Mechanism of Formation of Surface Protective Layer in Heat-Resistant HP40NbTu Alloys during High-Temperature Oxidation

The process of high-temperature oxidation of multicomponent multiphase HP40NbTi alloy at 1150°C with duration of up to 500 hours in air is studied experimentally at the micro level using SEM and x-ray mapping. It is shown that the process occurs under the influence of various interrelated mechanisms. The changes in the composition and structure of the surface multilayer scale and of the subsurface diffusion zone in the alloy after oxidation of various durations are studied consistently. It is demonstrated that after a long-term exposure, conditional equilibrium is established in the alloy subsurface area; the formation of scale and the internal oxidation deplete the diffusion zone of chromium and silicon. This is compensated by the diffusion of Cr, Ni and Fe from the bulk of the metal to the surface. The process slows down with time due to the increase in the depth of the diffusion zone and formation of a “barrier” layer of silicon oxides.