered phases with the η framework structure were determined experimentally. The solubility range of the η phase at elevated temperature does not differ remarkably from that in previous studies, but this phase is found to undergo complicated phase transformations upon cooling. Four phases are present, namely η', η", η"' and η m, with higher-order atomic orderings in the c-axis chain sites of the orthorhombic crystal structure of the parent η phase. The η" and η"' phases form on the Al-poor and Al-rich sides, respectively, in equilibrium with the ζ-FeAl2 phase below ~415°C and θ-Fe4Al13 phase below ~405°C. The η' and η m phases become stable below 312°C and 343°C with the peritectoid reactions η' → η m + η"' and η m → η + η", respectively. The η phase is not stable below 331°C with the eutectoid reaction of η m + η"' → η. On the basis of these findings, we unraveled the phase equilibria among the η-Fe2Al5 phase and its higher-ordered phases with the η framework structure.