Localized amorphism after high-strain-rate deformation in TWIP steel

The microstructural features of shear localization, generated by a high-strain-rate deformation (∼105 s-1), of a twinning-induced plasticity (TWIP) steel containing about 17.5 wt.% Mn were well characterized by means of optical microscopy, transmission electron microscopy and electron backscatter diffraction. The high deformation rate was obtained by a ballistic impact penetration test on the TWIP steel sheet. In addition to the deformation twins observed as the main microstructural characterization in the matrix, some shear bands consisting of complex microstructures were also evidenced in the highly deformed area. Inside the shear band, there exist a large region of amorphous phase and a smooth transition zone that also contains nanocrystalline phases. The grain size decreases gradually in the transition zone, changing from a coarse scale (>100 nm) to a fine scale (<10 nm) adjacent to the amorphous region. The coexistence of the amorphous state and the fine-scaled nanocrystalline phase clearly suggests that melting inside the shear bands occurred, which is corroborated by calculations showing a very high rise in temperature due to localized plastic deformation and extremely rapid cooling by heat dissipation into the specimen.