Ch 8

Question 1

Discuss how plastic deformation occurs. Connect this to the structure of metals.

Answer 1

plastic deformation occurs by motion of dislocations (edge, screw, mixed).

Metals have metallic bonding (Non-directional) and also are closed packed in certain directions/planes. This means that atoms are relatively close requiring less energy to break and reform bonds. Also, lots of directions/options for slip.

Question 2

Discuss how plastic deformation occurs. Connect this to the structure of metals.

Answer 2

plastic deformation occurs by motion of dislocations (edge, screw, mixed).

Metals have metallic bonding (Non-directional) and also are closed packed in certain directions/planes. This means that atoms are relatively close requiring less energy to break and reform bonds. Also, lots of directions/options for slip.

Question 3

Compare the plastic deformation of metals to that of ceramics, Give reasons for the differences.

Answer 3

Ceramics either have covalent bonding or ionic bonding.
Covalent bonding is highly directional and much stronger than metallic bonding.
Similarily, ionic bonding is much much stronger AND while it isn’t directional, the motion of nearby ions of LIKE charge is restricted by electrostatic repulsive forces

All reasons why ceramics are so brittle and don’t deform well

Question 4

Slip systems.

What is a slip system, list the most preferred slip systems for SC, BCC, FCC

Answer 4

Slip system: preferred direction and plan that a material will deform along. High planar density in the high linear density direction

Question 5

What is the applied tensile stress?

Answer 5

Shear stress component when slip plane is oriented neither perpendicular nor parallel to stress direction

Question 6

Why is Dislocation Motion & Plastic Deformation in Metals & Ceramics important?

Answer 6

Dislocations are intimately involved in plastic deformation. To control the mechanical properties (e.g. yield strength, UTS, toughness, high temperature properties) you control the ease with which dislocations move in a material. Understanding dislocation interactions with obstacles such as solutes, grain boundaries, and other dislocations, helps engineers to design better alloys and to achieve desired performance.

Question 7

Why is this important Interaction of Dislocations with Obstacles & Annealing?

Answer 7

Pure metals are soft, so to increase the strength of metals, engineers can use dislocation interactions with obstacles to increase yield strength: by solid solution hardening (alloying), grain boundary refinement (decreasing grain size), and increasing dislocation density (cold work). These design schemes form a key basis for alloy design everywhere throughout technology.

During plastic deformation strength increases (which is generally good), yet ductility decreases (which is generally bad). Annealing is a controlled heat treating process by which ductility is restored at the expense of yield strength. This is an important manufacturing process, which engineers can use to broaden the range of useful properties in alloys, and tailor their properties to specific applications.

Question 8

What angles is Tr the highest?

Answer 8

1/2 yield strength

Question 9

Discuss how grain boundaries play a role in polycrystalline materials

Answer 9

polycrystalline materials have many grains, and often random crystallographic orientations and therefore slip directions vary from grain to grain

There will be one (at least) plane that is oriented favorably and that is where the slip with begin.

slip first fail first

Question 10

What happens when you roll a material?

Answer 10

rolling of grains takes a random orientation and results in one direction

Question 11

Define anisotropic

Question 12

How are dislocations used as a strengthening mechanism?

Answer 12

If you want a material to not deform, you need to stop dislocations from moving

Question 13

What are the three mechanisms used to stop dislocations from moving around?

Answer 13

1. grain size reduction
2. solid solution strengthening
3. strain hardening (cold working)

Question 14

How do you use grain size to stop dislocations from moving around?

Answer 14

because grain boundaries act as barriers to dislocation motion, an increase in boundaries (decrease in grain size) will increase strength of material

Question 15

What are the consequences of reducing grain size? (or more generally any of the three mechanisms)

Answer 15

increase yield strength
increase tensile strength
increase hardness

Question 16

What is solid solution strengthening? Alloying?

Answer 16

When dislocations are present, the result is areas of compression and tension.

You can add a smaller atom to an area of compression to pin the line dislocation. This is favorable because the line dislocation has a ‘choice’ between well fitting (eg small atoms) and an area of compression

OR

You can add a larger atom to an area of tension. Similarily, edge location ‘preferrs’ to not move on

Question 17

What is strain hardening?

Answer 17

Plastically deforming most metals at room temperature makes them harder and stronger (because you are generating LOTs and LOTS os dislocations) which in turn get closer together which in turn get tangled up

Question 18

As %CW increases what other properties are affected?

Answer 18

Increase yield strength
Increase tensile strength
DECREASES %EL or %AR ductility

Question 19

so, now that you have generated all kinds of dislocations, what happens if you heat treat a metal?

Answer 19

heat treating will fix the defects by moving them around. Defects are not the lowest energy state and added energy through heat ‘allows’ defects to be reduced

Question 20

How does annealing (increase in temp) affect other properties?

Answer 20

Decreases tensile strength
Increases %EL ductility

Question 21

What are the three stages of annealing?

Answer 21

Recovery: dislocation annihilation
Recrystallization: create new small, perfect grains with few defects
Grain Growth: grain size grows

Question 22

What is the ideal temperature for recrystallization?

Answer 22

Tr = recrystalization temp

0.3Tm < Tr < 0.6Tm

in kelvin

Question 23

What does the recrystalization temp depend on?

Answer 23

%CW – TR decreases with increasing %CW
Purity of metal – TR decreases with increasing purity

Question 24

What is the relationship between cold working and hot working?

Answer 24

Cold working: strain hardening (deformation below Tr)
Hot working: minimal strain hardening (deformation above Tr)

Question 25

What is strain hardening

Answer 25

plastically deforming most metals at room temperature makes them harder and stronger

generates lots and lots of dislocations during this process

Question 26

What is the relationship between grain size and other properties?

Answer 26

Small grains (strong, tough, NOT DUCTILE)

Large grains (good cree resistance at hight Ts, DUCTILE)

Question 27

How can annealing treatment be used to change grain size and affect yield strength and ductility?

Answer 27

heat treating will ‘fix’ defects by moving them around

Question 28

Do brittle polymers deform plastically?

Give examples of specific polymer types.

Answer 28

DO NOT plastically deform

cross-linked and network polymbers

Question 29

Do plastic polymers deform plastically?

Answer 29

behave like metals b/c of secondary bonding between chains which are easy to break, easy to form.

Chains will align in the direction of the pull

Question 30

What properties of polymers affect tensile strength? Why?

Answer 30

chain length (long = more entanglement increases TS)
degree of polymerization
mol. weight

Question 31

What is one way to strengthen a polymer?

Answer 31

Drawing: form polymer with alighned chains results in

Increase E
Increase TS
decrease ductility

Question 32

After drawing a polymer, if you anneal the same polymber, what are the consequences?

Answer 32

decrease chain alignment
reverses the effects of drawing

decrease E
decrease TS
increase %EL

Question 33

Small interstitial atoms go in the
[ Select ]
strain region of an edge dislocation.

Answer 33

tensile