FIGURE 14 Yttrium-concentration/depth profiles in a cobalt-45 atomic percent chromium alloy after low (2×10¹⁴ Y/cm²) and high (2×10¹⁶ Y/cm²) dosages of yttrium ion implantation at 70 keV, followed by oxidation in 1 atm oxygen at 1273 K. From Przybylski.²⁶ Reprinted with permission.

15;²⁶ the higher-dose specimen (whose oxidation is controlled by O^2– diffusion) is much more resistant to oxidation than is the lower-dose specimen (whose oxidation is controlled by Cr³⁺ diffusion). Another surprising observation is that microanalytical measurements (by energy-dispersive x-ray analysis) demonstrates that the yttrium in the scale is located almost exclusively in the oxide grain boundaries without detectable presence of second phases.²⁷ It is tempting to speculate that, in the higher-dose specimens, the implanted yttrium is present in sufficient concentration along the oxide grain boundaries to inhibit the Cr³⁺ ion diffusion that would otherwise select these short-circuiting paths, and so the rate-controlling oxidation mechanism is then shifted to the slower O^2– diffusion.

Clearly, there are new challenges in these findings, not only from the standpoint of ion implantation research per se, but also as a technique for understanding and using the potent oxidation resistance contributed by small percentages of elements like yttrium in high-temperature alloys.

FIGURE 15 Isothermal oxidation kinetics of a cobalt-45 atomic percent chromium alloy after various dosages of yttrium ion implantation at 70 keV. From Przybylski.²⁶ Reprinted with permission.