Segregation of Phosphorus and Precipitation of MNP-Type Phosphide at the Grain Boundary of IN706 Superalloy

The effect of phosphorus content and heat treatment on the segregation of phosphorus and precipitation of phosphide at the grain boundary of IN706 alloy has been investigated. The phosphide had a stoichiometry of MNP (M = Nb, Ti and N = Ni, Fe, Cr) and an orthorhombic crystal structure. The solubility of phosphorus in the grain matrix of IN706 alloy was determined to be between 0.008% and 0.013%. When soaked at 980 °C for 5 min, the phosphide was noticeably precipitated at grain boundaries with the grain size unchanged. When soaked at 980 °C for 3 h, the grains grew significantly, but the phosphide stayed at original grain boundaries which outlined the grain shape before soaking. Soaking at 1060 °C for 2 h fully dissolved the pre-existing phosphide but phosphorus atoms were not distributed uniformly in the grain matrix. When the holding time at 1060 °C was extended to 10 h, the phosphorus atoms were distributed uniformly in the grain matrix by sufficient diffusion, and the phosphide could only be formed at grain boundaries during the 980 °C soaking. The precipitation of phosphide mainly relied on the phosphorus segregation which was built up by diffusion.