Room temperature precipitation in quenched Al–Cu–Mg alloys: a model for the reaction kinetics and yield strength development

Abstract

The microstructural evolution during low temperature ageing of two commercial purity alloys (Al-1.2Cu-1.2Mg-0.2Mn and Al-1.9Cu-1.6Mg-0.2Mn at.%) was investigated. The initial stage of hardening in these alloys is very rapid, with the alloys nearly doubling in hardness during 20 h ageing at room temperature. The microstructural evolution during this stage of hardening was investigated using differential scanning calorimetry (DSC), isothermal calorimetry and threedimensional atom probe analysis (3DAP). It is found that during the hardening a substantial exothermic heat evolution occurs and that the only microstructural change involves the formation of Cu-Mg co-clusters. The kinetics of cluster formation is analysed and the magnitude of the hardening is discussed on the basis of a model incorporating solid solution hardening and modulus hardening originating from the difference in modulus between Al and clusters.

VPPA welds of Al-2024 alloys: Analysis and modelling of local microstructure and strength

“…3D atom probe work has evidenced that in the present alloys room temperature hardening and hardening at 150 ºC is due to the formation of Cu-Mg coclusters [8,38,39], which become unstable at higher temperatures. Thus the endothermic effect up to about 230 to 250ºC is attributed mainly to the dissolution of these clusters [8,25,38]. For position 3, the dissolution of GPB2/S" phase will also contribute to the endothermic effect.…”

“…The two main strengthening contributions to the strength variations across the weld are the increment in critical resolved shear stress (CRSS) due to co-clusters, Δτ cl , and the CRSS increment due to S phase precipitates, Δτ S . Detailed analysis of strengthening and composition of the co-clusters has revealed that Δτ cl ,can be approximated well as [8,26]:…”

“…A further key part of the work is an attempt to predict the local strength of the weld based on the temperature history and nano/microstructure development of the material. This work part of the work is based on new insights into the relation between nano sized co-clusters that was recently developed [8,9]. The obtained hardness and microstructural data will be compared to a model that predicts the microstructural changes and resulting strength of Al-Cu-Mg alloys during ageing.…”

“…Recent work [8,18] using three-dimensional atom-probe indicates that Cu-Mg co-clusters are responsible for rapid hardening in Al-Cu-Mg alloys. This stage accounts for approximately 60% of the total hardness increase during ageing [26].…”

“…[25,8,45], and experimentally verified for a number of precipitation reactions in heat treatable Al based alloys [46,47,48];…”