A review of recent developments in Fe 3 Al-based alloys
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A review of recent developments in Fe3Al-based alloys C. G. McKamey, J. H. DeVan, P. F. Tortorelli, and V. K. Sikka Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6115 (Received 11 March 1991; accepted 15 April 1991)
Fe3Al-based iron aluminides have been of interest for many years because of their excellent oxidation and sulfldation resistance. However, limited room temperature ductility (900 °C). Indeed, much greater characterization of scale behavior under conditions where thicker products are formed is needed for these aluminides. B. High temperature sulfidation Effects of chromium and aluminum additions on the sulfidation of iron alloys have been examined by several investigators.146"150 These studies have shown that aluminum additions to iron and iron-chromium alloys significantly improve sulfidation resistance in sulfur vapor or H 2 S/H 2 mixtures and that the corrosion rate decreases as the Al/Cr ratio increases. In reviewing the design of sulfidation resistant alloys, Strafford and Datta151 noted that aluminum is a very beneficial alloying element because of (1) the thermodynamic stability of aluminum sulfide, (2) the relatively low rate of sulfidation of aluminum compared to iron and chromium, and (3) the relatively large molecular volume of aluminum sulfide (that is, a large Pilling-Bedworth ratio). These properties would be expected to support the selective sulfidation of aluminum to form a barrier sulfide layer with an acceptably low reaction rate at elevated temperatures. Several studies147'149-150'152 have verified that binary iron aluminide alloys containing ^ 1 8 % Al afford relatively good resistance in sulfur vapor or H 2 S/H 2 mixtures up to ~750 °C. Reaction rates are parabolic and are below 1 mg 2 c m " 4 h " 1 at 800 °C at an effective ps2 ^ 1 Pa. However, at 800-1000 °C, the ability of the aluminum to suppress iron sulfide formation at sulfur pressures >133 Pa (1 Torr) is lost and sulfidation rates increase rapidly with time. Depending on the sulfur and aluminum activities, the scales formed are multilayered, composed of A12S3 at the alloy surface, FeS at the gas/scale interface, and a transition product of (Fe,Al) 3 S 4 between these layers. Internal A12S3 precipitates sometimes mixed with FeAl 2 S 4 were also observed below this external scale in alloys containing 18-20% Al at 750 and 900 °C in sulfur vapor at 10~ 3 Pa.150 However, under the same conditions, an alloy containing 28% Al formed only an A12S3 external scale. (An analysis of the aluminum content required to suppress internal sulfidation in iron-aluminum alloys has been reported by Smeltzer and
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