Phase Transformation in the Rare Earth-Cobalt Alloys

Abstract

The phase relationship and the phase transformation in the rare earth-cobalt (R-Co) alloys were investigated by using a pair approximation. The structures of the alloys investigated were RCo₅ (CaCu₅ type), R₂Co₁₇ (Th₂Zn₁₇ type), and R₂Co₇ (Gd₂Co₇ type), It was proposed that RCo₅ and R₂Co₁₇ phases coexist stably with each other because the value of ordering energy depends on the distance between atoms. The phase separation of RCo₅→RCo₅+R₂Co₁₇ in Sm-Co and Gd-Co alloys follows spinodally if ordering of the Th₂Zn₁₇ type occurs. The composition of the eutectoid transformation RCo₅→R₂Co₇+R₂Co₁₇ shifts to the Co-rich side with decreasing ordering temperature and increasing eutectoid temperature.