Limitations on the Use of Ion Implantation for the Study of the Reactive Element Effect in β-Nial

Numerous investigations have used the ion implantation of reactive elements (RE) such as Y or Ce, to study their effect on the growth of external oxide scales on alloys. Ion implantation has, nevertheless, some specific limitations, especially in Al₂O₃-forming alloys. The most notable limitation occurs at temperatures >1000°C where, owing to the shallowness of implantation, any effects of the ion-implanted RE are short-lived and differ significantly from those observed for an RE alloy addition or an RE oxide dispersion. Additionally, in alumina-forming alloy systems, particularly β-NiAl, implanted Y stabilizes the first-forming, metastable θ-Al₂O₃. The retention of the θ-Al₂O₃ scale on Y-implanted substrates is a chemical effect of the high concentration of Y near the substrate surface and is not a result of the implantation process itself. The loss of the RE effect at high temperatures and long times for implanted alloys is related to the outward diffusion of the RE and the formation of RE-rich oxides near the gas interface of the scale.