The Co-Ni-Ti system (Cobalt-Nickel-Titanium)

Abstract

At temperatures above 422 °C, the Co-Ni system is a simple isomorphous system consisting of a single fcc γ phase [91Nas] (Fig. 1). At the Co end of the fcc solid solution, the fcc γ phase transforms to cph ω phase at temperatures below 422 °C. The Co-Ti system was reviewed by [87Mur] (Fig. 2) and shows the presence of five intermediate phases (Co3Ti, Co2Ti(h), Co2Ti(c), CoTi, and CoTi2, where h and c represent hexagonal and cubic, respectively) of which the Co3Ti, Co2Ti(h), Co2Ti(c), and CoTi2 phases form through peritectic reactions occurring at ~1190, 1253, 1210, and 1058 °C, respectively. Two eutectic reactions occur between the Co3Ti and Co2Ti(h) phases and between the CoTi2 and βTi terminal solid solution phases at 1170 and 1020 °C, respectively. The βTi terminal solid solution undergoes a eutectoid transformation at 685 °C. The CoTi phase melts congruently at 1325 °C. The Ni-Ti system reviewed by [87Mur] (Fig. 3) shows the presence of three intermediate phases (Ni3Ti, NiTi, and NiTi2) of which the Ni3Ti and the NiTi phases melt congruently at 1380 and 1310 °C, respectively. The NiTi2 phase forms through a peritectic reaction at 984 °C. Three eutectic reactions occur in the Ni-Ti system between the fcc γ and Ni3Ti, the Ni3Ti an NiTi, and the NiTi2 and βTi terminal solid-solution phases at 1118, 1025, and 942 °C, respectively. The βTi solidsolution phase undergoes a eutectoid reaction at 770 °C.