What is a substitutional alloy definition structure rules and examples
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 in Solid State Chemistry
1. What is a substitutional alloy?
A substitutional alloy is an alloy in which atoms of one metal replace or substitute the atoms of another metal in the crystal lattice. In this type of alloy, the solute metal atoms occupy the normal lattice positions of the host metal.
- The two metals must have similar atomic radii (usually within 15%).
- They often have the same crystal structure.
- Example: In brass, zinc (Zn) atoms substitute copper (Cu) atoms in the copper lattice.
2. How is a substitutional alloy formed?
A substitutional alloy is formed by melting two compatible metals together and allowing them to solidify into a single-phase solid solution. During solidification, atoms of the solute metal replace host metal atoms in the crystal lattice.
- Step 1: Heat the metals above their melting points.
- Step 2: Mix thoroughly in the liquid state.
- Step 3: Cool the mixture to form a solid solution.
3. What is the difference between substitutional and interstitial alloys?
The main difference is that substitutional alloys replace host atoms in the lattice, while interstitial alloys place smaller atoms in the spaces (interstices) between metal atoms.
- Substitutional alloy: Similar-sized atoms replace each other (e.g., brass: Cu–Zn).
- Interstitial alloy: Small atoms fit into gaps of a metal lattice (e.g., steel: carbon in iron).
- Substitutional alloys usually require similar atomic radii.
- Interstitial alloys require much smaller solute atoms.
4. What are some examples of substitutional alloys?
Common examples of substitutional alloys include brass, bronze, and sterling silver.
- Brass: Copper (Cu) and zinc (Zn).
- Bronze: Copper (Cu) and tin (Sn).
- Sterling silver: Silver (Ag) and copper (Cu).
- Cupronickel: Copper (Cu) and nickel (Ni).
5. Why must atomic sizes be similar in substitutional alloys?
Atomic sizes must be similar to prevent distortion of the crystal lattice. If the difference in atomic radii exceeds about 15%, the structure becomes unstable and a solid solution is unlikely to form.
- Large size differences create lattice strain.
- Excessive strain reduces alloy stability.
- This requirement is part of the Hume–Rothery rules.
6. What are the Hume–Rothery rules for substitutional alloys?
The Hume–Rothery rules are guidelines that predict whether two metals will form a substitutional solid solution.
- Atomic radius difference should be less than about 15%.
- Both metals should have the same crystal structure (e.g., FCC, BCC).
- Electronegativity values should be similar.
- Valency should be the same or compatible.
7. How does a substitutional alloy affect the properties of a metal?
A substitutional alloy typically increases strength and hardness while often reducing ductility. The substituted atoms disrupt the regular metal lattice and hinder dislocation movement.
- Increased tensile strength.
- Improved corrosion resistance (e.g., Cu–Ni alloys).
- Modified electrical conductivity.
- Enhanced mechanical durability.
8. Is brass a substitutional alloy?
Yes, brass is a substitutional alloy in which zinc atoms replace copper atoms in the copper lattice.
- Main components: Copper (Cu) and zinc (Zn).
- Zinc atoms occupy lattice positions normally filled by copper.
- Brass is stronger and harder than pure copper.
9. What is meant by a solid solution in substitutional alloys?
A solid solution is a homogeneous solid mixture in which solute atoms are uniformly distributed within the solvent metal lattice. In substitutional alloys, solute atoms replace solvent atoms at regular lattice points.
- The alloy has a single crystalline phase.
- Composition may vary within certain limits.
- No separate phases are visible under normal conditions.
10. Can substitutional alloys form between non-metals?
Substitutional alloys generally form between metals, not non-metals. Alloys are defined as mixtures containing at least one metal, and substitutional alloying specifically involves metal atoms replacing other metal atoms in a metallic lattice.
- Non-metals typically form covalent or ionic compounds instead.
- Metal–non-metal combinations may form interstitial alloys (e.g., carbon in iron).
- True substitutional alloys require metallic bonding.
