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APSC 278 (Engineering Materials) > Recrystallization > Flashcards

Recrystallization Flashcards

1
Q

What is the homologous temperature?

A

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

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2
Q

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

A
  • 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

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3
Q

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

A

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

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4
Q

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

A

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

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5
Q

Is there work hardening in hot working?

A

No

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6
Q

Is work hardening present in cold working?

A

Yes

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7
Q

How does cold working impact strength and ductility?

A

An increase in strength corresponds to a decrease in ductility

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8
Q

What is the internal energy from work hardening?

A

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.

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9
Q

How does work hardening impact internal energy?

A

Work hardening increases the internal energy within a material

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10
Q

What is an annealed treatment?

A

Treatment at elevated temperature.

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11
Q

Why are recrystallized grains softer than deformed grains?

A

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

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12
Q

How does annealing heat treatment affect work hardening?

A

Annealing heat treatment can reverse the
effects of work hardening

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13
Q

What is the driving force during work hardening for recrystallization?

A

Stored energy

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14
Q

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

A
  1. Recovery
  2. Recrystallization
  3. Grain Growth
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15
Q

How is energy expended during cold working?

A
  • About 95% of the work done in deforming a
    metal is converted to heat
  • About 5% is stored in the metal
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16
Q

What is the thermodynamic stability of a plastically deformed material?

A
  • A plastically deformed material is
    thermodynamically unstable, and wants to
    return to a lower energy state
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17
Q

What is recovery in annealing heat treatment?

A

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

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18
Q

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

A
  • Restoration of physical properties (electrical and thermal
    conductivities) to pre-cold worked state begins
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19
Q

What is recrystallization during annealed heat treatment?

A

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

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20
Q

What is equiaxed grain?

A

Grains having approximately equal dimensions in
all directions

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21
Q

What is the energy state of grains after recovery?

A

Grains are still in high strain energy state after recovery

22
Q

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

A

Internal energy

23
Q

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

A
  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.
24
Q

What is the relationship between dislocations and strain?

A

Dislocations induce strain within the crystal lattice.

25
Q

Where do repulsive interactions result based on dislocations?

A

Repulsive interactions result between strain fields of adjacent dislocations.

26
Q

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

A
  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.
27
Q

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

A

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

28
Q

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

A

Pure metals: TH < 0.4
Alloys: TH < 0.6

29
Q

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

A

Pure metals: TH > 0.4
Alloys: TH > 0.6

30
Q

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

A

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

31
Q

What is the relationship between annealing temperature and grain size?

A

Increase in annealing temperature = increase in grain size

32
Q

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

A

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

33
Q

What is the significance of the critical deformation to recrystallization?

A

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

34
Q

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

A

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

35
Q

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

A

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

36
Q

What type of motion does recrystallization require?

A

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

37
Q

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

A

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.
38
Q

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

A
  1. Alloy atoms diffuse to dislocation to pin it down
  2. Strain field interactions between a moving
    dislocation and substitutional atom
39
Q

What are strain fields associated with

A

Substitutional atoms and dislocations.

40
Q

How do substitutional atoms impede dislocation motion?

A

By imposing lattice strain

41
Q

Why do alloy atoms pin down dislocations?

A

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

42
Q

What is required for alloy atoms to pin down dislocations?

A

Energy is required to dissociate from low-strain regions

43
Q

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

A

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

44
Q

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

A

Number and density of dislocations
increase with plastic deformation

45
Q

Name 3 examples of dislocation formation sites

A
  • Existing dislocations
  • Internal defects
  • Surface defects (scratches and nicks)
46
Q

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

A

Dislocation-dislocation strain
interactions make dislocation
motion more difficult

47
Q

What happens to dislocations during recovery?

A

They reconfigure to have lower strain energy

48
Q

Does grain growth need to be preceded by recrystallization?

A

Grain growth does not need to be
preceded by recrystallization.

49
Q

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.?

A

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

50
Q

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

A

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

51
Q

What is the relationship between annealing temperature and recrystallization time?

A

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

APSC 278 (Engineering Materials) (30 decks)

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