Co corner of the system SmCoZr: decomposition of the phase 1:7 and equilibria at 850°C

Abstract In the Co range 100-75 at.% of the system SmCoZr, the phase equilibria at 850°C are determined by composition and crystal structure analysis of annealed alloys. Five three-phase fields centred on the phase R2:17, based on Sm 2 Co 17 , are detected. We observe the formation of ternary phases obtained by substitution of Zr for Sm in the binary compounds Sm n + 1 Co 5 n − 1 ( n = 2–4). The genesis of the multiphase fields centred on R2:17 involves the transformation of the high-temperature phase 1:7. This transformation is studied by microstructural investigation. As predicted by the phase equilibria, the depletion 1:7 → 1:5 + R2:17(+ 5:19) proceeds by continuous precipitation. The reactions 1:7 → R2:17 + Co + 6:23 and 1:7 → R 2:17 + 6:23 + 1:3 occur by discontinuous processes. The limiting step is the diffusion controlled growth. D 1:5 is evaluated at 30 × 10 −20 m 2 s −1 . For the reactions 1:7 → R2:17 + ( n + 1:5 n − 1), a transient step of continuous precipitation of the coherent metastable phases is proposed.