Diffusion bonding of AlCoCrFeNi2.1 eutectic high entropy alloy to TiAl alloy

High entropy alloys have special microstructure and remarkable properties. To explore their potential engineering application in high temperature structures, the microstructure evolution of bonding interface, the elemental diffusion behavior and mechanical property of the diffusion bonded joint between AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) and TiAl alloy were investigated. Four reaction layers (rodlike B2 phase, Al(Co, Ni)2Ti, τ3-Al3NiTi2 + TiAl, τ3-Al3NiTi2 + TiAl + Ti3Al) formed in the diffusion zone near FCC phase of EHEA, but three layers (Al(Co, Ni)2Ti, τ3-Al3NiTi2 + TiAl, τ3-Al3NiTi2 + TiAl + Ti3Al) formed near B2 phase. Al and Ni controlled the reaction diffusion of EHEA and TiAl alloy, coarsened the acicular precipitated B2 phase and turned TiAl phase into Al(Co, Ni)2Ti and τ3-Al3NiTi2 phases. All these reaction layers grew in a parabolic manner as a function of bonding temperature. Rodlike B2 phase has the lowest growth activation energy of 125.2 kJ/mol, and the growth activation energy of τ3-Al3NiTi2 + TiAl layer near B2 phase is much lower than that near FCC phase. The penetration phenomenon and convex structure formed in the diffusion zone, which resulted in interlocking effect and enhanced the strength of resultant joints. The highest shear strength of 449 MPa was achieved at 950 °C. And the brittle fracture generally initiated at the interface between Al(Co, Ni)2Ti and τ3-Al3NiTi2 + TiAl layers.