Precipitation behavior of AA2618

The precipitation behavior of AA2618 was studied by a multitude of characterization techniques: microhardness testing, lattice parameter measurement through X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and atom probe field ion microscopy (APFIM). The matrix lattice parameter increased during the first 20 hours of natural aging, due to the formation of Cu clusters and decreased over the next 24 hours, due to the formation of Mg-rich clusters. Prior natural aging weakened subsequent artificial aging hardening at 180 degrees C, 200 degrees C, and 230 degrees C, due to the cluster reversion that delayed the precipitation of strengthening phases. The matrix lattice parameter exhibited erratic changes during artificial aging that corresponded to the formation and partial dissolution of Guinier-Preston-Bagaryatsky (GPB) zones, the transformation of GPB zones to GPB2 zones, and the precipitation of S'. The structural changes during the artificial aging of AA2618 occur in this sequence: supersaturated solid solution -> clusters + GPB -> GPB + GPB2 -> GPB2 + S' -> S'+ S -> S.