High strength, low modulus, and ductile metastable beta Ti-Nb-Zr-Fe alloys by laser powder bed fusion in-situ alloying

A novel, high strength, low modulus, and ductile metastable β Ti-14Nb-6Zr-3Fe (TNZF) alloy was manufactured via laser powder bed fusion (LPBF) in-situ alloying. Process parameter optimization resulted in near-fully dense samples (< 0.1% porosity) under optimal conditions. The microstructure and mechanical properties of TNZF alloy were then tailored using two chessboard scanning strategies and a simple scanning strategy. Samples fabricated using the chessboard scanning strategy (CS-TNZF) benefited from a lower thermal gradient, the high growth restriction factor associated with the addition of Fe, and constitutional supercooling around residual Nb particles, resulting in a near-equiaxed grain structure (average aspect ratio: 2.0) compared to the AS-TNZF (fabricated using the simple scanning strategy). The microstructure consisted of a β-Ti matrix with 2–5 nm sized ω precipitates. Due to the addition of Zr, the transition from β to ω was suppressed during thermal cycling associated with LPBF, resulting in the presence of a partially collapsed ω phase. Without post-processing heat treatments, the AS-TNZF condition exhibited a yield strength (YS) of 1091 MPa, an elastic modulus (E) of 68 GPa, and an elongation (EL) of 16.8%, while the CS-TNZF counterpart achieved a higher YS of 1147 MPa and E of 71 GPa, but a lower EL of 10.8% due to a higher volume fraction of ω phase. Both AS-TNZF and CS-TNZF demonstrated high elastic admissible strain values (YS/E ratio) of 1.60 and 1.62, respectively, achieving high strength, low modulus, and good ductility, which indicates their suitability for biomedical implant materials.