Al Alloys

Question 1

What is a light alloy, examples and applications?

Answer 1

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

Question 2

What are the advantages and disadvantages of aluminium?

Answer 2

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

Question 3

What are some of the physical properties of aluminium?

Answer 3

Face centred cubic

Melting point 660C

Density 2700kg/m3 - steel is around 8000

Several alloy additions capable such as Cu, Mn, Si, Mg

Question 4

What are the two forms of alloys?

Answer 4

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

Question 5

What denotes an aluminium alloy?

Answer 5

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)

Question 6

What are the 3 strengthening mechanisms of aluminium alloys?

Answer 6

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

Question 7

What are the Hume-Rothery rules?

Answer 7

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.

Question 8

What is the strength of solid solution hardening proportional to?

Answer 8

Amount of solute

Solute misfit

Elastic modulus of matrix

Question 9

Explain the slip system

Answer 9

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.

Question 10

Describe the slip family system and brackets

Answer 10

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

Question 11

How does work or strain hardening affect stress?

Answer 11

Permanent increase in flow stress - due to the increase in dislocation density

Question 12

What happens if you remove and reapply a load to a material?

Answer 12

Some element of elastic strain recovery when removed but when reapplied, no yielding until it reaches same point when load was removed.

Question 13

What equation shows the relationship that yield stress is proportional to the dislocation density?

Answer 13

Oo = Oi + 1/2 Gbp^1/2

B = Burgers vector of the dislocation

Question 14

What are the stages of dynamic recovery?

Answer 14

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

Question 15

Explain jerky flow

Answer 15

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

Question 16

What is solute drag?

Answer 16

In very low solute concentrations, solute atoms reduce recovery by interaction with cells and subgrain boundaries.

Question 17

What do we mean by a periodic shearing force in a lattice?

Answer 17

Two planes moving relative to each other, force is periodic for each atom that passes - they want to return to original position.

Question 18

What’s the equation for the shear stress required to move two planes relative to each other?

Answer 18

Tm = G/2pi (very high)

Question 19

How do theoretical values for yield strength in alloys compare to typical values?

Answer 19

3 orders of magnitude higher than typical values

Question 20

What are the requirements for an alloy to be age hardened?

Answer 20

Needs to have high solubility at high temp. And low solubility at low strength (dramatic solubility change from high to low temp is good)

Question 21

What is the process for age hardening?

Answer 21

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

Question 22

What are the precipitate strengthening mechanisms?

Answer 22

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

Question 23

How does precipitate shearing cause stress concentrations and reduces toughness/ductility?

Answer 23

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.

Question 24

What is Orleans looping?

Answer 24

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

Question 25

What shows the maximum strength that can be developed in an alloy?

Answer 25

The transition from cutting to bowing on a stress v particle diameter graph

Question 26

What is the main difference in a cast alloy to a wrought alloy?

Answer 26

Cast contain high levels of alloying elements such as Si and Cu.

Question 27

What is the issue with the high amount of alloying elements in cast alloys?

Answer 27

They are coarse, sharp and brittle which can create harmful internal notches and nuclear cracks.

Question 28

On a phase diagram, where is the eutectic point and which way is hyper and hypo eutectic?

Answer 28

Eutectic is at the valley point, hyper is to the right and hypo is to the left.

Question 29

What is the Si content in Al-Si alloy and why is it eutectic?

Answer 29

12wt%Si - eutectic is lowest temp to melt it so cheaper manufacturing

Question 30

What are the benefits of Al-Si alloy for casting?

Answer 30

High fluidity and low shrinkage so complex shapes such as thin walls can be made.

Question 31

What are the effects of slow and rapid cooling of Al-Si casting?

Answer 31

Slow - very coarse microstructure with large needles of Si in Al matrix

Rapid - microstructure is greatly refined, improves ductility and tensile strength

Question 32

What element exists in Al alloys that we want to reduce and how much is a max?

Answer 32

Iron - 0.2% max

Question 33

What are the two methods of refining Al-Si cast alloy?

Answer 33

1) Inoculation - increased nucleation so finer and more evenly distributed primary Si

2) Modification - changed growth mechanism of eutectic Si

Question 34

What are the two types of Silicon in alloys?

Answer 34

Primary - forms in hyper eutectic compositions (over 12.2wt%Si)

Eutectic - forms via the eutectic reaction

Question 35

How does inoculation refine Si alloys?

Answer 35

Phosphide is added, aluminium reacts to make aluminium phosphide particles which act as nucleation sites for primary Si to grow.

Question 36

How does modification help refine eutectic Si alloys?

Answer 36

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.