L11: Strengthening Of Metals, Annealing Of Metals Flashcards
Which two methods are the only two which can be used to strengthen pure metals
Increased number of dislocations
Presence of grain boundaries
How does cold work (strain) hardening happen
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
And what processes does work hardening take place
Rolling
Forging
Drawing extrusion
Forming
What properties are formed due to work hardening increasing grain length
Anisotropic forces
What is required for two dislocations to interfere with each other slip
Same direction and slip plane
Why can dislocations interfere with each other
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
How does grain size strengthening work
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
How can you grain size be controlled and cast metals
By use of a inoculants that encourage heterogeneous nucleation
How does solid solution strengthening work?
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
What properties does Solid solution strengthening cause
Increase in yield stress
Tensile strength
Hardness
Creep resistance
Reduced ductility
Electrical conductivity
How does solute atoms segregating strengthen the material
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
When does dispersion strengthening take place
When there are second phase particles or regions that are dispersed in the matrix
How does dispersion strengthening work?
What shape should precipitate particles be?
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
How is dispersion strengthening different from precipitation (age) hardening
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
What is over ageing and what does it mean for industry
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
What are the 3 stages of annealing that depend on temp and time?
Recovery
Recrystallisation
Grain growth
What is annealing used for?
Increase ductility of metal (after cold working)
Remove strain hardening
Remove stresses from cold working
What is recovery also called and what does it do?
Stress releif anneal
Doesn’t change mech properties but changed electrical and thermal ones
How does annealing at low temperature encourage recovery
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
What does recrystallisation temp depend on?
Amount of cold work already done
Original grain size
Alloy composition and melting temp
What is the structure of the alloy after recrystallisation
Fine grain structure
Very few dislocations- ductile (similar to pre-cold worked level)
How does recrystallisation (high temp + long time annealing) work?
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
Why is it important to stop the annealing process after recrystallisation?
Further annealing to higher temp encourages grains to grow in size.
As grain size increases strength decreases
What is the main consequence of annealing
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
