Strength-ductility synergy in medium-entropy alloys via harnessing trace air in additive manufacturing

Conventional additive manufacturing (AM) of metallic materials demands costly high-vacuum or ultra-pure inert atmospheres to suppress impurity-induced embrittlement. Here, we overturn this paradigm by demonstrating that ambient trace O and N in an inert atmosphere can be turned into potent in-situ alloying species so that the strength and ductility of the material can be simultaneously enhanced. In a Ti₅₆Zr₃₀Nb₁₄ medium-entropy alloy (MEA) additively manufactured with optimized air doping, the yield strength rises by 67% to ≈1 GPa and the tensile ductility increases by 64% to ≈18%, achieving a simultaneous gain that defies the classical strength-ductility trade-off. Atom-probe tomography, enhanced by a machine-learning workflow, identifies two distinct families of nanoscale ordered interstitial complexes (OICs): O-rich OIC1 (O-Zr-Ti) and N-rich OIC2 (N-Zr-Ti). These complexes act as potent dislocation-pinning sites while promoting extensive cross-slip of dislocations and activating Frank-Read sources during plastic deformation. The resultant wavy slip and sustained work-hardening capacity give rise to exceptional strength-ductility synergy. Eliminating the need for high-purity inert gas, this air-alloying route delivers a low-cost, scalable pathway to strong-yet-ductile AM metallic materials.