Enhanced mechanical properties and refined microstructure induced by micron Fe for additive manufactured Ti-Fe alloys

The unique thermal history inherent to additive manufacturing (AM) process typically produces titanium alloys with coarse columnar prior β grains. This study investigates the effectiveness of Fe micro-alloying in modifying the microstructure and enhancing mechanical properties of laser melting deposited (LMD) titanium alloys. A series of Ti-xFe (x = 1, 2, 3, 4, 5 wt%) alloys were fabricated to systematically examine Fe content effects on grain refinement, phase evolution, and mechanical performance. The results demonstrated that increasing Fe content induces progressive microstructural refinement, transitioning from coarse columnar grains (∼ 500 µm width in Ti-1Fe) to fully equiaxed β grains in Ti-5Fe with an average grain size of 109.9 µm. Quantitative phase analysis demonstrates a positive correlation between Fe content and β-Ti phase fraction, accompanied by significant α lath width reduction from 3.62 µm (Ti-1Fe) to 0.91 µm (Ti-5Fe). Microstructural characterization reveals non-monotonic dislocation density variation with Fe addition, peaking at intermediate concentrations. X-ray diffraction analysis indicates expanded β-Ti interplanar spacing with increasing Fe content, confirming solid solution effects, while transmission electron microscopy reveals Fe-induced ω phase formation during AM processing. The optimized Ti-5Fe alloy exhibits superior mechanical properties with a 301 MPa yield strength enhancement over Ti-1Fe, achieved through synergistic strengthening mechanisms including β grain refinement, α-phase dimensional reduction, and Fe solute strengthening in the β matrix. These findings demonstrate that strategic Fe alloying effectively overcomes AM-induced microstructural limitations while maintaining favorable strength-ductility balance in titanium alloys.