Dislocation densities in a low-carbon steel during martensite transformation determined by in situ high energy X-Ray diffraction

Highlights: • Dislocation densities of martensite and austenite during the martensite transformation determined by in situ HEXRD experiment. • The dislocation density in the newly formed martensite is calculated, obtaining the distribution of dislocation densities. • TThe dislocation density distribution is found to be a first order contribution of the martensite mechanical behavior. • High dislocation densities are found in the residual austenite pointing out that it is probably as hard as martensite laths. The evolution of the dislocation densities in martensite and in austenite during the quench of a low-carbon (0.215 wt% C) steel is investigated in situ by the mean of a High Energy X-Ray Diffraction experiment on a synchrotron beamline. The line configuration offers an excellent time resolution well adapted to the studied martensitic transformation kinetics. The mean density of dislocations in martensite increases as the transformation proceeds confirming that dislocations are not homogeneously distributed between the laths in agreement with some recent post-mortem observations. The resulting spatial distribution of dislocations and the associated strain-hardening support the views assuming that lath martensite is a heterogeneous microstructure and behaves as a "multiphase" aggregate. In austenite, the increase in dislocation densities is even more significant meaning that austenite in martensite is also a hard phase, contradicting some recent theories attributing to films of retained austenite a major role in the plasticity of martensite.