The influence of sulfur on stress-rupture fracture in inconel 718 superalloys

The effects of sulfur, with content variations of 15 to 175 ppm, on the stress-rupture and tensile properties in nickel-base alloy 718 are reported. The stress-rupture life dramatically decreased with increasing sulfur content. This was especially noticeable in the ductility loss at 650 °C. Auger electron spectroscopy of stress-rupture tested specimens provided direct evidence of sulfur and phosphorus segregation to grain boundaries and carbide/matrix interfaces. The stress-rupture life and fracture morphology were both found to correlate with the segregation of sulfur at grain boundaries in alloy 718. Sulfur was also preferentially segregated at the carbide matrix surfaces, and phosphorus was found to be distributed on grain boundaries. However, the phosphorus segregation did not correlate with stress-rupture behavior. Sulfur contents in the range of 15 to 50 ppm had little effect on the stress-rupture life. However, the stress-rupture life decreased dramatically with increasing sulfur content above 50 ppm.