Effects of nanocrystalline and ultrafine grain sizes on constitutive behavior and shear bands in iron

The mechanical behaviors of consolidated iron with average grain sizes from tens of nanometers to tens of microns have been systematically studied under uniaxial compression over a wide range of strain rates. In addition to the well-known strengthening due to grain size refinement, grain size dependence is observed for several other key properties of plastic deformation. In contrast with conventional coarse-grained Fe, high-strength nanocrystalline and submicron-grained Fe exhibit diminished effective strain rate sensitivity of the flow stress. The observed reduction in effective rate sensitivity is shown to be a natural consequence of low-temperature plastic deformation mechanisms in bcc metals through the application of a constitutive model for the behavior of bcc Fe in this strain rate and temperature regime. The deformation mode also changes, with shear localization replacing uniform deformation as the dominant deformation mode from the onset of plastic deformation at both low and high strain rates. The evolution and multiplication of shear bands have been monitored as a function of plastic strain. The grain size dependence is discussed with respect to possible enhanced propensity for plastic instabilities at small grain sizes.