Origin of the initial rapid age hardening in an Al-1.7 at. % Mg-1.1 at.% Cu alloy

The origin of the rapid hardening which occurs during the initial ageing st age of an Al-1.7 at. % Mg-1.1 at. % Cu alloy has been investigated by the t hree-dimensional atom-probe (3DAP) and transmission electron microscopy tec hniques. Although the rapid hardening reaction occurs within 1 min at 150 d egrees C, no evidence for solute clusters or Guinier-Preston-Bagaryatsky (G PB) zones was detected using the 3DAP after this ageing time. When a short- term aged specimen is deformed and then aged at 150 degrees C, further rapi d hardening occurs. This suggests that the rapid hardening is associated wi th a solute-dislocation interaction. Uniform dispersions of Cu-Mg co-cluste rs do not occur until closer to the end of the hardness plateau and it is t hought that these evolve into GPB zones during the second stage of the age- hardening process.