Coldworking

Question 1

What type of deformation provides strengthening of a material?
What happens to tensile strength and yield strength?
What happens to Ductility?

Answer 1

-plastic deformation
- They both increase
-Ductility decreases

Question 2

What does the application of stress greater than the yield strength provide?

Answer 2

It increases dislocations, and dislocation density increases.

Question 3

What should we expect for the strain hardening exponents of metals?

Answer 3

It should be low doe HCP metals, and higher for BCC and FCC metals

Question 4

What does plastic deformation do to thermoplastic polymers?

Answer 4

It strengthens them

Question 5

When stress applied is greater than yield strength, what force between polymer chains is impaired?

Answer 5

Van der waals force

Question 6

What does strain hardening do to polymer chains?

Answer 6

The chains straighten and become aligned in the direction of the applied stress.

Question 7

What happens to dislocations in ceramics?

Answer 7

Dislocations are usually not mobile

Question 8

What characteristic do ceramics have, and how does it make them act?

Answer 8

Ceramics contain voids, and it makes brittle

Question 9

What is cold working?

Answer 9

It is the deformation of metal below recrystallization temperature.

Question 10

What are advantages of cold working?

Answer 10

• Allows metals to be strengthened & at the same time the desired shape to be generated
• Offers excellent dimensional tolerances and surface finish
  -It is a cheap method of manufacturing small parts in bulk
• Some deformation processing techniques can only occur if the material is strain hardened or cold worked.

Question 11

What are some disadvantages of coldworking

Answer 11

• Some materials can be cold worked
  only up to a small degree such as HCP
  magnesium
• Ductility, electrical conductivity &
  corrosion resistance are reduced
• Lack of control over residual stresses
  can impair the material properties
• Cold working for strengthening can’t
  be used for parts exposed to high
  temperatures during operation

Question 12

What is annealing?

Answer 12

Heating to a specific elevated temperature & cooling the material very slowly.

Question 13

What are the three stages in annealing?

Answer 13

Recovery, Recrystallization, and Grain growth

Question 14

What occurs during recovery?

Answer 14

Low temperature process
- Heat causes the dislocations to move
- Dislocations move and form the
boundaries of a polygonised sub grain
structure
- Residual stress is removed due to
rearrangement of dislocations
- This process is sometimes called stress
relief anneal
- Dislocation density is unchanged

Question 15

What does recovery do to electrical conductivity, and corrosion?

Answer 15

It restores high electrical conductivity, and improves corrosion resistance.

Question 16

What is recrystallization? What happens to dislocation density?

Answer 16

• Process of nucleation of new small grains at the cell boundaries of the polygonised structure by heat treatment
• Dislocation density is reduced.

Question 17

What is recrystallization temperature?

Answer 17

Temperature at which new small grains with low dislocation density appear

Question 18

What is grain growth?

Answer 18

It is the phenomenon of increased grain size at high enough temperature.

Question 19

What do favored grains do to smaller grains?

Answer 19

They consume them

Question 20

What happens to grain boundary area?

Answer 20

It decreases

Question 21

Is it a desirable or undesirable phenomenon?

Answer 21

It is undesirable

Question 22

Are recovery and recrystallization necessary for grain growth?

Answer 22

They are not necessary

Question 23

What is hot working? Cold working? Warm working?

Answer 23

• Deformation process conducted above 0.6 times the melting temperature of a metal in Kelvin
• Deformation below 0.3 times
• Deformation between 0.3-0.6 times melting temp

Question 24

What does repeated cold work and annealing cycle allow us to do?

Answer 24

Increase the total amount of deformation

Question 25

What happens to the effect of strain hardening at high temperature?

Answer 25

The effect is lost.

Question 26

What does welding do the the joining regions?

Answer 26

It causes the regions near the joints to be heated above the recrystallization and grain growth temperature.

Question 27

What are characteristic features of ductile fractured surfaces?

Answer 27

• Necking for thick samples
• Flat fractured surface where micro voids nucleate & coalesced
  -Shear lip at 45° angle to the applied stress giving a cup & cone appearance of the fractured surfaces.

Question 27

What is a ductile fracture?

Answer 27

Microvoid formation at grain boundaries & interface between grain/impurity particles

Question 28

What are some characteristics about brittle fracture?

Answer 28

• Crack initiation at small flaws
• Stress concentrates at crack tip
  -In some cases, crack propagates along specific crystallographic plane by cleavage (transgranular path)
• Cracks may also propagate along the grain boundaries (intergranular path)

Question 29

What are microstructure characteristics of brittle fractures?

Answer 29

• Fractured surface is flat & perpendicular to the applied stress
• Crystalline appearance since each fractured grain is flat & they have different orientation
• V-shaped chevron markings form pointing back to the crack initiation or lines radiate in a fanlike pattern from the crack
  origin

Question 30

What happens to cracks in plastic deformation?

Answer 30

Cracks become blunted in metals and polymers

Question 31

What does the strength of ceramic or glass depend on?

Answer 31

It depends on the probability of finding a flaw that exceeds a certain critical size

Question 32

For ceramics and glasses, what can you tell me about the distribution of strength?

Answer 32

The distribution of strength is wide and follows the Weibull distribution.