Revisiting the role of substrate microstructure and short-circuit diffusion in the oxidation of Ni-Cr alloys

International audienceThe growth of a protective oxide scale on a heat-resisting alloy leads to subsurface depletion of the protective element (most commonly Cr or Al), and requires a sufficient flux of this element from the bulk to the surface. Selective oxidation critically depends on the alloy microstructure, as shown by Giggins and Pettit in the case of Ni-Cr alloys at 900-1100 °C : in cold-worked specimens, subsurface defects generate new grain boundaries by recrystallization upon heating, which provides accelerated diffusion to the surface and favors the establishment of a Cr2O3 scale.Similar processes operate at lower temperatures, where recrystallization is absent or much slower. Depletion profiles observed in Ni-base alloys exposed to pressurized water 340-360 °C can reach dozens of nm from the surface, intergranular oxides or cracks. The diffusivity of Cr is not known at these temperatures, but the extrapolation of high temperature tracer data would suggest that lattice diffusion alone cannot account for these diffusion distances. Instead, dislocations and subgrain boundaries may act as high diffusivity paths.A difficulty in modeling these processes arises from the fact that the organization and density of 1D and 2D defects evolve during heat treatments as recovery, recrystallization and grain growth proceed. In addition, vacancy fluxes are generated in the alloy as a result of both the oxidation reaction and the Kirkendall effect. The extent to which vacancy supersaturations may develop depends on the alloy capacity for vacancy annihilation (at grain boundaries, interfaces, free surfaces or in the bulk by dislocation climb), and therefore on the density of the various types of defects. The resulting vacancy concentration will in turn affect diffusion.The present work aims at better understanding the influence of the microstructure on Cr depletion and selective oxidation, in order to better integrate microstructural evolutions in oxidation kinetics modeling. It combines oxidation experiments and measurements of base diffusion data in the same temperature range, using annealed and cold-worked substrates