Difference Between Substitutional and Interstitial Alloys

Main Difference – Substitutional vs Interstitial Alloys

An alloy is a mixture of metals or a mixture of metals plus some other elements. There are two main types of alloys as substitutional alloys and interstitial alloys. When a molten metal is used in the production of alloys, the size of the atoms will determine the type that is going to be formed. If the atoms of the metals have relatively similar sizes, a substitutional alloy type is formed, but if one type of metal atoms are smaller than the other type, an interstitial alloy is formed. The main difference between substitutional alloys and interstitial alloys is that substitutional alloys are formed by substituting one metal atom for another metal atom of similar size whereas interstitial alloys are formed by inserting smaller atoms into holes of the metal lattice.

Key Areas Covered

1. What are Substitutional Alloys
     – Definition, Formation, Examples
2. What are Interstitial Alloys
    – Definition, Examples, Mechanism of Formation
3. What is the Difference Between Substitutional and Interstitial Alloys
     – Comparison of Key Differences

Key Terms: Alloy, Atoms, Bronze, Interstitial Alloys, Interstices, Metal, Metal Lattice, Substitutional Alloys

What are Substitutional Alloys

Substitutional alloys are metal alloys formed by substituting one metal atom for another metal atom of similar size. To form an alloy, the two substances (metals) should be mixed. The type and size of the substances that are mixed may determine the type of alloy that is formed. In order to form a substitutional alloy, the atoms of the two metals should be of similar size.

Figure 1: Comparison Between Pure Metal and Substitutional Alloy

A substitutional alloy is formed via atom exchange mechanism. Here, some metal atoms of a metal lattice are replaced or substituted by metal atoms of the other metal. But both types of metal atoms should be in similar or nearly similar sizes for this substitution to occur. The percentage of the difference between atoms should not exceed 15%.

A well-known alloy that is a substitutional alloy is Bronze. Bronze is an alloy that mainly consists of copper and tin metal. But sometimes, other elements are also mixed with copper to make bronze. Such elements include arsenic, phosphorous, aluminium, manganese and silicon. Copper and tin metal atoms are nearly similar in their sizes.

What are Interstitial Alloys

Interstitial alloys are metal alloys formed by inserting smaller atoms into holes of the metal lattice. A metal lattice is a metallic structure that consists of a giant lattice of positively charged ions and delocalized outer electrons. An interstitial alloy is formed when a metal is mixed with another substance composed of small atoms (having small radii) that can reside in the holes of the metal lattice.

Figure 2: Comparison of a Pure Metal with an Interstitial Alloy

Some examples of such small atoms include hydrogen, carbon, boron, and nitrogen. The holes in a metal lattice are the spaces between the metal atoms. Steel is a good example of an interstitial alloy. Steel is a metal alloy containing iron, carbon and some other elements. Here, the small carbon atoms fill the spaces between large iron atoms. Because of this tightly packed structure of steel, it is hard and very strong.

The formation of an interstitial alloy is via the interstitial mechanism. Here, one type of atom is considerably smaller than the other type. Since these atoms are much smaller, they cannot replace atoms of the metal lattice as for substitutional alloys. Instead, they get trapped in spaces the large atoms of the metal lattice. These spaces are called interstices.

Difference Between Substitutional and Interstitial Alloys

Definition

Substitutional Alloys: Substitutional alloys are metal alloys formed by substituting one metal atom for another metal atom of similar size.

Interstitial Alloys: Interstitial alloys are metal alloys formed by inserting smaller atoms into holes of the metal lattice.

Sizes of Atoms

Substitutional Alloys: Substitutional alloys contain metal atoms having similar or nearly similar sizes.

Interstitial Alloys: Interstitial alloys contain large metal atoms and small atoms of different element(s).

Mechanism of Formation

Substitutional Alloys: Substitutional alloys are formed via atom exchange mechanism.

Interstitial Alloys: Interstitial alloys are formed via interstitial mechanism.

Examples

Substitutional Alloys: A well-known example of a substitutional alloy is bronze.

Interstitial Alloys: Steel is an interstitial alloy.

Conclusion

Alloys are substances that are formed by mixing two or more metal components along with some other compounds. There are two types of alloys based on the formation of the alloy: substitutional alloys and interstitial alloys. The main difference between substitutional and interstitial alloys is that substitutional alloys are formed by substituting one metal atom for another metal atom of similar size whereas interstitial alloys are formed by inserting smaller atoms into holes of the metal lattice.

References:

1. “Chemistry.” Annenberg Learner, Available here.
2. “6.7A: Substitutional Alloys.” Chemistry LibreTexts, Libretexts, 12 Dec. 2016, Available here.
3. “6.7B: Interstitial Alloys.” Chemistry LibreTexts, Libretexts, 21 July 2016, Available here.

Image Courtesy:

1. “Alloy atomic arrangements showing the different types” By Zaereth – Own work (CC0) via Commons Wikimedia