Ductility and plasticity of ferritic-pearlitic steel after severe plastic deformation

A ferritic-pearlitic steel was subjected to severe plastic deformation (SPD) through Equal Channel Angular Pressing (ECAP) at room temperature, obtaining strengths greater than 1 GPa. Steel constituents were identified by light microscopy displaying similar grain sizes around 14 mu m for the pearlite, and 13 mu m and 18 mu m for the ferrite before and after heat treatment, respectively. Texture changed from a rolling type to a simple shear texture with higher intensity after 4 ECAP passes. After different severe plastic deformation magnitudes, an ultra fine grain structure was obtained with grain sizes between 0.9 mu m-0.36 mu m. The substantial grain size reduction was related to heterogeneous Geometrically Necessary Dislocations (GNDs) distribution in the as-received material with pearlite showing higher GNDs densities than ferrite. The remarkable strength increase after ECAP processing was found to be dependent on both small grain sizes and high dislocation densities. On the other hand, the low ductility of the ultrafine-grained (UFG) material was associated with a high annihilation rate of mobile dislocations at deformations greater than 0.39. Additionally, excellent plasticity properties were associated with a high density of immobile dislocations as well as high GND densities inside the deformed grains.

Automated summary

ECAP processing leads to grain size reduction and strength increase; grain sizes range from 0.9µm to 0.36µm; the low ductility of UFG material may be associated with a high annihilation rate.