Early stages of oxidation of yttrium-implanted Ni-20% Cr

Abstract

The effect of ion-implanted yttrium on oxidation at elevated temperatures (500 to 1000° C) has been studied in Ni-Cr alloys using high-voltage electron microscopy (HVEM) ofin situ oxidized specimens, electron microscopy of scales oxidizedex situ, and Rutherford back scattering. The presence of yttrium enhances nucleation and growth of Cr2O3 at the alloy surface. Although the radiation damage induced by the ion implantation accelerates oxide nucleation and the initial rate of oxidation at 500° C, the growth of a mature oxide scale is slowed down, and the main influences seem to come from the chemical characteristics of the implanted yttrium. A Cr2O3 layer forms first on the yttrium-implanted alloys. Outward diffusion of Ni2+ cations through this layer forms an outer NiO scale. The initial growth process on unimplanted alloys is opposite. Here, NiO is the predominant initial oxide. As it thickens, a porous spinel and Cr2O3-enriched layer is formed between alloy and NiO. The inner oxide layer on yttrium-implanted alloys is fully dense and contains more than 50% Cr2O3. On the unimplanted alloy, the inner spinel layer is porous and contains a lesser enrichment in Cr2O3. The porous spinel delays formation of a protective Cr2O3 layer and gives poor scale adherence. The oxide growth mechanisms are discussed in the light of TEM results.