TEM EVIDENCE FOR DISCLINATIONS IN PLASTICALLY DEFORMED METALS

V. Klemm, P. Klimanek and M. Seefeldt

Freiberg University of Mining and Technology, Institute of Physical Metallurgy\\ Gustav-Zeuner-Str. 5, D-09596 Freiberg, Germany
Phone: ++49-(0)3731-39-3153, Fax: ++49-(0)3731-39-3657, e-mail: klimanek@ww.tu-freiberg.de

Keywords: Disclinations, TEM, Kikuchi line patterns, triple junctions, dislocation rotation boundaries.

Disclinations are the rotational equivalents of the translatational dislocations. They are line defects that can be generated by means of a Volterra process: If the two shores of the cut are twisted instead of shifted against each other, a disclination instead of a dislocation is produced [1]. Since disclinations can be translated into (more or less complex) groups of dislocations, they have not yet found very much interest among crystallographers and materials scientists. However, schematizing the micro- and substructures at large strains and modelling their evolutions in terms of disclinations simplifies the description considerably. Such an approach is able to reproduce the development of the mean misorientation and the flow behaviour at large strains, especially the transition from stage III to stage IV behaviour, in qualitative agreement with experimental results [2].

For the present work, mono- and polycrystalline copper have been rolled to 50 \% thickness reduction at room temperature to search for disclination structures by means of TEM. Especially, it has been checked whether the triple junctions of dislocation rotation boundaries are compensated or noncompensated nodes, that means whether the sum of the misorientation vectors across the three boundaries meeting in the junction gives zero or not. If not, one can guess at a long-range strain field of the rotational type due to the presence of a disclination in the node [1].

An analysis of Kikuchi line patterns proves triple junctions at the splitting of dense dislocation walls (DDW's) into first generation microbands (MB1's) [3] to be such noncompensated nodes. For example, misorientations of about 0.61^o around a [5 27 0] axis across the DDW and of about 0.92^o and 2.72^o around a [1 0 0] and a [9 11 0] axis, respectively, across the MB1 boundaries as well as continously varying orientations of the two adjoining cell blocks have been found in the surroundings of such a triple junction. This corresponds to a disclination with wedge and twist contributions and a strength of about 2.8^o- or to the insertion and rotation of a wedge of matter between the two cell blocks resulting in rotational straining of the cell blocks - similar to the Volterra process for the generation of disclinations.

1. A.E. Romanov and V.I. Vladimirov: Disclinations in Crystalline Solids, in: Dislocations in Solids, vol. 9, ed.: F.R.N. Nabarro, North Holland, Amsterdam 1992, p. 191.
2. M. Seefeldt and P. Klimanek, accepted for publication in Modelling and Simulation in Materials Science and Engineering (1998).
3. V.S. Ananthan, T. Leffers and N. Hansen, Mater. Sci. Technol., {\bf 7}, 1069 (1991).