Transmission electron microscopy, differential scanning calorimetry and hardness tests have been used to study the precipitation sequence on artificial ageing of a stretched Al–Cu–Mg alloy. The samples were aged for different times at 150 and 190 °C, respectively. Some orthorhombic GPB2/S? is present in samples aged at 150 °C for 48 h, which is at the very start of the second stage of hardening. The combined experiments clearly show that the second stage hardening is dominated by S phase, which forms a dense precipitate structure at the peak hardness stage, whilst no significant amounts of other phases or zones are detected
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