ENVIRONMENTAL EMBRITTLEMENT IN FEAL ALUMINIDES

In this paper we review experimental and theoretical findings related to the recently discovered mechanism of moisture-induced environmental embrittlement in FeAl-based alloys. We show that when low aluminum content FeAl alloys (35 and 36.5% Al) are tested in air, the aluminum in the alloys reacts with moisture in the air, producing atomic hydrogen. This atomic hydrogen enters the metal in the vicinity of the crack tips and embrittles the FeAl aluminides. As a result, when the alloys are tensile tested in air, it is commonly found that they fracture with limited ductility by transgranular cleavage. When this embrittlement mechanism is suppressed (e.g., by testing in dry oxygen), ductility is found to increase dramatically (to as much as 17-18%), and the fracture mode changes to intergranular. The intrinsic resistance to fracture is therefore quite high in these alloys. First-principles calculations confirm that the intrinsic cleavage strength and energy of FeAl are indeed quite high (comparable to or slightly higher than that of a ductile alloy like Ni3Al). The calculations also show that absorbed hydrogen can significantly reduce the cleavage strength and energy of FeAl (by as much as 20-70%, depending on the hydrogen concentration), consistent with the proposed embrittlement mechanism. In higher Al content FeAl alloys (40 and 43% Al), there is an additional cause of brittle fracture, namely intrinsically weak grain boundaries. In these alloys, the grain boundaries have to be first strengthened (by the addition of boron) before the moisture-induced environmental embrittlement mechanism becomes evident. Thus, the ductility of Fe-40Al alloys is approximately the same in air and dry oxygen, whereas the ductility of B-doped Fe-40Al alloys is significantly improved when tested in dry oxygen instead of air (from 4.3 to 16.8%). With increasing Al concentration, the grain boundaries in FeAl become increasingly more brittle and, in Fe-50Al, boron is unable to suppress intergranular fracture.