Nanophase strengthening Ti₂AlNb diffusion bonding joint using refractory high entropy interlayer and post-bond heat treatment

The brittle intermetallic compounds (IMC) are easily formed at diffusion bonding (DB) of Ti₂AlNb which seriously damages the ductility of joints. In this study, a novel refractory high entropy alloys was used as the interlayer to prevent the formation of IMC. Further on, the post-bonded heat treatment (PBHT) was applied to the Ti₂AlNb/RHEA/Ti₂AlNb joint to further increase the bonding strength. The microstructure evolution and mechanical properties of joints were comprehensively investigated by scanning and transmission electron microscopy (SEM and TEM). The microstructure of bonding interface after PBHT was composed of disordered bcc-type solid solution, in which nano-sized O phase precipitated uniformly. As the aging time extended from 5 h to 20 h, the volume fraction of O phase at the DB joint increased to 32.4%. The shear strength of joints also increased from 449 MPa to 582 MPa with the extension of aging time, which increased by 29.6% compared with the as-weld joint. The precipitation of O phase is driven by the low Gibbs free energy in thermodynamics. Meanwhile, short-range atomic diffusion provided the transformation pathway for O phase. The basketweave morphology of O phase was mainly attributed to the selection of variants of O phase. This work provides a new design idea of diffusion bonding interlayer and experimental basis for the design of post-weld heat treatment of joints.