材料期刊网

Nitrogen has a significant effect on superalloy mechanical properties, almost without exception in a negative sense. All of the alloys contain strong nitride formers (Ti, V, Nb, Hf, Ta) and will, because of the nitride-former concentration, produce nitride precipitates not only during solidification but also when the alloy is fully liquid and significantly superheated. In this review we present an examination of the problems involved in removing nitrogen from the alloys and also of the mechanisms by which nitrides influence mechanical behaviour. It is concluded that one of the most effective means of control of the nitrogen content lies in the choice of raw materials for the alloy make-up, but that the subsequent nitride formation can be minimised by correct choice of process routes in VIM, ESR, VAR and EBCHM.