Dynamic deformation response of a high-strength, high-toughness Fe–10Ni–0.1C steel

The results of an investigation of microstructural evolution during quasi-static and dynamic deformation of a Fe–10Ni–0.1C steel containing small amounts of V and Mo are reported. In the as-received condition, the steel has a lath martensite microstructure with a mean lath size of ∼150 nm, and MC carbides dispersed in it with a mean size of ∼20 nm. During dynamic deformation (strain rates of 1 × 103 s−1 to 4 × 103 s−1), depending on strain and strain rate, shear localization occurs and is accompanied by an optically visible shear band (∼20 μm wide). In the situation where the localization process is in its early stages (lower rates, lower global height strains, or both), the microstructure shows severe local deformation within the band but the initial microstructure is still discernible. With progression in severity of localization, there is clear and reproducible evidence for a central region (∼6–8 μm wide) within the shear band composed of ∼300 nm size equiaxed grains. Electron microscopy characterization of these grains confirms the presence of both, austenite, with a low dislocation content, and heavily dislocated and/or twinned ferrite. Composition measurements from the individual grains confirm partitionless transformation. When the test conditions are further intensified, a crack “chases” the shear band, with the crack running either partway or all the way through the band and therefore through the sample. Examination of the resulting fracture surfaces provides evidence for the presence of a thin liquid film layer (perhaps 10–20 nm or less) at various locations within the shear band that presumably has no shear resistance. Together, these observations provide a microstructural footprint for how deformation progressed during shear localization, a sense for the accompanying thermal profile within the shear band, and evidence for the intensity of localization in this alloy in the condition it was examined.