Gas phase embrittlement of nickel by sulfur

Abstract

Nickel and nickel-base alloys are extremely sensitive to embrittlement and enhanced crack propagation due to sulfur, which may contaminate during melting, processing, or service. This paper reports the results of a systematic study of the effect of sulfur partial pressure and exposure temperature on intergranular penetration and tensile embrittlement in Ni270. For exposures between 450 °C and 900 °C, a maximum embrittlement was observed at about 700 °C. Partial pressures of sulfur exceeding about 5 × 10-8 atm were embrittling. Kinetic measurements, based on the depth of postexposure intergranular fracture, gave an activation energy of 74 kJ/mol in the temperature range 450 °C to 800 °C. This value is considerably less than the value reported for matrix sulfur diffusion and also less than that reported for intergranular oxygen diffusion. Auger spectroscopy confirmed high local concentrations of sulfur at grain boundaries. In only one case, under the most severe sulfur exposure, was indirect evidence of sulfide formation observed; clearly, the embrittlement was a consequence of intergranular diffusion and segregation of elemental sulfur. A simple composite model for the tensile strength and ductility of partially embrittled specimens, in terms of sulfur penetration distance, satisfactorily accounted for the experimental measurements.