The Co-Nb-Si (Cobalt-Niobium-Silicon) System

Abstract

The Co-Nb system [Massalski2] shows the presence of three intermediate phases Co3Nb (k3), Co2Nb, and Co6Nb7 (l), of which the Co6Nb7 and Co2Nb phases melt congruently at 1402 and 1480 C, respectively. The Co2Nb phase is known to have two polymorphic forms, Co2Nb (k2), a Cu2Mg type Laves phase and Co2Nb (k1), a MgZn2 type Laves phase, the latter phase has been reported to exist above 1225 C [1964Kuz2]. The k3 phase, a MgNi2 type Laves phase, forms through a peritectic reaction L + Co2Nb $ k3 at 1247 C. Three eutectic reactions L $ a þ l;L $ l þ Co2Nb and L $ c þ k3 occur at 1374, 1378, and 1237 C, respectively. The a and c phases are the terminal solid solutions (Nb) and (aCo), respectively. The Co-Si system [Massalski2] has five intermediate phases, Co3Si (/), aCo2Si (q1), bCo2Si (q2), CoSi (f), and CoSi2 (p) of which the bCo2Si, CoSi, and CoSi2 phases melt congruently at 1334, 1460, and 1326 C, respectively. The Co3Si and aCo2Si phases form through peritectic reactions Lþ aCo2Si $ Co3Si at 1214 C and Lþ bCo2Si $ aCo2Si at 1320 C. The cph (eCo) phase is stabilized to higher temperatures with addition of Si to Co and at 1250 C a peritectic reaction Lþ ðaCoÞ $ ðeCoÞ occurs. The Co3Si and the bCo2Si phases exist only at higher temperatures and decompose through eutectoid reactions Co3Si $ ðeCoÞ þ aCo2Si at 1193 C and bCo2Si $