Predicting Interstitial Solutes in Refractory Complex Concentrated Alloys via a Combined Experimental and Computational Workflow: A Case Study on Oxygen in NbTaTiHf

Refractory complex concentrated alloys (RCCAs), composed of multiple principal refractory elements, are promising candidates for high-temperature structural applications due to their exceptional thermal stability and high melting points. However, their mechanical performance is often compromised by interstitial impurities—particularly oxygen, nitrogen, and carbon—which segregate to grain boundaries and promote embrittlement. In this study, we investigate the solubility and thermodynamic behavior of oxygen interstitials in a model Nb 45 Ti 25 Hf 15 Ta 15 RCCA system. We synthesized (Nb 45 Ti 25 Hf 15 Ta 15 ) 100-x O x alloys with varying oxygen contents (x=0–5 at.%) via plasma arc melting and characterized their phase evolution and microstructure using XRD, SEM, and TEM. Complementary computational modeling was performed using machine-learning interatomic potentials (MLIPs) integrated with Monte Carlo simulations to probe oxygen interactions at the atomic scale. Our results reveal an effective solubility limit for oxygen between 0.8 and 1.0 at.%, beyond which HfO₂ formation is energetically favorable. This combined experimental–computational framework provides a predictive approach for managing interstitial behavior in RCCAs, enabling improved alloy design strategies for enhanced mechanical performance.