Interfacial microstructure and mechanical properties of Al0.3CoCrFeNi high-entropy alloy joints vacuum brazed with Ni/Nb/Ni laminated filler metal

Vacuum brazing of Al 0.3 CoCrFeNi high-entropy alloys (HEAs) is investigated at 1220 °C by using a laminated Ni/Nb/Ni filler metal (FM). The effects of Ni/Nb/Ni FM composition and the holding time on the microstructure and mechanical properties of the joints are investigated. With increasing the Ni content in the Ni/Nb/Ni FMs, the size of the Laves/γ phase eutectic colonies in the brazing zone is decreased while the lamellar spacing of the Laves phase is increased. The shear strength of Al 0.3 CoCrFeNi joint is gradually elevated to 408 MPa due to the increased volume fraction of the eutectic γ phase. As the holding time increased from 1 min to 360 min, the microstructure of brazed joint evolved through the following sequence: blocky Laves phase with γ phase, to lamellar Laves/γ eutectic, and finally to rod-like Laves/γ eutectic. The average shear strength of the joint is sharply increased from 78 MPa at 1 min to 395 MPa at 2 min, and then gradually improved to the maximum value of 422 MPa at 60 min. A drop in the joint shear strength at 180 min and 360 min is due to the formation of crack in the rod-like Laves phase. The shear strength and shear displacement of the Al 0.3 CoCrFeNi joint at -196 °C reach 937 MPa and 1.62 mm, respectively. Its excellent cryogenic strength and ductility is mainly ascribed to the formation of stacking faults (SFs) and the nano-spacing SFs network in the γ(γ´) matrix during shearing deformation.