Mikrostrukturen i valset kobber

Henrik Christoffersen

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    The present thesis, submitted in partial fulfilment of the requirements for a Ph.D. degree from the University of Copenhagen, describes the microstructure in copper rolled to low and intermediate strains. The emphasis is on the “high wall density” component of the micro-structure.

    The experimental techniques used in the thesis are transmission electron microscopy and scanning electron microscopy. In transmission electron microscopy a slightly modified ver-sion of the hollow-cone technique has been used. In the field of scanning electron microscopy the resolution and the reliability of the back-scattering images has been investigated, for instance with comparison with transmission images.

    In connection with the general study of the development of the microstructure in rolled copper a number of quantitative investigations have been conducted as listed below.

    • The orientation of the dislocation walls It was found that the dislocation walls have no preference for orientations parallel to the {111} slip planes. Viewed in the longitudinal section (parallel to the rolling direction and the normal direction) the walls form angles of approximately ± 45° with the rolling direction. According to simple geometry this angle should decrease with increasing rolling reduction. A certain reduction was observed, but it is much smaller than the reduction expected theoretically. This must mean that the dislocation walls rotate in a direction opposite to the geometrically im-posed rotation.

    • The orientation distribution of the dislocation walls To a first crude approximation the walls are parallel to the transverse direction. However, as seen in transverse sections and sections parallel to the rolling plane (perpendicular to the rolling direction and the normal direction, respectively) this is a very crude approximation. A geometrical model with added rotation about axes parallel to the rolling and normal directions gives a better description of the wall orientations – even though it is not possible to reconcile all observations in the different sections. The ultimate statistical description of the wall orientations, independent of geometrical models, is the two-dimensional orientation distribution of the plane normals. A method for the synthesis of such an orientation distribution is developed. The resulting distribution reflects a rather wide spread in the orientations of the walls.

    • Microstructure and Crystallographic orientation An attempt was made to correlate the type of microstructure in the different grains (“high wall density” structure or the alter-native “low wall density” structure) to the crystallographic orientation of the grains. There was a certain, but not a very convincing, correlation. For a third not very com-mon, type of microstructure there was a very clear correlation with crystallographic orientation. In a parallel investigation of rolled brass, a clear correlation was found between the type of microstructure (twin lamellae or no twin lamellae) and the crystal-lographic orientation.

    Even though a number of details have been elucidated in the present work, the mechanics and the micromechanics behind the microstructures are not really understood. However, cer-tain preliminary conclusions may be drawn:

    • The initial formation of the high wall density structure is governed by specific disloca-

    tion interactions.

    • The subsequent development may be seen as subdivision of the grains into regions with different strains, different slip patterns and a number of active slip systems smaller than the five required in the Taylor model.

    • Grain-to-grain interaction, as modified by the specific neighbour relations plays a sig-nificant role in the formation of microstructure.
    Original languageDanish
    Place of PublicationRoskilde
    PublisherForskningscenter Risø
    Number of pages147
    ISBN (Print)87-550-2260-3
    Publication statusPublished - 1997
    SeriesDenmark. Forskningscenter Risoe. Risoe-R

    Note re. dissertation

    The present thesis is submitted in partial fulfilment of the requirements for a Ph.D. degree from the University of Copenhagen.

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