A novel CoCrNi medium entropy alloy enhanced AgCuTi composite filler to braze Kovar alloy and sapphire

A novel composite filler was prepared by introducing CoCrNi medium-entropy alloy (MEA) into the AgCuTi filler and it was then used to enhance the Sapphire/4 J33-Kovar alloy brazed joints. The effects of CoCrNi MEA on the interfacial microstructure and mechanical properties of brazed joints were discussed by comparatively studying the brazed joints with single AgCuTi and composite filler. The results demonstrated that the porous CoCr2 phase surrounded by the Ni3Ti layer was in-situ synthesized in the brazing seam with the addition of CoCrNi MEA, which relieved the local residual stress during the cooling process. Moreover, the effect of porous structure on retarding the diffusion of metal atoms limited the reaction of Ti with Fe, which promoted the sufficient interfacial reaction between brazing filler and sapphire. The maximum shear strength of 95 MPa was achieved when 3 wt% CoCrNi MEA was added into the AgCuTi filler, which was ∼57.5% higher than that of the joint brazed with the single AgCuTi filler. The reaction mechanism at the brazing seam with different CoCrNi MEA amounts was also illustrated. •CoCrNi MEA reinforced AgCuTi filler metal was prepared to braze sapphire and Kovar alloy.•The effects of CoCrNi MEA contents on the microstructure and mechanical properties of the joints were investigated.•The strengthening mechanism of CoCrNi MEA during brazing was discussed.