Defects in Crystals

Question 1

What are the types of defects?

Answer 1

0D, 1D, 2D, 3D

Question 2

What is a 0D defect?

Answer 2

A point defect: atoms missing or in irregular places within a lattice i.e. lattice vacancies

Question 3

What is a 1D defect?

Answer 3

Linear defect: groups of atoms in irregular positions i.e. screw and edge dislocations

Question 4

What is a 2D defect?

Answer 4

Planar defects: the interfaces between homogenous regions of the material i.e. grain boundaries and external surfaces

Question 5

What is a 3D defect?

Answer 5

Volume defects: extended defects i.e. pores and cracks

Question 6

What is always present (absent) in crystals?

Answer 6

Vacancies

Question 7

Why are there more vacancies at higher temperatures?

Answer 7

Atoms are frequently and randomly changing position leaving behind empty lattice sites.

Question 8

What is the Boltzmann distribution?

Answer 8

The equation used to estimate the number of vacancies in a lattice in relation to temperature.

Question 9

What is the Boltzmann equation?

Answer 9

Vacancies = nominal vacancies * e^(-Q/KbT)

Question 10

What is Q?

Answer 10

The energy needed to form a vacant lattice site in a perfect crystal (J/mol)

Question 11

What is Kb?

Answer 11

The Boltzmann constant, 1.38*10^-23 J/atomK

Question 12

What is T?

Answer 12

Temperature (Kelvin)

Question 13

What are the types of point defects?

Answer 13

vacancy, self-interstitial (uncommon), interstitial impurity, substitutional impurities (smaller and larger atoms)

Question 14

What defines a very pure metal?

Answer 14

1 impurity per million atoms

Question 15

What is added to iron to produce steel?

Answer 15

Carbon

Question 16

What is added to silicon to change its electrical properties?

Answer 16

Boron

Question 17

What are alloys?

Answer 17

Deliberate mixtures of metals

Question 18

What is a factor for high solubility of substitutional impurities?

Answer 18

1. Crystal structures of solute and solvent should be the same
2. Electro-negativities of solute and solvent should be similar
3. Atomic size factor - atoms need to “fit” ⇒ solute and solvent atomic radii should be within ~ 15%.

Question 19

What is a factor for interstitial impurities?

Answer 19

1. Atomic radius of solute should be significantly smaller than solvent
   (For fcc, bcc, hcp structures the voids (or interstices) between the host atoms are relatively small ⇒ atomic radius of solute should be significantly less than solvent)
2. Maximum solute concentration =< 10%

Question 20

What happens if too much of a solute is added to a lattice?

Answer 20

The second phase: new compounds/structures are formed or form local precipitates

Question 21

What is dislocation core?

Answer 21

The area of distortion created in the lattice structure

Question 22

What is dislocation?

Answer 22

The defect that produce distortion in the lattice situated around a line

Question 23

What are the types of dislocation?

Answer 23

Edge (Taylor), screw (Burgers), mixed

Question 24

What causes the formation of a dislocation?

Answer 24

concentration of vacancies, plastic deformation of crystal

Question 25

What is the burgers vector?

Answer 25

The magnitude and direction of the distortion within a lattice dislocation. Edge Dislocations (or TAYLOR)

Question 26

Which dislocation is parallel to the direction of crystal dislocation?

Answer 26

Screw.

Question 27

What is grain boundaries?

Answer 27

The atomic mismatch within the region where grains meet.

Question 28

What properties does a 3d defect affect?

Answer 28

External changes i.e Mechanical changes and optical properties

Question 29

Why are defects important?

Answer 29

Defects have a profound impact on the various properties of materials

Question 30

What is processing?

Answer 30

Processing allows the required properties to be achieved without changes in the composition of the material, but just by manipulating the crystal defects. Control (and intentional introduction) of defects is in the core of many types of material processing.

Question 31

What is the definition of impurities?

Answer 31

Atoms which are different from the host. All real solids are impure.

Question 32

What are solid solutions made from?

Answer 32

Solid solutions are made of a host (the solvent or matrix) which dissolves the minor component (solute). The ability to dissolve is called solubility.

Question 33

Give an example of 3D defect

Answer 33

Pores and cracks