Abstract: This study demonstrates how high-pressure thermal treatment achieves a breakthrough in the trade-off relation between strength and ductility in Ti-55511 alloy, providing critical insights for aerospace materials design. When Ti-55511 alloy was processed at 5 GPa across temperatures (room temperature, 700 °C, 900 °C), temperature-dependent gradient microstructures were formed and governed by stress heterogeneity. This heterogeneity arises from friction constraints induced by high-pressure stress at the contacting surfaces. The surface region exhibited stress concentration with higher hardness values, while the center region displayed a lower stress state with lower hardness values. A gradient distribution of the shear bands was induced at room temperature, with shear band-assisted phase transformation occurring in the surface region. Comparing the α-phase distributions emerging in the specimens treated in single-phase and dual-phase regions, the stress was found to promote α precipitation for the former one and suppress α precipitation for the latter one in the surface region. Notably, a unique α-phase fragmentation mechanism was identified in high-temperature-high-pressure treatment, with the nucleation of the β phase within the α phase, induced by α-to-β phase transformation. When the interior β phase grew up to connect the α/β interfaces on both sides of the α precipitates, the fragmentation process was completed. Besides, dynamic recrystallization occurred in the center of the specimen treated by high-pressure treatment in the single-phase region with limited α precipitation. This led to a notable enhancement in the mechanical properties, with plasticity increased by over 11 % and strength increased by ∼160 MPa compared with the original solid-solution state. These findings demonstrate the uniqueness of the high-pressure thermal treatment process and its potential as a preparation technique for titanium alloys with high-strength and high-ductility.

Key words: Ti-55511 alloy, High-pressure treatment, Shear bands, Phase transformation, Dislocations