Formation of eta phase during aging at 750？850 °C for Ni-base superalloys with different Ti/Al ratios

Abstract

The formation of the eta phase in two nickel-based superalloys that have similar amounts of titanium and aluminum but different Ti/Al ratios was investigated. The aging was carried out at 750 degrees C, 800 degrees C, and 850 degrees C for 10000 h. The microstructures were observed using SEM and TEM, and equilibria phases were calculated using a Thermo-Calc. equipped with TCNI11 database. Results showed that the number and size of eta phase increased with aging temperature and Ti/Al ratios. The size of the gamma ' phase was also found to be increased with aging temperature, but it was not significantly affected by the Ti/Al atomic ratio. However, the eta phase was not predicted to be the thermodynamic equilibrium phase in the alloy using Thermo-Calc. with the TCNI11 database. The eta phase was expected to be the equilibrium phase with Ti/Al ratios higher than 1.85 at 850 degrees C for Ni-27.0Cr19.1Co-0.91Nb-0.29Mo-0.14C-xTi-yAl alloys. However, the result does not match the experimental observations or other literature. It is suggested that the effect of titanium on the formation of the eta phase may be underestimated in the TCNI11 database. Therefore, it is necessary to enhance the accuracy of predicting the formation of the eta phase by modifying at least one of the thermodynamic parameters, including the phase equilibrium of the ternary Ni-Ti-Al system.