上海科技大学知识管理系统

Near-beta 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-beta 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 beta 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 alpha phase structure, characterized by uniformly distributed tertiary alpha phases within equiaxed beta 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 m(1)/(2). This study elucidated the mechanisms by which multi-stage heat treatments govern the formation of nano-scale secondary alpha phases (width similar to 50 nm), sub-micron alpha phases (width similar to 500 nm), and primary alpha phases (width similar to 1 mu m), creating a hierarchical microstructure that enhances both strength and toughness. The observed improvements in fracture toughness were attributed to the optimized distribution of alpha 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.