Deformation behavior and microstructure development of ultrafine grained iron

Work hardening behavior and microstructure development during deformation by cold rolling were investigated in pure iron with coarse grains (mean grain size; 20μm) and that with ultrafine grains (0.25μm). In the coarse grained iron, dislocations are introduced inhomogeneously in the early stage of deformation, and then the dislocation cell structure is formed with the increase in dislocation density during the deformation. On the other hand, the microstructure development in the ultrafine grained iron is significantly different from that in the coarse grained iron; that is, dislocations are more densely introduced by the deformation and form no dislocation cell structure. However, the ultrafine grained iron never exhibits work hardening, and the proof stress is kept constant even after 40% cold rolling. This suggests that the strength of the deformed ultrafine grained iron can not be given by the addition of dislocation strengthening and grain refinement strengthening even when the iron has a quite high dislocation density.