Defects of crystalline materials

Question 1

Point defects

Answer 1

Position of atoms
Vacancies
Self intersitials
Impurities in solid (substitutional and intersitial)

Question 2

Linear defects

Answer 2

one dimensional defects
edge dislocations
Screw dislocations

Question 3

Vacancies

Answer 3

Atom missing from lattice site
crystalline materials may contain many vacancies which increases the randomness of the crystal lattice

Question 4

How do vacancies influence material properties

Answer 4

Controls rate of diffusion of atoms in crystals
slows down the flow of electrons in a material
controls propagation of the edge dislocations

Question 5

Self interstitial defects

Answer 5

Atom crowded into an interstitial site
An atom at interstitial site substantially larger then space causing distortions in the surrounding lattice
rare in comparison to vacancies

Question 6

Impurities in solids-> alloys

Answer 6

Impurities are intentionally added to modulate the properties of the material
ex. Cu added to Au and Ag,
Cu-Zn brass
Cu-Sn bronze
Cr added to Fe to make steels stainless

Question 7

How does adding impurity atoms affect metal atoms

Answer 7

Forms a solid solution

Question 8

What are solid solutions made of?

Answer 8

Solvent: Major Concentration
Solute: Minor Concentration

Question 9

Rules of solid solubility

Answer 9

atoms must have a similar atomic radii (+_15%)
similar electronegativity
similar crystal structure
Same charge
solute (impurity) with higher valence dissolves more easily in solvent atoms
increased temp increases solubility

Question 10

Substitutional impurities

Answer 10

Solute /impurity replaces or substitutes host atom

Question 11

Interstitial Impurities

Answer 11

Solute/Impurity fills voids or interstices among host atoms
Interstitial atoms smaller than host atoms

Question 12

What happens if the interstitial atom is larger than the interstitial space?

Answer 12

Lattice strains on adjacent host atoms

Question 13

What are linear defects or dislocations?

Answer 13

-One-dimensional defects where some atoms are misaligned

Question 14

What happens when load is applied to linear defects

Answer 14

movement of dislocations causes plastic deformations

Question 15

How does the type movement of dislocations effect a material

Answer 15

Free movement- Ductile (metals)
Restricted- Brittle (Ceramic)

Question 16

What happens when dislocations move to the grain boundaries

Answer 16

material breaks

Question 17

Edge dislocations

Answer 17

Extra half-plane of atoms which distorts the nearby planes of atoms

Question 18

Where are edge dislocations located

Answer 18

perpendicular to the line of dislocation

Question 19

How do edge dislocations affect grain boundaries?

Answer 19

Moves to grain boundaries as a result of applied shear stress
Causes plastic deformation and ultimately fracture

Question 20

Screw dislocations

Answer 20

A shift of atoms parallel to the line of dislocation
Looks like steps in a crystal structure
moves in a spiral or helical path due to unzipping motion

Question 21

Mixed dislocations

Answer 21

Combination of both edge and screw dislocations

Question 22

How do dislocations affect metals

Answer 22

Dislocations move easily due to non-directional metallic bonds
Metals are more ductile and break easily

Question 23

How can you heal dislocations of metals

Answer 23

Annealing
makes material soft and ductile

Question 24

How do dislocations affect ceramics?

Answer 24

Ceramics are covalently or ionically bonded
highly directional strong bonds
restricted movements of dislocations
Brittle

Question 25

Interfacial defects

Answer 25

two-dimensional defect that occurs between boundaries
boundaries: two separate regions in a material with different crystal structures

Question 26

What are the areas where interfacial defects can occur?

Answer 26

External surfaces
grain boundaries
phase boundaries

Question 27

How do impurities at a grain boundary affect a material?

Answer 27

Makes the material brittle