Temporal Evolution of GP Zones in an Al-Ag Alloy

Summary from only given. In the phase-separating Al-Ag system, intermediate metastable precipitates (Guinier Preston zones) are formed. The metastable miscibility gap is characterized by a large asymmetry with a gap extending to ~55 at.% Ag below 175degC and to ~30 at.% Ag above 220degC. The exact structure of the GP zones (ordering, Ag concentration) and the existence of temperature-dependent phases (ordered at low T and disordered at high T) is still a matter of debate and the experimental studies published on this system have yet to concur. Previous atom probe studies have confirmed the variations of GP zone concentration with temperature and a more recent work by Al-Kassab et al. showed the coexistence of two types of precipitates. Recent TEM observations have also indicated that in the initial stages of decomposition, GP zones form with a surprising morphology consisting of a Ag-rich shell surrounding an Al-rich inner core. This paper discusses our recent transmission electron microscopy and atom probe tomography measurements of the three-dimensional Ag concentration in precipitates, including the temporal evolution