Formation mechanism and annealing behavior of nanocrystalline ferrite in pure Fe fabricated by ball milling

Abstract

Nanocrystalline ferrite formation by ball milling in pure Fe (0.004 mass% C) has been studied through morphology observation and microhardness measurements. It was found that nano-ferrite first formed at the outer surface area in the powders at the early stage of ball milling, which was attributed to the deformation localization. The boundaries between the nano-ferrite and work-hardened regions are quite clear under SEM and the hardness of nano-ferrite region is more than 3 GPa higher than that of the work-hardened region. Further milling led to the particle refinement and formation of nano-ferrite of the whole particles. It has been suggested that the nanocrystalline ferrite formed through a transition from dislocation cell wall created by work hardening during ball milling to grain boundary, which is regarded to contribute to the hardness gap between the work hardened structure and the nanocrystalline ferrite. A relatively high stability against temperature was observed in the nano-ferrite formed by ball milling. The irregular grain boundaries of nano-ferrite annealed at high temperature was attributed to that the grain growth of nanocrystalline ferrite takes place by coalescence of neighboring grains.