What are Alloys? (Definition, Examples, and Metallurgy)

Assume that you mix two elements together: A and B.

It’s possible that A atoms like each other much better than B atoms. In that case, the material will split into two phases (like oil and water). This minimizes the amount of A atoms that touch B atoms.

It’s also possible that A atoms like B atoms much more than they like other A atoms. This situation results in an ordered compound (like sodium and chloride). This maximizes the number of A atoms that touch B atoms.

It’s also possible (and quite likely, in metals), that A and B atoms like each about as much as they like themselves. This results in a solid solution (like most metals). If A and B are the same size, they can randomly substitute for each other. If A and B are different sizes, the smaller one rests in interstitial sites between the larger atoms.

The atoms arrange randomly because it maximizes entropy. Entropy is basically chaos, and the universe always tries to increase entropy. That’s the 2nd law of thermodynamics.

This “entropic driving force” can actually change what kind of alloy results. For example, suppose A and B don’t really like each other. Normally, they’d form separate phases. But if there is 99% A and 1% B, there is a huge entropy gain from spreading those B atoms among all the A atoms. So you would probably get a solid solution.

However once there are roughly even amounts of A and B, the entropy gain from spreading B atoms is not large enough to overcome B’s dislike of A. Separate phases result!

This may be a bit hard to understand, so imagine that you have a clean room. You come home, take off your clothes, and throw them on the floor. Your room is much messier than before! But imagine your room is already messy. Does one more dirty shirt really affect the overall messiness? Odds are that by that point, you’ve developed a pile of clothes on the ground and the shirt will go onto the pile.

A material’s strength is its resistance to deformation.

Metals move by dislocations, so a stronger metal means it’s more difficult to slide atoms past each other. Although A-B bonds are weaker than A-A or B-B bonds (making sliding easier), the fact that A and B atoms are different sizes means there is a strain field.

This strain field impedes dislocation motion, resulting in an overall stronger metal. We call this solid-solution strengthening, and it is one of the most reliable ways to make stronger metals.

Substitutional or interstitial alloys. Most of the composition is made of a single element, with small additions of other elements. If there are two elements, it’s a “binary alloy,” three elements is “ternary,” four elements is “quaternary,” and five elements is “quinary.” In practice, alloys with more than three different elements will be called “multi-component” alloys.

Steels are not fundamentally different from other alloys, except that they are perhaps the most advanced material metallurgy can offer. Highly engineered steels may have a dozen elements in small amounts (in addition to Fe and C).

Superalloys are another pinnacle of metallurgy. Superalloys are like micro-scale composites. They have a matrix (solid solution) phase, and a precipitate (intermetallic) phase. Like steels, modern superalloys may have 10-20 alloying elements.

High-Entropy Alloys (HEAs) take solid solution strengthening to the extreme. Remember how I said that most alloys are primarily composed of a single element? That’s because intermetallics may form if two elements occur in similar amounts. Instead, HEAs combine 5 or 6 metal elements in equal ratios. Because of the entropy of mixing, the alloy stays as a single phase. HEAs are a new type of alloy and are not commercially widespread.

Bulk Metallic Glasses (BMGs) are metals arranged in an amorphous structure. That means there is no orderly crystal structure. These are slightly more developed than HEAs, but they are still rare.

Intermetallics are not considered alloys by every materials scientist (see my argument here). I prefer to call them “intermetallic compounds” instead of “intermetallic alloys,” because the atoms are arranged in an ordered structure like ceramics, rather than in solid solution.  In general, intermetallics have terrible properties and are completely useless. However, there are two notable exceptions. The strengthening phase of superalloys is an intermetallic, and NiTi is an intermetallic shape memory alloy.

To me, the word “alloy” suggests a compositionally engineered metal. Add carbon to iron, and get the stronger alloy, steel. Add chromium to steel, and get the corrosion-resistant stainless steel.

Alloying a metal means adjusting its composition to achieve certain properties. But can “dealloying” achieve the same thing? I believe it can.

Imagine that I wanted a material with the highest melting point possible. We’ve already discussed that having extra atoms in your metal decreases the melting point. So in this case, I would want to take pure tungsten and purify it further.

“Pure tungsten” may be 99.99% tungsten, and 0.01% other things that the manufacturer couldn’t remove. So what if I intentionally design a process to make my tungsten 99.99999% pure, to achieve the highest melting point possible?

I no longer have a default pure element, I have an intentionally dealloyed material. Does this mean it’s an “alloy” again?

I think so, but so far no one else agrees with me!

Sterling silver is an alloy used in jewelry. In jewelry, it’s defined as having 92.5 wt% silver, with the rest usually being copper. The copper is added to increase the silver’s strength (pure silver is usually too soft to wear as jewelry). As a trade-off, sterling silver has lower oxidation/corrosion resistance than pure silver.

Ti64 (pronounced Tie 6 4) is the most-used titanium alloy. It has excellent strength-to-density, very good corrosion and oxidation resistance, and is mostly biocompatible. Ti64 dominates the aerospace industry and so much is produced that– even though there are more biocompatible alloys–Ti64 is also frequently used in the medical field for implants. Any time you see the buzzword “made of titanium” such as on the SR-71, they probably mean Ti-6Al-4V.

Stainless Steel is a steel alloy (iron and carbon) with chromium, which provides oxidation and corrosion resistance. This is useful whenever you don’t want steel to rust such as kitchen utensils, surgical instruments, water pipes, and jewelry.

Solders are low melting point, conductive alloys. They are usually made of lead, tin, and/or bismuth. Because they have a low melting point, plumbers, jewelers, and electricians melt solders between other metals to join them together (like hot glue). Solders are most well-known for connecting wires to a circuit board.

Bronzes are copper alloys. Apart from copper, they consist of tin or other elements like aluminum, silicon, nickel. They are usually used for their combination of strength and high corrosion resistance.

Brass is another commonly-used copper alloy. They consist of copper and zinc. Brasses are widely used in everyday applications, for example as jewelry, coins, musical instruments, cartridge casings, etc.

Vitreloy was the first (and currently only?) commercial bulk metallic glass (BMG). That means that the atoms are not in a crystal structure, but are disorganized like regular glasses. This increases corrosion resistance, strength, and elasticity, but almost completely eliminates ductility at room temperature. BMGs are often used in sporting equipment because they are very stiff.

50% Ni and 50% Ti forms an intermetallic compound that is biocompatible, extremely elastic, and can restore its shape when heated. Normally, intermetallic compounds are very strong and brittle because dislocations are blocked, but NiTi can deform by twinning instead. This is also what allows the shape memory process. NiTi is used in wire glasses frames, braces wires, and stents.