Our recent work on Al-Cu-Mg-based alloys with Cu:Mg ratio close to unity showed that the rapid hardening at room temperature and the substantial heat evolution arising from the formation of Cu-Mg co-clusters. Here, it is shown that the measured enthalpy of formation of clusters (similar to 0.3 eV per Mg atom) is in reasonable agreement with expectations based on the similarity with Mg-vacancy clusters. The origin of the term GPB zones, as applied to the rapid hardening in Al-Cu-Mg-based alloys, is investigated. It is shown that current knowledge on the nanostucture and microstructure development during rapid hardening can be described without recourse to this alloy-specific term. Analysis of the kinetics of Cu-Mg co-clusters formation by DSC indicates that the formation of Cu-Mg co-cluster formation during a fast water quench can be sufficiently suppressed to cause substantial nucleation of co-clusters to occur in subsequent natural ageing and artificial ageing at low temperatures
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