Uniaxial ratchetting of 20 carbon steel: Macroscopic and microscopic experimental observations

The uniaxial ratchetting of polycrystalline 20 ordinary carbon steel (denoted as 20CS), a body-centred cubic (BCC) crystal structure metal, was investigated by macroscopic and microscopic observations at room temperature. The microscopic observations were performed by transmission electron microscopy (TEM) to the samples of 20CS obtained at different stages of monotonic tension, strain-controlled cyclic tension–compression and uniaxial ratchetting deformation. Main attention was paid on the dislocation patterns and their evolution during the ratchetting deformation. The results show that the dislocation patterns evolve from some low density modes such as dislocation lines and networks to those with high density, i.e., dislocation tangles, walls and cells. And then, sub-grain structures are formed by the re-arrangement of dislocation after certain number of cycles during the ratchetting deformation. The uniaxial ratchetting of 20CS is qualitatively explained by considering the observed dislocation patterns and their evolution with the increasing number of cycles and comparing with the patterns observed during the monotonic tension and strain-controlled cyclic tension–compression tests.