Effect of yttrium on the oxidation behavior of cast Ni-30Cr alloy

Cast Ni-30Cr and Ni-30Cr-0.5Y alloys were oxidized at 1000°C in pure O₂ for various times, then were either furnace cooled to room temperature, or thermally cycled between 1000°C and different lower temperatures. The isothermal oxidation rate of the Ni-30Cr alloy was reduced by about a factor of 3.6 by the addition of 0.5% Y. Acoustic emission signals, which are generated by scale fracture events, were collected during isothermal oxidation, during continuous furnace cooling and during thermal cycling. These data showed, as others have shown, that the scale formed on Ni-30Cr-0.5Y was significantly more resistant to fracture than that on Ni-30Cr. This advantage of the Y-containing alloy was evident for comparisons based on equal oxidation times, and more importantly, at equal scale thicknesses. SEM and EDAX analyses show that continuous Cr₂O₃ scales were formed on both Y-bearing and Y-free alloys after a short time of oxidation (2 h), but after a longer period of oxidation and thermal cycling, a NiO or NiCr₂O₄ outer layer was found. This outer scale created a new interface with the Cr₂O₃ scale where thermal stresses will be generated during cooling due to the thermal expansion difference between Cr₂O₃ and NiO or NiCr₂O₄. Spallation at the inner scale/outer scale interface, as well as at the metal/scale interface, was observed. X-ray measurements of scale strains at equal scale thicknesses showed that the growth strains (at the end of the isothermal oxidation period) were larger on the Y-containing alloy, and that this alloy also sustained larger residual strains upon cooling to room temperature. Using a model based on elastic strain energy, estimates of the surface energy for scale fracture (a measure of scale adhesion) were significantly higher for the Y-containing alloy at equal scale thicknesses. Both the AE and the strain measurements are consistent with the proposal that Y improves the inherent strength of the metal/scale interface. The smaller rate of scale cracking for Y-containing alloys, combined with their slower scale growth rate, offers the further benefit of delaying the onset of NiO or NiCr₂O₄ overgrowth layers, which themselves may degrade the integrity of the scale.