Abstract:

This work demonstrates significant improvements in both the aging kinetics and precipitation hardening of an Al-Li-Cu alloy with the minor addition of Cd (0.06 at.%). The precipitation hardening effect of T₁ precipitates in casting Al-Li-Cu alloys has long been ignored since it is difficult to achieve a high number density of fine precipitates without a large number of dislocations. A detailed transmission electron microscopy investigation shows that the Cd addition has changed the distribution of T₁ precipitates from the conventional uneven distribution near dislocations or grain boundaries to a more homogeneous manner. Most of the Cd-rich nanoparticles were observed at the broad face and/or terminal of the T₁ platelets. It is highly likely that these nanoparticles act as heterogeneous nucleation sites, which consequently leads to a higher number density of T₁ precipitates. Moreover, Cd atoms were preferentially segregated within δ′ precipitates, which can be attributed to the strong bonding between Li and Cd. The interactions between Cd and the T₁ (Al₂CuLi) and δ′ (Al₃Li) precipitates in Al-Li-Cu alloy are first reported. The present study may propose a new mechanism to effectively improve precipitation kinetics and therefore the age-hardening effect of Al-Li-Cu alloys.

Key words: Aluminum alloys, Three-dimensional atom probe (3DAP), High-angle annular dark-field (HAADF), Precipitation, Cadmium