Wr. Sun et al., EFFECTS OF PHOSPHORUS ON THE DELTA-NI3NB PHASE PRECIPITATION AND THE STRESS RUPTURE PROPERTIES IN ALLOY-718, Materials science & engineering. A, Structural materials: properties, microstructure and processing, 247(1-2), 1998, pp. 173-179
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.