Alloying Effects on the Competition Between Discontinuous Precipitation Versus Continuous Precipitation of δ/η Phases in Model Ni-Based Superalloys

Alloying effects on the competition between discontinuous and continuous precipitation of δ-Ni₃Nb (D0_a) phase and η-Ni₃Ti (D0₂₄) phase were investigated in model Ni-Cr-Fe-Nb and Ni-Ti-based alloys, respectively, to aim at designing polycrystalline Ni based superalloys with better temperature capability. Discontinuous precipitation tends to occur at lower temperatures, while continuous precipitation dominates at higher temperatures in the two alloy systems. The addition of Mo promotes continuous precipitation with respect to discontinuous precipitation while that of Ti promotes discontinuous precipitation rather than continuous precipitation in δ phase precipitation alloys. A replacement of Ti with Mo promotes continuous precipitation with respect to discontinuous precipitation in η phase precipitation alloys at 800 °C. The observed alloying effects are discussed in terms of chemical driving force, interfacial energy between the matrix phase and the δ(η) phases, and coherency strain caused by the formation of the metastable phases prior to the formation of δ/η phases.