Mult-Component Intergranular and Interfacial Segregation in Alloy 718 with Correlations to Stress Rupture Behavior

The effects of sulfur in the range of 15ppm to 175ppm on the stress rupture fracture and tensile properties of Alloy 718 were investigated. Stress rupture life dramatically decreased with increasing sulfur content. Auger analyses provided direct evidence of sulfur segregation to grain boundaries and carbide/matrix interfaces, and further indicated that stress rupture fracture was mainly controlled by the segregation of sulfur at grain boundaries. However, almost no sulfur segregation was found when the sulfur content was lower than 1 Sppm. Phosphorous was also found segregated on the grain boundaries. The experimental results suggested that there existed attractive segregation interactions between P and the metal elements MO, Nb and Cr at grain boundary. The influence of the addition of Mg on mechanical properties was also investigated. Auger results showed that no Mg segregation was found at grain boundaries or carbide/matrix interfaces. However, small particles of Mg sulfide and MgO were found. This may have been associated with the negative effect of Mg on stress rupture life. Superalloys 718,625,706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals &Materials Society, 1997 553