Environmental effects in iron aluminides

Iron aluminides based on the stoichiometric compositions of Fe3Al and FeAl exhibit poor room temperature ductilities due to hydrogen embrittlement (HE), The presence of surface passive films reduces HE. The reduction is due to the lower rate of hydrogen liberation on the surface of iron aluminides with a passive layer. Theoretical and experimental verification for this idea are provided. The effect of addition of passivity-inducing elements Ti, Zr, V, Nb, Ta, Cr, No, W, Si and Ni to Fe,AI on the thermomechanical and electrochemical behaviour has been outlined. The Cr- and Ti-alloyed intermetallics exhibited significant room temperature ductilities. Microstructural studies of the alloyed intermetallics revealed that when the addition of passivity-inducing element results in the precipitation of brittle phases with Fe and Al, they crack during the processing operation. The addition of oxygen-active elements on the embrittlement behaviour is also discussed. The effect of these :additions on the potentiodynamic polarization behaviour and high temperature oxidation behaviour is also briefly addressed. Methods to minimize:HE by the addition of elements that irreversibly trap hydrogen and that prevent recrystallization have also been discussed.