Role of dislocations in the stress–strain behaviour of as-quenched lath martensitic steel

As-quenched lath martensitic steels exhibit a low elastic limit followed by high-strain-hardening without reasonable dislocation multiplication. The role of numerous dislocations in the unique stress–strain behaviour is unclear. Here, we investigated the obstacles to dislocation motion and evolution of dislocation properties (density, fraction of screw dislocations, and distribution parameter) using 0.1 mass%C steel with and without ausforming. X-ray line profile analysis revealed that these dislocation properties were barely changed after tensile tests. Stress relaxation tests indicated that the internal stress, arising from long-range obstacles such as dislocation tangles, constantly accounts for more than 90% of the applied stress. The effective stress, arising from short-range obstacles, was negligibly small at the elastic limit and increased proportionally with the internal stress. This trend implies that the short-range obstacles are dependent on dislocation density, such as dislocation intersections. These findings support two key conclusions: (1) At the elastic limit, dislocations are sufficiently mobile that the distance a dislocation moves is too short to cause dislocation intersections and (2) the strain hardening results from the sequential yielding of soft (low) to hard (high dislocation density) regions. Thus, the elevated flow stress observed in ausformed specimens is attributed to a higher initial screw dislocation density, which intensifies both internal and effective stresses.