High Resolution Lattice Images of Ordered Structures in Al–Li Alloys

Ordered structures formed in Al–Li alloys during the decomposition were investigated by means of high resolution electron microscopy. Image calculations were also performed to interpret the obtained images.In the initial stage of decomposition (323 K, 86.4 ks), extremely fine domains which had already an ordered structure of the L12-type were formed densely. The domains were separated by the vague and complicated boundary areas with incompletely ordered structure. In the proceeded stage of decomposition (473 K, 86.4 ks), spherical δ′ particles were formed and the atomic arrangement of a L12-type ordered structure was clearly observed within δ′ particles. The δ′/matrix interfaces were very clear and have a step-like morphology. It appeared that small segments with locally ordered structure were formed in the matrix during the electron microscope observation. Image calculations for the Al–Li alloys proved that atom sites were revealed as bright contrast dots under the proper defocus conditions, and that the image contrast was not changed drastically with the total specimen thickness at least up to ∼40 nm and depth of the ordered region. This is useful in discussing the formtion mechanism and spatial distribution of ordered domains in this alloy by using high resolution electron micrographs.