Importance of trace element control on mechanical and foundry properties of cast superalloys

Abstract

The properties principally affected by trace elements in Ni-based alloys are creep life and ductility. Results of experimental research have shown that the most detrimental element is Bi, which can reduce creep life by 20% at a contamination level of 0·2 ppm. This element is followed in the severity of its effect on these properties by Te, Se, Pb, and Ag, in that order. Gases in alloys affect cleanness and castability. Oxygen affects alloy cleanness by forming oxides, and increases the severity of metal-crucible reactions. Nitrogen is believed to affect the degree of microporosity formation in vacuum-cast Ni-based alloys. This element in Cobased alloys reduces ductility, but can also significantly increase tensile strength through strain aging. The paper concludes with a case study of Bi contamination in cast turbine rotor blades, which illustrates the importance of maintaining stringent trace element control.