| Summary: | TC4 (Ti–6Al–4V) titanium alloy and Al5(HfNbTiZr)95 refractory high-entropy alloy were successfully diffusion bonded at a temperature range of 800–1000 °C for 60 min under 5 MPa. The microstructure and mechanical properties of the diffusion bonded joints were investigated. The diffusion bonding seam of the joint contained two different zones, namely: a reaction zone with acicular titanium structures (Zone I); and a continuous solid solution reaction zone (Zone II) which inherited the orientation of the matrix Al5(HfNbTiZr)95. The highest microhardness of reaction zone reached 6.82 GPa, located in the boundary of TC4 and the acicular titanium structure zone. The effects of bonding temperature on the microstructure and mechanical properties of the joints were revealed. As bonding temperature increased, all the reaction zones grew in a parabolic manner with an increase in the bonding temperature. The shear strength of the joints ranged from 55.56 MPa to 777.06 MPa as the bonding temperature increased. The maximum shear strength of 770.06 MPa was obtained at a bonding temperature of 950 °C for 60 min under 5 MPa, and the fracture located in the TC4 substrate illustrated a typical ductile fracture mode.
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