Phosphorous behavior and its effect on secondary phase formation in high refractory content powder-processed Ni-based superalloys

Abstract

The behavior of phosphorous and its effect on the microstructure of a powder-processed, high refractory content, Ni-based superalloy was studied and compared to a P-free variant. Transmission electron microscopy and atom probe tomography were used to identify the crystal structure of the five topologically close packed (TCP) and geometrically close packed (GCP) phases that formed, assess phase compositions, and determine any elemental segregation to phase boundaries. The results revealed strong phosphorous segregation to grain boundaries, moderate solubility in TCP and MC phases, limited solubility in the γ matrix and no solubility in the γ′ phase or any other Ni3X type GCP phases. The phosphorus segregation to the grain boundaries of the P-doped alloy lowered the incipient melting temperature, thereby eliminating the processing window, and led to the formation of (Ni,Co,Cr,P)2(Nb,Mo) laves phases along grain boundaries during supersolvus heat treatment. Interestingly, APT revealed segregation of P to Al2O3 oxide surfaces, explaining the frequent observation of laves precipitates nucleating on such impurities. Due to the low stability of the C14–Ni2Nb laves phase, it was prone to phase separation, and served as the nucleation point for μ, intergrown η/δ, and Nb3P phosphide phases.