Duralumin is an aluminium alloy containing 4wt% copper, as well as smaller amounts of other elements. The impurities in the material changes its properties by changing the microstructure, and since the distribution of the copper atoms can be varied using heat treatments, a variety of microstructures, and hence properties can be produced.
In the samples used in this experiment, the copper forms precipitates of CuAl2 within an aluminium matrix - see image below. These precipitates hinder the movement of dislocations and substantially strengthens the alloy. This process is widely used to make strong aluminium alloys for structural purposes, and is known as precipitation hardening .
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