The influence of microstructure on the ductility of iron aluminides

Considerable effort has been devoted over the last decade to the development of iron aluminides as materials for high temperature applications, where their good oxidation and corrosion resistance, combined with reasonable strength, may be utilised. Poor formability and ductility, however, particularly at room temperature, has hampered the exploitation of these materials. The present review examines the present state of understanding of the factors which influence the ductility. Recent research has made clear the important influence of testing environment, the role of Al content and minor additions of B, as well as the effect of quenched-in vacancies. The extent to which other factors, such as alloying additions and microstructural features, affect the ductility has not received the same attention, and is examined in the present study. Alloy strengthening, by almost any mechanism, is seen to lead to a dramatic loss of ductility. The only parameter allowing both strength increase and ductility improvement for a given set of Al/B/vacancy/environment conditions is the grain size. The best ductility for a given alloy, which should have as low an Al content as compatible with other requirements, is obtained by refining the grain size and by maintaining the alloy in the softest possible state. For the most part these conclusions are drawn from analysis of the behaviour of B2 ordered FeAl alloys, although similar trends seem also to apply to alloys of slightly lower Al content where DO3 ordering can occur. The observations drawn can be understood in terms of the mechanisms leading to the nucleation and propagation of brittle fracture, either as transgranular cleavage cracks or as grain boundary cracks. The possible role of additional factors, such as the texture, or grain and grain boundary distribution, surface layers producing protective stress effects, and strain homogenising or crack arresting dispersions, has not been sufficiently evaluated to determine whether any further improvements of ductility are possible.