Recrystallization

Question 1

What is the homologous temperature?

Answer 1

The ratio of the
deformation temperature to melting temperature
(in Kelvin

Question 2

Why is the homologous temperature at which hot
working begins higher for alloys than for pure metals? (Written Response)

Answer 2

• Solutes strengthen materials by inducing strain within the crystal structure, as a result of atomic size differences
• Strain impedes dislocation motion, thereby strengthening the material
• Higher energy input (ie. higher temperature) is required to relieve strain (higher temp needed) in the crystal via dislocation motion
• Therefore, alloys can exhibit work hardening (cold working) over a

larger temperature range than pure metals

Question 3

What is the range for homologous temperature for cold working and hot working for pure metals?

Answer 3

For cold working (Th < 0.4) and for hot working (Th > 0.4)

Question 4

What is the range for homologous temperature for cold working and hot working for alloys?

Answer 4

For cold working (Th < 0.6) and for hot working (Th > 0.6)

Question 5

Is there work hardening in hot working?

Answer 5

No

Question 6

Is work hardening present in cold working?

Answer 6

Yes

Question 7

How does cold working impact strength and ductility?

Answer 7

An increase in strength corresponds to a decrease in ductility

Question 8

What is the internal energy from work hardening?

Answer 8

Internal energy from work hardening = energy per unit length of
dislocation multiplied by the dislocation density;

The driving force for recovery and recrystallization with annealing
treatment.

Question 9

How does work hardening impact internal energy?

Answer 9

Work hardening increases the internal energy within a material

Question 10

What is an annealed treatment?

Answer 10

Treatment at elevated temperature.

Question 11

Why are recrystallized grains softer than deformed grains?

Answer 11

The recrystallized grains are much softer
(lower yield strength) because they have
a lower dislocation density

Question 12

How does annealing heat treatment affect work hardening?

Answer 12

Annealing heat treatment can reverse the
effects of work hardening

Question 13

What is the driving force during work hardening for recrystallization?

Answer 13

Stored energy

Question 14

What are the three stages (in consecutive order) of annealing treatment?

Answer 14

1. Recovery
2. Recrystallization
3. Grain Growth

Question 15

How is energy expended during cold working?

Answer 15

• About 95% of the work done in deforming a
  metal is converted to heat
• About 5% is stored in the metal

Question 16

What is the thermodynamic stability of a plastically deformed material?

Answer 16

• A plastically deformed material is
  thermodynamically unstable, and wants to
  return to a lower energy state

Question 17

What is recovery in annealing heat treatment?

Answer 17

Recovery: release of stored energy by dislocation motion
(without applied stress), due to enhanced atomic
diffusion at elevated temperature

Question 18

What happens to physical properties during recovery stage of annealing heat treatment?

Answer 18

• Restoration of physical properties (electrical and thermal
  conductivities) to pre-cold worked state begins

Question 19

What is recrystallization during annealed heat treatment?

Answer 19

The formation of new strain-free and
equiaxed grains with low dislocation densities

Question 20

What is equiaxed grain?

Answer 20

Grains having approximately equal dimensions in
all directions

Question 21

What is the energy state of grains after recovery?

Answer 21

Grains are still in high strain energy state after recovery

Question 22

What is the driving force for recovery and recrystallization with annealing
treatment?

Answer 22

Internal energy

Question 23

How is internal energy “stored” in a material as a result of cold working?

Answer 23

1. Repulsive interactions result from strain fields between adjacent dislocations
2. Cold working results in the breaking of atomic bonds, and the creation of defects in the material.

Question 24

What is the relationship between dislocations and strain?

Answer 24

Dislocations induce strain within the crystal lattice.

Question 25

Where do repulsive interactions result based on dislocations?

Answer 25

Repulsive interactions result between strain fields of adjacent dislocations.

Question 26

Based on the graph that showcases softening as a function of annealing temperature, what are 3 major differences between the curves represented?

Answer 26

1. Initial hardness is greater with the increased degree of cold work. 2. The magnitude of decrease in hardness is greater with the increased degree of cold work 3. Recovery and recrystallization begin at a lower temperature for materials with an increased degree of cold work.

Question 27

Compare the presence of work hardening for cold working and hot working

Answer 27

There is work hardening in cold working and there is NO work hardening in hot working.

Question 28

What is the homologous temperature range for pure metals and alloys in cold working?

Answer 28

Pure metals: TH < 0.4 Alloys: TH < 0.6

Question 29

What is the temperature range for pure metals and alloys in hot working?

Answer 29

Pure metals: TH > 0.4 Alloys: TH > 0.6

Question 30

What is the relationship between % cold work and yield strength, ultimate tensile strength, and ductility?

Answer 30

Increase in % cold work = increase in yield strength = increase in tensile strength = decrease in ductility (% elongation)

Question 31

What is the relationship between annealing temperature and grain size?

Answer 31

Increase in annealing temperature = increase in grain size

Question 32

How and why is the magnitude of reduction in hardness related to % cold work?

Answer 32

Due to increased stored energy within the material, which acts as the driving force for recovery and recrystallization

Question 33

What is the significance of the critical deformation to recrystallization?

Answer 33

When a crtitical deformation exists, recrystallization doesn't occur.

Question 34

What is the relationship between the recrystallization rate at a given temperature and cold work?

Answer 34

Thus, the rate of recrystallization at a given temperature increases as cold work increases

Question 35

What is the relationship between activation energy for recrystallization and cold work?

Answer 35

The activation energy Q for recrystallization is a decreasing function of the amount of cold work

Question 36

What type of motion does recrystallization require?

Answer 36

Recrystallization involves motion of grain boundaries as new grains nucleate and grow

Question 37

Does recrystallization occur more rapidly in pure metals or metal alloys? Why?

Answer 37

Pure metals have a faster recrystallization rate; * Impurity atoms (solute) interact with grain boundaries to reduce the mobility of alloys * This reduces recrystallization rate and increases recrystallization temperature of metal alloys.

Question 38

What are the two types of interaction between substitutional atoms and dislocation?

Answer 38

1. Alloy atoms diffuse to dislocation to pin it down 2. Strain field interactions between a moving dislocation and substitutional atom

Question 39

What are strain fields associated with

Answer 39

Substitutional atoms and dislocations.

Question 40

How do substitutional atoms impede dislocation motion?

Answer 40

By imposing lattice strain

Question 41

Why do alloy atoms pin down dislocations?

Answer 41

Alloy atom diffuses to a location to reduce strain imposed by dislocation

Question 42

What is required for alloy atoms to pin down dislocations?

Answer 42

Energy is required to dissociate from low-strain regions

Question 43

How does work hardening impact dislocation density and dislocation-dislocation interaction?

Answer 43

Work hardening involves an increase in dislocation density and dislocation-dislocation interaction

Question 44

What is the relationship between the number and density of dislocations and plastic deformation?

Answer 44

Number and density of dislocations increase with plastic deformation

Question 45

Name 3 examples of dislocation formation sites

Answer 45

* Existing dislocations * Internal defects * Surface defects (scratches and nicks)

Question 46

What is the relationship between dislocation-dislocation strain interactions and dislocation motion

Answer 46

Dislocation-dislocation strain interactions make dislocation motion more difficult

Question 47

What happens to dislocations during recovery?

Answer 47

They reconfigure to have lower strain energy

Question 48

Does grain growth need to be preceded by recrystallization?

Answer 48

Grain growth does not need to be preceded by recrystallization.

Question 49

Why may there be differences between specimens 1 and 2 with similar-sized equiaxed grains and specimen 3 with elongated grains, all oriented in the same direction, with no signs of recrystallization at the grain boundaries.?

Answer 49

The differences in grain morphology between the three specimens may be due to differences in annealing time and chemical composition of the metal alloys.

Question 50

What is the relationship between grain growth, time, and temperature?

Answer 50

As temperature and/or time increases, the grain growth increases.

Question 51

What is the relationship between annealing temperature and recrystallization time?

Answer 51

Increasing the annealing temperature decreases the time required for recrystallization to be complete.