Precipitation hardening is responsible for the strongest aluminium alloys currently in service. It usually involves the precipitation of metastable phases from a supersaturated solid solution. A textbook case is the precipitation of Guinier-Preston (GP) zones in Al-Cu alloys [1] and the subsequent formation of θʺ ″and θʹ′ metastable phases [2]. More than 75 years after the original work by Guinier and Preston, the atomic-scale mechanisms behind the solid-solid phase transformations associated with the nucleation and growth of those phases remain unknown. One aspect of phase transformations that was proposed many decades ago [3] but never demonstrated, is the direct nucleation of a phase from its coherent precipitate precursor. In this contribution we show that this particular phase transformation is indeed possible and can be brought about in different ways.

We used an aberration-corrected FEI Titan3 80-300 at 300 kV in high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) mode to image solid-state precipitates in Al-1.7 at.% Cu, Al-1.7 at.% Ag and Al-1.7 at.% Cu-0.02 at.% Au alloys. The images shown were not processed apart from minor brightness-contrast adjustments.