Al Alloys Flashcards
What is a light alloy, examples and applications?
Potential for weight reduction because of their low density
Eg. Ti, Al, Mg
Used for transport, especially aerospace, construction, and packaging industry
Al gradually replacing steel and cast iron in cars
What are the advantages and disadvantages of aluminium?
Good corrosion resistance
Excellent electrical and thermal conductivities
Very abundant material and infinitely recyclable
Relatively high production costs due to energy used to extract primary aluminium from bauxite ore - however recycled aluminium uses 5-10% of energy
What are some of the physical properties of aluminium?
Face centred cubic
Melting point 660C
Density 2700kg/m3 - steel is around 8000
Several alloy additions capable such as Cu, Mn, Si, Mg
What are the two forms of alloys?
Wrought alloys - cast structure is changed by mechanical working and thermal treatments (sheet, foil, extrusions etc)
Casting alloys - no attempt to change structure - used in sand castings, permanent mould and pressure die casting - advantage of low melting point and fluidity but can shrink on solidification
What denotes an aluminium alloy?
Series number
First digit - principal alloy constituent like Cu, Mn, Si etc.
Second digit - variation of initial alloy
Third and fourth digit - individual alloy variations (number has no significance but is unique)
What are the 3 strengthening mechanisms of aluminium alloys?
Solid solution strengthening - adding alloy additions (substitutional solute atoms) which are bigger or smaller than the lattice which forms a strain field and dislocations
- if atom is smaller than lattice, you get a tensile strain
- if atom is bigger than lattice, you get a compressive strain
Age (precipitation) hardening - solubility is higher at higher temperatures (like sugar in tea)
Work hardening - for alloys that can’t be precipitation strengthened - reduces ductility
What are the Hume-Rothery rules?
Denote whether an atom will substitute into a lattice
1) If atomic size is +-15% of parent lattice, then it will likely have low solubility. SIZE FACTOR is said to be unfavourable.
2) Big difference in electronegativity when compared with the host is more likely to form a compound.
3) Lower valency solutes are more likely to dissolve in higher valency hosts.
What is the strength of solid solution hardening proportional to?
Amount of solute
Solute misfit
Elastic modulus of matrix
Explain the slip system
Dislocations don’t move with equal ease on all planes and directions
A preferred plane and direction are combined to make the SLIP SYSTEM. The atomic distortion of the slip system is the minimum for that crystal structure.
SLIP PLANE will have most dense atomic packing (more close packed means less movement necessary for deformation)
SLIP DIRECTION will have the highest linear density.
Describe the slip family system and brackets
In FCC system:
SLIP PLANE is the family of {111} planes - 4 unique (111) planes
SLIP DIRECTION is the family of <110> directions - 3 unique [1-10] directions
How does work or strain hardening affect stress?
Permanent increase in flow stress - due to the increase in dislocation density
What happens if you remove and reapply a load to a material?
Some element of elastic strain recovery when removed but when reapplied, no yielding until it reaches same point when load was removed.
What equation shows the relationship that yield stress is proportional to the dislocation density?
Oo = Oi + 1/2 Gbp^1/2
B = Burgers vector of the dislocation
What are the stages of dynamic recovery?
1) Dislocation Tangles (random array of dislocations)
2) Cell formation by rearrangement of dislocations
3) Annihilation of dislocations of opposite sign (Up and Down T’s cancel each other out)
4) Subgrain formation
Explain jerky flow
Dislocations are moving through material
Solute atoms are catching up and pinning them in place
Stress is increased until the solute atoms break away and the dislocations carry on moving
Solute atoms catch up and pin so the jerky flow is formed by a pinning/unpinning which alters the stress
What is solute drag?
In very low solute concentrations, solute atoms reduce recovery by interaction with cells and subgrain boundaries.
What do we mean by a periodic shearing force in a lattice?
Two planes moving relative to each other, force is periodic for each atom that passes - they want to return to original position.
What’s the equation for the shear stress required to move two planes relative to each other?
Tm = G/2pi (very high)
How do theoretical values for yield strength in alloys compare to typical values?
3 orders of magnitude higher than typical values
What are the requirements for an alloy to be age hardened?
Needs to have high solubility at high temp. And low solubility at low strength (dramatic solubility change from high to low temp is good)
What is the process for age hardening?
1) Heat to 550C
2) Quench to room temperature
3) Age at 150-200C which causes precipitation of a fine dispersion of alloy
If you cool slowly, you’ll get coarse precipitates which is poor strengthening as dislocations can move in gaps
If you quench and age at a low temp, the precipitates will be fine and homogeneously distributed
What are the precipitate strengthening mechanisms?
When precipitates uncleared and grow, they intersect with slip planes and their respective dislocations
A dislocation must either CUT (through) or BOW (go around) between the precipitate - will adopt path of least resistance.
Critical parameter for this is inter precipitate spacing - related to size and amount of precipitate
How does precipitate shearing cause stress concentrations and reduces toughness/ductility?
Once particles are sheared, dislocations continue on active slip plane.
Deformation becomes localised, leading to planar slip and pile ups of dislocations at grain boundaries.
What is Orleans looping?
Precipitates that can’t be sheared.
As force is applied, area between precipitates can move and creates a loop - as loop moline loop next to it it pinches and joins
Creates loops which is not good
T = Gb/lambda
What shows the maximum strength that can be developed in an alloy?
The transition from cutting to bowing on a stress v particle diameter graph
What is the main difference in a cast alloy to a wrought alloy?
Cast contain high levels of alloying elements such as Si and Cu.
What is the issue with the high amount of alloying elements in cast alloys?
They are coarse, sharp and brittle which can create harmful internal notches and nuclear cracks.
On a phase diagram, where is the eutectic point and which way is hyper and hypo eutectic?
Eutectic is at the valley point, hyper is to the right and hypo is to the left.
What is the Si content in Al-Si alloy and why is it eutectic?
12wt%Si - eutectic is lowest temp to melt it so cheaper manufacturing
What are the benefits of Al-Si alloy for casting?
High fluidity and low shrinkage so complex shapes such as thin walls can be made.
What are the effects of slow and rapid cooling of Al-Si casting?
Slow - very coarse microstructure with large needles of Si in Al matrix
Rapid - microstructure is greatly refined, improves ductility and tensile strength
What element exists in Al alloys that we want to reduce and how much is a max?
Iron - 0.2% max
What are the two methods of refining Al-Si cast alloy?
1) Inoculation - increased nucleation so finer and more evenly distributed primary Si
2) Modification - changed growth mechanism of eutectic Si
What are the two types of Silicon in alloys?
Primary - forms in hyper eutectic compositions (over 12.2wt%Si)
Eutectic - forms via the eutectic reaction
How does inoculation refine Si alloys?
Phosphide is added, aluminium reacts to make aluminium phosphide particles which act as nucleation sites for primary Si to grow.
How does modification help refine eutectic Si alloys?
Sr or Na are added which are adsorbed on the surfaces of Si and make needle shaped growth difficult - refines the microstructure as needle particles are bad.
