Oxide inclusions are present in nearly all iron-based alloys as a direct consequence of the steelmaking process, and are known to affect the mechanical properties of steel products. While the relation between steelmaking practice and the characteristics of oxide inclusions is currently receiving strong attention, much remains to be learned about the local structure and properties of the oxides themselves, and the influence that those properties exert on properties of the steel in which they are contained. In this work, oxide inclusions are produced by melting and deoxidizing high purity iron under a controlled atmosphere. The stiffness and hardness of oxide inclusions thus produced are probed by nanoindentation and analysed using a strategy developed to measure the elastic modulus and hardness of particles embedded in a matrix having a different stiffness than the particles. Data correction is based on theoretical considerations complemented with results from finite element simulations, and leads to accurate determinations if indentation data are collected and averaged over a large number of particles (~30 or more). Results of this work show that the local inclusion stiffness and hardness vary strongly within narrow compositional ranges in inclusions based on Si-Fe-Mn oxide, or alternatively remain relatively unchanged as a function of the oxide chemistry in inclusions based on Si-Al-Ca oxide. Stiffnesses of inclusions probed in this study remain lower than that of iron for all inclusions except Mn-rich Si-Fe-Mn oxides, for which the elastic modulus is observed to be close to that of iron. This suggests that such inclusions might be more benign with respect to high-cycle fatigue resistance due to the lower matrix/inclusion elastic mismatch. Hardness values for inclusions analysed in this work, remaining in the range of 4 to 11 GPa, are all far above typical values for metal alloys, while also showing an upward jump in Mn-rich Si-Fe-Mn-O inclusions.

This work was sponsored by the Swiss National Science Foundation, Grant No. 200021_182557.