Joint clearances must be tight for effective Ni-brazing. 1. Nickel-based brazing filler metals (BFM) can leave a hard, non-ductile eutectic phase in the middle of a brazed joint.
The hard, non-ductile metallurgical phase-structures that form upon solidification of Ni-brazed joints must be carefully controlled, or else they can, and will, result in cracks inside the joint in stressful mechanical or thermal-cycling service.
The last phases to solidify when brazing with nickel-based brazing filler metals (BFMs) will be those phases that are the lowest-melting, i.e., those phases rich in the temperature-lowering, eutectic-forming, elements (meaning those that are rich in boron, silicon, or phosphorus). Remember, “eutectic” refers to the composition of an alloy that is the lowest melting point portion of the BFM. Thus, eutectic phases will not only be the first composition to start melting during heating of the BFM but also will be the last to solidify during cooling. Thus, during cooling these eutectic-phases will “migrate” towards the center of the joint as the “solidification-front” of the BFM moves from the base-metal/BFM interface toward the center of the joint, and will be forced to solidify right at the center of the joint.
Unfortunately, all of these temperature-lowering, eutectic-forming, elements in nickel-based BFMs are also hardeners, that is, the phase-structures resulting from solidification of these elements have virtually zero ductility! Thus, the last phases to solidify (in the center of the joint) will be hard, and non-ductile. If the joint is thicker than only about 0.004” (0.10mm) max., these hard centerline eutectics can actually form a continuous line down the center of the joint, and cause the joint to become very prone to cracking under any kind of thermal or mechanical stress or strain in service.