Environmental embrittlement of single crystal and thermomechanically processed B2-ordered iron aluminides

Abstract

Environmental embrittlement contributes to the poor ductility of iron aluminides. Elongations in dry environments, such as vacuum or dry oxygen, are considerably greater than in air. Moisture in air is the source of the embrittlement and it is believed that the water vapor reacts with aluminum atoms at the crack tip to produce hydrogen, which then causes embrittlement. In order to expand the use of iron aluminides commercially, this poor room temperature ductility has to be overcome. This has been accomplished in part by alloying with chromium and by thermomechanical processing (T/M-P) schemes. In fact, significant increases in ductility in air have been reported for an alloy containing 5% Cr which had been warm rolled to produce a partially recrystallized microstructure. The effects of chromium are not well understood as of yet, but it has been postulated that the T/M-P effect is caused by the presence of the partially recrystallized microstructure, which reduces the environmental susceptibility by decreasing the number of transverse grain boundaries, thus reducing hydrogen penetration into the material. This argument assumes that grain boundaries have a large effect on embrittlement by controlling hydrogen ingress into the material. The objectives of this work are to determine the rolesmore » grain boundaries have in the environmental embrittlement of iron aluminides and in the T/M-P effect observed in these materials. Alloys of several compositions were thermomechanically processed, heat treated and then tensile tested in environments of dry oxygen and moist air. Single crystals containing 35% Al were also tested in oxygen and air.« less