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Advancing Materials Research
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 O2– 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 O2– 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.