Achieving high oxygen tolerance in Ti6Al4V: Copper-oxygen co-doping strategy for ultrahigh strength-ductility balance

Conventional α + β Ti6Al4V alloys lack sufficient strengthening mechanisms, limiting strength. While oxygen (O) offers a cost-effective strengthening route, exceeding ∼ 0.33 wt% causes significant embrittlement. Here, we explored how to efficiently utilize interstitial oxygen to enhance the mechanical properties of Ti6Al4V. The copper oxide (CuO) was innovatively employed as a precursor to completely dissolve into Ti6Al4V matrix, interstitial O and substitutional Cu atoms were simultaneously utilized to strengthen the primary α-phase (α_p) while inducing the abundant secondary-α (α_s) nanoprecipitates. Surprisingly, the introduction of Cu element facilitated control of lattice distortion and redistributed oxygen between α_p and β-transformed (β_trans) structure, resulting in the Ti6Al4V-2.5CuO (wt.%) alloy with high oxygen tolerance (0.62 wt%) and an ultra-high ultimate strength of ∼ 1635 MPa and a favorable ductility of ∼ 5.3 %. The dual effect of interstitial solid solution strengthening and α_s precipitation strengthening were achieved under the Cu/O interaction. Additionally, the addition of Cu promoted the oxygen redistribution and activation of the basal < a > and pyramidal < c + a > slip systems, thereby ensuring improved ductility. This study presented a novel strategy for high-strength Ti alloys using interstitial oxygen, maximizing strengthening while mitigating embrittlement.