Strengthening effects of different alloying elements in CoNiCr‑Bbased filler metals on brazed joints of single crystal superalloy DD6

Series of CoNiCr based filler metals were designed for vacuum brazing single crystal superalloyDD6. The influences of strengthening elements (W, Mo, Nb) in CoNiCr-B based filler metal on joint microstructure and boride formation were investigated using SEM and EDS. The results reveal that W, Mo and Nb participate in boride formation within the brazing seam. As the strengthening-element content increase, the boride morphology was changed from discrete vein-like dendrite to blocky, acicular of fragmented brittle phases. Excessive addition of the strengthening element leads to coarse aggregregated borides, which act as potential crack-initiation sites. Molybdenum exerts the strongest morphological modification effect, producing acicular borides at high content. High-Nb addition yields fragmented blocky borides. Tungsten mainly enlarges original vein-like boride dimension before destroying dendritic features. This work provides fundamental support for composition optimization of brazing fillers for single crystal superalloy components.