Interstitial and Substitutional Alloys

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 and Substitutional Alloys Interstitial and Substitutional Alloys

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Table of contents

    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.

    1. 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.

    2. 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!

    Substitutional and Interstitial Alloys-explain interstitial and substitutional alloys atomic arrangement-StudySmarterFig. 1: The atomic arrangement of interstitial steel and substitutional bronze alloy, StudySmarter 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, StudySmarter 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!

    Substitutional and Interstitial Alloys-bronze benefits of substitutional alloys -StudySmarterBronze 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

    1. “6.7A: Substitutional Alloys.” Chemistry LibreTexts, Libretexts, 12 Dec. 2016
    2. Brown, Theodore L, H E. LeMay, Bruce E. Bursten, Catherine J. Murphy, Patrick M. Woodward, and Matthew Stoltzfus. Chemistry: The Central Science. , 2018.
    3. 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
    Frequently Asked Questions about Interstitial and Substitutional Alloys

    What is a substitutional alloy?

    A substitutional alloy forms when elements with atoms of similar atomic radii are mixed.  The atoms of one element will substitute into the crystal lattice structure of the other element. 

    What are interstitial alloys? 


    Interstitial alloys form when the elements being mixed have different atomic radii. The smaller atoms fill in the interstitial spaces (holes)  in the lattice structure of the larger atoms.  

    How to tell if an alloy is interstitial or substitutional?

    To tell if an alloy is interstitial or substitutions, look at the atomic arrangement, size of atomic radii,  and the type of elements being mixed. Interstitial alloys are usually composed of metals and nonmetals and the arrangement is large metal atoms with smaller atoms between them. Substitutional alloys usually are two metals and the atomic arrangement shows different atoms of the same size in the lattice structure. 

    What is one example of interstitial alloys?

    An example of an interstitial alloy is steel. Steel is composed of iron and carbon. Iron atoms are significantly larger than carbon atoms, so carbon atoms fill in the spaces between the crystal lattice of iron atoms. 

    What is the difference between substitutional and interstitial alloys?

    The main difference between substitutional and interstitial alloys is how they form. Interstitial alloys form via small atoms filling in the interstitial spaces (holes) of larger metal atoms in a crystal lattice. Substitutional alloys form via atom substitution when the metals being mixed have the same size atoms and substutite for one another in the crystal lattice structure. 

    Test your knowledge with multiple choice flashcards

    A substitutional alloy forms between elements with ____ size atoms, and an interstitial alloy forms between elements with ___ size atoms. 

    True or False. Interstitial alloys usually form from a mixture of exclusively metals. 

    True or false. Substitutional alloys form when elements have similar sized atoms

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