Authors’ Reply

Oxidation Behavior of ODS Fe?Cr Alloys

“…7. However, the benefit of adding Y cannot compensate for the lower Cr in the alloy, thus ODS Fe-21Cr was oxidation resistant at 1100 • C, 18 while FCW-Y (14%Cr) failed in less than 100 hr and FCW-YT failed after 7,000 hr at 900 • C. The formation of FeO x nodules on FC-Y at 700 and 800 • C (Figs. 3b and 5c) was likely due to the lower Cr content in this alloy, and suggests that these materials are near the minimum Cr content necessary to provide good high-temperature oxidation resistance.…”

“…This limited the performance of FCW-Y at 1000 • C and FCW-YT and FC-Al at 900 • C. Previous work on ODS Fe-21%Cr showed good oxidation resistance at 1100 • C when a Y 2 O 3 or La 2 O 3 dispersion was present. 18 Otherwise, breakaway oxidation was observed. This observation is analogous to the results for 13-14%Cr ODS alloys at 900 • C where the presence of a reactive element (Y) reduced the scale growth rate, which slowed the consumption of Cr and prevented accelerated oxidation attack, Fig.…”

“…Oxide dispersions not only improve the creep strength, but also have been found to be beneficial to the oxidation resistance of both Fe-and Ni-base alloys. [15][16][17][18][19][20][21][22][23] The oxidation behavior of ODS Fe-Cr alloys has been previously investigated, 1,17,18,24,25 however, the materials in this study have lower Cr contents (13-14 at.%Cr) than several of the earlier studies. At these low Cr contents, Fe-Cr alloys are considered marginal chromia-formers and, as the Cr-consumption rate increases with temperature, can be limited at higher temperatures in their ability to form a protective, external Cr 2 O 3 -rich scale.…”

“…It is well known that an addition of Y reduces the growth rate of Cr 2 O 3 by a factor of 10-100 at this temperature. 17,18,36,37,42 Because the scale grew faster on FC-Al (compare the mass gain after one cycle to FCW-Y), more Cr was consumed, likely causing Cr depletion in the metal near the metal-scale interface. No scale spallation was measured after the 500 hr cycle but any cracking of the scale could have caused rapid reoxidation during the next cycle, sending the alloy into breakaway oxidation.…”

Grain Boundary Segregation of Yttrium in Chromia Scales

“…These effects are observed when the yttrium is present as an alloying addition or as an oxide dispersion (1)(2)(3)(4)(5)(6). In addition, the mechanism of growth of Cr203 is changed from one involving predominant cation diffusion to one involving predominant anion diffusion, and the microstructure of the scale is changed.…”