Neutron Diffraction Analysis of Dislocation Behaviour and Substructures of Si-alloyed and Interstitial-free BCC Steels

The effect of Si solid solution on the dislocation behaviour of body-centred cubic (BCC) steels was studied by performing neutron diffraction line profile analysis (LPA) on Fe–4wt%Si (4%Si steel) and interstitial-free (IF) steels strained to 11.7% nominal strain. Dislocation parameters were quantitatively determined: density (ρ), character (q), arrangement parameter (M*) and crystallite size (D). Plastic deformation substantially increased dislocation density in both steels, with the increment in the 4%Si steel being more than twice that in the IF steel. In the 4%Si steel, plastic deformation also remarkably increased the screw dislocation fraction. This behaviour can be attributed to cross-slip suppression by Si, which confines the screw dislocations to their original slip planes and reduces their annihilation probability. This leads to the formation of a planar dislocation substructure characterised by spatially isolated screw dislocations, which fragment the crystal into relatively small coherent domains and high M* values. By contrast, the active cross-slip in the IF steel promotes dislocation annihilation and develops a cell substructure with interacting dislocations concentrated in the cell walls, resulting in relatively large coherent domains and low M* values. Overall, neutron diffraction LPA reveals that Si addition alters dislocation cross-slip behaviour and the resulting substructural development in BCC steels.