Recrystallization kinetics and microstructure evolution during annealing of a cold-rolled Fe-Mn-C alloy

The recrystallization behavior of a 50% cold-rolled Fe-21.6% Mn-0.38% C alloy during annealing at temperatures between 560 and 700 deg. C was investigated. Microhardness tests were used to characterize the recrystallization kinetics. X-ray diffraction, optical microscopy, electron back-scatter diffraction measurements and transmission electron microscopy were used to characterize the grain microstructure and texture evolution during annealing. The obtained experimental data were evaluated in terms of the Johnson-Mehl-Avrami-Kolmogorov model. The obtained values of the Avrami exponent ranged from 0.70 to 1.37. The nucleation of new grains was found to be site saturated. The growth rate of nuclei decreased with annealing time and was mainly responsible for the low Avrami exponents. This decrease in the growth rate was due to a reduction in the driving force essentially because of recovery. On the basis of the experimentally obtained growth rate, the recovery behavior as well as the grain boundary migration was analyzed. The activation energies found suggest pipe-diffusion-controlled dislocation climb and grain boundary diffusion as recovery and boundary migration mechanisms, respectively. Shear bands, grain boundaries and triple junctions were identified as preferential nucleation sites. The inhomogeneous grain microstructure after recrystallization is attributed to non-randomly distributed nuclei and an inhomogeneous distribution of the stored energy. The annealing texture above 630 deg. C was very weak and determined by the orientation of recrystallization nuclei, whereas after annealing at 560 deg. C, the α-fiber (<1 1 0>//RD) character of the texture was enhanced, apparently as a consequence of selective growth of grains with α-fiber orientations.