Interstitial-mediated spinodal decomposition pathways leading to strengthening in TiNb

Materials engineered to endure extreme environmental conditions face a challenging balance between temperature resistance and vexing strength-toughness trade-offs. Body-centered cubic refractory alloys are attractive for their exceptional strength at elevated temperatures, yet at ambient conditions, they tend to exhibit ceramic-like behavior characterized by low toughness and ductility. In this work, we demonstrate a metastability alloy design approach using oxygen interstitials to generate a hierarchical microstructure in equiatomic TiNb with a strength exceeding 2 GPa in tension while retaining moderate initiation fracture toughness. These exceptional properties, measured site-specifically using nanoindentation and micro-tensile tests, are linked to phase decomposition pathways arising from oxygen-induced immiscibility, including spinodal decomposition with nanoscale compositional undulations and the simultaneous emergence of a dual-phase lamellar structure. These microstructures feature nanoscale domains that can be described via a structural evolution along the Burgers pathway, including intermediate orthorhombic structures, which act in concert to provide obstacles to dislocation glide at multiple length scales. In situ tensile experiments demonstrate that dislocation-mediated plasticity is difficult in the spinodal-like regions, whereas dislocation glide can occur readily within the Nb-rich BCC lamellae, facilitating more uniform plasticity. The interstitial engineering approach shown here integrates nanostructured architectures, strength, and toughness reminiscent of advanced steels with the potential for high-temperature structural applications.