model for the reaction kinetics and yield-strength development.”

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 arise from the formation of Cu-Mg co-clusters. Here, it is shown that the measured enthalpy of formation of clusters (~0.3eV 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-cluster formation by DSC indicates that the formation of Cu-Mg co-clusters 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. In this reply to [1], which contains some comments on our paper [2], we would like to discuss a number of issues pertinent to points raised by Zahra et al. in the course of their paper. Before we deal with these we first want to note that much of [1] is taken up by a literature review and by