Difference Between Substitutional and Interstitial Alloys

The key difference between substitutional and interstitial alloys is that the substitutional alloys form when one metal atom substitutes another metal atom of similar size in the metal lattice whereas interstitial alloys form when small atoms insert into the holes of the metal lattice.

An alloy is a mixture of metals. However, sometimes this mixture could contain non-metals as well. The production of metal alloys involves the mixing of molten metals. There, the size of the metal atoms determines the type of alloy formed; that is, if the metal atoms are of the same size, then the alloy formed is substitutional. If the metal atoms have different sizes, then the resulting alloy is interstitial.

CONTENTS

1. Overview and Key Difference
2. What are Substitutional Alloys
3. What are Interstitial Alloys
4. Side by Side Comparison – Substitutional vs Interstitial Alloys in Tabular Form
5. Summary

What are Substitutional Alloys?

Substitutional alloys are metal alloys that form from atom exchange mechanisms. Here, the metal atoms of a different metal (the other metal mixed to form the alloy) substitute the metal atoms of a metal lattice.

This substitution occurs only if the metal atoms are of similar sizes. Some common substitutional alloys include brass, bronze, etc.  There, copper atoms of the metal lattice substitute with either tin or zinc metal atoms.

What are Interstitial Alloys?

Interstitial alloys are metal alloys that form from interstitial mechanism. Furthermore, this mechanism involves the insertion of small atoms into the holes of metal lattices. A metal lattice contains large metal atoms in a network structure. There are also delocalized electrons surrounding the metal atoms. Therefore, when a molten metal mixes with a different metal having small atoms, an interstitial alloy forms. However, these small atoms should be small enough to insert into the holes of the lattice.

Some examples of small atoms that are capable of inserting into a metal lattice include hydrogen, carbon, boron, and nitrogen. A common example of an interstitial alloy is steel. Steel contains iron, carbon and some other elements. There are no substitutions occurring when forming an interstitial alloy because the atoms mixed are not large enough to replace a metal atom.

What is the Difference Between Substitutional and Interstitial Alloys?

Substitutional vs Interstitial Alloys
Substitutional alloys are metal alloys formed from atom exchange mechanisms.	Interstitial alloys are metal alloys formed from interstitial mechanism.
Formation Mechanism
Forms via atom exchange mechanism.	Forms via the interstitial mechanism.
Size of the Atoms
In this alloy formation, a molten metal is mixed with another molten metal having similar atomic sizes.	In this alloy formation, a molten metal is mixed with a compound having small atoms that are capable of inserting into the holes of metal lattice.
Common Examples
Brass and bronze	Steel

Summary – Substitutional vs Interstitial Alloys

Alloys are mixtures of metals and other non-metals. These alloys have improved properties than individual metals. There are two types of alloys namely, substitutional alloys and interstitial alloys. The difference between substitutional and interstitial alloys is that substitutional alloys form when one metal atom substitutes another metal atom of similar size in the metal lattice whereas interstitial alloys form when small metal atoms insert into the holes of the metal lattice.

Reference:

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

Image Courtesy:

1.’Alloy Substitutional’By John Aplessed – Own work, (Public Domain) via Commons Wikimedia 
2.’Alloy Interstitial’By John Aplessed – Own work, (Public Domain) via Commons Wikimedia