Oxygen-dislocation interaction-mediated nanotwinned nanomartensites in ultra-strong and ductile titanium alloys

High specific-strength lightweight titanium (Ti) alloys, in the absence of interstitial strengthening of oxygen (O) atoms to avoid O-embrittlement, are mainly strengthened via densely semi-coherent nanoprecipitates in the β-matrix that act as dislocation obstacles and often result in high-stress concentrations, contributing to their strength-ductility trade-off. Here, using a low cost Ti-2.8Cr-4.5Zr-5.2Al duplex alloy as a model material, we present a counterintuitive O-doping strategy to create topologically coherent, interstitial-O α′ nanotwinned nanomartensites (NTNMs) with good interfacial strain compatibilities. The interstitial atoms tailor the stress field of edge dislocation cores from planar to non-planar, facilitating multiple variants nucleate simultaneously along O-rich edge dislocations to construct interstitial-O NTNMs. The interstitial-O NTNMs endow our duplex Ti alloys with superior strength of 1.64 gigapascals and large uniform elongation of 11.5%, surpassing all previously reported bulk Ti alloys. This unprecedented combination of mechanical properties is conferred mainly by the interstitial NTNMs, which serve as a sustainable ductility source via a self-hardening deformation mechanism and utilize the pronounced interstitial strengthening of concentrated O atoms. As such, the coherent interstitial NTNMs engineering strategy efficiently combines interstitial solid solution strengthening, and coherent interface strengthening mechanisms, that provides new insights into designing high-strength and large ductility O-tolerant alloys for cost-effective and lightweight applications.