Relationship between flow stress and atomic order in the FeCo alloy

Abstract

Stress–strain curves in compression were obtained as a function of temperature for both the disordered and fully ordered FeCo–B2 type superlattice only. Supplementary observations were made of the surface slip markings after various strains, as well as of the dislocation configurations existing after these strains, using transmission electron microscopy techniques. It has been found that the deformation process occurring in the disordered alloys is similar at all strains. Furthermore, the flow stress is strongly dependent on temperature and nearly independent of strain. The profuse stepped slip lines observed on the surface of these specimens suggests that thermally activated cross-slip is the controlling mode of deformation in the disordered alloys. The ordered alloys, on the other hand, show three distinct deformation stages similar to those observed in face-centred cubic metals and alloys. The flow stress in Stage I is independent of strain. On the other hand, the flow stress in Stage II increases rapidly with strain and is virtually independent of temperature. The flow stress in this stage is satisfactorily accounted for in terms of the non-conservative motion of jogs in screw dislocations. Furthermore, the slip lines are straight in Stage II because of the difficulties associated with cross-slip in the ordered alloy. Finally, Stage III is reached at some early constant stress level, τ_III. During this stage, the slip lines become stepped and the change in flow stress with temperature becomes identical to that for the disordered alloy. This has been interpreted in terms of the fact that at the stress level, τ_III, many of the individual dislocations which constitute the superlattice dislocation are able to cross-slip more or less independently with the creation of antiphase boundaries. Since the deformation behaviour of the disordered FeCo alloy appears to be essentially the same as that in many other body-centred cubic metals and alloys in general, it is suggested that the deformation mechanism associated with them is the same as that in disordered FeCo, namely the thermally activated cross-slip of screw dislocations.