The Co-Mo-Ta (Cobalt-Molybdenum-Tantalum) system

Abstract

The Co-Mo system [Massalski2] (Fig. 1) shows the presence of four intermediate phases: Co9Mo2, Co3Mo, Co7Mo6 ( ), and . The and phases form through peritectic reactions L + ↔ at 1620 °C and L + ↔ at 1510 °C. is the bcc terminal solid solution of Co in Mo. A eutectic reaction L ↔ + occurs at 1335 °C, being the fcc terminal solid solution of Mo in Co. Three peritectoid reactions + ↔ Co9Mo2, Co9Mo2 + ↔ , and possibly + ↔ , where is near Co3Mo and is the cph terminal solid solution of Co, occur at 1200, 1025, and 700 °C, respectively. The Co9Mo2 and phases undergo eutectoid decomposition Co9Mo2 ↔ + Co3Mo at 1018 °C and ↔ + at 1000 °C. The Co–Ta phase diagram is given by [1967Ram] and [1986Bar], and adopted by [Massalski2] (Fig. 2) and [1996Gar]. While [1996Gar] (Fig. 3) gives only a partial diagram at the Co end, the phase diagram given by [Massalski2] shows the presence of six intermediate phases: (Co7Ta2, three Laves phases ( 1, 2, and 3), Co6Ta7, and CoTa2. [1967Ram] also showed the presence of the same six phases given by [1986Bar], but also indicated the presence of two more high temperature (T > 1300 °C) phases near 60 at.% Ta. According to the Co-Ta diagram given by [Massalski2], the CoTa2 and the 3 phases form through peritectic reactions L + ↔ CoTa2 at 1800 °C, where is the bcc terminal solid solution of Co in Ta, and L + 2 ↔ 3 at 1450 °C. The 2 and 2 phases melt congruently at 1620 and 1700 °C, respectively. The 1 and Co7Ta2 phases form through peritectoid reactions 2 + 2 ↔ 1 at 1540 °C and 3 + ↔ Co7Ta2 at 950 °C. Three eutectic reactions L ↔ 2 + CoTa2, L ↔ 2 + 2 and L ↔ + 3 occur at 1670, 1570, and 1280 °C, respectively. The 1 phase undergoes eutectoid transformation 1 ↔ 2 + 2 at 1130 °C. Neither [Massalski2] nor [1996Gar] give any reaction through which the fcc phase changes to the cph phase. [1967Ram] suggested a peritectoid reaction + Co7Ta2 ( ) ↔ at 1000 °C. But according to [Massalski2], the Ta7Co2 phase is stable only below 950 °C, suggesting that the phase formation temperature possibly is below 950 °C. The Mo-Ta system is a simple isomorphous system [Massalski2] (Fig. 4) giving rise to a bcc solid solution phase extending from Mo to Ta.