Softening and Melting of Wüstite: Insights from a Multiscale Study

Wüstite (FeO) has been extensively studied within the context of ironmaking metallurgy and the recycling of industrial waste, owing to its crucial role in high-temperature systems that exhibit softening and melting behaviors. Understanding these behaviors is vital for advancing multi-phase transport and chemical reactions in metallurgical processes. In this work, the Tammann temperature of FeO was identified to be approximately 826 K, a finding confirmed by molecular dynamics simulations and experimental validation. Below this threshold, the atoms' thermal vibration led to a volumetric expansion of the material. Conversely, surpassing 826 K triggered solid-state sintering, resulting in a noticeable shrinkage of FeO granules and the compaction of packed beds under mechanical stress. During softening, the reorganization of FeO grains was observed, with bonding commencing at contact points and the formation of sintering necks as surface atoms migrated and diffused. As temperatures rose further, this mass transfer and atomic diffusion intensified, facilitating the outward migration across grain boundaries, and culminating in the coalescence of smaller grains into larger formations.