Guinier-Preston zones observed by high-angle annular detector dark-field scanning transmission electron microscopy

We have examined Guinier-Preston (GP) zones in aged Al-Cu alloys using atomic-resolution high-angle annular detector dark-field (HAADF) scanning transmission electron microscopy (STEM), together with conventional high-resolution transmission electron microscopy (HRTEM). Because the imaging of coherent Cu precipitates in an Al matrix by HAADF-STEM is less sensitive to the imaging conditions that would severely limit HRTEM, densely populated GP-I zones are clearly observed. From the HAADF-STEM observations, we have found the GP-I zones to consist of two Cu layers, in addition to the commonly known monolayered GP-I zones. The structure of GP-II zones, in which Cu monolayers are separated by three Al layers, was identified, whereas a variant of GP-II zones does exist, for example double Cu layers separated by a single Al layer. In addition, in the GP-I stage, we have occasionally found Cu monolayers separated by three Al layers, which can be the incipient stage of GP-II zones. These results are in general agreement with previous observations by small-angle scattering and field ion microscopy. Also, our results indicate that the GP-II zone can act as a nucleation site for the θ′ phase.