Precipitation processes during the early stages of ageing in Al-Cu-Mg alloys

Atom probe field ion microscopy (APFIM), transmission electron microscopy (TEM), and microbeam electron diffraction have been used to study the early stages of precipitation in an Al-1.1Cu-1.7Mg (at%) alloy. This alloy exhibits a large and rapid hardening reaction upon ageing for as short as 60 s at temperatures higher than 130 degrees C. Results from APFIM suggest that the formation of Cu-Mg co-clusters, not resolved in TEM, is responsible for the rapid hardening reaction. A period of constant hardness follows the rapid initial reaction, during which the clusters develop into ultra fine rod-like precipitates, considered to be GPB zones and precipitation of the S phase (Al2CuMg) occurs heterogeneously on dislocations. Like the initial clusters, the GPB zones were found to be rich in Cu and Mg. Peak hardness was associated with the further dispersion of S precipitates in the matrix and further growth of GPB zones, suggesting the following precipitation sequence: solid solution --> pre-precipitate stage --> GPB zones + S --> S.