Reply to the Comments on “ Room - temperature precipitation in quenched Al - Cu - Mg alloys : a 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 CuMg 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 a review of calorimetry data, which do not contradict our work. We will not comment on this but we do need to stress that the final interpretation by Zahra et al. [1] “[..] plate-like GPB zones which nucleate on clusters of irregular shape are responsible for the first hardening stage” is inconsistent with our three-dimensional atom probe (3DAP), differential scanning calorimetry (DSC) and hardness data on the two alloys we studied (Al-1.2Cu-1.2Mg and the Al-1.9Cu-1.6Mg (at%)): our 3DAP data shows that neither of these alloys contain substantial numbers of platelet or rod-like structures such as GPB zones during several days ageing at room temperature, whilst the hardness increase and heat evolution is completed within about one day. We will show that all the data and theoretical considerations expressed in [1], as well as data in papers cited in [1], are fully consistent with our conclusions. Specifically we need to note that nowhere in [1], or in any of the papers cited therein, is any nano/microstructural data presented that show platelet or rod-shaped GPB zones within Al-Cu-Mg alloy samples with compositions and heat treatments near the ones we have studied. We believe that there is no conclusive microstructural data having the necessary resolution on Al-Cu-Mg alloys with both Cu and Mg contents in excess of 1at%, heat treated to times up to 5 × * Corresponding author: Tel: 023 80595094; Fax: 023 80593016; Email:[email protected]