A constitutional diagram of the yttrium-cobalt system was determined by optical microscopy, differential thermal analysis, X-ray diffraction and electron microprobe techniques. This system exhibits (i) seven peritectic reactions forming following intermetallic compounds: Y_3Co, Y_8Co_5, YCo, Y_2Co_3, YCo_2, YCo_3 and Y_2Co_7 respectively; (ii) two peritectoid reactions forming Y_9Co_7 and Y_6Co_7 respectively, the Y_6Co_7 is a new phase which had not been reported before; (iii) two congruently melting phases Y_2Co_(17), YCo_5; (iv) three eutectic reactions occurring isothermally at 738 ℃, 1334 ℃ and 1340 ℃ with eutectic containing 37at%Co, 87at%Co and 93at%Co respectively. The solid solubility of yttrium in cobalt is about 0.04wt% (0.03at%)Co at 1200 ℃ and negligible at the room temperature. Both Y_2Co_(17) phase and YCo_5 phase have a large homogeneity region at the high temperature. There is one metastable phase Y_3Co_2 in as-cast alloy containing 40at% Co. The Y_3Co_2 decomposes into Y_8Co_5 and Y_9Co_7 at 700 ℃.
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