Chapter 10 + 11 + 12 - Plasticity + Defects in Crystal Structure + Influences of Defects

Question 1

What is plastic deformation?

Answer 1

When materials changed shape permanently after a certain load was applied to it and then removed.

Question 2

How can we use this to our advantage?

Answer 2

Extrusion, forging, rolling and sheet forming (all cold forming)

Question 3

What is cold forming?

Answer 3

Applying plastic deformation without heating it up to the melting temperature. So cold means relatively cold (500 degrees celsius is cold if the melting point is at 2000 degrees)

Question 4

Does the energy get absorbed when plastic deformation happens or where does it go? Give an example.

Answer 4

When plastic deformation happens it’s almost fully converted into heat. This is why crumple zones in a car are so good at absorbing the impact, instead of absorbing the impact and releasing it by backwards force (which releases force forwards again causing more injury to the passengers and driver)

Question 5

What are brittle materials?

Answer 5

Materials that can’t undergo plastic deformation before breaking.

Question 6

Give an example of a brittle kind of material.

Answer 6

Ceramics

Question 7

What are ductile materials?

Answer 7

Materials that can deform elasticly to a certain extend. But then start to deform plasticly before they break.

Question 8

Give an example of ductile materials

Answer 8

A lot of metals.

Question 9

What is an Ashby diagram (what does it show)?

Answer 9

A diagram where 2 properties (for example strength and density) for different types of materials are shown. It shows the different alloys and what range they are usually found

Question 10

What is the upper and lower yield point?

Answer 10

The upper is the the point where the stress stops going up and starts going down instead. The lower is the point where fluctuation takes place.

Question 11

What are Luder’s bands?

Answer 11

During yielding, plastic deformation occurs in different bands, this is visible and we call it Luder’s bands

Question 12

What is yielding of steel?

Answer 12

Elongation under the suddenly reduced resistance

Question 13

What is work hardening?

Answer 13

When we plasticly deform a rod, and we look at the graph, the material suddenly can handle more stress. This means that the material is stronger in that case. We call this work hardening.

Question 14

What is the tensile strength?

Answer 14

The point at which the work hardening can’t resist any further deformation anymore. it will start necking (becoming thinner in 1 plant ) causing less stress to be necessary to break the rod.

Question 15

What is toughness?

Answer 15

The area under the whole curve.

Question 16

What is resilience?

Answer 16

The area under the elastic part of the curve.

Question 17

Check the labs for different hardness tests.

Answer 17

// not a question, but make sure to at least read through it.

Question 18

Crystallographic slip

Answer 18

When a metal undergoes plastic deformation, Crystallographic slip is what causes the deformation

Question 19

Draw the structure of a HCP (hexagonal close packed) lattice

Answer 19

// ABABABABAB

Question 20

Draw the structure of a FCC (face centred cubic) lattice

Answer 20

// ABCABCABC

Question 21

What do ABCABC and ABABABA have in common?

Answer 21

They are both the closest possible stacking.

Question 22

How much space does the closest possible stacking use and is there a difference for HCP and FCC?

Answer 22

74% in both case. We call it the atomic packing factor

Question 23

Then why is there BCC (Body centred cubic)

Answer 23

Not all metals have the closest packed structure. Some form a square grid but the individual atoms do not touch each other. The space that the atoms take in is 68% (= atomic packing factor)

Question 24

Deviations from perfect stacking

Answer 24

In reality it isnt always perfect:

• sometimes an atom is missing. We call this vacancies. They do not influence the strength of the material.
• Sometimes there is a foreign molecule on the place where another atom of the material is supposed to be: substitutional solid solution
• Other small foreign atoms can be in between the atoms (so in the cavities): we call this interstitial solid solution

Question 25

substitutional solid solution

Answer 25

When a foreign atom takes a spot where a non-foreign atom would be

Question 26

interstitial solid solution

Answer 26

When a foreign atom takes a spot in between the non-foreign atoms

Question 27

Why are alloys stronger than pure metals?

Answer 27

dissolved (foreign) atoms disturb the normal structure a bit. This means that the layers cannot slide over each other as well as they can in a pure metal. So the imperfections are a good thing in this case actually.

Question 28

What is edge dislocation?

Answer 28

When metal solidifies, there’s no way to avoid imperfections in the lattice. Above the slip plane, there will be an extra row compared to the ones below the slip plan. This makes the metal soft and ductile

Question 29

Wat are grain boundaries?

Answer 29

Because solidifying happens in all directions at the same time, this creates grains, where each grain has its own orientation.

Question 30

Why doesn’t plastic deformation stetch or break all bonds simultaneously?

Answer 30

The energy, and thus stress needed to do that is way higher than the values that we measure in tests.

Question 31

How does plastic deformation work?

Answer 31

When stress is applied, this moves the dislocation, so only a single row of bonds has to be broken at a time

Question 32

What is a slip system?

Answer 32

The combination of the slip plane and its direction

Question 33

What is critical shear stress?

Answer 33

The shear stress required to achieve slip.

Question 34

Why is the FCC structure the easiest to deform?

Answer 34

Densely packed structures are easier to deform since shear is most easily achieved in the densest directions.

Question 35

Why are HCP metals not good for cold forming?

Answer 35

The most densily packed slip direction and slip planes are not sufficient for plastic deformation.

Question 36

Why do ceramics not undergo plastic deformation

Answer 36

Ionic bonds resist sliding, thus no plastic deformation can be used.

Question 37

How can we make a pure metal without any defects stronger?

Answer 37

We can make the movement of dislocations more difficult: Solution hardening; Precipitation hardening; Work hardening; Grain boundary reinforcement.

Question 38

Solution hardening

Answer 38

Alloying (“legeren”): We add foreign atoms to the metal to disturb the lattice a bit and make it harder to for planes to slide over each other.

Question 39

Dispersion and precipitation hardening

Answer 39

Hard particles that are allowed to precipitate (precipitation) or are already solid (dispersion) through the crystal structure. It is achieved by adding hard, solid particles in liquid metal that is then allowed to solidify.

Question 40

Work hardening:

Answer 40

Uses the property that plastic deformation increases the stress that it can withstand. There is more stress needed after it has already deformed plastically.