Effects of phosphorus on the δ-Ni3Nb phase precipitation and the stress rupture properties in alloy 718

The effects of phosphorus on the phase transformation and stress rupture properties of alloy 718 were investigated. The nucleation of δ-phase, which does not contain phosphorus, was suppressed by the enrichment of phosphorus at grain boundaries. A low level of phosphorus resulted in the formation of faults-containing film-like δ-phase along the grain boundaries, while a higher level of phosphorus favored the long lath-like δ-phase precipitation. Phosphorus greatly prolonged the stress rupture life of the alloy in the range of 0.0008–0.013 wt.%, while it reduced the stress rupture life in the range of 0.013–0.049 wt.%. The effect of phosphorus on the stress rupture properties was closely related to its interaction with oxygen. Phosphorus atoms, in the range of 0.0008–0.013 wt.%, enhanced the resistance to oxygen intrusion along the grain boundaries, protected the grain boundaries from decohesion by oxygen atoms and oxidation, and subsequently prolonged the rupture life of the alloy. The protection effect of P is clearly demonstrated by the phenomenon that the crack initiation site was shifted from the surface to the center in the stress-ruptured samples with increasing addition of P. Over 0.013 wt.%, the protection effect of phosphorus is excessive and phosphorus began to display its inherent effect of damaging the grain boundary strength; the stress rupture life of the alloy was reduced accordingly. Maximum stress rupture life was thus obtained at ≈0.013 wt.% P.