Discussion
During the ageing of a homogenized Al-Cu alloy (1 to 4 wt.% Cu) below the GP zone solvus, hardness of the alloy:
Answer: C
The hardness first increases and then decreases.
Ageing a supersaturated Al-Cu solid solution forms coherent GP zones and then the transition phases \(\theta''\) and \(\theta'\). These fine, closely spaced precipitates obstruct dislocations and raise the hardness up to a peak. With longer ageing the precipitates coarsen, lose coherency and move towards the equilibrium \(\theta\) (CuAl2); dislocations can then bow around them more easily and the hardness falls (overageing).
A monotonic change in either direction misses one of these two stages, and an initial drop is not part of the age-hardening curve.
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