Understanding the Substitutional Alloy Definition
When a molten metal is mixed with another substance, one of two mechanisms can occur, resulting in the formation of an alloy:
Atom exchange or
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)
FAQs on Substitutional Alloy
1. What is a substitutional alloy as defined in the CBSE Class 12 Chemistry syllabus?
A substitutional alloy is a type of metallic alloy where atoms of one element are replaced by atoms of another element within the crystal lattice. For this to happen, the atoms of the two metals must be of similar size. The new atoms occupy the original lattice sites (or atomic positions) of the host metal, forming a solid solution.
2. What are some common examples of substitutional alloys and their uses?
There are several important substitutional alloys used in various applications. Some key examples include:
- Brass: An alloy of copper and zinc, used in plumbing fittings, musical instruments, and decorative items due to its corrosion resistance and acoustic properties.
- Bronze: An alloy primarily of copper and tin, known for its hardness and durability. It is used in sculptures, bearings, and medals.
- Sterling Silver: An alloy of silver and copper. The copper atoms substitute silver atoms, making the material much harder and more suitable for jewellery and cutlery.
- Cupronickel: An alloy of copper and nickel, highly resistant to corrosion from seawater, making it ideal for marine hardware and coins.
3. What is the primary difference between how substitutional and interstitial alloys are formed?
The primary difference lies in the placement of the alloying atoms. In a substitutional alloy, atoms of the solute element are large enough to replace the solvent atoms in their lattice positions. In contrast, an interstitial alloy is formed when very small atoms (like carbon, hydrogen, or nitrogen) fit into the empty spaces, or interstices, between the larger metal atoms in the crystal lattice without displacing them.
4. What conditions must be met for two metals to form an ideal substitutional alloy?
For two or more metals to form a stable substitutional alloy, certain conditions, often referred to as the Hume-Rothery rules, should be met:
- Similar Atomic Radii: The difference in the atomic radii between the two types of atoms should be less than 15%.
- Same Crystal Structure: The metals should crystallise in the same type of lattice structure for better solubility.
- Similar Electronegativity: The electronegativity of the elements should be very close to prevent the formation of an intermetallic compound.
- Same Valence: Metals with the same number of valence electrons are more likely to form a substitutional solid solution.
5. How does replacing atoms in a metal lattice affect the properties of a substitutional alloy?
Replacing atoms disrupts the perfect, uniform arrangement of the original metal's crystal lattice. The presence of different-sized atoms creates internal stress and makes it more difficult for layers of atoms to slide past one another. This change typically results in increased hardness, greater tensile strength, and reduced malleability and ductility compared to the pure parent metals.
6. Is steel a substitutional or an interstitial alloy? Explain with an example.
This is a common point of confusion. Basic carbon steel is primarily an interstitial alloy because small carbon atoms occupy the voids in the iron crystal lattice. However, many modern steels are more complex. For example, stainless steel is both interstitial and substitutional. It contains carbon in its interstices (interstitial) and also has chromium and nickel atoms that substitute for iron atoms in the lattice (substitutional). This combination gives it both strength and corrosion resistance.
