Severe Plastic Deformation to Achieve High Strength and High Ductility in Fe-Ni Based Alloys with Lattice Softening

In the present article, recent reports on microstructure and mechanical properties in SPDed Fe-Ni-Co-Ti and Fe-Ni-Al-C alloys are overviewed. The chemical compositions in these alloys have been determined to have lattice softening where the elastic constant CA goes to a very small value. The alloys with lattice softening were processed with severe plastic deformation, SPD, to raise the strength. Here, SPD includes severe cold working by rotary swaging, cold rolling and high-pressure torsion. These alloys have been reported to have ultrahigh strength along with good tensile ductility and the balance of strength and ductility is better than that in conventional high-strength steels. Microstructural development during the SPD process and the relation between microstructure and mechanical properties in SPDed alloys are summarized. The phase stability of gamma has an important role in the microstructural development during SPD process and deformation behavior in the subsequent mechanical testing. The untransformed gamma has a potent capability for plastic deformation where strain-induced transformation and deformation twinning can be expected to occur during mechanical tests after SPD processing. Simultaneous activation of these deformation mechanisms during plastic deformation would be a common feature in the lattice softened alloys, but the effects of alloying elements and phase stability on the deformation behavior have not been understood yet.