Reply to the comments on “Room-temperature precipitation in quenched Al–Cu–Mg alloys: a model for the reaction kinetics and yield-strength development”

Abstract

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 (∼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.

Acknowledgements

Ms Laury Davin is acknowledged for additional discussions and APFIM analysis.

Notes

¹ It is noted that in 3DAP only the larger co-clusters are detected (the limit of 6–7 atoms in the reconstruction used in 2 means that due to the 50% overall detection efficiency clusters with a minimum of 12–14 are detected). Thus APFIM cannot provide data on the amount of Mg atoms involved in the co-clustering process, and we have to rely here on estimates from a regular solution model.

² “GPI zones” compete with Cu–Mg co-clusters for Cu atoms, and a shift in the relative proportion of the two structures on adding substantial Mg to Al–Cu alloys indicates that the formation enthalpies of the two structures are similar. The enthalpy of formation of “GPI zones” (we could also call them sub-nanometre platelets of θ″ (Al₃Cu) phase) at temperatures near room temperature has been reported to be about 13 kJ per mol Cu in the zones (0.13 eV) 23. This is similar to the enthalpy of formation for Cu–Mg co-clusters calculated here, and this comparison provides indication that the enthalpy of formation for Cu–Mg co-clusters calculated here must be at least reasonable.