The mechanisms of scale and subsurface diffusion zone formation of heat-resistant HP40NbTi alloy at long-term high-temperature exposure

Surface multilayer scale and the subsurface diffusion zone of heat-resistant HP40NbTi alloy were characterized by SEM and X-ray mapping after high-temperature exposure for different durations. After long-term exposure, conditional equilibrium is established in the alloy subsurface area: scale formation and internal oxidation deplete the diffusion zone of Cr and Si, which is compensated for by their diffusion along with Ni and Fe diffusion from the bulk of the metal to the surface. The process slows over time owing to the increase in the diffusion zone depth and the formation of a silicon oxide "barrier" layer.