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CRYSTAL STRUCTURES

CRYSTAL STRUCTURES
• Have you ever wondered how atoms assemble into solid structures? • How does the density of a material depend on its structures?

CRYSTAL STRUCTURES
• Solid materials can broadly be classified as crystalline and non crystalline (amorphous) solids. • In crystalline solid the arrangement of atoms is in a periodically repeating manner whereas no such patterns are found in a non-crystalline solid.

CRYSTAL STRUCTURES
• 2 types of crystalline solids: a) Single crystal : the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption. b) Polycrystalline solid : a collective aggregate of many crystals separated by well defined boundaries.

CRYSTAL STRUCTURES
• As a general rule, most metals are crystalline, while ceramics and polymers may be either crystalline or non-crystalline.

Differences between crystalline and non-crystalline solids
Characteristic
Atomic arrangements

Crystalline
Regular and orderly manner in all three dimensions

Non-crystalline
Irregular

Fracture mechanism Ductile manner. Solids Brittle manner. behave elastically up to Solids do not their yield points behave elastically

Tensile strength
Dislocation defects

High
Possible

Low
Not possible

Unit cell
• A crystalline solid consists of a number of crystals. A crystal structure can be considered as consisting of tiny blocks which are repeated in three dimensional pattern. • Each of the tiny block (called a unit cell) is actually made from the arrangement of a small group of atoms.

Crystal Structure
• Types of crystal structure: 1) Simple cubic 2) Body centered cubic (BCC) 3) Face centered cubic (FCC) 4) Hexagonal closed packed (HCP)

Simple Cubic

BODY-CENTERED CUBIC CRYSTAL STRUCTURE

FIGURE 3.2 For the body-centered cubic crystal structure,…...

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