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 stage of an Al-1.7 at. % Mg-1.1 at. % Cu alloy has been investigated by the three-dimensional atom-probe (3DAP) and transmission electron microscopy techniques. Although the rapid hardening reaction occurs within 1 min at 150 degrees C, no evidence for solute clusters or Guinier-Preston-Bagaryatsky (GPB) 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 rapid hardening occurs. This suggests that the rapid hardening is associated with a solute-dislocation interaction. Uniform dispersions of Cu-Mg co-clusters do not occur until closer to the end of the hardness plateau and it is thought that these evolve into GPB zones during the second stage of the age-hardening process.