1. What is crystal structure ?
Most solid materials, especially metals, have atoms arranged in a regular, repeating three-dimensional pattern known as a crystal structure. Crystal structure arrangement is mainly responsible for the properties Ex. Strength, Hardness. of the solid. Solids exist in nature in two principal forms: crystalline and non-crystalline (amorphous), which differ substantially in their properties. Most crystalline solids are made up of millions of tiny single crystals called grains which constitute what is called microstructure and are said to be polycrystalline. These grains are oriented randomly with respect to each other.
Table of Contents
2. Crystalline and Non-Crystalline Structure
2.1 Crystalline Structure
A crystalline structure is the regular, repeating three-dimensional arrangement of atoms, ions, or molecules in a solid. This orderly arrangement forms a crystal lattice, in which each particle occupies a fixed position relative to its neighbors.
Crystalline materials exhibit long-range order, meaning the atomic arrangement repeats throughout the entire material.
2.2 Amorphous (Non-Crystalline) Structure
- No long-range order
- Example: Glass, plastics
3. Space Lattice and Unit Cell
3.1 Space Lattice
A space lattice is a three-dimensional arrangement of points representing atomic positions.
3.2 Unit Cell
- The smallest repeating unit of a crystal lattice
- Repetition of unit cell forms the entire structure
Types of Unit Cells
- Primitive cell
- Body-centered cell
- Face-centered cell
4. Crystal Systems
There are seven crystal systems based on axial lengths and angles:
| System | Axes Relation |
|---|---|
| Cubic | a = b = c, ฮฑ = ฮฒ = ฮณ = 90ยฐ |
| Tetragonal | a = b โ c |
| Orthorhombic | a โ b โ c |
| Hexagonal | a = b โ c, ฮณ = 120ยฐ |
| Monoclinic | a โ b โ c, ฮฒ โ 90ยฐ |
| Triclinic | a โ b โ c, ฮฑ โ ฮฒ โ ฮณ |
| Rhombohedral | a = b = c, ฮฑ = ฮฒ = ฮณ โ 90ยฐ |
5. Common Crystal Structures of Metals
5.1 Body-Centered Cubic (BCC)
4
Characteristics
- Atoms at 8 corners + 1 atom at center
- Atoms per unit cell = 2
Examples
- Iron (ฮฑ-Fe)
- Chromium
- Tungsten
Properties
- Strong but less ductile
5.2 Face-Centered Cubic (FCC)
4
Characteristics
- Atoms at 8 corners + 6 face centers
- Atoms per unit cell = 4
Examples
- Aluminum
- Copper
- Gold
Properties
- Highly ductile
- Good formability
5.3 Hexagonal Close-Packed (HCP)
4
Characteristics
- Hexagonal arrangement
- Atoms per unit cell = 6
Examples
- Magnesium
- Zinc
- Titanium
Properties
- Less ductile than FCC
- Strong structure
6. Atomic Packing Factor (APF)
- Fraction of volume occupied by atoms
| Structure | APF |
|---|---|
| BCC | 0.68 |
| FCC | 0.74 |
| HCP | 0.74 |
7. Coordination Number
Number of nearest neighboring atoms:
| Structure | Coordination Number |
|---|---|
| BCC | 8 |
| FCC | 12 |
| HCP | 12 |
8. Crystal Defects
8.1 Point Defects
- Vacancy defect
- Interstitial defect
8.2 Line Defects
- Dislocations
8.3 Surface Defects
- Grain boundaries
9. Grain Structure
- Polycrystalline materials consist of many grains
- Grain size affects strength and hardness
10. Importance of Crystal Structure
- Influences deformation behavior
- Determines mechanical properties
- Affects electrical and thermal properties
11. Applications
- Material selection in engineering
- Heat treatment processes
- Alloy design
- Failure analysis