The development of Fe3Al aluminides has been restricted in the past by poor ductility at ambient temperatures, and it is only recently that possible solutions to this problem have been found. It was shown a few years ago (1) that the addition of 2-6% Cr to a Fe3Al base alloy led to good ductility and this improvement was explained by a reduction of the APB energy, increasing the separation of the superpartial dislocations and thereby allowing easier dissociation of these dislocations, easier cross slip and a reduced tendency to stress and strain concentrations. However, at a later stage, an alternative explanation was proposed (2,3) based on examinations of both FeAl and Fe3Al alloys under different environments, and the ductility change was explained in terms of chemical attack at the tip of a crack leading to local hydrogen embrittlement.
The present study re-examines the behaviour of a Fe3Al alloy both with and without the addition of Cr. Strength, work hardening behaviour and failure ductility are examined under conditions where environmental effects should not be important, and the mechanical behaviour is interpreted in terms of significant variations in the type of order, the ordered domain structure and the resulting dislocation structures. It is seen that the addition of Cr can lead to a better ordered material and the differences in ordered state between the two materials can significantly affect dislocation behaviour and mechanical properties.