Phase relations in the mischmetall-cobalt system

Abstract

While phase relations in binary systems of pure rare earths with cobalt have been studied extensively, only limited information is available on the MM-Co system, where the mischmetall (MM) contains cerium, lanthanum, neodymium and praseodymium as primary constituents. In the mischmetallcobalt system, six phases MMCo₃, MM₂Co₇, MM₅Co₁₉, MMCo₅, MM₂Co₁₇ and β-Co were identified in the composition range 55-84 wt.% Co. The MMCo₃, MM₂Co₇ and MM₅Co₁₉ phases have very similar X-ray patterns and hence they were identified by electron probe microanalysis and thermomagnetic analysis. The MM₂Co₇ phase exists in both hexagonal and rhombohedral modifications. Of the various phases, only the MMCo₅ phase has an observable homogeneity region extending over 2 wt.% Co at 900 °C. The MMCo₅ phase is unstable at temperatures below 700°C and the addition of iron increases its instability, leading for example to a nearly complete disappearance in a sample with 12 wt.% Fe in 30 days at 700 °C. The decomposition products of MMCo₅ occur at grain boundaries and inside the MMCo₅ grains of alloys with low iron contents but they occur as separate grains in alloys with high iron contents.