Precipitation behavior of an Al-Cu alloy during isothermal aging at low temperatures

The relationship between microstructures and the thermal stability of precipitates appearing during isothermal aging at 403 K in an Al-Cu alloy was investigated, using Vickers microhardness tests, differential scanning calorimetry (DSC), high-resolution transmission electron microscopy (HRTEM) and high-angular annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations. Using a new terminology, we suggest a modified interpretation that the phase decomposition in the Al-Cu alloy follows the sequence: supersaturated solid solution→quenched clusters→G.P.(I)→fG.P.(II)→θʺ (independent of G.P.(II))→θ′→stable θ. The present HRTEM and HAADF-STEM observations revealed that a quenched cluster is a monolayer Cu-rich plate approximately a few nm Cu in size, G.P.(I) is also a monolayer copper platelet but slightly larger than the cluster. G.P.(II) is bilayer and θʺ comprises multiple (at least three) copper-layers, respectively. The Vickers microhardness increased mainly due to the formation of quenched clusters and G.P.(II).