CH 7 - Deformation & Strengthening Mechanisms

Question 1

How does plastic deformation occur in metals?

Answer 1

slips (edge dislocations)

Question 2

What directions do dislocations move in?

Answer 2

• moves along slip plane in a slip direction perpendicular to the dislocation line
• slip direction is the same as the burgers vector direction

Question 3

What is a slip plane?

Answer 3

• plane on which easiest slippage occurs
• highest planar densities
• large interplanar spacing

Question 4

What is a slip direction?

Answer 4

• directions of movement

- highest linear densities

Question 5

What is the slip system for FCC?

Answer 5

• {111} planes in directions

- total of 12 slip systems

Question 6

What is resolved shear stress?

Answer 6

results from applied tensile stresses

Question 7

What is critical resolved shear stress?

Answer 7

• condition for dislocation motion: resolved shear > critical
• ease of dislocation motion depends on crystallographic orientation

Question 8

Why are polycrystals stronger than single crystals?

Answer 8

grain boundaries are barriers to dislocation motion

Question 9

The grain with the largest ______ yields first

Answer 9

resolved shear stress

Question 10

What are the four strategies for strengthening?

Answer 10

• reduce grain size
• form solid solutions
• precipitation strengthening
• cold working

Question 11

How does reducing grain size strengthen a material?

Answer 11

• grain boundaries are barriers to slip
• barrier “strength” increases with increasing angle of misorientation
• smaller grain size = more barriers

Question 12

How does forming solid solutions strengthen a material?

Answer 12

• impurity atoms distort the lattice & generate lattice strains
• strains act as barriers to dislocation motion
• small impurities tend to concentrate at dislocations; reduces mobility of dislocations and increases strength

Question 13

How does precipitation strengthening strengthen a material?

Answer 13

• large shear stress needed to move dislocation toward precipitate and shear it
• dislocations “advance” but precipitates act as “pinning” sites

Question 14

How does cold working strengthen a material?

Answer 14

• deformation at room temp
• reduce cross-sectional area
• dislocations entangle with one another making dislocation motion more difficult
• dislocation density increases

Question 15

What are some benefits of cold working?

Answer 15

• yield and tensile strength increase

- ductility decreases

Question 16

What are the three stages during heat treatment?

Answer 16

(1) Recovery
(2) Recrystallization
(3) Grain Growth

Question 17

What happens during recovery (from heat treatment)?

Answer 17

Scenario 1:
- atoms diffuse to regions of tension
- dislocations annihilate and form a perfect atomic plane
Scenario 2:
-dislocations “blocked”
-atoms leave by vacancy diffusion allowing dislocations to “climb”
-“climbed” dislocations can now move on new slip plane
- opposite dislocations meet and annihilate

Question 18

What happens during recrystallization (from heat treatment)?

Answer 18

new grains are formed that:

• have low dislocation densities
• are small in size
• consume and replace parent cold-worked grains

Question 19

What happens during grain growth (from heat treatment)?

Answer 19

• at longer times, average grain size increases
• small grains shrink & eventually disappear
• large grains continue to grow

Question 20

What is the recrystallization temperature?

Answer 20

temp at which recrystallization just reaches completion in one hour

Question 21

What is the relationship between hot/cold work and the recrystallization temp?

Answer 21

Hot working = deformation above Tr

cold working = deformation below Tr

Question 22

What is the influence of grain size on mechanical properties?

Answer 22

• metals with small grains are relatively strong and tough at low temps
• metals with large grains have good creep resistance at relatively high temps