Review of Trace Element Effects on High-Temperature Fracture of Fe- and Ni-Base Alloys

Trace elements can have harmful or beneficial effects on elevated-temperature mechanical properties. In iron and low-alloy steels, S at high concentrations forms sulfides which are potent nucleation sites for intergranular creep cavities. As a result, ductility decreases and fracture becomes increasingly intergranular with increasing S concentration. Oxides and nitrides nucleate cavities only in the presence of segregated S; by themselves they are relatively benign. The harmful effects of S can be ameliorated by the addition of carbon and phosphorus. It is unclear whether C and P have beneficial effects of their own, or whether they act merely by counteracting the negative effects of sulfur. Carbides almost never nucleate cavities. In Ni-base alloys, too, S is deleterious, and alloying with strong sulfide getters increases ductility and rupture life. Boron and P interact synergistically and increase stress rupture lives in some alloys but not in others. The detailed mechanism of this interaction is not well understood.