Enhancement of element diffusion and microstructural evolution in transient liquid phase bonding of AlCoCrFeNi_2.1 high entropy alloy using a Ni-Cr based alloy interlayer

This study focuses on accelerating element diffusion during Transient Liquid Phase (TLP) bonding of AlCoCrFeNi_2.1 by incorporating a Ni-Cr-based alloy interlayer. The microstructure, mechanical properties, and their evolution of TLP joints were comprehensively investigated. The addition of the interlayer significantly enhances the element diffusion coefficient during the TLP process. Interdiffusion between the interlayer and the BM results in the formation of an Isothermal Solidification Zone (ISZ) and a Diffusion-Affected Zone (DAZ). The ISZ predominantly consists of (1, 1¯, 1) L12 and (2¯, 3, 1¯) B2 phases in a semi-coherent relationship, with the presence of nanoscale BCC phases. The DAZ is defined by the diffusion of unique elements from the interlayer into the BM. The mechanical properties of the joint are largely influenced by its microstructure. With an increase in bonding time from 10 min to 120 min, the athermal solidification zone gradually transforms into the ISZ. During this process, the increase in alloying degree, recrystallization, and B2 phase content within the ISZ leads to a greater contribution from solid solution strengthening, grain refinement, enhanced dislocation hardening, and improved lamellar structure. Consequently, the mechanical properties improve. When bonding at 1160 °C − 4 MPa − 90 min, the joint shows the maximum elongation (16.5%) and tensile strength (1062 MPa), matching the values of the base material, with fracture occurring in the BM. However, at 120 min, the grain size up to 40 μm in ISZ promotes crack initiation and propagation along the interface easily under tensile load. The fracture surface exhibits a mixed fracture mode.