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.