The origin of different morphology of internal oxide precipitates in ferritic and austenitic steels

The internal oxide precipitates were supposed to be spherical in Wagner's original theory, while the following research demonstrated that this assumption is an exception rather than the truth, which caused deviations in the application of this theory. In this study, the internal oxide precipitates have a needlelike and a near-spherical morphology in a Fe-9Cr ferritic and a Fe-17Cr-9Ni austenitic steels after exposure to 600 degrees C deaerated steam for 600 h, respectively. The nano-to-atomic scale characterization shows that the morphology of the internal oxide precipitates is controlled by the structure of the interfaces between the metal matrix and the internal oxide, while the interface structure is mainly affected by the crystallographic structure of the two phases and their orientation relationship. In addition, the internal oxide precipitation-induced volume expansion and the outward Fe diffusion-induced volume shrink occur simultaneously during the oxidation process. The stress status in the internal oxidation zone (IOZ) is the competing result of the two factors, which could dynamically affect the high-temperature oxidation. The results obtained in this study suggest that there is potential to control the distribution, morphology, and interface structure of the internal oxide precipitates by selecting appropriate base metal and internal oxide-forming element, in order to obtain better high-temperature oxidation-resistant materials.(c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

This study reveals that the internal oxide precipitates in Fe-9Cr ferritic steel and Fe-17Cr-9Ni austenitic steel exposed to 600°C deaerated steam for 600 hours have needle-like and near-spherical morphology. The morphology of these precipitates is controlled by the interface structure between the metal matrix and the internal oxide, which is influenced by the crystallographic structure and orientation relationship of the two phases. The results suggest the potential to manipulate the distribution, morphology, and interface structure of internal oxide precipitates for better high-temperature oxidation resistance.