Evolution of dislocation structure determined by neutron diffraction line profile analysis during tensile deformation in quenched and tempered martensitic steels

The role of the dislocation structure on the work-hardening behavior during the tensile deformation of quenched and tempered martensite was studied. The evolution of the dislocation structure during tensile deformation at room temperature in ultralow-carbon 18 mass%Ni martensitic steels under the conditions of as-quenched by sub-zero-treatment (SZ) and quenched-and-tempered at 573 and 773 K (T573 and T773, respectively) was monitored using in situ time-of-flight neutron diffraction combined with the convolutional multiple whole profile (CMWP) procedure. The changes in the dislocation parameters due to tempering and deformation obtained by the CMWP procedure were explained by the metallurgical phenomena of body-centered cubic iron. The elastic limit increased in the order SZ, T573, and T773, whereas the dislocation density decreased in the opposite order, indicating that the elastic limit is not always dependent on the total dislocation density of martensite. The dislocation density in SZ, which showed a high level of work hardening after yielding, hardly changed during tensile deformation, whereas that in T573 and T773 increased with tensile straining. The dislocation arrange-ment parameter that represents the interaction among the dislocations was high before deformation and decreased during deformation in materials SZ and T573, whereas the parameter was maintained at a low value during the entire deformation in material T773. Large and small values of the dislocation arrangement parameter indicate weak and strong interactions, respectively. The dislocation arrangement parameter is considered helpful for estimating the increment in strength of martensitic steels by dislocation strengthening, as the coefficient alpha in Taylor's equation, for both the as-quenched and tempered conditions.