Substitutional Alloy

When a molten metal is mixed with another substance, one of two mechanisms can occur, resulting in the formation of an alloy:

1. Atom exchange or

2. Interstitial mechanism.

The relative size of each element in the mixture determines which mechanism will occur. When the atoms are of comparable size, the atom exchange method is usually used, in which some of the atoms composing the metallic crystals are substituted with atoms of the other constituent. This is referred to as a substitutional alloy. Bronze and brass are examples of substitutional alloys in which some of the copper atoms are substituted with tin or zinc atoms.

Definition of Alloy with an Example

An alloy is defined as a substance formed by the combination of two or more metals. Metals and other elements can also be combined to form alloys. The properties of an alloy are frequently quite different from the properties of its constituents. Alloys frequently have greater strength and hardness than pure metals. Red gold is an example of an alloy formed by the combination of copper and gold. White gold is another important gold alloy formed by the combination of silver and gold.

Types of Alloys

There are mainly two types of Alloys. They are substitutional and interstitial alloys. These two types are discussed in the following content.

Substitutional Alloy

The relative size of each element in the mixture determines which mechanism will occur. When the atoms are of comparable size, the atom exchange method is usually used, in which some of the atoms composing the metallic crystals are substituted with atoms of the other constituent. This is referred to as a substitutional alloy. Bronze and brass are examples of substitutional alloys in which some of the copper atoms are substituted with tin or zinc atoms.

Interstitial Alloy

Since one atom is usually much smaller than the other in the interstitial mechanism, it cannot successfully replace an atom in base metal crystals. The smaller atoms become trapped in the interstices, which are the spaces between the atoms in the crystal matrix. This is known as an interstitial alloy.

Steel is an example of an interstitial alloy because the very small carbon atoms fit into the iron matrix's interstices. Since the carbon atoms fit into the interstices, stainless steel is a combination of interstitial and substitutional alloys, but some of the iron atoms are replaced with nickel and chromium atoms.

Characteristics of Alloys

Every alloy has unique properties. The properties of an alloy differ from those of the individual metals from which it is made. Some alloy properties are listed below.

• Alloys have a higher hardness than their constituent metals.

• Alloys are more corrosion resistant than pure metals.

• Alloys are more durable than the metals from which they are made.

• Alloys have lower electrical conductivity than pure metals.

• Alloys have a lower melting point than the metals they are made from.

• Alloys are more ductile than the constituent metals.

Important Questions

1. Aluminium is a reactive metal but is still used for packing food articles. Explain

Ans. Aluminium is a strong and inexpensive metal. It is also a good heat conductor. However, it is extremely reactive. When exposed to moist air, it forms a thin impervious layer of aluminium oxide on its surface (Al₂O₃). This layer prevents moist air from coming into contact with the fresh metal, protecting it from further damage or corrosion. As a result of the formation of this protective layer of Al2O3

, aluminium becomes corrosion resistant. Because of this, despite being a highly reactive metal, aluminium is still used in food packaging.

2. What are the properties of copper alloy?

Ans. Copper is a durable, ductile, and malleable metal. Copper's properties make it ideal for tube forming, wire drawing, spinning, and deep drawing. Copper and its alloys have the following properties:

• Outstanding heat conductivity

• Outstanding electrical conductivity

• Excellent corrosion resistance

• Excellent biofouling resistance

• Excellent machinability

• At cryogenic temperature mechanical and electrical properties are retained

• Non-magnetic

Conclusion

Alloys are more corrosion resistant than pure metals. Metals in their pure form are chemically reactive and easily corroded by atmospheric gases and moisture. Alloying a metal increases its inertness, which increases its corrosion resistance. Alloys are used in our daily lives to improve our quality of life and products. For example, 7075 alloy is used in the manufacture of aeroplanes. It is made up of copper, magnesium, and zinc for added strength.

Multiple Choice Questions

1. Which of the following is an alloy of iron?

a) Vitallium

b) Brass

c) Invar

d) Solder

Answer: (c)

2. What are alloys with two components called?

a) Binary alloy

b) Ternary alloy

c) Quaternary alloy

d) There is no name given to an alloy with two components

Answer: (a)

3. Alloy is an example of

(a) Colloidal Solution

(b) Emulsion

(c) Solid Solution

(d) Heterogeneous solution

Answer: (c)