Enhanced mechanical properties of laser directed energy deposition (LDED) pure titanium with trace interstitial oxygen solute

Laser directed energy deposition (LDED) commercially pure titanium (CP-Ti) is frequently limited by a relatively lower strength. The introduction of more interstitial oxygen (O) solute has enabled significant improvement in strength but leads to reduced plasticity. This study systematically investigates the effect of trace oxygen addition (∼0-495 ppm) on the microstructure and mechanical behavior of LDED Ti-O alloys via in-situ powder blending. The results revealed that the as-deposited samples exhibited refined prior-β grains and acicular α-lath with increasing oxygen addition. Yield strength (YS) and ultimate tensile strength (UTS) increased monotonically with oxygen addition, whereas elongation (EL(%)) exhibited a non-monotonic dependence. LDED Ti-O alloy at an intermediate oxygen addition (∼165 ppm, named Ti2) possessed the optimal strength-ductility synergy with an UTS of 568 MPa and an EL(%) of 27.9%, compared with LDED CP-Ti with an UTS of 463 MPa and an EL(%) of 18.7%. A quantitative strengthening analysis elucidated that α-lath refinement and solid-solution strengthening dominated the improvement of strength. EBSD analysis of fractures indicated that the ductility improvement of LDED Ti2 alloy was associated with the activation of {10 2} deformation twinning that enhanced strain hardening and delayed the necking. At higher oxygen addition (∼495 ppm), twinning was suppressed and dislocation mobility became more constrained, promoting strain localization and reducing ductility. These results provide guidance for designing LDED Ti-O alloys with enhanced mechanical properties for structural and biomedical applications.