Acceptable Aluminum Additions for Minimal Environmental Effect in Iron-Aluminum Alloys
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ACCEPTABLE ALUMINUM ADDITIONS FOR MINIMAL ENVIRONMENTAL EFFECT IN IRON-ALUMINUM ALLOYS*
t VINOD K. SIKKA, t SRINATH VISWANATHAN, AND SANJAY VYAS* 'Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6083 *Carnegie Mellon University, 5000 Forbes Avenue, Pittsburghs, PA 15213
ABSTRACT A systematic study of iron-aluminum alloys has shown that Fe- 16 at. % Al alloys are not very sensitive to environmental embrittlement. The Fe-22 and -28 at. % Al alloys are sensitive to environmental embrittlement, and the effect can be reduced by the addition of chromium and .through the control of grain size by additions of zirconium and carbon. The Fe-16 at. % Al binary, and alloys based on it, yielded over 20% room-temperature (RT) elongation even after high-temperature annealing treatments at 1100*C. The best values for the Fe-22 and -28 at. % Al-base alloys after similar annealing treatments were 5 and 10%, respectively. A multicomponent alloy, FAP, based on Fe-16 at. % Al was designed, which gave an RT ductility of over 25%. INTRODUCTION The Fe3Al-based alloys are low-cost materials with excellent resistance to hightemperature oxidation and sulfidation. They suffer from low ductility at RT and poor hightemperature strength.' The low RT ductility has been associated with environmental 4 embrittlement,'- which results from the reaction of aluminum in the alloy with moisture in the air. The nascent hydrogen that is generated diffuses into the metal and causes loss in ductility. The hydrogen ingress can be reduced by a simple barrier such as oil film between the metal surface and the moisture. When hydrogen is generated (e.g., on a bare surface), its embrittling effect can be reduced'.4. 5 by producing an elongated microstructure with minimum transverse boundaries for diffusion paths. However, neither the surface barrier nor the elongated microstructure is an acceptable commercial solutions to the environmental effect. The present paper will describe a systematic study of environmental effects in binary iron-aluminum alloys over a range of Fe-16 to -28 at. % Al. (All percentages in the remaining text are in atomic percent..) This study established an acceptable aluminum limit for minimal environmental effects and developed an alloy composition with RT ductility values exceeding 25%. EXPERIMENTAL DETAILS This study is based on binary iron-aluminum alloys selected from three different regions of the phase diagram: disordered ox(Fe-16% Al), disordered a plus ordered D03 (Fe-22 % Al), and ordered D03 (Fe-28% Al). The binary alloys were also systematically modified by additions of Cr; Cr and C; and Cr, Zr, and C. A final iron-aluminum alloy containing Cr, Zr, C, and Mo was investigated for structural applications. The nominal composition of the alloys investigated is shown in Table I. Alloys of 500 g were prepared by nonconsumable arc melting in a copper crucible and casting into a 12.7 x 25.4 x 127-mm copper mold. Each ingot was cut into two pieces and processed into 0.76-mm-thick sheet by forging 50% at 1000TC, hot roll
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