L11: Strengthening Of Metals, Annealing Of Metals Flashcards

1
Q

Which two methods are the only two which can be used to strengthen pure metals

A

Increased number of dislocations

Presence of grain boundaries

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2
Q

How does cold work (strain) hardening happen

A

Plastically deformed the metal causing dislocations to slip meaning more dislocations are formed

The more dislocations the more likely they are to interfere with each other during slip are you i.e. two dislocations of the same type cannot slide past each other

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3
Q

And what processes does work hardening take place

A

Rolling
Forging
Drawing extrusion
Forming

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4
Q

What properties are formed due to work hardening increasing grain length

A

Anisotropic forces

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5
Q

What is required for two dislocations to interfere with each other slip

A

Same direction and slip plane

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6
Q

Why can dislocations interfere with each other

A

Some atoms around the dislocation are in tension well others are in compression

As dislocations approach each other the buildup of tension and compression in the region around the dislocations causes too much distortion of the lattice and more force is required to push them together i.e. the dislocations are repelled from each other

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7
Q

How does grain size strengthening work

A

Orientations of the slip planes are usually very different at grain boundaries
By reducing the size of the greens in the metal the number of grain boundaries can be increased and the dislocation movement is included
This increases toughness and strength

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8
Q

How can you grain size be controlled and cast metals

A

By use of a inoculants that encourage heterogeneous nucleation

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9
Q

How does solid solution strengthening work?

A

Solute atoms within the metal hinder the slip of dislocations
Large diff in size of solvent vs solute atoms and large quantity of solute atoms helps the strengthening process

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10
Q

What properties does Solid solution strengthening cause

A

Increase in yield stress
Tensile strength
Hardness
Creep resistance

Reduced ductility
Electrical conductivity

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11
Q

How does solute atoms segregating strengthen the material

A

They segregate to dislocations to reduce the amount of distortion
If slip was to take place this would increase the distortion again which is an unfavourable process

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12
Q

When does dispersion strengthening take place

A

When there are second phase particles or regions that are dispersed in the matrix

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13
Q

How does dispersion strengthening work?

What shape should precipitate particles be?

A

Precipitation of the second phase material or hard intermediate compound hinders the slip of dislocations
The precipitate particles should be small and numerous to maximise interference and round rather than needles to avoid stress raising

(In eutectic lamellar structure, a+b layers hinder the slip process

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14
Q

How is dispersion strengthening different from precipitation (age) hardening

A

Age hardening relies on heat treatment to enhance it
The second phase is normally present as relatively large particles within the material so does not contribute to metal hardening
Can heat treat the alloy to form a more highly dispersed second phase that interferes with the slip and promotes strengthening
2nd phase can be solid solution or intermediate (intermetallic) phase

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15
Q

What is over ageing and what does it mean for industry

A

At Hyatt raging temperatures and longer times the particles grow too large and they become less effective at preventing dislocation slip

This means that precipitation Hardened alloys cannot be used in high temperature environments as the second phase tends to resolve all grow in size and the strengthening mechanism is lost

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16
Q

What are the 3 stages of annealing that depend on temp and time?

A

Recovery
Recrystallisation
Grain growth

17
Q

What is annealing used for?

A

Increase ductility of metal (after cold working)
Remove strain hardening
Remove stresses from cold working

18
Q

What is recovery also called and what does it do?

A

Stress releif anneal

Doesn’t change mech properties but changed electrical and thermal ones

19
Q

How does annealing at low temperature encourage recovery

A

Enables dislocations which were tangled to move
Increased temp increases atom diff and vacancy formation
Dislocations arrange to minimise interaction - some destroyed, mainly #stays same
Distorted grain structure remains the same

20
Q

What does recrystallisation temp depend on?

A

Amount of cold work already done
Original grain size
Alloy composition and melting temp

21
Q

What is the structure of the alloy after recrystallisation

A

Fine grain structure

Very few dislocations- ductile (similar to pre-cold worked level)

22
Q

How does recrystallisation (high temp + long time annealing) work?

A

Distorted grains in the alleyway and replaced by a new set of strain free smaller grains
The very strained crystal structure encourages formation of lower energy ordered regions -these new nucleation sites grow within the metal and contain similar numbers of dislocations to the pre-cold worked metal

23
Q

Why is it important to stop the annealing process after recrystallisation?

A

Further annealing to higher temp encourages grains to grow in size.
As grain size increases strength decreases

24
Q

What is the main consequence of annealing

A

At high temperature the material will anneal slowly becoming weaker and more prone to yielding
Some metal joining processes e.g. welding heats the metal in certain regions which can cause local recrystallisation and grain growth