Defect contrast under weak-beam (WB) conditions was understood in the late sixties. Until then, details on dislocation dissociation below a resolution of about ξg/3 was only matter of speculation. WB elucidated a number of problems on dissociation at a resolution of 2 to 6nm. WB became much less fashionable in the late seventies, leaving considerable domains almost undocumented. At this time, WB remained used mostly by materials scientists in the area of semiconductors and ceramics. A renewed WB activity took place in the mid eighties with the initiation of R & D projects on ordered intermetallics in several countries. Ordered metal alloys are attractive since they offer high specific strengths that may increase with increasing test temperature above ambient up to 600°C-850°C. Unfortunately, most of these systems are so brittle that none of the expected substitutes for superalloys has been designed so far. Presently, Ti-Al is the system that receives the largest attention. Dislocation analysis is important in intermetallics since, excepted in multicomponent systems, the mobility of dislocations is dictated (i) by dissociation symmetries, (ii) by the transitions that dissociation may undergo dynamically and (iii) by the role of temperature on these. In view of the splitting distances that are currently observed, WB is perfectly adapted to this prospect. As a matter of facts, the potential of WB was originally exemplified by a report on four-fold dissociation in ordered Fe3Al. Upon request of the organizers of this session, the present contribution focuses on instrumental electron microscopy and on some aspects of contrast analysis under WB conditions. General information on the implications of core structure analysis on the mechanical properties of alloys together with limitations of dislocation analysis in TEM are reviewed elsewhere.
Diffraction contrast analysis of dislocations in 2D materials using true dark-field and 4D-STEM in SEM
