Co corner of the system Sm-Co-Zr: Decomposition of the phase 1:7 and equilibria at 850 degrees C

In the Co range 100-75 at.% of the system Sm-Co-Zr, the phase equilibria at 850 degrees C are determined by composition and crystal structure analysis of annealed alloys. Five three-phase fields centred on the phase R2:17, based on Sm2Co17, are detected. We observe the formation of ternary phases obtained by substitution of Zr for Sm in the binary compounds Smn+1CO5n-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 --> R2:17 + 6:23 + 1:3 occur by discontinuous processes. The limiting step is the diffusion controlled growth. <(D)over tilde (1:5)> is evaluated at 30 x 10(-20) m(2) s(-1). For the reactions 1:7 --> R2:17 + (n + 1:5n-1), a transient step of continuous precipitation of the coherent metastable phases is proposed.