Characterization of precipitates in an aged 7xxx series Al alloy

Precipitates in a 7050 Al alloy under two-step ageing treatments were characterized using an energy-compensated optical position-sensitive atom probe and transmission electron microscopy. Selected-area diffraction analyses indicated that the early-stage precipitates present after a stabilization ageing treatment at 121 °C for 4 h have a crystallographic structure similar to η', but probably rich with defects. After T6 treatment ageing at 121°C for 24 h followed byl2 h at 154°C, η' and η coexist within the Al matrix. Energy-compensated optical position-sensitive atom probe analysis was used to measure the partitioning of Zn, Mg and Cu to the precipitates. Early-stage precipitates in the alloy after stabilization treatment have a Zn/Mg ratio of ∼1.3 and a (Zn + Cu)/Mg ratio of 1.4. The T6-aged alloy contained η precipitates with an Zn/Mg ratio of ∼1.2 and a (Zn + Cu)/Mg ratio of 1.3 coexisting with η' precipitates with a Zn/Mg ratio of ∼0.7 and a (Zn + Cu)/Mg ratio of ∼1.