Characterization of Guinier-Preston Zones by High-Angle Annular Detector Dark-Field Scanning Transmission Electron Microscopy

Age-hardened alloys are everywhere: in aeroplanes, aluminum sashes, electrical connections for computer devices, and so on. The materials that have made possible the very existence of these human inventions are manufactured based on a single materials-science principle known as ageing. Here, supersaturated impurity atoms that dissolved in an otherwise pure metal gather around inside the metal as time goes by, and precipitates, sometimes in the form of a spherical cluster, sometimes in the form of a disc aligned in parallel to a crystallographic plane of the metal matrix. These crystallographically coherent precipitations are collectively known as Guinier-Preston (GP) zones after the pioneering scientists who revealed the structure of the zones in the Al-Cu system by X-ray diffraction technique [1, 2]. Since these precipitate phases are highly coherent with the matrix, it is very difficult to identify their structure unambiguously even with high-resolution transmission electron microscopy (HRTEM). For example, in the Al-Cu system, the Cu atoms precipitate in parallel to the {100} planes of the Al matrix, and thus the phase contrast is highly sensitive to a number of experimental conditions, including the thickness of the specimen and defocus of the objective lens. This can be evidenced by a number of simulation works that have been reported so far in order to elucidate the structure [3-7]. High-angle annular detector darkfield scanning transmission electron microscopy (HAADF-STEM), on the other Toyohiko J. Konno , Masahiro Kawasaki and Kenji Hiraga