LARGE ANGLE CONVERGENT BEAM ELECTRON DIFFRACTION OF GRAIN BOUNDARIES. J. P. Morniroli, Laboratoire de Métallurgie Physique de l'URA CNRS 234, Université de Lille I, 59655 Villeneuve d'Ascq Cédex, France

The transmitted disc (Bright Field disc) of Large Angle Convergent Beam Electron Diffraction (LACBED) patterns exhibits three main features which are very well-adapted to the analysis of grain boundaries:

- it is composed of many dark lines called Bragg lines or Bragg contours. The lines with large extinction distances are very thin.

- most of the inelastic scattering is removed meaning that the Bragg lines have a very high contrast with respect to the background.

- it displays, at the same time, an image of the illuminated area.

Therefore, LACBED patterns can be used to analyse with great accuracy the crystal orientation with respect to the incident beam. For a grain boundary, they can be used to measure the misorientation. LACBED experiments performed on many types of grain boundaries (twins, [[Sigma]]9 and [[Sigma]]25 coïncidence boundaries, subgrain boundaries and high misorientation boundaries) present in various types of specimens (alloys, semi- conductors, minerals) have shown that the misorientation can be measured with an accuracy (about 0.01 to 0.05deg.) better than the one obtained with other conventional techniques (Kikuchi lines, ECP, EBSD..).

In addition, since the LACBED technique also allows the characterization of the Burgers vector of dislocations [1], we have analysed dislocations present in [[Sigma]]3 and [[Sigma]]9 silicon coincidence grain boundaries as well as dislocations present in subgrain boundaries of the [[sigma]] phase in steels. The results obtained are in good agreement with theoretical models [2].

1 D. Cherns and A. R. Preston, J. Electr. Micr. Tech. 13 (1989) 111

2 J. P. Morniroli and D. Cherns, to appear in Ultramicroscopy