Understanding Eutectics: A Guide for Heat-Treating Professionals

Understanding the Basics

• Introduction to Eutectics
• Eutectics 101
• Everyday Examples

At the University of Health Sciences and Pharmacy in Saint Louis, the athletic mascot stands out: Mortarmer “Morty” McPestle, the Fighting Eutectic. While he represents the school’s blend of rigorous academics and competitive sports, the term eutectic has a much different—and more scientific—meaning for students in the chemistry lab.

In chemistry, eutectics refers to the way two substances can interact to create a lower melting point than either would have on its own. The word eutectic comes from Greek roots meaning “well-melting” and was coined in 1884 by British physicist and chemist Frederick Guthrie.

Eutectic reactions are all around us. A common example is adding salt to water, which lowers the freezing point—helpful for making ice cream or clearing icy roads. Inkjet printers rely on eutectic materials to function at lower temperatures, and some topical anesthetics use a eutectic mixture of lidocaine and prilocaine that turns into a liquid at room temperature.

The Role of Eutectics in Metallurgy

When combined correctly at certain temperatures, eutectic chemical reactions create important metal alloys. Metallurgists can optimize these alloys for strength, hardness, ductility, cleanliness, density, and a variety of other important qualities through a series of thermal processing steps. But eutectic reactions must be carefully managed.

When an unexpected eutectic event occurs in a vacuum furnace, the process can warp parts, create brittleness or unexpected hardness, melt your parts and fixtures into a puddle at the bottom of your hot zone, or cause any of a number of other issues ranging from mildly inconvenient to catastrophic.

That’s why understanding a phase diagram is essential.

Reading a Basic Phase Diagram

Phase transformations are a key part of the heat-treating process. A basic understanding of eutectic reactions and phase diagrams helps furnace operators make sense of what’s happening behind the sealed, water-cooled walls of a vacuum furnace. (for those already familiar with phase diagrams, you can skip to the next section.)

To better understand eutectics, let’s review the salt and ice scenario and take a look at how that phase diagram works.

In this example from the U.S. Department of Transportation, they compare two different salts that can be used to de-ice snowy winter roads – NaCl (Table Salt) and CaCl₂ (calcium chloride salt). Note that both table salt and calcium chloride are solids at room temperature (70 °F/20 °C).

In both situations, the reaction and documented results can occur either when water starts as a liquid or a solid. That’s why salt is an effective tool for melting ice on roads, driveways, sidewalks, and bike paths.

While the freezing temperature of pure water is well known (32 °F/0 °C), we can see that dissolving table salt into water lowers the melting point of water significantly, up to a concentration of about 23 percent salt. In an ideal solution at that concentration, the freezing point drops (-4 °F/-21 C). Adding more salt to that concentration won’t lower the freezing point any further, it will only leave undissolved salt residue behind. Calcium chloride salts, however, can be dissolved into water at a much higher concentration, creating a solution that has up to 29 percent dissolved CaCl₂. As a result, that solution has an even lower freezing point (-60 °F/-52 °C).

Reading the phase diagram, each salt’s chart reveals a V shape that separates out the state of the solution. Inside the V is a fully liquid solution of salt and water—the solution at these percent concentrations and at these temperatures is fully melted and dissolved. To the left of the V you see where a mixture of ice and solution can be found—partially melted ice. To the right of the V you have undissolved salts and a liquid solution—think of it like the residue of salt left behind when the now-melted ice flows down the curb and into the storm drain. Below the dashed line you have solid salt and solid ice—the system is too cold for the salt to dissolve into the ice.

The V, then, reflects the eutectic zone—the part of the system where combining two substances creates a phase shift from solid to liquid at a given temperature that neither substance would experience without the other’s presence.

Phase diagrams can also include additional reaction data; for example, some graphs show how atmospheric pressure affects phase changes. This information is especially important when evaluating reactions in a vacuum furnace.

What does this have to do with metallurgy?

When metals are heated up to high-enough temperatures, they will melt into a liquid form. The temperature at which a metal will melt with no remaining solids suspended in the molten liquid is called the liquidus temperature. 

Images of molten metal being poured into molds—whether for casting swords or manufacturing components—often depict a mixture of iron and carbon. This combination forms a eutectic reaction that produces cast iron or steel with specific, desirable properties.

From stainless steel knives to cast iron pans, forged and cast metals all depend on these eutectic interactions to both determine their liquidus points, as well as the quality of the metal once the piece has been cooled to room temperature. The key differences in their characteristics are determined by the concentration of carbon in the iron, and the speed with which the item was quenched (or cooled).

Eutectics and Brazing

In the vacuum furnace world, one great example of intended eutectics comes from brazing. Much like your typical workbench solder, brazing filler alloys are designed to have a lower melting point than the work pieces they’re affecting, even if they share a primary metallic element.

By melting the filler alloy, the material uses capillary action to flow into the seams and hollows of the metal parts, creating a clean, complete, and visually appealing connection or seal.

Vacuum furnaces are exceptional at brazing processes that involve materials which are highly susceptible to oxidation, like aluminum and titanium. Precision temperature controls and monitoring systems within a vacuum furnace ensure that the process melts the brazing material, while leaving the part materials unchanged.

Consider that aluminum brazing frequently involves using an aluminum-silicon alloy. This phase diagram shows that aluminum and silicon, when alloyed at roughly an 88 percent Al to 12 percent Si solution, have a eutectic melting point of 1070 °F/577 °C—almost 100 °C cooler than pure aluminum’s melting point. 

Using aluminum-silicon brazing material on a pure aluminum part will ensure a consistency of metallic qualities for further manufacture and operational purposes. The brazing process produces a corrosion-resistant, medium strength bond. Adjusting the concentration of silicon within the solution can create a variety of benefits and limitations, depending on the intended final use of the product.

Controlling Eutectic Reactions in an Industrial Furnace

In this section, we take a closer look at why furnace operators are so cautious about unwanted eutectic interactions during a heat-treating process and how to prevent them.

Eutectic Reactions Gone Wrong

As you can see in the phase diagram above, nickel and titanium have many, many eutectic points, all of which can produce a sudden, unwanted liquidus stage. Thermal processors who have experienced new operators putting a titanium part into a nickel-alloy basket will often share horror stories of melted metals pooled on the bottom of their hot zone.

It doesn’t take much to set off a eutectic reaction, either. Just two parts touching at the right temperature can trigger a reaction that will run its course until the metals have either fused together or met chemical equilibrium. In the case of the nickel-titanium eutectic diagram, there are several eutectic equilibrium states—so much so that the diagram looks a bit like a bouncing ball. These various eutectic points are a reminder that chemical volatility affects solids too, just like when salt is spread on ice.

The nickel-titanium reaction isn’t alone in creating unexpected problems for vacuum furnace operators. Other metals that should be processed with care when adjacent within a vacuum furnace include: nickel and carbon, iron and titanium, carbon and titanium, and iron and carbon. Using eutectic barriers and referring to the compatible materials temperature chart in cases involving these materials is recommended.

Many thermal processing companies will have metallurgists review the composition of everything from parts to fixtures, selecting the correct furnace settings and hot zone design to optimize for the parts they most frequently process. They’ll consult phase diagrams and look for resources like eutectic barriers to protect the investment they made when purchasing the furnace. 

Furnace operators need to be trained to understand the consequences of failing to follow protocols. And furnace system maintenance teams need to be good at monitoring and reporting signs that an unwanted chemical reaction is happening within the chamber.

Eutectic Barriers

Heat-treating operators don’t necessarily need to know how to read a phase diagram to know that they’re having problems with parts sticking to the fixtures. Whether they’re using common nickel-alloy baskets or graphite fixtures, sometimes users need to find a quick answer to the problem and move on. The most common solution to the problem is: ceramics.

Here are three useful products to know about when looking for a ceramic solution to a eutectic problem.

1. Anti-eutectic ceramic coatings – Creating a buffer between the components that you’re treating and their fixtures, ceramic coatings can survive very-high temperatures and create a completely inert barrier between the two. While a ceramic coating can create a complete barrier for any point where a part could touch the fixture, the cost of coating your carbon-carbon (C/C) fixtures may be high, and the fixture would need to be inspected periodically for chips or cracks that come from part loading and unloading.

• Ceramic alumina plates – Stacking parts on a carbon fiber composite (CFC) tray or basket may create unwanted carbon infiltration in certain alloys. Ceramic alumina plates can create a nice, flat barrier between the parts and the basket, particularly useful for powder metal parts.

• Ceramic fiber cloth – An inexpensive disposable cloth that can be cut from a roll like a gigantic sheet of paper, ceramic fiber cloth can be produced as thin as 1/16^th inch thickness. Flexible for use in a variety of fixtures across multiple operations, it is a single-use product that should always be replenished well ahead of the consumption of the last roll. Operators should also take care when loading/unloading the fixture that the blanket covers all areas where parts may encounter the fixture, even if they shift while being loaded into the furnace.

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If you have had an unexpected eutectic reaction occur in your hot zone and you need service on your furnace, contact 1-844-GO-IPSEN to schedule a field service call.