EFFECTS OF PHOSPHORUS ON THE DELTA-NI3NB PHASE PRECIPITATION AND THE STRESS RUPTURE PROPERTIES IN ALLOY-718

The effects of phosphorus on the phase transformation and stress ruptu re properties of alloy 718 were investigated. The nucleation of delta- phase, which does not contain phosphorus, was suppressed by the enrich ment of phosphorus at grain boundaries. A low level of phosphorus resu lted in the formation of faults-containing film-like delta-phase along the grain boundaries, while a higher level of phosphorus favored the long lath-like delta-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 0.013-0.04 9 wt.%. The effect of phosphorus on the stress rupture properties was closely related to its interaction with oxygen. Phosphorus atoms, in t he range of 0.0008-0.013 wt.%, enhanced the resistance to oxygen intru sion along the grain boundaries, protected the grain boundaries from d ecohesion by oxygen atoms and oxidation, and subsequently prolonged th e rupture life of the alloy. The protection effect of P is clearly dem onstrated by the phenomenon that the crack initiation site was shifted from the surface to the center in the stress-ruptured samples with in creasing addition of P. Over 0.013 wt.%, the protection effect of phos phorus is excessive and phosphorus began to display its inherent effec t of damaging the grain boundary strength; the stress rupture life of the alloy was reduced accordingly. Maximum stress rupture life was thu s obtained at approximate to 0.113 wt.% P. (C) 1998 Elsevier Science S .A. All rights reserved.