Formation of pegs during hightemperature oxidation of Fe 3 Al containing yttrium
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(b) Fig. 1—SEM micrographs showing (a) spallation of Al2O3 scale formed on Fe-14. 3 wt pct Al and (b) adherent oxide scale with occasional Y-rich second-phase oxide in Fe-14.1 wt pct Al-0.3 wt pct Y alloys oxidized in air at 1100 8C.
yttrium to Fe3Al alloy significantly improves the adhesion of oxide scale to the alloy substrate. Figure 1(b) also shows the formation of second phase in alumina scale, and the second phase is found to be enriched with yttrium. To clarify the mechanism for the oxide adhesion improvement in Fe3Al-Y alloy, microstructure of the alloy oxidized at 1100 8C for 5 days was examined. One of the main microstructural characteristics was the formation of pegs. Figure 2 shows a typical peg formed at the substrate/alumina scale interface, and the peg seems to grow inward through the substrate. As shown in Figure 2, the peg anchors the oxide scale to the alloy, leading to the improved adhesion of scale to the alloy substrate. Figure 3(a) shows grain boundaries of the Y-doped Fe3Al alloy before oxidation. It was observed that yttrium was already segregated to the grain boundaries of the alloy before oxidation. After oxidizing the alloy for 5 days in air at 1100 8C, the cross section of the alloy specimen was analyzed. Figure 3(b) is for the oxidized alloy showing grain boundaries of the alloy substrate and pegs. As shown in Figure VOLUME 31A, JUNE 2000—1685
Fig. 2—SEM micrograph showing the formation of peg at Al2O3/Fe3Al interface.
Fig. 4—SEM and X-ray maps showing the composition of pegs formed at alumina scale/substrate interface: (a) SEM image, (b) oxygen X-ray map, (c) aluminum X-ray map, (d ) yttrium X-ray map, and (e) iron X-ray map.
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(b) Fig. 3—Optical micrographs showing the cross section of Fe3Al-Y (a) before oxidation and (b) after oxidation in air at 1100 8C for 5 days. 1686—VOLUME 31A, JUNE 2000
3(b), pegs seem to be connected to the grain boundaries of the alloy substrate, implying that pegs may be grown inward through the grain boundaries of the substrate. Figure 4 shows an SEM micrograph and X-ray maps of the substrate containing pegs formed along the alumina scale/substrate interface, indicating that the composition of pegs is different from that of the alloy substrate. The pegs are enriched in O, Al, and Y and deficient in iron. From this result, it can be concluded that pegs are developed from grain boundaries of substrate near the scale/substrate interface due to internal oxidation. In other words, oxygen diffuses inward through the alumina scale and reacts with Al and Y at the grain boundaries of the substrate, which result in the formation of new complex oxides such as pegs in the substrate. No pegs were observed in the Y-free Fe3Al oxidized under the same experimental conditions. Therefore, it can be deduced that the formation of pegs due to the presence of yttrium is indicative of the change of oxide growth mechanism from predominant outward diffusion of Al to predominant inward diffusion of oxygen. A large number of studies have been performed to investigate the
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