UC Santa Barbara

Niobium oxides and alloys are of growing interest to both battery engineers and high temperature structural alloy designers in recent years. The niobium oxide Wadsley-Roth crystallographic shear phases are promising next generation Li-ion battery anode materials while a new class of refractory multi-principal metals alloys is of interest for extreme temperature aerospace applications. The phase stability of niobium and oxygen was originally studied during alloy development efforts in the 1950s and 1960s, but has not been recently revisited with modern first principles and statistical mechanics tools. Both niobium and other refractory metals used in these alloys can dissolve significant quantities of oxygen and readily react to form complex and often highly non-protective oxide scales. It is thus of great interest to revisit and evaluate the phase stability and defect behavior of niobium alloys and related oxides.

Throughout this work, a computational approach was applied to determine not only thermodynamic phase stability, but to illuminate the underlying structural and chemical factors which drive this behavior.From first principles and statistical mechanics, a revised phase diagram of the Nb-O system is proposed. This includes a solubility curve determined from a Bayesian cluster expansion approach which allowed the uncertainty of the solubility curve to be directly probed. We also determined a systematic way to classify and enumerate Wadsley-Roth block structures and then identified electrostatic repulsion and lattice relaxations as the key factors determining phase stability. At the other compositional extreme, the multifaceted interactions between interstitial oxygen atoms dissolved in niobium were analyzed.

Notably, closed-shell hybridization between oxygen-oxygen pairs drives strong repulsion at short distances which leads to the suppression of high energy pair clusters which persists to high temperatures.Similar closed-shell hybridization is also observed in some repulsive oxygen-solute interactions in dilute alloys, though frequently electrostatic interactions are dominant in this scenario. Finally, through first principles calculations we found that the topology of generalized stacking fault energy curves in BCC Nb are drastically altered when interstitial oxygen atoms are contained within a glide plane. These findings suggest that mechanical deformation mechanisms may be impacted by interstitial oxygen in unexpected ways.