Investigation of the partition coefficients in the Ni-Fe-Nb alloys: A thermodynamic and experimental approach

The Nb containing Ni-based superalloys used for the production of large castings are highly sensible to the formation of macrosegregation defects like freckles and white spots. Production cost can be reduced by better prediction of the freckle formation. The most accepted freckling criterion is the one based on the Rayleigh number, which requires accurate determination of the partition coefficients for the solute elements. In this work, the partition coefficients of Nb and Fe on the ternary system Ni-Fe-Nb were studied by experimental measurements and by thermodynamic calculations. Modified DTA experiments were performed for model alloys, and a thermodynamic database for the Ni-Fe-Nb ternary system was developed. Most of the calculated partition coefficients using the developed model agree well with the experimental data. One exception to this, as will be discussed later, is the partitioning coefficient of Fe for the alloy containing 36 wt. %Fe.