Alloys

You might have seen many gold ornaments with intricate designs, sometimes embedded with precious stones. Gold is definitely one of the most precious metals on the earth. But, is the gold that is being used to craft jewellery 100% pure?

The answer is no. The gold we use in ornaments is not a pure metal but is a mixture of several other metals.

But, why can't we use pure gold instead? Pure gold is extremely soft which is an advantage to making wonderful designs as it moulds into any shape, but is also a disadvantage as it can be easily deformed. To bring stability, firmness and strength to gold, it is mixed with copper, nickel and zinc, thus producing an 'alloy' of metals. Alloys are thus stronger than individual metals.

In a 22karat gold ornament, around 91.6% is gold while the remaining 8.4%(approx) is usually copper, zinc and nickel. Each of these metals has a specific purpose that adds up to the unique properties of an alloy.

Fig. 1: Gold bullion bars | Public domain | Wikimedia Commons

Let us now look into alloys, how they are made, types and examples.

• We will first explain to you what an alloy is and how they are prepared.
• Then we will go over the types of alloys classified based on the arrangement of atoms.
• We will then look into a few examples of alloys
• We will conclude by describing the properties of alloys.

Alloy Definition

Just like metals, most alloys have metallic bonding.

Alloys are prepared by melting the metals, then mixing them and allowing them to cool, and solidify.

An alloy doesn't necessarily contain only metals. It can also be formed in combination with non-metals like carbon, but a general rule of thumb is at least one of the combining substances must be a metal.

Alloys are stronger and harder as compared to pure metals. Pure metals have a crystal structure just like ionic solids, but the particles in pure metals are not locked in their positions, unlike ionic solids. Cations of metals are arranged in layers among a sea of electrons. These layers slide over one another. Hence, when external pressure is applied to pure metals, they cannot withstand the pressure and bend because of the movement of particles. This property is called the malleability of metals.

Fig. 2: Structure of metal with cations(in red) in sea of electrons | Created using images from Wikimedia Commons

To offer extra strength and hardness to metals, they are alloyed with other substances. Doing so will result in the distortion of the metal's crystal structure locking the particles in place. The particles will sit in their places when external pressure is applied, making them a rigid solid. Hence, the properties of alloys are superior as compared to pure metals.

Types of Alloys

Based on how the atoms are arranged in an alloy, they are classified into two types:

• Substitutional alloy.
• Interstitial alloy.

Substitutional Alloy

Imagine a box of red apples. Pick a few apples randomly and replace them with green apples. The result should look something like this which is similar to the atomic arrangement that you might find in a substitutional alloy.

Fig. 3: Pure metal vs substitutional alloy | Created using the images from wikimedia commons (Shared under public domain)

A substitutional alloy is formed when the size of the atoms of the metal that are being replaced is roughly similar to that of the size of the atoms of the element that is replacing them.

Bronze, an alloy of 88% copper and 12% tin, is an example of a substitutional alloy. Both copper and tin have similar-sized atoms in them. The primary metal is copper whose atoms are replaced by tin atoms.

Interstitial Alloy

Again imagine a box of red apples. Even if they are tightly packed, small spaces will be left between them. If you drop in a handful of blueberries, they occupy the spaces between the apples.

Can you observe the difference between the substitutional and interstitial alloys? The element with a smaller atomic radius will not have to replace the atoms of primary metal. Instead, they just occupy the spaces between the atoms of the primary metals-Interstitial spaces.

Steel is an example of interstitial alloy. Steel contains iron and some amount of carbon. The carbon atoms are smaller than the iron atoms hence slip into the interstices of the iron matrix.

Now that we have understood the types of alloys, let us move on to some examples of alloys.

Examples of Alloys

Given below are a list of some common alloys and their applications.

Properties of Alloys

As discussed in previous sections, alloys are superior in quality compared to individual metals. Let us have a brief look into the properties of alloys:

1. Better strength : The alloy steel which is made up of iron and carbon is tougher than the parent element iron. Alloys are stronger and harder than the parent metals because of the disruption of the metal's crystal structure as discussed in the previous sections.

2. Corrosion-resistance: Another reason to employ alloys is their better performance against corrosion, which is a natural and unwanted process of decaying elements. If you take the example of the alloy bronze, which is a mixture of copper and tin, it is less likely to corrode than the parent element copper itself. Thus, alloys are resistant to corrosion, hence more durable.

3. Low malleability: Although malleability is a useful property of a metal, it can be disadvantageous at times. Consider gold, for instance, an extremely malleable metal so much so that if you make a ring out of its pure form, it easily gets deformed. Extreme malleability will not help retain the shapes of the materials we design. Hence, alloying reduces the malleability to some extent so that the material stays in shape

4. Low Melting points: Alloys have low melting points than the corresponding pure metals. Hence, moulding a metal is easier than moulding a pure metal, thus making the manufacture of goods easier and more economical.

With this, we conclude the topic-Alloys. Now, have a look around your home and school and make a note of the alloys you find. Interview your history teacher about the Bronze Age and how bronze was invented.