Harnessing Bifunctional Nitrogen-Dislocation Interactions for a Record Ultra-Strong-and-Ductile Duplex Titanium Alloy

Duplex (α+β) Ti alloys often manifest limited uniform elongation (ε_u) mainly originating from the lack of <c+a> dislocations for insufficient work hardening capability and semi-coherent α/β interfaces for strain incompatibility. The strength–ductility trade-off of duplex Ti alloys is further amplified by interstitial atoms-poisoning effects (e.g., N and O). Here, by selecting N atoms with the strongest hardening ability in Ti alloys, a counterintuitive strategy is proposed that harnesses bifunctional N-dislocation interactions in a model duplex Ti–Cr–Zr–Al alloy to construct a heterogeneous lamella structure, involving the elongated α_p grains decorated with N-rich low-angle grain boundaries (LAGBs) and densely coherent interstitial-N α′-nanotwinned martensites in β-grains. This structural heterogeneity achieves extremely high yield/tensile strength of ≈1532/1869 MPa in our alloys, which in turn promotes the emission of massive <c+a> dislocations from N-rich LAGBs and coherent interfaces through stress-activated bow-out and cross-slip processes for relatively large ε_u ≈10.2%. This work thus opens an avenue, via bifunctional interstitial atom-dislocation interactions, to construct a unique microstructure, toward ultrahigh strength and large ductility in interstitial-strengthening Ti alloys.