Chapter 7

Question 1

What is cold working?

Answer 1

The plastic deformation of a metal at a temperature below that at which it recrystallizes.

Question 1

What is critical resolved shear stress?

Answer 1

The shear stress, resolved within a slip plane and direction, required to initiate slip.

Question 2

What is dislocation density?

Answer 2

The total dislocation length per unit volume of material; alternatively, the number of dislocations that intersect a unit area of a random surface section.

Question 3

What is grain growth?

Answer 3

The increase in average grain size of a polycrystalline material; for most materials, an elevated-temperature heat treatment is necessary.

Question 4

What is lattice strain?

Answer 4

Slight displacements of atoms relative to their normal lattice positions, normally imposed by crystalline defects such as dislocations, and interstitial and impurity atoms.

Question 5

What is recovery?

Answer 5

The relief of some of the internal strain energy of a previously cold-worked metal, usually by heat treatment.

Question 6

What is recrystallization?

Answer 6

The formation of a new set of strain-free grains within a previously cold-worked material; normally, an annealing heat treatment is necessary.

Question 7

What is recrystallization temperature?

Answer 7

For a particular alloy, the minimum temperature at which complete recrystallization occurs within approximately 1 h.

Question 8

What is resolved shear stress?

Answer 8

An applied tensile or compressive stress resolved into a shear component along a specific plane and direction within that plane. Formula: τR = (F/A)*cosλcosΦ

Question 9

What does slip mean?

Answer 9

Plastic deformation as the result of dislocation motion; also, the shear displacement of two adjacent planes of atoms.

Question 10

What is a slip system?

Answer 10

The combination of a crystallographic (slip) plane and, within that plane, a crystallographic (slip) direction along which slip (i.e., dislocation motion) occurs. Both have high density.

Question 11

What is solid-solution strengthening?

Answer 11

Hardening and strengthening of metals that result from alloying in which a solid solution is formed. The presence of impurity atoms restricts dislocation mobility.

Question 12

What is strain hardening?

Answer 12

The increase in hardness and strength of a ductile metal as it is plastically deformed below its recrystallization temperature.

Question 13

What are the three types of dislocations?

Answer 13

• Edge
• Screw
• Mixed

Question 14

What direction do edge dislocations move in?

Answer 14

Parallel to the applied shear stress

Question 15

What direction do screw dislocations move in?

Answer 15

Perpendicular to the applied shear stress

Question 16

What are some dislocation characteristics of metals?

Answer 16

• Dislocation motion occurs relatively easily
• Have non-directional metallic bonding
• Closed-packed planes and directions for slip

Question 17

What are some dislocation characteristics of ceramics?

Answer 17

• Dislocation motion is relatively difficult
• Covalent bonding is directional
• Ionically bonded have few slip systems

Question 18

What are the characteristics of an FCC slip system?

Answer 18

• {plane} <direction></direction>
• Dislocation motion on {1 1 1} planes
• Dislocation motion on {1 1 0} directions
• Total of 12 independent slip systems

Question 19

How is the required tensile stress to cause yielding calculated?

Answer 19

σy = τCRSS/(cosλcosΦ)

Question 20

What are the characteristics of plastic deformation in polycrystalline materials?

Answer 20

• Grains change shape due to slip
• Grains are rolled, they go from equiaxed and randomly oriented (isotropic) to elongated in the rolling direction (becomes slightly anisotropic)

Question 21

What are three mechanisms to strengthen/harden metals or to decrease dislocation mobility?

Answer 21

• Grain size reduction
• Solid solution strengthening
• Strain hardening (cold working)

Question 22

What does reducing grain size do?

Answer 22

• Increases grain boundary area
• More barriers to dislocation motion
• Increases yield strength, tensile strength, and hardness

Question 23

What is solid solution strengthening?

Answer 23

Adding small interstitial (impurity) atoms introduces tensile stress which changes to compression and partially counteracts. The opposite occurs when large interstitial atoms are added. Higher shear stress is required to cause dislocation motion

Question 24

What is strain hardening (cold working)?

Answer 24

Plastically deforming metals at room temperature makes them harder and stronger. Deformations often cause reductions in cross-sectional area

Question 25

What happens during cold working?

Answer 25

• Yield and tensile strength increase
• Ductility decreases

Question 26

How is dislocation density calculated?

Answer 26

Total dislocation length/unit volume

Question 27

How is dislocation density affected by deformation (cold work)?

Answer 27

It increases, causing the distance between dislocations to decrease, which hinders dislocation motion through repulsive forces

Question 28

What happens during annealing?

Answer 28

Reverses effects of cold working
1) Recovery (reduction of dislocation density)
2) Recrystallization (new, small grains form)
3) Grain growth (consume and replace
Decreases tensile strength and increases ductility

Question 29

What is recrystallization temperature TR?

Answer 29

• The temperature at which recrystallization reaches completion in 1 hour.
• TR decreases with increasing cold working
• TR decreases with increasing purity

Question 30

What is the difference between cold working vs hot working?

Answer 30

• Hot working is deformation above TR
• Cold working deformation below TR

Question 31

How does grain size influence properties?

Answer 31

• Small grains are relatively strong and tough at low temperatures.
• Large grains have good creep resistance at high temperatures.