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

The microstructural evolution during low temperature ageing of two commercial purity alloys (Al1.2Cu-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. ‖ Author for correspondence: Tel: 023 80595094; Fax: 023 80593016; Email:[email protected]