Did you know that in the same way bakers bake cakes, a metallurgist can make metal alloys? Okay, so maybe not in the exact same way, but metallurgists do have the unique and necessary skill of combining metals to make alloys such as bronze and steel. We will walk in the shoes of a metallurgist and scratch the surface of the world of “metal baking” in this overview of the two main types of alloys: interstitial and substitutional alloys!
Interstitial alloys form when the mixed elements in the alloy have different atomic radii, and the smaller atoms fill in the interstitial spaces (holes) in the lattice structure of larger atoms.
Substitutional alloys form when the mixed elements in the alloy have similar atomic radii. The atoms of one element can substitute into the crystal lattice structure of the other element.
- This article is about substitutional and intersticial alloys.
- We are going to begin by explaining what interstitial and substitutional alloys are
- Then, we will compare interstitial and substitutional alloys and look at the essential differences between them.
- Next, we will do a deep dive into the benefits of substitutional alloys
- Building on our foundation of knowledge of these alloys, we will then investigate the properties of interstitial and substitutional alloys
- Finally, we will look at substitutional alloy and interstitial alloy examples
If you aren’t familiar with the crystal lattice structure of metals and alloys, head over to Structure of Metal and Alloys for a review before continuing!
Interstitial and Substitutional Alloys Explained
When a metallurgist mixes metals, is it just by chance that they form useful alloys? No! If the metals and other elements have been melted down and are being mixed in their liquid phases, the relative size of their atoms can be used to determine what type of alloy will form.
If the atoms of the elements being mixed have different radii, interstitial alloys will form. This is the case with steel composed of iron and carbon; the much smaller carbon atoms fill in the tiny spaces between the crystal lattice of iron atoms.
If the atoms of the metals being mixed have similar radii, substitutional alloys will form. The oldest example of a substitutional alloy is bronze, made up of copper and tin metal. Some of the copper atoms in the lattice get replaced by the tin atoms!
But, what does this atomic size difference look like in the atomic arrangement of these alloys? Let’s look at the atomic makeup of steel compared to bronze to see!
Fig. 1: The atomic arrangement of interstitial steel and substitutional bronze alloy, Vaia Original.
Remember, even though alloys have specific ratios of elements, there is often no fixed atomic composition, which is why the tin and carbon atoms are randomly distributed in both pictures!
Compare Interstitial and Substitutional Alloys
Even though interstitial and substitutional alloys are both types of alloys, we’ve already seen a significant difference in how they form! The table below lists the other essential differences between the alloys.
| Substitutional Alloys | Interstitial alloys |
Formation | Atom substitution | Interstitial filling |
Size of Atoms | Relatively the same size atoms | Different sized atoms There must be at least a 15% difference in size |
Types of Elements | Usually occurs between metals | Usually, one metal has large atoms and another smaller element that fills the interstitial spaces. |
Properties compared to metals *we go more in detail at the end | Corrosion-resistant, lower melting point, increased hardness | Less malleable, Less ductile, increased hardness, higher melting point |
Table 1. Comparing Substitutional and Interstitial Alloys, Vaia Original.
An easy way to remember the difference between interstitial and substitutionary alloys is to look at their names!
Substitutionary = Substitute Same Size Atoms
Interstitial = Inconsistent Sized Atoms In spaces
Benefits of Substitutional Alloys
Substitutional alloys are not only chemical concepts, but they played a massive role in shaping human history. Did you know the earliest recorded substitutional alloy has a whole age of history named after it? It’s the Bronze Age! During the Bronze Age, the combination of tin in copper to create bronze proved to be stronger and harder than copper by itself, leading to an explosion in bronze weapons and tools. In modern times, bronze is used in things ranging from screws to statues!
Bronze can be used in statues, weapons, and instruments.
In modern times, the benefits of substitutional alloys can be seen in sterling silver (silver and copper) jewelry that fills up jewelry shops and bronze instruments, statues, and screws. In all of these examples, the broad benefits of substitutional alloys are that they are harder, stronger, and more durable than pure metals! We will investigate these further as we look at the properties of both types of alloys.
Properties of Interstitial and Substitutional Alloys
When alloys are created, they maintain and enhance metallic properties, and these properties can be easily manipulated! So far, we have hinted at the various properties of interstitial and substitutional alloys, such as malleability, durability, and strength. Now, we will look at these properties and their chemical basis in more detail!
Interstitial alloys tend to be:
Less malleable and ductile.
Harder.
More corrosion resistant.
Have increased melting point.
Substitutional alloys tend to:
Remain malleable and ductile.
Be harder.
Be more corrosion resistant.
Have a lower melting point.
Most metals are not as useful in pure form. For example, pure 24 karat gold is much too soft and bendable for jewelry, so usually, gold is alloyed with silver, copper, nickel, and more to strengthen it.
We can see that alloys, regardless of type, often are harder and more corrosion resistant than the pure metal base they have. This is one of the main reasons alloys are formed. Pure metals are chemically reactive and corrode fast. Both alloys can be designed to resist corrosion. An example of this is iron. Iron is very soft, malleable when hot, and quick to rust (corrode). When alloyed with nickel and chromium, it becomes stainless steel which is significantly harder and stronger and does not rust.
Because interstitial alloys contain different-sized atoms inside, the crystal lattice structure is more rigid, making it harder for the atoms to move around. This causes alloys to be harder and less malleable and ductile.
Remember, malleability refers to a metal's ability to be hammered into sheets without breaking, and ductility refers to a metal's ability to be stretched into a thin wire. Both properties essentially show how soft and moldable a metal is.
Substitutional Alloy and Interstitial Alloy Examples
Throughout this explanation, we have mentioned some examples of different alloys! We will end by doing a deep dive into the most common examples of substitutional and interstitial alloys and their uses!
Some common substitutional alloys are:
Bronze, which is typically about 78-95% copper and about 5-22% tin, is used in award metals, musical instruments, screws, and statues.
Sterling silver, usually about 93% silver and 7% copper, is used in jewelry, instruments, and fine dining utensils.
Brass, usually about 60-90% copper and 10-35% zinc, is used in types of musical instruments, doorknobs, and locks.
Some common interstitial alloys are:
Steel, 99% copper and 1% carbon, used in everyday life items such as utensils, roads, bridges
Cast Iron, 96-98% iron and 2-4% carbon, used in cast iron skillets
We have finished our look at alloys. By now, you should be able to differentiate between substitutional and interstitial alloys on an atomic level and their properties! Hopefully, you have learned that many of our everyday life items are created from alloys!
Substitutional and Interstitial Alloys - Key takeaways
- Substitutional alloy forms when elements with atoms of similar atomic radii aremixed, and the atoms of one element replace the atoms of another element.
- Usually made up of metals.
- Bronze and Brass are examples.
- Interstitial alloys form when the elements being mixed have different atomic radii and the smaller atoms fill in the interstitial spaces (holes) in the lattice structure of the larger atoms.
- Usually made up of metals and nonmetals.
- Steel is a prime example.
- Interstitial alloys tend to be less malleable and ductile, while substitutional alloys remain malleable and ductile.
- Both types of alloys can be created to be corrosive resistant, harder, and stronger than the pure metal base.
References
- “6.7A: Substitutional Alloys.” Chemistry LibreTexts, Libretexts, 12 Dec. 2016
- Brown, Theodore L, H E. LeMay, Bruce E. Bursten, Catherine J. Murphy, Patrick M. Woodward, and Matthew Stoltzfus. Chemistry: The Central Science. , 2018.
- AP Chemistry course and exam description, effective fall 2020. (n.d.). May 31st, 2022, from https://apcentral.collegeboard.org/pdf/ap-chemistry-course-and-exam-description.pdf?course=ap-chemistry
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