Interfacial segregation, pore formation, and scale adhesion on NiAl alloys

Ni-40 and 50at% Al alloys were oxidized at 1000degreesC for various times in oxygen. Auger electron microscopy was used to study the interface chemistry after scale spallation in ultra high vacuum. The interfacial failure stresses were determined using a tensile pull tester and related to the interface chemistry, pore area and density. Results showed that sulfur started to segregate to areas of the Al2O3/Ni40Al interface, where the scale was in contact with the alloy after a complete layer of alpha-Al2O3 developed there; the concentration then gradually increased to a steady level of similar to 2 at%. However, sulfur did not segregate to similar areas of the Al2O3/Ni50Al interface even after extended oxidation when it was amply present on interfacial void faces. This behavior demonstrated a strong dependence of interface segregation on NiAl alloy composition. Interfacial failure stress was found to decrease with increasing sulfur content between voids and with higher interface porosity. The level of porosity was strongly related to the sulfur content in the alloy. When Ni40Al was doped with excess sulfur, the segregation behavior did not change, but the interfacial pore density increased significantly. The detrimental effect of sulfur on scale adhesion is 2-fold: to weaken the interface and to enhance interfacial pore formation.