On the role of yttrium during high-temperature oxidation of an Ni-Cr-AI-Fe-Y alloy
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nominal chemical composition is given in Table I. Above about 1000 ~ the alloy derives its oxidation resistance from a thin tenacious layer of Y-modified A1203 scale. For example, after 1008 hours of exposure in air at 1150 ~ the thickness of the oxide scale is about 4/~m. tz] Although it is well known that the protective nature of A1203 is improved by critical Y additions, a number of mechanisms, sometimes opposing, have been proposed to explain the Y effect. Several extensive reviews have dealt with this subject, t3-8] To provide a basic understanding of the Y effect in a given alloy, it is essential to determine its distribution within the oxide scale on the atomic level. Thus, it was the objective of this investigation to examine the role of Y during high-temperature oxidation of alloy 214 by the various techniques of analytical electron microscopy (AEM). Emphasis was placed upon the state of Y presence and its distribution throughout the oxide scale. Some of the results were examined with reference to various proposed mechanisms. Table I lists the chemical composition of the heat investigated. Sheet samples initially 1.0-mm thick were reduced to about 0.5 ram, annealed in quartz capsules under argon atmosphere ( - 1 0 -6 torr) at 1095 ~ as recommended by the manufacturer, and finally water quenched. For AEM work, disc-type specimens 3 mm in diameter were machined off the annealed sheets, ground to about 0.3-mm thick, and then metallographically polished. Isothermal oxidation of those specimens was con-
ducted in still air at 1150 ~ for up to 1000 hours using a resistance-heated furnace. Also, coupons (25.4 • 25.4 x 1 mm) were oxidized under the same conditions to examine fracture sections of the scale in a scanning electron microscope. To prepare thin foils near the oxidemetal interface, oxidized specimens were electropolished on one side t91 in a solution consisting of 30 pct nitric acid in methanol at about - 2 0 ~ until perforation occurred. Oxide films left behind were thinned in an ion beam mill at 5 KV. All foils were examined in an ana.lyrical electron microscope operating at 200 KV and equipped with an ultra-thin window X-ray detector capable of detecting elements down to C and an advanced condenser lens system which promotes the X-ray signal at a small probe diameter. Typical features of the grain structure of ml203 formed after 1000 hours of exposure at 1150 ~ are summarized in Figure 1. Parallel to the plane of oxidation, the grains were nearly equiaxed, as illustrated in the bright-field transmission electron microscopy (TEM) image of Figure 1(a). Although no attempt was made to determine the exact nature of fine parallel bands of alternating conWast observed within the grains, they could either be twins or stacking faults. A corresponding selected-area diffraction (SAD) ring pattern is shown in Figure l(b). All measured d-spacings were in agreement with those of a-AlzO3 (hexagonal; a = 0.476 and c = 1.299 nm).
(a)
H.M. TAWANCY, Research Engineer, is with the Materials Characterization Laborator
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