Diffusion bonding of AlCoCrFeNi_2.1 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 AlCoCrFeNi_2.1 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.