In situ characterization of microstructural instabilities: Recovery, recrystallization and abnormal growth in nanoreinforced steel powder

An in situ X-ray diffraction experiment was set up to study the microstructural evolution of a nanostructured oxide dispersion-strengthened ferritic steel produced by high-energy ball milling. Dislocation density and grain growth between 20 and 1100 °C were quantified by coupling-modified Williamson-Hall and Warren-Averbach methods. During the early stages of heating, recovery through the rearrangement of dislocations increases the coherent domain size from 23 to about 60 nm. Once the annealing temperature reaches 800 °C, recrystallization starts. Using a specific analysis of 2-D detector signal, it has been possible to grasp the occurrence of abnormal growth leading to bimodal grain size distribution with both ultrafine grains and coarser micronic grains. The grain growth kinetics upon heating were determined for both populations and separately quantified. Ultrafine grains exhibit a continuous moderate growth rate, leading to continuous recrystallization, whereas specific grains experience a rapid abnormal growth up to their final size after a short incubation time.