Dislocation patterns in graphite

A detailed analysis is given of the dislocation patterns observed by means of electron microscopy in graphite single crystal flakes. It is found that the dislocations are ribbons consisting of two partiale of the Shockley type separated by a strip of stacking fault. All patterns can be interpreted in a consistent way on the basis of this model and on the assumption that only small energy stacking faults occur, i.e. faults consisting of a lamella of rhombohedrally stacked material. The stacking fault energy is obtained from the width of the dislocation ribbons and from the shape of extended nodes of partials with known Burgers vector. The dependence of the width of the ribbon on the angle between the direction of the ribbon and the Burgers vector is checked, and it is shown that this can be used to determine an effective Poisson's ratio. The possible interactions between ribbons are discussed from a theoretical point of view and compared with observed configurations. The Burgers vectors were determined experimentally by making use of contrast effects. Graphite quenched from 3000° C and annealed at 1200° C is found to contain vacancy loops. The annealing peak at 1200-1300° C, found previously by means of electrical measurements and measurements of stored energy, is therefore associated with the formation of vacancy loops. It is shown that from a determination of the Burgers vector a conclusion as to the nature of the loops can be reached. The interaction between vacancy loops and glissile dislocations is discussed from a theoretical point of view. It is shown that vacancy loops tend to form preferentially in the stacking fault ribbons. The formation mechanism and the structure of dislocation networks are discussed as well as the nature of the twin boundaries. Dislocation movement and the formation of constrictions in the ribbons are discussed.