Embrittlement of B2 iron aluminide by water vapor and by hydrogen
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I.
INTRODUCTION
SEVERAL recent studies on the mechanical properties of iron aluminides have confirmed that the tendency toward brittleness exhibited by these materials when tested in air is due to the presence of water vapor. I1-71 For example, tensile tests conducted in low moisture environments exhibit very significant increases in ductility over those observed in air. This has led some researchers to advance the hypothesis that the water vapor in air is reduced by aluminum atoms in the alloy to form atomic hydrogen, which penetrates into the region ahead of the crack tip to cause hydrogen embrittlement; tl,2] however, this has not been proven. Studies which show that the ductilities of iron aluminides in air are strain rate sensitive lend credence to the hypothesis that hydrogen embrittlement is responsible for the low ductility commonly observed in air. For example, Fe-45AI exhibits ap2Proximately 2 pct elongation at strain rates below 10- s -1 and about 9 pct elongation at strain rates above 10-1 s-l. I8] Fe-28A1 with and without 4 pct Cr exhibits a monotonic increase in elongation from less than 2 pct to over 8 pet for strain rate increases from 3.3 • 10 -4 s -1 to 3.3 • 10~ S-1.tgj These strain rate effects suggest that a rate process, possibly hydrogen diffusion, is important in the embrittlement of iron aluminides. Overall, other processes which may be involved include transport of water vapor to the crack tip reaction site, the water reduction reaction, and hydrogen influenced dislocation generation and motion at the crack tip. I1~ This study was undertaken to gain information about the rate of the embrittlement process in air by measuring crack growth velocities under constant load conditions and by determining the effect of strain rate on the tensile ductility. Further, to learn about the influence of internal hydrogen, p e r se, tensile properties were characterized for samples cathodically charged with hydrogen. In addition, the kinetics of the recovery of ductility were studied by measuring the response of the charged materials L.A. HELDT, Professor, is with the Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931. D B . KASUL, formerly Graduate Research Assistant, Department of Metallurgical and Materials Engineering, Michigan Technological University, is Group Leader, Materials Science, Associated Spring Corporation, Barnes Group, Bristol, CT 06010. Manuscript submitted September 7, 1993. METALLURGICALAND MATERIALS TRANSACTIONSA
to baking treatments. Based on these results, consideration is given to the question of whether crack growth in iron aluminides in air can be supported by diffusion of hydrogen in advance of the crack front.
II.
EXPERIMENTAL PROCEDURE
The Fe-35 at. pct A1 alloy used in this study was prepared by repeated arc melting (seven times in high-purity argon) the necessary weights of electrolytic iron (99.9 pct) and aluminum shot (99.999 pct). Analysis indicated impurity levels of (wt pct) C (0.005), S (0.003), N (0.002),
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