The bi-layered precipitate phase zeta in the Al-Ag alloy system

The Al-Ag system is thought to be a well-understood model system used to study diffusional phase transformations in alloys. Here we report the existence of a new precipitate phase, zeta, in this classical system using scanning transmission electron microscopy (STEM). The zeta phase has a modulated structure composed of alternating bilayers enriched in Al or Ag. Our in situ annealing experiments reveal that the zeta phase is an intermediate precipitate phase between GP zones epsilon and gamma prime. First-principles calculations show that z is a local energy minimum state formed during Ag clustering in Al. The layered structure of zeta is analogous to the well-known Ag segregation at the precipitate-matrix interfaces when Ag is micro- alloyed in various aluminium alloys.