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

Abstract

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, herethe first experimental evidence to directly observe individual oxygen atoms in a highly concentrated bcc solid solution-the (TiNbZr)86 O12 C1 N1 medium-entropy alloy (MEA)-isprovided, wherebythe interstitial sites in whichtheoxygenatoms are locatedare 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 concentrationsisshown. 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.