Chapter 14.3, Problem 3RQ

Interpretation Introduction

Interpretation: The difference between substitutional alloy and interstitial alloy needs to be explained.

Concept introduction: Because of the nature of the metallic crystal, other elements can be

inserted into the crystal lattice relatively easily to produce substances called alloys.

An alloy can be defined as a substance that consists of two or more elements and has superior metallic properties. There are mainly two types of alloys (a) substitutional alloy and (b) interstitial alloy.

Answer to Problem 3RQ

The distinction between substitutional alloy and interstitial alloy on the basis of size of the atom introduces in metallic crystal to make the alloy.

Explanation of Solution

In a substitutional alloy, other metal atoms of similar sizes replace some of the host metal. As a general rule atomic radius of two metals should not differ by more than 15%. For example, brass is composed of zinc and copper where one- third of the atoms in the host copper metal have been replaced by zinc atoms.

An interstitial alloy is formed when atoms much smaller than the host atoms occupies the interstices (holes) of the closely packed metal atoms. Steel is an example of interstitial alloy which contains carbon atoms in the “holes” of an iron crystal.

The smaller carbon (a nonmetal) atoms forms covalent bond with relatively higher size iron metal atoms and thus increases the hardness and strength of an alloy but decreases the ductility.

Conclusion

The distinction between substitutional alloy and interstitial alloy on the basis of size of the atom introduces metallic crystal to make the alloy.