Shown here in Fig. 2 is a more complex binary phase diagram, this one for the Nickel/Boron (Ni/B) alloy system, in which more than one eutectic composition exists. Notice that in this system there are four (4) eutectic compositions possible. Each such eutectic is characterized by being the lowest-temp liquid-composition within its composition range, and each will go from solid-to-liquid at a single temperature, i.e., the solidus temperature and liquidus temperature for that alloy composition are identical. Thus, there is no “melt-range” for that particular chemistry, i.e., no significant difference between the solidus temperature and liquidus temperature. They are technically one and the same at that composition.
There are many thousands of binary phase diagrams in existence, most of which will display eutectics; thus there are thousands of potential eutectics to consider in the world of brazing, depending on the BFMs and base-metals involved.
During brazing, eutectics can be very important. The can play a major role when heating a brazing filler metal (BFM) up to the brazing temperature, as well as when cooling the BFM after brazing.
During heat-up and melting of a BFM, I like the fact that the eutectic composition will be the first to melt (since it is the lowest melting composition of the alloy system), and because of its narrow melting range (solidus and liquidus are the same), it will flow into the braze joint with no liquation apparent (see my earlier article on liquation elsewhere on this website). To this day, whenever I am looking to select a BFM for a particular brazing application, I will usually begin with the eutectic (or nearly-eutectic) composition within any given BFM alloy system, because of these advantages.
Another thing about eutectics to remember is that, being the lowest melting point composition of a particular BFM alloy system, it should then be the last composition to freeze when cooling down from the brazing temperature.
As the BFM cools down from brazing temperature, the brazed joint will generally begin to solidify from the outside edges of the joint toward the center. The center of the braze joint is usually the last to freeze/solidify. Therefore, based on the information presented so far, one would logically expect that because the eutectic composition should be the last to freeze, the eutectic material should then solidify at the center of the braze joint. And yes, this is what one sees in most brazed-joint microstructures.
For most BFMs, the eutectic compositions consist of metals whose various alloy components are usually quite ductile. BFM compositions containing silver, copper, gold, nickel, chromium, etc., are usually quite ductile in nature.
Sometimes metallurgists will look at these eutectic-forming combinations, as just described, and tell themselves that they can, by adding additional elements into the alloys, create complex systems that will begin to melt at even lower temperatures. These added elements are often called “temperature depressants”. Elements added into the silver-copper (Ag/Cu) alloy systems for this purpose, such as zinc and/or cadmium, don’t significantly alter the ductile behavior of these Ag/Cu systems. This is pretty much true for most BFM families, in that the addition of temperature-depressing elements can nicely lower the melting temps of the BFMs without really affecting the hardness and/or ductility of the overall systems.
However, the nickel-based BFM family is an exception to this. Experience has shown that the most effective temperature depressants for the nickel-based family of alloys are phosphorus, boron, and silicon. Phosphorus is a non-metallic, and both boron and silicon are semi-metallics. Unfortunately, when they are added into an alloy of nickel or cobalt, these temperature depressants also become hardeners, the resulting metallurgical phases upon cooling being complex borides, phosphides, or silicides, with high hardness and zero ductility. Thus, because they are added as temperature depressants, creating the lowest-temp liquid-phases within that nickel-based alloy system, those phases will be the last to solidify upon cooling from brazing temperature.