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