Adiabatic shear banding in ultrafine-grained Fe processed by severe plastic deformation

We have investigated the plastic deformation behavior of ultrafine-grained (UFG) bulk Fe. The UFG microstructure (grain size ∼300 and ∼150 nm) was obtained through combined equal channel angular extrusion and low-temperature rolling, and confirmed using transmission electron microscopy. Mechanical properties were measured under both quasi-static (strain rate in the range of 10^-4-10⁰ s^-1) and dynamic (strain rate up to ∼10³ s^-1 using the Kolsky bar technique) compressive loading. Dynamic stress-strain curves of the UFG Fe exhibit significant flow softening, in addition to a significant reduction in the strain rate sensitivity of the flow stress. Consequently, under dynamic loading the plastic flow mode changes from the uniform deformation of conventional coarse-grained Fe to substantial localized deformation in UFG Fe. Both in situ high-speed camera movies and post-loading optical and scanning electron microscopy show the development of adiabatic shear bands, observed for the first time in pure Fe under compressive loading conditions. The morphology of the shear bands and the increased propensity for flow localization due to the ultrafine grain structure are explained using established models for adiabatic shear banding.