The formation and evolution of a novel Cu-Ag-Cd cluster and its effect on microstructures and mechanical properties of an Al-Cu-Ag-Cd alloy

Abstract

In this work, a novel Cu-Ag-Cd cluster was observed in an Al-Cu-Ag-Cd alloy aged at 165 °C. The evolution of Cu-Ag-Cd clusters is divided into nucleation, growth and transformation stages, as revealed by atom probe tomography (APT). First-principles density functional (DFT) calculation results show the Cu-Ag-Cd cluster prefers to form from the segregation of Cu, Ag and Cd atoms directly. Scanning transmission electron microscopy (STEM) analysis proposes the presence of these Cu-Ag-Cd clusters promotes the precipitation of θ″, leading to a θ″ phase dominated microstructures with hybrid θ″+ θ′ precipitates in the peak-aged alloy. The contribution of these clusters is also quantified during aged at 165 °C. Our calculation results suggest order strengthening is the main strengthening mechanism of these clusters.