The influence of microstructure on the ductility of iron aluminides

Considerable effort has been devoted over the last decade to the developmen t of iron aluminides as materials for high temperature applications, where their good oxidation and corrosion resistance, combined with reasonable str ength, may be utilised. Poor formability and ductility, however, particular ly at room temperature, has hampered the exploitation of these materials. T he present review examines the present state of understanding of the factor s which influence the ductility. Recent research has made clear the importa nt influence of testing environment, the role of Al content and minor addit ions of B, as well as the effect of quenched-in vacancies. The extent to wh ich 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 a lloy, which should have as low an Al content as compatible with other requi rements, is obtained by refining the grain size and by maintaining the allo y in the softest possible state. For the most part these conclusions are dr awn from analysis of the behaviour of B2 ordered FeAl alloys, although simi lar 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 o f the mechanisms leading to the nucleation and propagation of brittle fract ure, either as transgranular cleavage cracks or as grain boundary cracks. T he possible role of additional factors, such as the texture, or grain and g rain boundary distribution, surface layers producing protective stress effe cts, and strain homogenising or crack arresting dispersions, has not been s ufficiently evaluated to determine whether any further improvements of duct ility are possible. (C) 1999 Elsevier Science Ltd. All rights reserved.