Measurement of the Critical Level of Moisture for Initiation of Water-Vapor-Induced Environmental Embrittlement in an Fe
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MEASUREMENT OF THE CRITICAL LEVEL OF MOISTURE FOR INITIATION OF WATER-VAPOR-INDUCED ENVIRONMENTAL EMBRITrLEMENT IN AN Fe3AI-BASED ALLOY C. G. McKAMEY AND E. H. LEE Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 378316114 ABSTRACT Iron aluminides based on Fe 3AI are of interest as structural materials because of their excellent corrosion resistance in many environments. However, studies have shown that one of the major causes of low room temperature tensile ductility, which so far has limited the use of these alloys, is an environmental reaction involving aluminum in the presence of water vapor. During this reaction, atomic hydrogen is released, moves into the sample ahead of the crack tip during stressing, and causes failure before the true ultimate tensile strength of the material is reached. This reaction is reduced significantly by testing in oxygen or vacuum. In the present study, an Fe 3Al-based alloy was tensile tested as a function of the level of water vapor in the test environment, from a vacuum of 10.' Pa to a water vapor partial pressure of 1330 Pa. The results show that a water vapor level of as low as 133 Pa (1 torr) can result in significant embrittlement. The fracture mode remains transgranular cleavage, but the scale of the cleavage facets changes with the water vapor level. INTRODUCTION Iron aluminides based on Fe3A] afford excellent corrosion properties at relatively low cost, making them candidates for use as structural material in corrosive environments [1,2]. However, these alloys have not yet found widespread use because they exhibit poor ductility at ambient temperatures, accompanied by brittle fracture [3]. Recently, efforts have been devoted to understanding the reason for their brittle behavior and to improving their ductility through control of grain structure, alloy additions, and material processing [3-5]. Recent studies of FeAl and Fe 3AI in various tensile testing environments have indicated that both alloy systems are more ductile at room temperature when tested in vacuum or dry oxygen [6-8]. Ductilities of 12-18% were attained in both iron aluminide systems in an oxygen pressure of 6.7 x 10' Pa, while only 2-4% ductility was achieved in normal laboratory air. The low ductility in air tests was attributed to environmental embrittlement involving generation of atomic hydrogen at the crack tip which is then transported into the specimen during stressing producing brittle cleavage failure. This atomic hydrogen is produced by the reaction of aluminum atoms at the crack tips with water molecules in the air. More recent studies have indicated that this type of environmental embrittlement also contributes to the low ductility of many other intermetallic systems [9-17]. The purpose of the present study is to determine the partial pressure of water vapor in the tensile test atmosphere necessary to produce severe embrittlement at room temperature.
Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
984
EXPERIMENTAL PROCEDURES A 500-g ingot of a
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