Understanding the oxidation behavior of high-entropy alloys (HEAs) is essential to their practical applications. Here we conducted in situ environmental transmission electron microscopy (E-TEM) observations of dynamic oxidation processes in CrMnFeCoNi and CrFeCoNiPd nanoparticles (NPs) near room temperature. During the oxidation of CrMnFeCoNi NPs, a favorable oxidation product was MnCr₂O₄ with the spinel structure. The surface nanoislands of MnCr₂O₄ underwent dynamic reconstruction, resulting in the thickened oxide layer with less crystallinity. In CrFeCoNiPd NPs, the reactive element Mn was replaced by the inert element Pd. As a result, a favorable oxide product was CrO₂ with the rutile structure. CrO₂ formed on the NP surface and was a result of Cr outward diffusion through the oxide layer. In addition, FeO nanocrystals formed at the oxide/metal interface and were a result of O inward diffusion through the oxide layer. We also performed first principles calculations to provide insights into the energetics and diffusion rates related to oxide formation. These results reveal the non-equilibrium processes of oxidation in HEA NPs that can be strongly influenced by small particle sizes and large surface areas. This work underscores the high tunability of oxidation behavior in nanoscale HEAs by changing their constituent alloying elements.

Oxidation is a universal process causing metals’ corrosion and degradation. While intensive researches have been conducted for decades, the detailed atomistic and mesoscale mechanisms of metal oxidation are still not well understood. Here using in situ environmental transmission electron microscopy (E-TEM) with atomic resolution, we revealed systematically the oxidation mechanisms of aluminum from ambient temperature to ~ 600 °C. It was found that an amorphous oxide layer formed readily once Al was exposed to air at room temperature. At ~ 150 °C, triangle-shaped Al₂O₃ lamellas grew selectively on gas/solid (oxygen/amorphous oxide layer) interface, however, the thickness of the oxide layer slowly increased mainly due to the inward diffusion of oxygen. As the temperature further increased, partial amorphous-to-crystallization transition was observed on the amorphous oxide film, resulting in the formation of highly dense nano-cracks in the oxide layer. At ~ 600 °C, fast oxidation process was observed. Lamellas grew into terraces on the oxide/gas interface, indicating that the high temperature oxidation is controlled by the outward diffusion of Al. Single or double/multi-layers of oxide nucleated at the corners of the terraces, forming dense γ’-Al₂O₃, which is a metastable oxide structure but may be stabilized at nanoscale.

Oxidation plays a tremendous role in the long-term performance of metals. As an important lightweight metal for industrial applications, magnesium suffers from its high reactivity with oxygen and increased evaporation at high temperatures. To understand the oxidation mechanism of magnesium at elevated temperatures, in situ environmental transmission electron microscopy (E-TEM) was performed on magnesium nanoparticles. At a relatively low temperature, the growth of a MgO lamellae via the outward diffusion of bulk magnesium atoms dominated the oxidation process. In contrast, a sublimation-enhanced oxidation via gas phase reaction occurred at 200 ℃, leading to the growth of MgO dendrites over the particle that finally leads to the degradation of the magnesium structure. This study provides a direct observation and model of the oxidation mechanism of a direct gas–gas reaction that improves our understanding of the oxidation mechanism at elevated temperatures.