Influence of Environment on Crack Growth Resistance of an Fe 3 Al,Cr Alloy
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INFLUENCE OF ENVIRONMENT ON CRACK GROWTH RESISTANCE OF AN Fe 3AI,Cr ALLOY A. CASTAGNA, P.J. MAZIASZ, AND N.S. STOLOFF
ABSTRACT The effect of hydrogen on the tensile and fatigue behavior of an Fe3AI, Cr type intermetallic alloy is examined. Hydrogen due to moisture in the air is found to be a major cause of embrittlement. The rates and mechanisms of the observed embrittlement appear to be temperature dependant. In addition, the alloy was found to have no notch sensitivity. INTRODUCTION Iron aluminides are currently being studied for use in applications where low cost, relatively low weight, and excellent corrosion resistance are desired. The main disadvantage of the alloys is low room temperature ductility in air due to moisture which causes hydrogen embrittlement [1-3]. This investigation examines changes in tensile and fatigue behavior of an Fe 3 AI,Cr type intermetallic compound in environments of various hydrogen content and at various temperatures. The alloy was studied in two ordered states, the DO 3 and the B2. The DO 3 is a highly ordered superlattice, while the B2 is a superlattice of imperfect order. EXPERIMENTAL MATERIALS AND PROCEDURE Material The material used in this study, designated FA129, was supplied by Oak Ridge National Labs and was composed of 28.6 at.%Al, 4.8%Cr, 0.21%C, 0.5%Nb, balance Fe. Material fabrication and annealing conditions to obtain B2 and DO 3 order are described in [4].
Notch Sensiti'i Notch sensitivity experiments were carried out on electro-discharge machined cylindrical specimens with a 22.6mm long by 4.6mm diameter gauge section. Unnotched specimens and specimens with a 1.0mm deep circumferential notch of 0.089mm radius were tested. This notch geometry resulted in a stress concentration factor of 3.9. The unnotched specimens were mechanically polished to a 0.3pnm finish. Tests were performed at a strain rate of 3.3x10-4 sec- 1 on an Instron servo-hydraulic machine fitted with a vacuum chamber. Two environmental conditions were studied in each of the two ordered states: dry oxygen and air. In the oxygen tests, the vacuum chamber was first evacuated to < 1.3x10-4 Pa before admitting constantly flowing oxygen at 1.01xl0 5 Pa. All tests were conducted at room temperature. Fatigue Crack Growth Room temperature fatigue testing was performed on compact tension specimens measuring 31.6mm x 30.5mm x 5.1mm thick with a machined notch of 60' angle and 0.127mm radius cut by EDM techniques. Specimen faces were polished to a 0.3gtm finish. Fatigue crack growth tests were performed on an Instron servo-hydraulic testing apparatus and crack length was measured via the d.c. potential drop method. Experimental details are as described in [4], except for a load shedding routine which was incorporated into the procedure to obtain more reliable threshold data. Five environmental conditions were previously studied in each of the two ordered states: dry oxygen, air, vacuum, hydrogen gas, and hydrogen charged. Fatigue tests in air at temperatures of 150'C, 300°C, and 450'C have been performed on DO
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