  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.
