High temperature oxidation behaviors of nickel base superalloys in air, helium, and vacuum environments

The oxidation characteristics of Alloy 230 and 617, for the candidate materials for the very high temperature reactor (VHTR) application, and Alloy 214, reference material for aluminum content effect were investigated. High temperature oxidation tests were conducted at 900 .deg. C and 1100 .deg. C in air, helium and vacuum environments and the results were analyzed. All three materials showed parabolic oxidation behaviors at 900 .deg. C. However, unstable oxidation behaviors were shown in alloy 230 and 617 at 1100 .deg. C, and these materials showed unstable oxide layer as the exposure time increased. The Cr-rich oxides were formed on the surface and the Al-rich discrete internal oxides were formed below the surface oxide layer in alloy 230 and 617. On the other sides, alloy 214 showed stable behaviors and very smooth and continuous Al-oxide layer even in a low oxygen containing helium environment at 1100 .deg. C. Depletion of grain boundary carbides was also observed during the oxidation test. Such decarburization would have detrimental effects on the high temperature mechanical properties, especially the creep resistance of superalloys for the VHTR application