The roles of P/B-addition in tailoring δ-phase-enhanced grain boundaries in a nickel-based superalloy

Phosphorus (P) and boron (B) as microalloying elements can play significant roles in improving the high-temperature performance of polycrystalline nickel-based superalloys, but their microscopic mechanism in modifying the alloys’ microstructures is still not clarified. Here, we report the distinct roles of P/B elements in tailoring the grain boundary precipitation microstructures of the alloy upon thermal aging. Using advanced electron microscopy techniques, we demonstrate that upon thermal aging P and B can depress development of continuous δ-phase (Ni ₃Nb) precipitates along grain boundaries by promoting formation of other much smaller grain boundary precipitates. It is identified that these nano-scaled grain boundary precipitates are Nb-B-rich MC carbides, P-rich C14 Laves phase (Ni, Fe, Cr) ₂(Nb, Mo) precipitates, and dispersed δ-phase precipitates. Electron tomography shows that the three nano-scaled grain boundary precipitates all have their own featured three-dimensional shapes. In comparison with the alloy without P/B-addition, our study reveals that P/B microalloying can trigger and lead to optimized grain boundary microstructures, enhancing the superalloys’ high-temperature properties.