Distribution of Phosphorus and Its Effects on Precipitation Behaviors and Tensile Properties of IN718C Cast Superalloy

The effect of phosphorus on the precipitations of γ″, γ′ and δ phases and associated tensile properties in IN718C alloy are investigated in this study. It is revealed that P atoms are dissolved in the grain interior to a relatively high degree and hence influence the precipitation behaviors in the grain interior and improve the tensile strength of IN718C alloy. γ″ and γ′ phases did not precipitate in the alloy without P addition during air cooling, while γ″ and γ′ phases precipitated in the grain interior during air cooling in the alloys with P addition, and the amounts of γ″ and γ′ phases increased with increasing P content. Therefore, the Vickers micro-hardness in the as-cast state increased gradually with increasing P content. In double-aging state, the sizes of γ″ and γ′ phases in the alloys with P addition were larger than that in the alloy without P addition, while the sizes were invariable when the P content (wt%) was higher than 0.015. Therefore, the micro-hardness and tensile strength of IN718C alloy treated by double aging increased first and then kept invariable with increasing P content. The precipitations of δ phases both in the grain interior and on grain boundaries were inhibited by P markedly. The inhibitory effect of P on δ phase enhanced gradually with increasing content of P, but the plasticity increased first and then decreased. What is more, the crack tended to propagate into the matrix around the particles (Laves phases and NbC carbides) in the alloys without P addition at the beginning of the tensile fracture, while it tended to propagate along the interfaces between the matrix and those particles in the alloys with P addition, which resulted from the synthetical effect of P on γ″, γ′ and δ phases.