Abstract:With the continuous advancement of the aerospace industry, increasingly stringent demands are being placed on the performance of titanium alloys, particularly requiring higher strength while maintaining sufficient ductility to ensure safety, reliability, and extended service life under extreme environments. Near-α titanium alloys, which are widely utilized in high-temperature structural components of aero-engines and airframes, typically exhibit four classical microstructural types: equiaxed, bimodal, widmanst?tten (lamellar), and basket-weave structures. Despite their wide application, these microstructures often encounter an inherent strength–ductility trade-off, which limits further improvement of comprehensive mechanical properties. To address this limitation and meet the evolving requirements of aerospace engineering, the development of novel microstructures that can simultaneously enhance strength and maintain or improve ductility has become an urgent research focus in the field of advanced titanium alloys. In this study, a cast near-α titanium alloy was selected as the research material, and a systematically designed thermomechanical processing route was employed to develop near-α titanium alloys with targeted microstructural configurations that aim to overcome the strength–ductility trade-off. Initially, through carefully controlled forging and solution treatment processes, appropriate bimodal microstructures were obtained, providing a foundation for subsequent microstructural tuning. Building upon these bimodal structures, a meticulously designed rolling process was applied to develop specific rolling-induced microstructures, followed by two distinctly different heat treatment strategies to ultimately obtain two types of tri-modal microstructures within the near-α titanium alloy system. Detailed microstructural characterizations were conducted using optical microscopy, electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) to compare the differences among the three microstructural configurations, including the baseline bimodal and the two developed tri-modal structures. The tri-modal structures consisted of elongated primary α (αp) phases, equiaxed αp phases, and secondary α (αs) phases, with the overall grain morphology and microstructural features differing significantly from those observed in the conventional bimodal structure. The tri-modal structure obtained through direct aging treatment (tri-modal-1) exhibited significantly refined grains, a higher volume fraction of αp phases, and an abundance of elongated, lath-like αp phases distributed along the rolling direction. Remarkably, even with a relatively high αp phase content, the tensile strength of the alloy increased from 956 MPa in the bimodal structure to 1070 MPa while maintaining a favorable elongation of 10.2%. The second tri-modal structure (tri-modal-2), which underwent a short-duration, high-temperature heat treatment, presented grain sizes and αp phase content similar to those of the bimodal structure but still exhibited an enhanced tensile strength of 1008 MPa. Further analysis revealed that the tri-modal-1 structure exhibited larger aspect ratios in the elongated αp phases, finer grain sizes in the αs phases, and higher dislocation densities within the matrix. This was primarily attributed to the processing route employed, where the absence of high-temperature recovery and recrystallization in tri-modal-1 helped retain high dislocation densities and refined grains. In contrast, the tri-modal-2 structure underwent more pronounced recovery and partial recrystallization during the high-temperature heat treatment, leading to changes in grain size, morphology, and a significant reduction in dislocation density. It is noteworthy that all three microstructures underwent the same aging treatment, which resulted in similar precipitation behavior of the primary strengthening phases within the alloy. This consistency in precipitation strengthening across the different microstructures indicates that the observed improvements in mechanical properties are primarily due to differences in the grain structure and dislocation configurations rather than differences in precipitate hardening. Through comprehensive comparative analysis of the microstructures and mechanical properties across the three configurations, it was demonstrated that dislocation strengthening and grain refinement are the critical mechanisms contributing to the enhanced strength of the near-α titanium alloy. Furthermore, the presence of elongated αp phases aligned parallel to the tensile loading direction was found to play a significant role in maintaining planar dislocation slip during deformation, thereby facilitating strain accommodation while preserving strength. This alignment effectively hinders the operation of cross-slip and the formation of detrimental stress concentrations, which are typically responsible for premature failure in high-strength alloys. Overall, the study highlights that the strategically designed tri-modal microstructures developed through systematic thermomechanical processing can significantly enhance the strength of near-α titanium alloys while maintaining sufficient ductility, offering a promising pathway for the development of next-generation high-performance titanium alloys for demanding aerospace applications.

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