In the previous section, we saw how a fine dispersion of GP zones or precipitates can be formed by heat treatment. In this section, we will go on to look at how these precipitates strengthen the alloy by inhibiting dislocation mobility.

Learning Outcomes for this Section

After completing this section, you should be able to:

• distinguish between coherency strain, chemical and dispersion hardening
• associate each of the above with different microstructures
• explain why coherency strain hardening is maximised at a critical zone/precipitate spacing
• list and explain 3 contributions to chemical hardening
• quote the Orowan bowing stress equation
• compare the nature of slip in 'underaged' and 'overaged' alloys and explain how this effects work hardening rate, fatigue and stress-corrosion properties
• show, using illustrations, how an overall ageing curve can be compiled from the individual strengthening contribution

Pre-Requisites

Before starting, it is important that you are familiar with the following terms: dislocation; solid solution; strain; stress;