Experimental Investigation and Thermodynamic Description of the Co–Nb–Zr System

The liquidus surface projection and isothermal sections at 1100 and 1000 °C of the Co–Nb–Zr system were constructed by analyzing the solidification morphology of as-cast alloys and annealed microstructure of the equilibrated alloys and diffusion couples. Six primary solidification areas were obtained in the liquidus surface projection, and four primary solidification areas were deduced from the binary phase diagram. Additionally, three three-phase regions at 1100 °C and five three-phase regions at 1000 °C were determined in the isothermal sections. Based on the experimental information, a thermodynamic description of the Co–Nb–Zr system was established using the CALPHAD (CALculation of PHAse Diagram) method. The thermodynamic models of intermetallic phases Co₁₁Zr₂, Co₂₃Zr₆, CoZr₂, Co₇Nb₂, λ₁, λ₂ and λ₃, were described as (Co)₁₁(Nb,Zr)₂, (Co)₂₃(Nb,Zr)₆, (Co,Zr)(Nb,Zr)₂, Co₇Nb₂, (Co,Nb)₂(Co,Nb), (Co,Nb,Zr)₂(Co,Nb,Zr) and (Co,Nb)₂(Co,Nb), respectively. CoZr₃ and μ were modeled as (Co,Zr)(Co,Zr)Zr₂ and (Co,Nb,Zr)₁(Nb,Zr)₄(Co,Nb,Zr)₂(Co,Nb,Zr)₆. Moreso, CoZr with the structure of B2 was described as an ordered phase of bcc_A2. Thus, a reasonable thermodynamic description of the Co–Nb–Zr system was obtained, and the experimental information was well reproduced.