Effect of sulfur removal on Al 2 O 3 scale adhesion
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I.
INTRODUCTION
THE purpose
of this study is to demonstrate quantitatively whether or not alumina scales can be made adherent to NiCrAI alloys just be removing the sulfur impurity. The only way to establish unequivocally the major role of reactive element dopants on adhesion is to perform controlled experiments in which only one of the proposed mechanisms may operate at any one time. A1203 scale adherence to MCrA1 alloys is enhanced dramatically by the addition of small amounts of reactive element dopants. "Reactive" is loosely defined as oxygenactive, meaning a stable oxide former including Sc, Y, Zr, Hf, Th, and the lanthanide group elements. This group can also be defined as sulfur-active for the same reasonsJ 1] The adhesion mechanisms proposed for these dopants include (1) mechanical keying, (2) vacancy sinks or eliminating interfacial voids, (3) reduction of growth stresses via changes in diffusion mechanisms, (4) enhanced scale plasticity via the maintenance of free-grained scales, and (5) the production of a strong scale-metal interface by gettering sulfur or other impurities known to segregate and weaken grain boundaries in transition metal alloys, t2,31 Unfortunately, one or all of these effects may occur simultaneously due to reactive element doping. Identification of the dominant mechanism for adhesion is therefore very difficult. If total adhesion could always be produced by just introducing pegs, eliminating voids, minimizing growth stress, maintaining fine-grained plastic scales, or removing sulfur impurities--without the addition of dopants--then the process producing the greatest degree of adhesion by itself could be singled out as the dominant mechanism. Observations made over the years help de-emphasize some of the mechanisms. Adherence without pegs, spalling without voids, adherence of coarse-grained scales, spalling without buckling, or
JAMES L. SMIALEK, Research Scientist, is with NASA Lewis Research Center, Cleveland, OH 44135. Manuscript submitted November 20, 1989. METALLURGICAL TRANSACTIONS A
spalling with pegs point to the need for an alternative adherence mechanism. [1'4'5'61 Chemical bond strengthening of the oxide-metal interface might result from the observed interfacial segregation of the reactive elements, tl'7-~~ However, recent data suggest that substantial adhesion occurs simply by removing sulfur impurities and eliminating the bond weakening that may result from sulfur segregation to the scale-metal interface, tS'6,n-161 To evaluate the critical role of sulfur on scale adhesion, the task is to quantify the amount of scale adhesion that occurs just by sulfur removal alone. From the engineering standpoint of scale retention in cyclic oxidation, weight loss due to spallation can be used as a first approximation of the degree of bond weakness. This assumes that spalling occurs at the oxide-metal interface and that comparisons are made between alloys possessing similar oxidation rates. Smeggil et al. reduced spallation for undoped NiCrA1 in an 1180 ~ 90-hour cyclic oxidation tes
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