The word “eutectic” is one that I use in each of my brazing seminars during our discussions about brazing filler metals (BFMs) as well as metallurgical phase-diagrams, but it is also a word used in describing some of the features of metallurgical-structures within a solidified brazed joint.
The word “eutectic” comes from the Greek word “eutektos” which means “easily melted”.
Simply put, a eutectic-composition is an alloy of two or more metals, which, when heated to its melting point (solidus temp.), will completely change from solid to liquid at the same temperature (i.e., isothermally). Technically, theoreticians prefer to define a eutectic-reaction in reverse, proceeding from the molten state to the solid. However, I believe my description will help personnel in the brazing world grasp the general concept more easily.
Thus, a eutectic-composition will be characterized by being the first alloy-composition to melt during heating, and also the last to freeze during cooling. This generalization is clearly seen in the attached phase diagram for the Silver-Copper (Ag-Cu) binary system, as shown in Fig. 1, where the eutectic composition occurs at 72%Ag and 28% Cu, with a eutectic-temperature of 780°C (1435°F).
For this binary system, when any Ag/Cu alloy is heated, it will start to melt at 780°C (1435°F), with the formation of a molten metal of composition 72%Ag/28%Cu, the Ag/Cu eutectic. By the way, if this liquid metal is located at the edge of any capillary juncture (such as overlapping metals forming a joint to be brazed), then that initial eutectic-liquid which forms should be quickly drawn into the joint by capillary action, assuming conditions support such action (i.e., proper joint cleanliness, gap-clearance, atmosphere quality, etc.).
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.
And, because they are the last to solidify, where will they be? That’s correct – in the center of the joint! Consequently, we will call that material a “centerline eutectic”, and example of which is shown in Fig. 3.
(Note the continuous centerline eutectic in the bottom joint, containing cracks, and the non-continuous centerline phases in the top of the joint. Those dark spots in the center of the top joint are not holes, but rather dark-etching centerline eutectic material.)
These centerline phases, rich in the eutectic former (temperature depressant), are not usually a problem, as mentioned earlier. However, a problem can indeed occur if these hard centerline eutectic phase-structures/particles are able to link up with each other and form a continuous matrix of this non-ductile material down the center of a brazed joint. Then, under severe service conditions involving shock, fatigue, vibration, etc., that brazed joint might crack right down the center (as shown in the bottom of Fig. 3, as well as in Fig. 4 below), resulting in weakening, or even failure of the part.
The primary reason for centerline-eutectic phases being able to form a continuous matrix, rather than remaining as discreet, separate particles in the center of the solidified joint, is the thickness of the gap itself. In nickel-brazing, when the gaps are kept very tight (0.001”-0.002” / 0.025-0.05 mm) the amount of the centerline eutectic forming materials is so minimal that continuous matrix of that eutectic material will not form. But, when the nickel-brazing gaps are much thicker than this, there is enough eutectic-forming phases present upon solidification to form a continuous centerline eutectic, which can put a brazed joint at severe risk in service.
Thus, in brazing, the word “eutectic” can be good (referring to the formation of a unique low-melting, free-flowing alloy composition that does not liquate), or it can be bad (when referring to the existence of a continuous band of hard, non-ductile eutectic phases down the center of a brazed joint).
The reader will hopefully now understand what a eutectic is, and how it can be used for good or bad in a given brazing application.
Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at [email protected], and his website can be visited at: http://www.kaybrazing.com/
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