Phase Equilibria Among γ/TCP/GCP Phases in Nb-Poor Region of Ni-Nb-Co Ternary System at Elevated Temperatures

Phase equilibria among the γ-(Ni, Co), topologically close-packed and geometrically close-packed phases in the Ni-Nb-Co ternary system were investigated using x-ray diffraction, transmission electron microscopy, and electron probe microanalysis for equilibrated bulk alloys at 1100 and 1200 °C. A two-phase equilibrium between the γ (A1) and D0_a phases (Ni₃Nb) is confirmed to exist on the Ni-rich side with the Ni/(Ni + Co) ratio above ~ 0.8, which agrees well with literature data. The phases in equilibrium with the A1 phase change into a ternary-compound D0₁₉ phase ((Ni, Co)₃Nb) and an mC18 phase (Co₇Nb₂) with increasing Co content. The C15 phase (Co₂Nb) is not in equilibrium with the A1 phase. The three-phase tie-triangle of the A1/mC18/D0₁₉ moves toward the Co-rich side with decreasing temperature at an equivalent Nb concentration, while that of A1/D0₁₉/D0_a almost remains at the chemical composition of Ni-20Co-20Nb in the temperature range investigated. Because the mC18 phase is in equilibrium with the A1 phase up to 1086 °C in the Co-Nb binary system, Ni addition should stabilize the mC18 phase and expand the mC18 phase region toward high temperatures.