Formation and compensation of internal stresses during internal nitridation of nickel-base alloys

The interactions between the internal nitridation behavior of Ni-base alloys, the precipitation-induced internal stresses in the near-surface area and the formation of metallic surface protrusions were investigated by exposing several alloys of the system Ni–Cr–Al–Ti to oxygen-free nitrogen atmosphere at 800–1100°C. Depending on alloy composition, exposure time and temperature, internal Cr, Al, and Ti nitrides precipitate, which are of a high specific volume as compared with the base metal. As a consequence of this volume increase, internal compressive stresses are generated giving rise to an outward diffusional flux of those metallic elements that do not form nitrides. As an explanation of the mechanism of this metal diffusion, which relieves the internal stresses and leads to the formation of surface protrusions, Nabarro–Herring creep as well as dislocation pipe diffusion controlled creep are discussed.