Imperfections and dislocations

Question 1

What are the types of point defects in crystals?

Answer 1

-Vacancies
-Interstitial defects

Question 2

What is the relationship between temperatures and the number of vacancies per unit volume?

Answer 2

As the temperature increases, the number of vacancies per unit volume increases.

Question 3

What phenomena is diffusion important in?

Answer 3

• Creep
• Recrystallization
• Grain Growth
• Phase transformations

Question 4

What does diffusion allow atoms to do?

Answer 4

It allows atoms to re-arrange
themselves to obtain lower
energy configurations

Question 5

What are dislocations?

Answer 5

Linear crystalline
defects whose motion gives rise to
plastic deformation in crystalline
materials

Question 6

What causes plastic deformation in crystalline solids?

Answer 6

Propagation (movement) of dislocations.

Question 7

When are dislocations introduced?

Answer 7

During solidification, plastic deformation,
and from thermal stresses.

Question 8

When were dislocations first experimentally observed?

Answer 8

They were observed experimentally by transmission
electron microscopy (TEM) in 1950s.

Question 9

What is slip?

Answer 9

The process by which plastic deformation is produced by
dislocation movement

Question 10

What happens at the atomic level during slip?

Answer 10

Severing and re-forming atomic bonds

Question 11

What is slip a result of?

Answer 11

Applied shear stress

Question 12

Define slip plane

Answer 12

The plane along which the dislocation moves

Question 13

Where can dislocations end and where can they NOT end?

Answer 13

Dislocations cannot end within a crystal or grain, but only at grain boundaries or at a free surface

Question 14

What is a slip system?

Answer 14

Describes the preferred plane (slip plane) and direction (slip direction) for dislocation movement.

Question 15

What does the slip system depend on?

Answer 15

Depends on the crystal structure

Question 16

Where does slip occur for a given crystal structure?

Answer 16

• Along the plane with the densest atomic
  packing (slip plane)
• Along the direction with the highest linear
  density (slip direction)

Question 17

What condition is required for each grain to have deformation occur?

Answer 17

For each grain, deformation occurs
along the slip system that has the
most favourable orientation.

Question 18

What are slip systems in a single crystal?

Answer 18

Slip systems in a single crystal refer to combinations of slip planes and slip directions.

Question 19

For a single crystal with multiple slip system, how many can be oriented favourably and what is unique about one of the slip systems?

Answer 19

One is usually oriented favorably and has
greatest resolved shear stress

Question 20

What is the critical resolved shear stress?

Answer 20

A material property that indicates when yielding
occurs

Question 21

Why does the direction of slip vary in polycrystalline materials (grains)?

Answer 21

Due to the random crystallographic orientations of the grains.

Question 22

Compare the strength between polycrystalline metals and their single crystal equivalent and explain your reasoning.

Answer 22

Polycrystalline metals are stronger than
their single crystal equivalent because
deformation cannot occur unless
dislocations must propagate along
adjacent, less favorably oriented grains

Question 23

What is required to break atomic bonds during slip simultaneously on a slip plane

Answer 23

A very high stress

Question 24

Are all bonds on a slip plane broken simultaneously? If not, what does happen?

Answer 24

Because not all bonds on a slip
plane are severed
simultaneously; instead it
involves breaking atomic
bonds in one plane at a time and reforming bonds.

Question 25

What types of fields do dislocations cause?

Answer 25

The presence of a dislocation
causes elastic stress and strain
fields in the surrounding region.

Question 26

What is the difference between edge dislocations and screw dislocations?

Answer 26

• Edge dislocations introduce tensile
  stresses and
  compressive stresses
• Screw dislocations introduce shear
  stress fields around them

Question 27

When do dislocations repel each other?

Answer 27

When the compressive & tensile stresses for both dislocations are in the same location (either above or below the slip plane).

Question 28

When are dislocations attracted to each other? What happens?

Answer 28

Two dislocations of opposite sign moving on the same slip plane exert an attractive force on each other, which causes them to attract each other and annihilate (perfect crystal).

Question 29

What is dislocation density?

Answer 29

dislocation density = total length of dislocation line / volume

Question 30

When do shear components of an applied stress NOT exist?

Answer 30

At parallel or perpendicular
alignments to stress direction

Question 31

What are resolved shear stresses?

Answer 31

Shear components of an applied stress.

Question 32

What do resolved shear stresses depend on?

Answer 32

Orientation of the slip plane and direction.

Question 33

What happens when a polished metal surface is plastically deformed? What does it indicate?

Answer 33

“Slip lines” appear on the surface. This indicates that atoms are shearing across each other.

Question 34

What is minimized in a slip system?

Answer 34

Atomic distortion is
minimized with dislocation movement

Question 35

When does slip occur along the slip system in a single crystal?

Answer 35

Slip occurs along this slip system when the
critical resolved shear stress is reached.