Effect of Phosphorus on the Phase Stability of a High Refractory Content Powder-Processed Ni‐Base Superalloy

The effect of P on the phase stability of a high refractory content powder-processed Ni-base superalloy was systematically studied with a variation of P addition. The microstructural evolution of nominally identical alloys with three levels of P additions (0.013, 0.026, and 0.041 wt pct) were investigated in the as-solutioned state and after thermal exposures at 800 °C for up to 1000 hours. Additions of P were found to segregate strongly at grain boundaries and depress the incipient melting temperature. The presence of P contributed to the formation of a C14 Laves as resolidification of incipiently melted liquid occurred. P additions also affected the solid-state phase stability of the alloys as aging at 800 °C led to the formation of C36 Laves phase precipitates. An increase in the fraction of C36 Laves phase correlated directly to the increases in the P content of the alloy. The extensive precipitation of C36 Laves phase eventually led to the formation of a basket-weave structure composed of an intertwined mixture of C14 Laves and sigma phase during the long-term thermal exposure at 800 °C. The stabilization of the Laves phase structure due to the minor additions of P was found to be consistent with density functional theory calculations and could be rationalized through structure maps that relate the valence electron concentration and relative size differences.