Crystal Superstructure Research Articles

The application of dark‐field electron microscopy to the study of short‐range order in alloys

SUMMARYThrough the comparison of electron diffraction and high resolution dark‐field imaging from two ‘model’ short‐range ordered alloys (quenched CuPt and quenched Ni4Mo) the different nature of short‐range order in these systems is made apparent. Dark‐field stereo microscopy is used to overcome the difficulties due to the projection of the local atomic arrangement, through the foil thickness, on to a plane perpendicular to the electron beam. It is thus possible to distinguish between three‐dimensionally bound scattering regions (called microdomains, when they are associated with a crystal superstructure and have a physically realistic size) and other scattering regions, apparently due to the superposition of scattering from several locally correlated groups of atoms. In this way and through the comparison of other dark‐field images it has been shown that local ordering in CuPt is of the form of microdomains of size 0ṁ7–10 nm, with an imperfectly ordered L ***11 structure, whilst a more appropriate description of short range order for Ni4Mo is that of static concentration wave packets. The results obtained on other short‐range ordered alloys are reviewed in the light of this application.

The crystal structure and superstructure of the K phase, Mn77Fe4Si19

The crystal structure and superstructure of the K phase, Mn77Fe4Si19