DISTRIBUTION OF ION-IMPLANTED YTTRIUM IN CR2O3 SCALES AND IN THE UNDERLYING NI-25WT-PERCENT-CR ALLOY

Implantation of yttrium and other reactive metals has been known to have significant effects on the oxidation behavior of Cr2O3-forming alloys. The oxide growth rate decreases by nearly a factor of ten, and the adhesion of oxide scales to alloys is greatly improved. To understand better the mechanisms by which these elements affect oxidation, it is important to know whether or not they are present in the metal ahead of the oxide layer. In this study, the diffusivity of implanted yttrium in an Ni-25wt.%Cr alloy and its distribution in the oxide scale after different oxidation times were evaluated using secondary ion mass spectroscopy. It was found that only 17 ppm yttrium was left in the alloy after just 20 min of oxidation at 1000-degrees-C. The implanted yttrium remained concentrated at the oxide-gas interface as the oxide thickened with time. Within the oxide layer, the yttrium concentration progressively dropped to zero at the scale-alloy interface. These results are discussed in relation to several important mechanisms proposed for the beneficial effects of reactive element additions on scale adhesion.