A novel high entropy composite interlayer for diffusion bonding of TC4 titanium alloy to 316L stainless steel

Titanium/steel hybrid structure exhibit broad prospects in the nuclear industry and aerospace applications. Achieving high-performance titanium/steel bonding is crucial. Herein, the novel AlCoCrNiCuAg/Cu high entropy composite interlayer was designed for vacuum diffusion bonding of TC4 alloy to 316L stainless steel. The effects of bonding temperature, holding time and assembly sequence on the interfacial microstructure and mechanical properties were investigated, and the fracture behaviors were researched. Typical microstructure of the joint bonded at 1010 °C for 90 min via AlCoCrNiCuAg/Cu sequence was TC4/β-Ti/β-Ti+TiFe/TiFe+χ+σ/α-Fe+χ+σ/316L. With bonding temperature increased, due to the dispersed distribution of the Cu phase in the Ti-Fe phases, the highest shear strength of 222.8 MPa was achieved at 1010 °C/90 min. The cleavage characteristics were confirmed in the fracture surfaces, suggesting the brittle fracture. The main cracks were located at the junction of the TiFe+χ+σ and the β-Ti+TiFe layers, further confirmed that the interface appeared immense strain, accelerating the joint fracture.