CALPHAD approach for prediction of local phase transformation at superlattice stacking fault in gamma prime precipitates in superalloys with multi-component system

Abstract

This manuscript investigated the possibility of CALPHAD (calculation of phase diagram) approach to predict the local phase transformation (LPT) accompanied by compositional transition on the superlattice stacking fault in the γ′ precipitates in multi-component superalloys. The method is basically parallel tangent construction using the concept of the LPT phase in the system of γ and γ′ phases to approximate the atomic structure of the stacking fault. Because the important issue for the strengthening by the LPT is whether the LPT phase is ordered or disordered, the ordering was judged by the size of the stacking fault energy to form the LPT phase from γ′ precipitates. In addition, since the solute partitioning ratio between LPT phase and γ′ precipitates is also significant to consider the LPT, the predicted ratio was verified using the experimental results in the previous reports. In the case of SISF (superlattice intrinsic stacking fault), Co-base superalloys tend to form ordered $\chi$ LPT phase, but formability of ordered $\chi$ and disordered $\epsilon$ LPT phases in Ni-base superalloys is calculated to be competitive. The predicted solute partitioning rate almost agrees with the experimental one except for Nb in multi-component superalloys. This discrepancy could originate from the accuracy of the physical properties of Ti and Nb in the database of $\chi$ LPT phase. In the case of SESF (superlattice extrinsic stacking fault), all considered alloys of Ni-base superalloys were predicted to have ordered $\eta$ LPT phase, judged by stacking fault energy, however, the predicted solute partitioning ratio was different from the experimental ones. This issue could originate from inaccurate physical properties between Ti and Ta in the database of $\eta$ LPT phase with the multi-component system.