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.

Highlights : Recovery of severely deformed ferrite was followed in situ by High Energy X Ray Diffraction during heating and isothermal holding experiments. Dislocation densities during annealing were determined by a modified Williamson Hall method. A saturation in recovery has been observed in all studied conditions whatever the holding temperature. The beginning of recrystallization is detected by a large density drop at 650 °C.

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