Crystal Structure and Peritectoid Reaction Temperature of Ta ₂ Co ₇ in the Co–Ta Binary System

Ta ₂ Co ₇ is considered a potential candidate for a crystalline “mille‐feuille” structure material, similar to Nb ₂ Co ₇ . However, the crystal structure of Ta ₂ Co ₇ remains unknown, and several structures have been proposed. In addition, although it is known that Ta ₂ Co ₇ forms via a peritectoid reaction, the invariant reaction temperature varies widely across studies, ranging from 950°C to 1033°C. In the present study, to elucidate the crystal structure and peritectoid reaction temperature of Ta ₂ Co ₇ , Co–Ta alloys were prepared with long isothermal heat treatments up to 10 000 h, and detailed experimental analyses were performed using electron‐probe microanalysis, X‐ray diffraction, and differential scanning calorimetry (DSC). Microstructural observation of long‐term aged samples reveals that the phase previously reported as Ta ₂ Co ₇ should be reinterpreted as β‐Co ₃ Ta, with a stoichiometric composition of 25 at% Ta and a rhombohedral BaPb ₃ ‐type (space group: 166, Rm) crystal structure. The formation of β‐Co ₃ Ta is considered very slow and absent in the as‐cast state. Detailed thermal analyses by DSC elucidate that the peritectoid reaction temperature of β‐Co ₃ Ta is (995 ± 1)°C.