Solidification of Al-Cu alloys page 1 of 5
Observant people may have noticed that the Al-Cu phase diagram shown only goes up to around 60%, by weight, of Copper. The Al-Cu phase diagram is split at around 54wt%Cu by a particular phase. This "split" means that the two parts of the diagram can be considered separately. The diagram up to the 54% point is very similar to the "standard" phase diagram.

When a phase diagram, such as this one, has an intermetallic phase it is not named alpha or beta but is assigned another Greek letter. Unfortunately there is no system to this naming, it is purely convention. Here the "right" hand phase is named theta but other than its name it is dealt with in exactly the same way as a beta phase.

This diagram is on p20 in your books.
Solidification of Al-Cu alloys page 2 of 5
Consider two Al-Cu alloys, one of composition 33wt%Cu and the other of 20wt%Cu.

Examining the phase diagram:

  • the 33wt%Cu alloy is of eutectic composition and
  • the 20wt%Cu alloy is hypoeutectic as it is to the left of the eutectic point.

So, first considering the Al-33%Cu alloy:
Solidification of Al-Cu alloys page 3 of 5
The eutectic reaction, at 33%Cu can be shown as:

Liquid > alpha + theta

So, the two phases grow simultaneously as an interconnected structure - the eutectic phase. The lamellar nature of the eutectic mixture as it solidifies ensures that diffusion fields ahead of the liquid-solid interface are limited (i.e. the atoms do not have to travel vast distances for the two phases to form simultaneously)

Micrograph of Al-33%wtCu alloy
Solidification of Al-Cu alloys page 4 of 5
Now, the Al-20wt%Cu alloy is hypoeutectic. The liquid alloy initially contains a higher percentage of Al than that corresponding to the eutectic composition; the primary solid phase that forms is therefore alpha.

Formation of the alpha phase, however, depletes the remaining liquid in Aluminium (Al) and the liquid composition shifts toward the eutectic composition. When the composition reaches eutectic (33%Cu for this phase diagram) the remaining liquid follows the eutectic reaction.

So, at room temperature, the microstructure consists of primary alpha dendrites surrounded by a finely divided eutectic mixture of two solid phases (alpha+theta).

Micrograph of Al-20%wtCu alloy
Solidification of Al-Cu alloys page 5 of 5
Now, the question presents itself : "Why do I want to know this ?"

Well, microstructural changes strongly influence the engineering properties of an alloy. The lamellar nature of the eutectic phase, for instance, impedes dislocation movement through the alloy. If dislocations cannot move easily then the material won't yield until a higher stress is applied, i.e. the strength of the alloy is increased.

So, by understanding how phase diagrams work we can predict and refine alloys to give us the properties we require.
Solidification of Al-Cu alloys more info
Micrograph of Al-33%wtCu alloy

This specimen has been metallographically prepared (mounted, ground and polished) and then etched in dilute (10%) nitric acid which stains the surface of the theta phase brown/black whilst leaving the alpha phase unattacked - thus appearing white. The specimen was then photographed under a microscope. The scale is shown in the bottom corner of the micrograph.
Solidification of Al-Cu alloys more info
Micrograph of Al-20%wtCu alloy

This specimen has been metallographically prepared (mounted, ground and polished) and then etched in dilute (10%) nitric acid which stains the surface of the theta phase brown/black whilst leaving the alpha phase unattacked - thus appearing white. The specimen was then photographed under a microscope. The scale is shown in the bottom corner of the micrograph.