Aluminium Copper Alloys

Question 1

What is the process of precipitation strengthening (hardening) of aluminium copper alloys?

Answer 1

1. Solution treatment at ~ 515C
   
   • All Cu is dissolved
   • Al and Cu atoms diffuse randomly into a uniform substitutional
     solid solution. (this is the α region in the phase diagram.
2. Quench (in water) to room temperature, or below.
   
   • All solute is trapped ie 4% Cu
   • Non equilibrium structure at room temp.
   • Produces a SuperSaturated Solid Solution (ssss) of Cu in Al (Cu
     better dissolved than for solutionising).
3. Artificially aged at a temperature between ~130 – 190C.

Question 2

What are the other phases of ageing?

Answer 2

1) GP 1 zones are formed homogeneously at low temperatures (ie.
130C or below).
* They consist of disks a few copper atoms thick (4 – 6 A) and about
80 – 100A in diameter.
* Cu atoms are about 11% smaller in diameter than Al atoms. Their
lattice parameter is less than that of the matrix and hence a strain
field is formed, though they remain coherent with the matrix.
* The zones, and their associated strain field inhibit dislocation
movement and increase the strength and decrease the ductility of
the alloy.

2) GP 2 zones (’’) form homogeneously at around 130C after quite
long ageing times.
* They are about 10 – 40 A thick and around 100 – 1000 A in diameter
and are also coherent. They produce a further increase in strength.

3) Theta prime phase nucleates heterogeneously on dislocations and they are incoherent with respect to the matrix.
* The size of the ’ particles depends on the ageing time and
temperature, typically they are 100 – 6000 A in diameter and 100 –
150 A thick.
* The alloy is now in the overaged condition and the strength will be
falling

4) Theta phase is precipitated by ageing for extended periods at high
temperatures(~ 190˚C).
* Theta can form from Theta prime or can form directly from the
matrix.
* This phase is present when the alloy is highly overaged.