Tetrahedral versus octahedral interstitial sites for oxygen solutes in NbTiZr medium-entropy alloy

Oxygen interstitial solutes have traditionally been thought to occupy only octahedral interstices in body-centered cubic (bcc) metals. However, the competition between tetrahedral and octahedral interstitial sites for oxygen solutes in multi-principal element alloys has become a topic of debate. The driving force and atomistic mechanism behind this observed preference switch remain unclear. In this study, we systematically investigate the competition between tetrahedral and octahedral sites for oxygen, nitrogen, and carbon solute atoms in bcc NbTiZr alloys using density functional theory calculations. At dilute concentrations, interstitial solutes exhibit a strong preference for octahedral sites, with 100 % of solutes initially placed in tetrahedral sites migrating to nearby octahedral sites after structural relaxation. Notably, a transition from octahedral site to tetrahedral site occupancy is observed under specific conditions, including lattice expansion, high interstitial solute concentrations, or significant spatial heterogeneity (reflecting locally aggregated oxygen interstitials), provided no phase transformation occurs. This transition is driven by a crossover in elastic strain energy between the two competing configurations of octahedral and tetrahedral interstices. Our findings provide mechanistic insights into the behavior of small interstitial solutes and their solid solution strengthening effects in bcc multi-principal element alloys, offering valuable guidance for alloy design.