Morphological confinement of secondary phases in entropy stabilized oxides

Four transition metal entropy-stabilized oxide samples were synthesized with varying processing methods to create four different average grain sizes: 90 nm, 200 nm, 430 nm, and 15 μm. All samples were heat treated to form a Cu-rich tenorite secondary phase, analyzed by scanning electron microscopy and energy-dispersive x-ray spectroscopy. Three secondary-phase morphologies were identified: Cu-rich tenorite needle-like particles, Cu-rich tenorite filled grain boundaries, and Cu-rich tenorite nano-grains. Average grain size and boundary layer thickness have a significant impact on the morphology of the secondary phase. The effect of grain size and boundary layer thickness on the secondary-phase morphology, termed morphological confinement, was simulated using Dream 3.D with a tetrakaidekahedron geometric model. These synthetic microstructure simulation results produced three confinement regimes (unconfined particles, confined particles, and morphological confinement). For coarse-grain samples, the geometric conditions allow for unconfined secondary-phase particles to form in the primary phase. For nanocrystalline grain samples, the geometric conditions restrict the secondary-phase formation to individual nanograins, distinct from primary-phase nanograins. For intermediate grain sizes, there is a diffuse transition between the morphological confinement regimes, allowing a sample to exhibit one or more secondary-phase morphologies simultaneously. Consequently, this study highlights the novel approach of using grain size to manipulate the morphology of secondary phases in multi-phase ceramics, establishing the foundation for future studies on the effect of the morphology variation on functional and mechanical behavior.