The stability and homogeneity range of the TCP phases in the Co-Fe-Ta system

Abstract

The topologically closed phases (TCP) in the Co-Fe-Ta system were studied. The phase equilibria of the Co-Fe-Ta system were investigated for the first time. The Co-Fe-Ta isothermal section at 1100 °C was measured through 23 alloys and 5 diffusion couples. The continuous μ single-phase region through the entire Ta compositions has formed. All the other binary phases show large homogeneity ranges in the Co-Fe-Ta system. The enthalpies of formation for all end-members of the topologically closed phases (TCP), such as C14, C15 and μ, were calculated by density functional theory (DFT). The DFT results were used to evaluate the thermodynamic parameters of the C14, C15 and μ phases. The isothermal section at 1100 °C for the Co-Fe-Ta ternary system was accurately predicted by applying the DFT results to the CALPHAD assessment. Statistical analysis of the entry of different atoms into the lattices of the C14 and μ phases revealed that only four end-members were critical to obtain a quantitative CALPHAD prediction. This significantly reduces the work of the DFT calculation. A comprehensive CALPHAD description was performed finally considering all the available data. The calculated isothermal section at 1100 °C is in good agreement with the experimental data. The metastable Co-Fe-Ta FCC-L12 phase diagram has been studied. The predicted results show that the solvus temperature was decreased by the addition of Fe.