Boron Depletion in a Nickel Base Superalloy Induced by High Temperature Oxidation

The conventionally cast Ni-base superalloy Rene 80 containing 90 wt ppm boron was oxidized in synthetic air at 850-1,050 A degrees C for times up to 100 h. Extensive microstructural studies using GD-OES and SEM/WDX/EBSD revealed formation of a porous chromia scale containing TiO2 precipitates at its outer surface and semi-continuous layers of BCrO3 and CrTi2O5 at its base. The development of B-rich oxide in the scale is related to its high thermodynamic stability and the high alloy boron diffusivity. Its enrichment in the oxide scale resulted in B-depletion from the alloy matrix, which was extremely rapid at 1,050 A degrees C, draining the boron content of a 2 mm thick alloy coupon in 100 h. After 100 h exposure at 950 A degrees C, the B-content dropped below 40 wt ppm, but only minor depletion was found after 100 h at 850 A degrees C. Predictions of B-depletion based on calculation of its diffusion from a thin, flat sheet were in good agreement with experiment at 1,050 A degrees C. Precipitation of B-containing compounds (borides) within the alloy is shown to account qualitatively for slower B-outdiffusion at lower temperatures.