Microstructural Observations on Oxidized Ni-Based Superalloy TMS-75 and PGM-doped Derivatives

Abstract

The isothermal oxidation behavior of experimental 3rd and 4th generation Ni-based superalloys was studied at temperatures between 900 and 1100ºC. It was found that the lower Cr-concentration in 3rd and 4th generation alloys compared to 2nd generation alloys negatively affects the ability of the alloy to form and maintain a protective alumina scale esp. at lower temperatures. An extensive internal reaction zone comprised of oxide and nitride phases is observed underneath a non-protective external scale. At higher temperatures, a continuous alumina scale forms underneath (Ni,Co)O and spinel layers. Due to the low level of reactive element additions in these experimental alloys, scale adhesion is not good though. Increased Cr content is necessary to ensure formation of a protective alumina scale at lower temperatures. Addition of Y and or Zr is recommended for improved oxide adhesion resulting in better overall oxidation performance.