Chapter 10 + 11 + 12 - Plasticity + Defects in Crystal Structure + Influences of Defects Flashcards
What is plastic deformation?
When materials changed shape permanently after a certain load was applied to it and then removed.
How can we use this to our advantage?
Extrusion, forging, rolling and sheet forming (all cold forming)
What is cold forming?
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)
Does the energy get absorbed when plastic deformation happens or where does it go? Give an example.
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)
What are brittle materials?
Materials that can’t undergo plastic deformation before breaking.
Give an example of a brittle kind of material.
Ceramics
What are ductile materials?
Materials that can deform elasticly to a certain extend. But then start to deform plasticly before they break.
Give an example of ductile materials
A lot of metals.
What is an Ashby diagram (what does it show)?
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
What is the upper and lower yield point?
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.
What are Luder’s bands?
During yielding, plastic deformation occurs in different bands, this is visible and we call it Luder’s bands
What is yielding of steel?
Elongation under the suddenly reduced resistance
What is work hardening?
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.
What is the tensile strength?
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.
What is toughness?
The area under the whole curve.
What is resilience?
The area under the elastic part of the curve.
Check the labs for different hardness tests.
// not a question, but make sure to at least read through it.
Crystallographic slip
When a metal undergoes plastic deformation, Crystallographic slip is what causes the deformation
Draw the structure of a HCP (hexagonal close packed) lattice
// ABABABABAB
Draw the structure of a FCC (face centred cubic) lattice
// ABCABCABC
What do ABCABC and ABABABA have in common?
They are both the closest possible stacking.
How much space does the closest possible stacking use and is there a difference for HCP and FCC?
74% in both case. We call it the atomic packing factor
Then why is there BCC (Body centred cubic)
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)
Deviations from perfect stacking
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
substitutional solid solution
When a foreign atom takes a spot where a non-foreign atom would be
interstitial solid solution
When a foreign atom takes a spot in between the non-foreign atoms
Why are alloys stronger than pure metals?
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.
What is edge dislocation?
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
Wat are grain boundaries?
Because solidifying happens in all directions at the same time, this creates grains, where each grain has its own orientation.
Why doesn’t plastic deformation stetch or break all bonds simultaneously?
The energy, and thus stress needed to do that is way higher than the values that we measure in tests.
How does plastic deformation work?
When stress is applied, this moves the dislocation, so only a single row of bonds has to be broken at a time
What is a slip system?
The combination of the slip plane and its direction
What is critical shear stress?
The shear stress required to achieve slip.
Why is the FCC structure the easiest to deform?
Densely packed structures are easier to deform since shear is most easily achieved in the densest directions.
Why are HCP metals not good for cold forming?
The most densily packed slip direction and slip planes are not sufficient for plastic deformation.
Why do ceramics not undergo plastic deformation
Ionic bonds resist sliding, thus no plastic deformation can be used.
How can we make a pure metal without any defects stronger?
We can make the movement of dislocations more difficult: Solution hardening; Precipitation hardening; Work hardening; Grain boundary reinforcement.
Solution hardening
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.
Dispersion and precipitation hardening
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.
Work hardening:
Uses the property that plastic deformation increases the stress that it can withstand. There is more stress needed after it has already deformed plastically.
