Direct Observation of Oxygen Atoms Taking Tetrahedral Interstitial Sites in Medium-Entropy Body-Centered-Cubic Solutions

Interstitial solutes, such as carbon in steels, are effective solid-solution hardening agents. These alloying elements are believed to occupy the octahedral interstices in body-centered-cubic (bcc) metals. Using deep-sub-angstrom-resolution electron ptychography, here the first experimental evidence to directly observe individual oxygen atoms in a highly concentrated bcc solid solution—the (TiNbZr)₈₆O₁₂C₁N₁ medium-entropy alloy (MEA)—is provided, whereby the interstitial sites in which the oxygen atoms are located are discerned. In addition to oxygen interstitials residing in octahedral sites, the first unambiguous evidence of a switch in preference to the unusual tetrahedral sites at high oxygen concentrations is shown. This shift away from octahedral occupancy is explained as resulting from the extra cost of strain energy when the requisite displacement of the host atoms is deterred in the presence of nearby octahedral interstitials.