Process-driven ω-phase precipitation and mechanical biofunctionality of low-modulus Ti-40Nb alloy fabricated by laser powder bed fusion

초록

beta-type Ti-Nb alloys have attracted considerable interest as orthopedic implant materials due to their low elastic modulus and excellent biocompatibility. Among various additive manufacturing (AM) techniques, laser powder bed fusion (L-PBF) is particularly well-suited for biomedical applications as it enables the fabrication of patient-specific components with tailored geometry and properties. In this study, a Ti-40 wt% Nb alloy was fabricated via L-PBF using a pre-alloyed powder feedstock, and its microstructural evolution and mechanical properties were systematically investigated. The use of pre-alloyed powder facilitated the formation of a uniform beta-phase microstructure while minimizing macroscopic compositional inhomogeneity. However, elemental segregation near the molten pool boundary (MPB) remained locally present. Due to the solidification behavior and repeated thermal cycling inherent in the L-PBF process, omega-phase precipitates were non-uniformly generated, particularly in MPB and interdendritic regions. The as-built alloy exhibited a high relative density of approximately 99.10 % and consisted of columnar and cellular dendrites with a strong < 100 > texture along the build direction. Room-temperature tensile tests revealed excellent mechanical performance, including a yield strength of 540 MPa, an ultimate tensile strength of 752 MPa, and a reduced Young's modulus of 57 GPa, confirming that the alloy exhibits favorable mechanical characteristics relevant to biomedical applications from a mechanical property perspective. Post-deformation EBSD analysis confirmed the activation of the {332}< 113 > twinning system and a dynamic Hall-Petch effect. While the omega phase contributed to strengthening, its excessive precipitation near the MPB induced localized embrittlement and acted as a crack initiation site. Based on these findings, the deformation behavior of L-PBF-built Ti-40Nb was interpreted in relation to its microstructural characteristics.

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