Materials- Metals: Defects

Question 1

What are defects?

Answer 1

Imperfections in the crystal lattice

Question 2

Are properties of materials influenced by the presence of defects?

Answer 2

Yes. They can change with the number of particular defects

Question 3

What are point defects and give 4 examples?

Answer 3

Defects associated with one or two atomic positions.

E.g vacancies, self-interstitial atoms, interstitial solute atoms, substitutional atoms

Question 4

What is a vacancy?

Answer 4

A lattice site that would normally be occupied from which an atom is missing.

Question 5

Formula for number of vacancies in a given quantity of material

Answer 5

Nv=Ne^(-Qv/kT)
v is subscript 
Nv is equilibrium number of vacancies
N is total number of atomic sites
Qv is energy required for the formation of a vacancy (J or eV)
T is absolute temperature (K)
k is Boltzmann’s constant (J/atom/K)

Question 6

What is a self-interstitial?

Answer 6

An atom from the crystal that is crowded into an interstitial site (a small void space that under ordinary circumstances is not occupied)

Question 7

Are vacancies or self-interstitials more common?

Answer 7

Vacancies

Question 8

What is a solid solution?

Answer 8

A homogeneous crystalline phase that contains two or more chemical species

Question 9

What is a substitutional solid solution?

Answer 9

A solid solution where the solute atoms replace or substitute for the host atoms. The two types of atoms are of a similar size

Question 10

What is an interstitial solid solution?

Answer 10

A solid solution where relatively small solute atoms occupy interstitial positions between the solvent or host atoms.

Question 11

How to calculate the size of the interstitial site at the (1/4,1/2,0) location of a BCC material

Answer 11

Worked out using right angle triangles. It is the gap above and between the two bottom atoms of the cube in a unit cell. The atomic radius plus interstitial radius squared is equal to have the unit cell length squared plus a quarter of the unit cell length squared.

Question 12

How to calculate the size of the interstitial site at the (1/2, 0, 0) location in FCC materials

Answer 12

It is between the nearest left two atoms where one is above the other. The interstitial radius doubled plus the atomic radius doubled equals the unit cell length.

Question 13

Give 4 examples of bulk defects

Answer 13

Bubbles, precipitates, pores, cracks

Question 14

Give an example of a line defect

Answer 14

Dislocation

Question 15

What is a dislocation.

Answer 15

A linear or one-dimensional defect around which some of the atoms are misaligned

Question 16

What is dislocation density in a material?

Answer 16

The total dislocation length per unit volume. Or equivalently, the number of dislocations that intersect a unit area of a random section. Units mm^-2

Question 17

How does number of dislocations present within the crystal vary with increasing plastic strain?

Answer 17

It increases

Question 18

What is an edge dislocation?

Answer 18

An extra half plane of atoms, the edge of which terminates within the crystal. It is a linear defect that centres on the line that is defined along the end of the extra half plane of atoms- the dislocation line.

Question 19

How does the dislocation line for an edge dislocation look on paper?

Answer 19

If the extra half plane of atoms appears as a vertical line going into a square plane of atoms, the dislocation line is perpendicular to the page so appears as an upside down T at the end of the half plane. Symbol is T either way up (not the letter but that shape)

Question 20

What is a screw dislocation?

Answer 20

A dislocation formed by a shear stress resulting in the upper front region of the crystal being shifted one atomic distance to the right relative to the bottom portion. The dislocation line passes through the centre of a spiral. Symbol of circular arrow

Question 21

What is a Burgess vector?

Answer 21

The magnitude and direction of the lattice distortion associated with a dislocation. Denoted b. The nature of a dislocation is defined by the relative orientations of the dislocation line and Burgess vector.

Question 22

What are the relative orientations of the dislocation line and Burgess vector for edge and screw dislocations?

Answer 22

Edge: they are perpendicular
Screw: they are parallel

Question 23

Describe the motion of an edge dislocation

Answer 23

Moves in response to a shear stress applied in a direction perpendicular to its line. If the extra half plane comes down from the top and the top part is pushed right and the bottom part of the material left: dislocation moves one atomic distance right as the EHP links up with the bottom part of the vertical plane to its right, the upper portion of this plane becomes the EHP, this process repeats such that the EHP moves right by successive and repeated breaking of bonds and shifting by interatomic distances of upper half-planes. The dislocation moves parallel to the shear stress direction.

Question 24

What is slip?

Answer 24

The process by which plastic deformation is produced by dislocation motion

Question 25

What is a slip plane?

Answer 25

The crystallographic plane along which the dislocation line traverses

Question 26

What is the slip direction?

Answer 26

The direction of movement a slip plane follows.

Question 27

What is a slip system?

Answer 27

The combination of the slip plane and slip direction

Question 28

What does the slip system depend on?

Answer 28

The crystal structure of the metal. It is such that the atomic distortion that accompanies the motion of a dislocation is a minimum.

Question 29

In which direction does slip occur?

Answer 29

Along a number of equivalent and most favourably orientated planes and directions at various positions along the specimen length.

Question 30

Describe the appearance of slip deformation of a cylindrical sample with diagonal slip planes under tension

Answer 30

Forms small diagonal steps that are parallel to each other and loop around the specimen. The steps appear as lines called slip lines. This is for a single crystal specimen.

Question 31

How and why do two edge dislocations close to each other with the same sign and identical slip plane interact?

Answer 31

They repel each other. The compressive and tensile strain fields both lie on the same side of the slip plane. The strain field interaction is that there exists between these two isolated dislocations a mutual repulsive force that tends to move them apart.

Question 32

Which side of the dislocation line of edge dislocations experience compressive strain and which experience tensile strain?

Answer 32

The atoms immediately on the side of the dislocation line with the extra half plane are squeezed together so experience a compressive strain relative to atoms positioned in the perfect crystal. The atoms immediately on the other side of the dislocation line to the extra half plane sustain an imposed tensile strain. The cross of the T shows the side in tension.

Question 33

What is a strain field?

Answer 33

The lattice distortions that radiate from the dislocation line so the strain extends into the surrounding atoms. The magnitude of strain decreases with radial distance from the dislocation.

Question 34

How and why do two edge dislocations close to each other with the opposite sign and identical slip plane interact?

Answer 34

They are attracted to one another. Dislocation annihilation occurs when they meet. The two extra half planes of atoms will align and become a complete plane.

Question 35

What is the direction of motion for a screw dislocation relative to the applied shear stress?

Answer 35

Perpendicular to the shear stress direction. Like tearing paper.

Question 36

How does the net plastic deformation for motion of screw and edge dislocations compare?

Answer 36

They are the same