On the Room-Temperature Deformation Mechanisms of Lamellar-Structured Fe30Ni20Mn35Al15

A eutectic alloy Fe30Ni20Mn35Al15 (in at. %), consisting of B2 (ordered b.c.c.) and f.c.c. phases, was prepared by directional solidification, drop-casting or quenching from 1623K in order to obtain different lamellar sizes and morphologies. The hardness of the individual phases, measured using nanoindentation, was 4.38 +/- 0.20 GPa for the B2 phase and 2.72 +/-0.14 GPa for the f.c.c. phase. The roles of these phases in mechanical deformation were investigated. In both the drop-cast and the quenched alloy, which contained refined discontinuous lamellae, the B2 lamellae showed little sign of plastic deformation and simply behaved as obstacles to moving dislocations. The yield strength increased with refinement of the lamellae, whereas the ductility decreased. In contrast, tensile tests performed along the growth direction of the directionally-solidified alloy, which contained lamellae several microns wide aligned with the growth direction, showed that the f.c.c. lamellae experienced plastic deformation by glide of <110> dislocations, whereas the B2 lamellae fractured elastically into 30 μm long segments. Tearing at interfaces occurred for B2 lamellae inclined to the tensile axis.