Enhanced Strength-Ductility Synergy in an Al–Cu Alloy Via Cd-induced Hybrid Θ″+θ′ Precipitation

In this work, trace Cd addition (∼0.2 wt%) was exploited in Al–Cu alloys to improve the mechanical properties by introducing the hybrid θ″+θ′ precipitates, as governed by the solid-solution temperature T. In detail, if the solid solution temperature is low (T = 500 °C), Cd microalloying merely initiates undesirable dual precipitates of θ″ and θ′ precipitates with similar size and homogenous distribution. In such case, the Cd-microalloyed Al–Cu alloy suffers from limited optimization on the strength-ductility synergy in comparison to the Cd-free Al–Cu alloy. However, elevating T to 530 °C results in the formation of dense Cd-rich nanoparticles, significantly promoting the hybrid θ″+θ′ precipitation with bimodal distribution by offering numerous heterogeneous Cd-rich nucleation sites. Such hybrid θ″+θ′ precipitation imparts the architecture of microstructural heterogeneity, which was found to be responsible for the rapid age-hardening response and enhanced strength-ductility synergy in the Cd-microalloyed Al–Cu alloy.