Fe25 mol.%Al2mol.%Cr, Fe28mol.%Al and Fe28mol.%Al2mol.%Cr alloys were prepared in order to investigate the effect of Cr addition on antiphase boundary (APB) types and tensile properties of D03 single-phase Fe3Al polycrystals. The highest elongation and the lowest work-hardening rate are obtained for Fe28Al2Cr. Fe25Al2Cr has the lowest elongation and the highest work-hardening rate. In addition to thermal APBs, many uncoupled superpartial dislocations dragging extended APBs can be seen in deformation microstructures for all the alloys. In Fe25Al2Cr B-type APBs are frequently observed with a small amount of D03-type APBs, whereas in Fe28Al and Fe28Al2Cr all of thermal or deformation-induced APBs are of D03-type. Uncoupled superpartial dislocations in deformed Fe25Al2Cr are blocked at thermal or deformation-induced APBs. By considering coordination configurations of neighbouring atoms at an APB, the blocking force against dislocation motions is shown to be higher for the B2-type APB than for the D03-type APB. This is related to the highest work-hardening rate of Fe25Al2Cr. In contrast, cross-slipping of uncoupled superpartials is more frequently observed in Fe28Al2Cr than in Fe28Al. The work-hardening rate of Fe28Al2Cr would be reduced by the occurrence of cross-slipping. Ductility improvement by the Cr addition is discussed in terms of an intrinsic change in the work-hardening rate of D03 Fe3Al alloys.
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