On the Microstructural Stability of Ultrafine-Grained Interstitial-Free Steel under Cyclic Loading

The microstructural stability of ultrafine-grained (UFG) interstitial-free (IF) steel under cyclic loading was investigated. The samples were extracted from material processed along two different equal channel angular extrusion (ECAE) routes (4C and 4E) at room temperature. Low-cycle fatigue tests were carried out in addition to electron and optical microscopy in order to characterize the microstructural evolution induced by cyclic deformation. The results revealed substantial differences in microstructure resulting from different processing routes. Specifically, the volume fraction of high-angle grain boundaries (HAGBs) and low-angle grain boundaries (LAGBs) varied significantly depending on the processing route. The different microstructural characteristics stemming from different ECAE routes expressively influence the fatigue response. Route-4C-processed material displays cyclic softening, while processing along route 4E leads to microstructural stability under cyclic loading. This highly route-dependent trend in the cyclic stress-strain response is attributed to the instability of the LAGBs and stability of HAGBs during cyclic deformation, which is further supported by electron backscattering diffraction results.