The metal after cold working has raised its internal energy and is therefore in a metastable, high energy state. It will try and lower its internal energy. In most cases it needs some external agent to help it to lower its internal energy. Recovery is the earliest onset of re-arrangement of crystal defects in the cold worked microstructure at elevated temperature, where some restoration of the original structure and properties may occur through annihilation of point defects and dislocations and, spatial re-arrangement of dislocations. Consequently, the metal will soften.

In the case of deformed metals, point defects and dislocations are created during deformation. Point defects anneal out at lower temperatures and do not usually constitute a separately identifiable stage of recovery.

In this module, we shall study in detail the microstructural changes that take place during recovery of Al alloys.

Learning Outcomes for this Section

After completing this section, you should be able to:

• describe microstructural changes that take place during recovery
• state the two types of recovery mechanisms
• explain the driving force for recovery
• explain what is meant by the term 'polygonisation'
• explain what is meant by sub-grains and sub-grain growth
• define the term Solute Drag
• state the factors that influence recovery kinetics
• explain the effect of strain on recovery
• explain why recovery affects material properties such as strength, dislocation density and electrical resistivity
• state the experimental techniques that can be used to study recovery

Pre-Requisites

Before starting, it is important that you are familiar with the following terms: climb; cross-slip; dislocation; glide;

You should be familiar with the concepts covered in: Solute Hardening; Cold Work and Stored Energy;