In this section we aim to develop an understanding of the processing-structure relationship in age-hardening aluminium alloys (i.e. the 2xxx, 6xxx and 7xxx alloys).

This involves looking at how a fine distribution of precipitates can be achieved by suitable heat treatment.

The effects of such precipitates on the alloy properties (esp. strength) are covered in the next section.

Learning Outcomes for this Section

After completing this section, you should be able to:

• describe the major requirement for an alloy system to respond to age-hardening
• explain the purposes of solution heat treatment, quenching and ageing
• explain why upon ageing, the formation of the equilibrium phase may be preceded by one or more metastable phases
• list and describe the possible phases for Al-4%Cu
• explain why the Al-4%Cu system is more of scientific than practical interest
• draw a set of hypothetical TTT curves for metastable phases (e.g. in Al-Cu)
• relate these curves to a set of metastable solvuses, and hence explain why an upper temperature limit exists above which a (metastable) phase is unstable
• explain how Precipitate Free Zones (PFZs) are formed and why they are detrimental to a material's properties
• explain how a heat treatment can be selected to minimise the widths of Precipitate Free Zones (PFZs)