Fracture toughness enhancement of a laser powder bed fusion manufactured Ti-55511 alloy with a heat-treatment-tailored hierarchical microstructure

Near-β titanium alloy Ti–5Al–5Mo–5V–1Cr–1Fe (Ti-55511), fabricated using laser powder bed fusion (LPBF), typically exhibits heterogeneous microstructures characterized by strongly textured columnar prior-β grains. These anisotropic microstructures often lead to reduced fracture resistance during crack growth, limiting its potential for high-performance applications. To address this challenge, a novel three-step heat treatment was developed, comprising solution treatment near the β transus temperature, followed by a double-stage aging process. This tailored heat treatment not only refined the microstructure but also induced the formation of a hierarchical α phase structure, characterized by uniformly distributed tertiary α phases within equiaxed β grains. Specifically, the LPBF-fabricated Ti-55511 samples demonstrated an excellent combination of mechanical properties, including a yield strength exceeding 1100 MPa, ductility greater than 10 %, and fracture toughness exceeding 72.9 ± 3.9 MPa m¹/². This study elucidated the mechanisms by which multi-stage heat treatments govern the formation of nano-scale secondary α phases (width ∼ 50 nm), sub-micron α phases (width ∼ 500 nm), and primary α phases (width ∼ 1 μm), creating a hierarchical microstructure that enhances both strength and toughness. The observed improvements in fracture toughness were attributed to the optimized distribution of α phases, which effectively suppressed crack propagation by promoting crack deflection and bridging mechanisms. This work provides new insights into the relationship between hierarchical microstructural evolution and mechanical behavior in LPBF-processed Ti-55511 alloys, offering a promising pathway for enhancing the fracture resistance of additively manufactured titanium alloys.