ENVIRONMENTAL EMBRITTLEMENT OF IRON ALUMINIDES

The effects of gaseous and liquid environments on the fracture behavio r of Fe3Al and FeAl are reviewed. The influence of variables such as c omposition, microstructure, loading rate and temperature are described . It is shown that the iron aluminides, when tested in dry environment s (oxygen, dry air, vacuum) are inherently ductile, but that the prese nce of very small levels of moisture is sufficient to cause severe emb rittlement. Hydrogen, whether released from water vapor or directly in contact with the iron aluminides, is the agent of embrittlement. A de tailed discussion of the mechanism of embrittlement is included. Allev iation of embrittlement by such techniques as alloying, surface protec tion and control of microstructure is outlined. Finally, the environme ntal embrittlement of iron aluminides is contrasted to that of other a luminides; the major difference is shown to be the propensity for clea vage fractures in FeAl and Fe3Al, compared to intergranular fracture i n L1(2) polycrystals.