β-Solidifying α₂-Based Ti–Al Alloys for Laser Powder Bed Fusion: Synchrotron X-ray Insight into Phase Retention and Transformation

Additive manufacturing (AM) offers a unique opportunity to revisit historically brittle α₂-Ti 3 Al alloys by exploiting rapid solidification to retain metastable, crack-tolerant phases during fabrication while enabling transformation to α₂ for service. Here, we develop a β-solidifying α₂-based Ti–Al alloy tailored for laser powder bed fusion (L-PBF) using a combined Mo-equivalent (Mo eq) and Thermo-Calc equilibrium design strategy. A target composition of Ti–28Al–2. 4Nb–0. 6Mo–1. 8 V (at%) (Mo eq ≈ 4. 2 wt%) was realized via in-situ alloying by blending two commercial powders (TNM-B1 and Ti-6Al-4V). Crack-free, near-fully dense components (~99. 9% relative density) were produced using low scanning speeds, demonstrating that metastable β retention can be achieved at substantially lower Mo eq than in conventional alloy design for β solidifying titanium alloys. Room-temperature synchrotron X-ray diffraction (SXRD) shows the as-built microstructure is dominated by retained β with weak B2 ordering (B2100/β110 ≈ 0. 05). In-situ high-temperature SXRD (300–600 °C) reveals a staged transformation pathway: β/B2 (partially ordered) → β (disordered) → α (disordered) → α/α₂ (partially ordered), with α₂ superlattice reflections emerging above ~500 °C. Post-build heat treatment (PBHT) at 600 °C/8 h eliminates β/B2 and yields an α/α₂-dominated microstructure. Hot tensile testing at 750 °C shows an ultimate tensile strength of 356. 9 MPa with 4. 7% total strain and an extended post-ultimate elongation to 17. 3% (σ fra ≈ 195 MPa), exceeding reported ductility for laser-AM TiAl. Finally, crack-free macro-functionally graded structures between Ti-6Al-4V and the developed alloy were demonstrated, highlighting a pathway for bimetallic high-temperature components and coatings. • Novel β-solidifying α 2 -Ti 3 Al enables crack-free L-PBF at low Mo eq. • SXRD: metastable β/B2 retained; α 2 ordering begins above ~500 °C. • PBHT 600 °C/8 h eliminates β/B2, forming service-relevant α/α 2. • High-T tensile (750 °C): UTS 357 MPa, total strain 17. 3%. • Crack-free Ti-6Al-4V/α 2 -Ti 3 Al functionally graded structure.