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Microstructure and mechanical properties of Ti-Ti2AlNb interface

Diffusion bonding of Ti2AlNb alloy using pure titanium (Ti) foil as an interlayer was carried out on superplastic forming and diffusion bonding special equipment by gas pressure loading method. The microstructure of Ti-Ti2AlNb interface was observed using scanning electron microscope and energy-dispersive spectrometer while the mechanical properties of the joints were evaluated by shear test. The results show that the thickness of Ti foil interlayer has a great influence on the microstructure and shear strength of the interface diffusion region. When the thickness of the intermediate layer is thin (25 µm), Ti, aluminum (Al), and niobium (Nb) elements are fully diffused with uniform element distribution through the diffusion region. The diffusion layer region presents uniform, fine, and disordered lamellar α-Ti + β-Ti dual-phase structure with high shear strength. When the thickness of Ti foil interlayer is thick (50 µm), the distribution of Al elements is relatively uniform through the diffusion region due to its smaller radius and faster diffusion speed, and Ti and Nb elements present gradient distribution from the middle to both sides. The diffusion layer region presents a coarse and long strip shape α-Ti + β-Ti dual-phase structure in the middle part and a fine needle-like or irregular α-Ti + β-Ti dual-phase structure in both side parts, with slightly lower shear strength. Temperature has a great influence on the microstructure and mechanical properties of the diffusion bonding joints. The diffusion region presents a black α-Ti strip area in the middle part with the width of about 10 µm at lower temperature (910°C) with poorer property, due to the grain growth of the parent metal, the property is slightly poorer when the temperature is too high (960°C), and the optimal temperature is 930°C with a higher shear strength.