A non-coherent (C15-Laves + Y₂O₃) encircling nano-unit and its strengthening role in a near-α Ti alloy at 650 °C

To clarify the origin of the Y-induced nanoscale structural units in near-α high-temperature titanium alloys, Ti-6Al-1.5Mo-1Ta-2.5Zr-0.4Si-xY (x = 0 and 0.2 wt%) alloys were prepared by vacuum arc melting, and systematically characterized.With the addition of 0.2 wt% Y, Y₂O₃ particles and nanoscale (C15-Laves + Y₂O₃) two-phase structural units formed near phase boundaries. Transmission electron microscopy (TEM)/High-resolution TEM (HRTEM) analyses show that the C15-Laves phase exhibits a well-defined orientation relationship with β-Ti and relatively favorable interfacial matching, whereas the interface between C15-Laves and Y₂O₃ is non-coherent. In combination with the observed compositional partitioning and spatial arrangement, this structural unit is interpreted as a diffusion-assisted, segregation-mediated sequential precipitation product rather than a simple encapsulated structure: Y first captures oxygen near the migrating α/β interface to form Y₂O₃, after which Ta, Zr, and residual Y redistribute to the adjacent β side, thereby promoting the peripheral precipitation of the C15-Laves phase. After tensile deformation at 650 °C, pronounced dislocation pile-ups are observed around the structural unit, and local strain inhomogeneity is concentrated near its internal interfaces. These findings indicate that the structural unit forms through phase-transformation-assisted solute diffusion and interfacial redistribution, and its composite interfacial character plays an important role in dislocation interaction and damage evolution.