Thermohydrogen-Induced α→β Phase Transition and Interfacial Aggregation: A Diffusion Bonding Mechanism for Ti65 Alloy at Low Temperatures

Diffusion bonding (DB) of near-α titanium alloys requires high temperatures, which can cause grain coarsening, structural deformations, and degradation of the workpiece's mechanical characteristics. In this study, thermal hydrogen processing (THP) was applied to Ti65, combined with selective magnetron sputtering of a Ti-Al thin film to promote hydrogen segregation, achieving DB of Ti65 at an ultralow temperature of 750°C, and the joint's shear strength reached 757.9 MPa. The typical microstructure of the hydrogenated Ti65 alloy at room temperature was composed of β-Ti, α-Ti, α′, and hydrides. A small fraction of hydrogen-induced α→β (β_H) phase transformation occurred preferentially along grain boundaries, refining the grain size by 19.9%. Because of hydrogen segregation at the diffusion-bonded interface, continuous β_H-phase structures formed throughout the interfacial region, contributing to the stability of the joint. TEM and EBSD results revealed a high density of lattice defects in β_H, whereas first-principles simulations demonstrated an increase in vacancy concentration in β_H. The increased lattice defect density significantly accelerated the atomic diffusion rate of the β_H-phase. Overall, the hydrogen-induced phase transformation and the stable segregation of the β_H-phase at the interface have been confirmed as key factors in achieving low-temperature DB of the Ti65 alloy.