Influence of Al₂O₃ particle pinning on thermal stability of nanocrystalline Fe

Second-phase particle pinning has been well known as a mechanism impeding grain boundary (GB) migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline (NC) materials at elevated temperatures. The pinning force exerted by interaction between small dispersed particles and GBs strongly depends on size and volume fraction of the particles. Since metallic oxides, e.g. Al₂O₃, exhibit great structural stability and high resistance against coarsening at high temperatures, they are expected as effective stabilizers for NC materials. In this work, NC composites consisting of NC Fe and Al₂O₃ nanoparticles with different amounts and sizes were prepared by high energy ball milling and annealed at various temperatures (T_ann) for different time periods (t_ann). Microstructures of the ball milled and annealed samples were examined by X-ray diffraction and transmission electron microscopy. The results show that the addition of Al₂O₃ nanoparticles not only enhances the thermal stability of NC Fe grains but also reduces their coarsening rate at elevated temperatures, and reducing the particle size and/or increasing its amount enhance the stabilizing effect of the Al₂O₃ particles on the NC Fe grains.