Enhanced mechanical properties of Ti-5Al-5Mo-5V-3Cr-1Zr by bimodal lamellar precipitate microstructures via two-step aging

The formation of ultra-fine alpha precipitates in beta-titanium alloys can significantly improve strength but may lead to rupture and low ductility. Here, we propose a simple two-step aging heat treatment to design a bimodal microstructure with distinctively different populations of fine-scale and coarse alpha precipitates in Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531, wt. %) by involving two transformation mechanisms, i.e., classical nucleation and growth, and pseudo-spinodal decomposition. Such a multi-scale alpha microstructure exhibits a synergistic combination of yield strength (similar to 1.1 GPa) and ductility (similar to 19.5% elongation). TEM characterization shows that the appearance of deformation twins in coarse alpha precipitates contribute to increased ductility, and the higher strength may be attributed to dislocation tangles in fine-scale alpha precipitates. Our work provides a new strategy to overcome the strength-ductility trade-off by designing a heterogeneous microstructure.