The aluminium-copper system represents the classical age hardening alloy, where copper atoms that are in solid solution go through a sequence of metastable-precipitation events that ultimately end in the equilibrium CuAl2 compound.
However, the CuAl2 is difficult to nucleate so the process begins with the formation of "GP1" zones, in which approximately 10 nm diameter copper-rich plates form on the {100}Al planes. These then develop into bigger GP2 zones that also are coherent.
The atom-probe images below were taken from Al-4Cu wt% solid solution that was aged at 130°C for 95 h to produce the GP zones illustrated by the Cu+ image.
Photographs courtesy of Dr Sally Waugh.
Field ion image showing the positions of all atoms, imaging atom-probe picture showing the copper atoms and corresponding aluminium atom image.
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