First-principles Analysis of the Effects of Oxygen, Vacancies, and Their Complexes on the Screw Dislocation Motion in Body-Centered Cubic Nb

Some solute atoms induce strengthening and embrittlement in body-centered cubic refractory metals. Especially, interstitial oxygen produces remarkable strengthening effects in Nb, wherein the yield stress of oxygen-doped Nb alloys is more than twice that of pure Nb. Conventional mechanisms cannot explain this oxygen-induced dramatic strengthening because the interaction between dislocations and oxygen atoms is not so significant. In a previous study, we found that the formation of vacancy–oxygen pairs enhances the attractive interaction with a screw dislocation and increases the energy barrier for dislocation motion associated with cross-kink nucleation in Nb–O alloys. However, the strengthening effect could not be described by the pinning model for dislocation motion. Herein, we focused on the atomic-level analysis of the fundamental process related to the dislocation motion around a vacancy, an oxygen atom, and a vacancy–oxygen pair. First-principles calculations revealed that the vacancy–oxygen pairs increase the energy barrier with respect to the dislocation motion more substantially than vacancies and oxygen interstitials owing to a unique oxygen-induced mechanism; an octahedral–tetrahedral shuffling process of oxygen is necessary for dislocation passing through vacancy–oxygen obstacles. Such event almost never happens in the real metallic materials. Instead, cross-kink nucleation occurs frequently to overcome the widely distributed vacancy–oxygen obstacles, which contributes to the dramatic strengthening.