Ultrahigh strength of nanocrystalline iron-based alloys produced by high-pressure torsion

Microstructural evolution of an Fe-18.1%Ni-34.9%Co-9.3%Ti (in at.%) in processing by high-pressure torsion (HPT) was investigated by electron backscatter diffraction. After 10 turns of HPT straining at room temperature with a rotation speed of 1 rpm under a pressure of 6 GPa, the alloy was composed of body-centered cubic-structured grains with sizes of 20-50 nm having high density crystal defects inside the grains. The mechanism of this significant grain refinement was discussed in relation to the stress-induced martensitic transformation and transgranular shear near ideal strength. The actual shear stress of the nanocrystalline iron-based alloy was estimated to be 1.37 GPa and as high as 38% of the ideal shear stress, which is similar to Gum Metal exhibiting deformation without dislocation activity. It is inferred from these results that the dislocation motion can be suppressed up to ultrahigh stress level near ideal strength by the formation of nano-sized grains.