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

Abstract

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.

Effect of B on microstructure and properties of low thermal expansion superalloy

“…Therefore, the intrinsic cohesion of GB in high B doped alloy 2 is expected to be much crucial. As has been widely accepted, B segregation at GBs can increase the GB cohesion, tie up vacancies and reduce GB diffusion reaction, and thus prevents a premature formation of cavities and micro-cracks at the GBs 13 – 15. In the case of cavities formed, it has been found that B has segregated on the nucleated cavity surface during creep test,16 which decreased the diffusivity along the cavity surface and suppressed creep cavitation.…”

Copper, Boron, and Cerium Additions in Type 347 Austenitic Steel to Improve Creep Rupture Strength

“…Trace elements in a material can influence the intergranular creep cavity nucleation and growth processes critically because of their strong segregation tendencies to the grain boundary and nucleated cavity surface. [8,9,12] Holt and Wallace [13] classified the most common trace elements according to whether they have detrimental or beneficial effects on creep rupture strength. Among them, oxygen and sulfur can cause the most severe embrittlement during creep even at low levels.…”