Microstructure and mechanical properties of CoCrNi/GH99 medium entropy alloy brazed joints: Formation of medium entropy brazing seam

Abstract

Brazing CoCrNi to GH99 is of great significance for manufacturing high-strength and lightweight structural components in thermal protection systems. BNi-5a amorphous filler was used to braze CoCrNi to GH99 in this study, and medium entropy brazing seams were obtained. The typical CoCrNi/BNi-5a/GH99 joint is composed of the brazing seam and diffusion zones on both sides of substrates. The main component of the brazing seam is a γ (Ni, Cr) solid solution, with CrB and borides on CoCrNi side and GH99 side, respectively. Increasing the brazing temperature and prolonging the holding time can promote atomic diffusion and form more precipitates in diffusion zones on both sides, but the microstructure of the joints tends to homogenize under excessively high temperature or longer holding time. The joints obtained at the parameter of 1210 °C/10 min reach the maximum shear strength of 557 MPa. The brazing seam has outstanding effects similar to those of medium and high entropy alloys, leading to excellent mechanical properties. Most joints exhibit a mixed mode of cleavage fracture and microvoid coalescence fracture, and fracture occurs at the brazing seam. EBSD analysis reveals that the deformation of CoCrNi base material is greater than that of the brazing seam after shear test. ∑3 twins and low-angle grain boundaries are formed in CoCrNi substrate, and cracks propagate along the grain boundaries in the brazing seam.