Defects in Crystals

There won’t be a crystal with 100% of perfect arrangement. Defects are are advantages in materials most of the time.

1. Point defects
2. Line defects / Dislocations
3. Planar defects
4. Bulk defects - Related to volumetric defects. Common for any materials. Example: crack, holes. Not covered in s1.

Point defects

Related to a single atom.

• Vacancy - Missing parent atom
• Self-interstitial atom - A parent atom entered into an interstitial site
• Interstitial impurity atom - A foreign atom entered into an interstitial site
• Substitutional impurity atom - A foreign atom replaced a parent atom or a vacancy

Line defects

Related to a line of atoms. Three types:

• Edge dislocation - only covered in s1
• Screw dislocation
• Mixed dislocation (combination of above 2)

Edge dislocation

Caused by removal of a half plane.

Half plane

TODO

Slip plane

Plane that is perpendicular to the half plane.

Dislocation line

Intersecting line of half plane and slip plane.

Planar defects

Related to a plane of atoms.

Solidification

The process of conversion liquid to solid. Occurs in 3 steps:

• Nucleation
• Growth of crystals
• Formation of grain structure

Tiny solid particles forms. Called as Nuclei (nothing related to atom’s nucleus). Each nucleus grows bigger and forms a crystal. And then forms grains.

Grain

A group of atoms packed in a particular orientation that is different from that of the neighbor ones.

Each grain is usually in micrometer in size. Grain structure can only be observed through a microscope after careful preparation of samples (microstructure).

Grain boundaries

Boundary between 2 adjacent grains is grain boundary.

Planar defects found in crystalline materials.