Influence of oxide volume fraction on abnormal growth of nanostructured ferritic steels during non-isothermal treatments: An in situ study

Nanostructured ferritic steels were mechanically alloyed with various contents of oxide-forming yttrium and titanium (0, 0.05, 0.3 and 1 wt%). The microstructure evolution of the milled powders during non-isothermal annealing treatments was studied using in situ synchrotron X-ray diffraction. Recrystallization and grain growth were quantified upon heating up to 1100°C, which is the typical consolidation temperature for nanostructured ferritic steels. The temperature where abnormal grain growth occurs is observed to increase with the volume fraction of oxide nanoparticles. This demonstrates the interest of increasing the amount of alloying elements to limit the formation of the bi-modal grain microstructure. Using the nanoscale characterization of the precipitation state, the size of retained ultrafine grains (UFG) in the bimodal microstructure was found to be in agreement with the modified Zener theory demonstrating that the microstructure of ultrafine-grained steels can be tailored by the amount and size of second-phase particles.