Imperfection in Solids

Introduction 

Existence of perfect crystals (without defects) are ideal. Instead, we have kinds of imperfection.

Ques: Why study imperfections in Solids?

• When pure metals alloyed (addition of impurities) like Sterling Silver ↣ 92.5 % Ag and 7.5 % Cu, Cartridge Brass ↣ 70 % Cu and 30 % Zn.
• These impurities play an important role in semiconductors properties, steel composition, etc.
• Atomic defects are responsible for reduction gas pollutants emissions in automobiles.

Ques: How polluting gases converted into non-pollutant gases?

Ans: Molecules of pollutant gases become attached to surface defects of crystalline material. Hence it convert into less-polluting substance.

Types of Defects (Imperfections) in Solids

There are four types of defects:

1. Point (or Zero-D) Defects
2. Linear (or 1-D) Defects 
3. Planar (or 2-D) Defects 
4. Bulk (or 3-D) Defects

1. Point Defects

It is defined as, "defects associated with one or two atomic position". Point defects are further divided into two types:

a. Vacancy & Self Interstitial Defects

• Vacancy Defect  ↣  "type of defect in which an atom is missing from a lattice site i.e normally occupied".
• All solids have vacancy defects, which increases entropy.
• Self Interstitial  ↣  "defect in which extra atoms positioned between atomic sites".

b. Impurities in Solids

It is defined as, "defects in which pure metal consist of more than one type of metal". These impurities are due to

• The extraction and solidification techniques.
• Addition of other metal atoms as impurities (called Alloys).

Solid Solution  ↣  "When a foreign atom is introduced in crystal structure of a metal", either of two cases occurs:

1. Foreign atom react with local atom and form inter-metallic compound.
2. Foreign material (solute or reinforcement material) form a mixture with host (solvent) to form solid solution.

Types of Solid Solution

 I. Substitutional Solid Solution

It is defined as, "Impurity atoms (solute atoms) replaces solvent atoms (host atoms) from lattice site". Factors that determines the degree of solute materials in solvent material are:

1. Atomic Size Factor  ↣  Difference between atomic radii is +- 15%. If difference exceeds 15%, lattice distortion occur.
2. Crystal Structure  ↣  Crystal structure of both metal types must be same (like cubic, hexagon, etc.).
3. Electronegativity  ↣  If electronegativity difference between atoms is greater, they form intermetallic bonds.
4. Valency  ↣  If solute metal has higher valency than solvent, so greater dissolving property.

II. Interstitial Solid Solution

It is defined as, "Solute atoms fills the interstitial sites (called voids)". Some other properties of Interstitial Solid Solution are:

• Atomic diameter of solute atoms must be substantially smaller than solvent atoms.
• If solute atom diameter > interstitial sites  ↣  they produce lattice strains.
• The maximum allowable concentration of solute atoms is 10%.
• If solvent atom has high APF  ↣  allow smaller size atoms.

2. Linear (or 1-D) Defects

It is defined as, "a defect around which some of atoms are misaligned".

• Due to slip (plastic deformation) occurs when stress level > shear strength of perfect crystal.
• Slip (dislocation) motion which is important to explain slip, strain hardening, yield point, fatigue, creep, brittle fracture.

Types of Dislocations

a. Edge Dislocation

When an extra half plane exist in crystal structure, the edge of which terminates within the crystal is called Edge Dislocation.

• Burger Vector  ↣  represent magnitude and direction of distortion by dislocation and is perpendicular to dislocation line.
• Upper part (where extra plane is present) is under compression, lower part is under tension.

b. Screw Dislocation

The dislocation in which lattice itself shear and become offset.

• Burger vector is parallel to dislocation/distortion.

c. Mixed Dislocation

The dislocation in which extra half plane distortion and offset distortion due to applied stresses occur.

• Most in practical cases.

3. Planar (or 2D or Interfacial) Defects

It is defined as, "which occur normally in separate regions of material that have different crystal structure or crystallographic orientation".

• Unit cell  ↣  Building block of crystal.
• Crystal  ↣  aggregates of unit cell translated along coordinate axes.
• Grains  ↣  regions of polycrystalline material where unit cell and crystallographic orientation remain same.

Types of Planar Defects

a. External Surface Defects

External surface of crystalline material itself is considered as a defect because surface atoms react with impurities around (because of having high energy states).

b. Grain Boundary Defects

Grain boundaries exert influence on plastic deformation behavior of polycrystalline material.

Types of grain boundary defects are:

1. High Angle Grain Boundary Defects  ↣  having high angle of misalignment (or mis-orientation).
2. Low Angle Grain Boundary Defects  ↣  having low angle of misalignment (or mis-orientation).
3. Tilt Grain Boundary Defects  ↣  having multiple edge dislocation that naturally causes mis-
   orientation.

c. Twin Grain Boundary

It is a special type of grain boundary across which there is specific mirror lattice symmetry.

Miscellaneous Defects

1. Stacking Faults  ↣  ABC ABC AC faults/defect.
2. Phase Boundary Defect  ↣  a region in which orientation are same but physical and chemical properties are different (so same structure).

References:

• Material from Class Lectures + Book named Materials Science and Engineering: An Introduction by Callister and Rethwick + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube.