REVIEW OF TRACE-ELEMENT EFFECTS ON HIGH-TEMPERATURE FRACTURE OF FE-BASE AND NI-BASE ALLOYS

Trace elements can have harmful or beneficial effects on elevated-temp erature mechanical properties. In iron and low-alloy steels, S at high concentrations forms sulfides which are potent nucleation sites for i ntergranular creep cavities. As a result, ductility decreases and frac ture becomes increasingly intergranular with increasing S concentratio n. Oxides and nitrides nucleate cavities only in the presence of segre gated 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 wh ether 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 ducti lity and rupture life. Boron and P interact synergistically and increa se stress rupture lives in some alloys but not in others. The detailed mechanism of this interaction is not well understood.