The influence of ion-implanted yttrium on the selective oxidation of chromium in Co-25 wt.% Cr

Abstract

Specimens of Co-25 wt.% Cr, Co-25 wt.% Cr-1 wt.% Y, and yttriumimplanted Co-25 wt.% Cr alloy were oxidized at 1000°C in 1 atm O₂. The implantation dosage ranged between 10¹⁶ to 10¹⁸ ions/cm². The unimplanted binary alloy oxidized to a duplex Co-rich scale, but the Y-containing ternary alloy formed a continuous Cr₂O₃ layer. When the implantation dosages were lower than a nominal 10¹⁸ ions/cm², the alloy failed to develop a similar continuous Cr₂O₃ layer as that observed with the Y-containing alloy. A temporarily stable external Cr₂O₃ scale was formed on the most heavily implanted specimen (1×10¹⁸ Y⁺/cm²). This Cr₂O₃ scale consisted of very fine-grained oxide, which is permeable to the outward transport of Cr and Co. Internal oxidation pretreatment of the ion-implanted specimens converting the Y metal to its oxide prior to the oxidation experiment, can enhance the development of an external Cr₂O₃ scale, but this scale is also unstable. Results suggest that the selective oxidation of chromium in an ordinarily non-Cr₂O₃ -forming alloy can be due to the reactive element oxides acting as preferential nucleation sites on the alloy surfaces, but the subsequent growth of these scales may require a continuous supply of reactive elements in the alloy.