Date of Award

1985

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Transmission electron microscopy has been used to study grain boundary structure and dislocation reactions in annealed pure nickel sheets. In slightly annealed nickel, the dislocation content and distribution in grain boundaries were found to be random. After annealing at 800(DEGREES)C, a temperature within the recovery and recrystallization stage, the majority of grain boundaries contained periodic arrays of dislocations. The hexagonal dislocation networks with Burgers vectors of the type of a/6 were found in a large number of twin boundaries. The origin and the formation mechanisms of this configuration are discussed. The incorporation of absorbed lattice dislocations in the boundary during annealing was found to occur either by a dissociation reaction or by the dislocation strain field interacting with the pre-existing DSC dislocation networks followed by a rearrangement of their line directions parallel to the pre-existing DSC dislocations. By both mechanisms the long range strain fields of the absorbed lattice dislocations can be reduced. The dislocation structure of several near-coincidence boundaries was characterized in detail including the misorientation axis/angle, the orientation of the boundary plane, the line directions and the Burgers vectors of the grain boundary dislocations. The boundary structure is interpreted by the CSL theory and the DSC model. Fringes parallel to a direction were always observed in the near (SIGMA)3 boundaries with closely spaced hexagonal dislocation networks (<15 nm). The fringes were resulted from the overlap of strain contrast and interference strain contrast and not related to the real boundary structure. The overwhelming majority of the grain boundaries in nickel annealed at 1000(DEGREES)C were coherent twin boundaries. Twin boundary planes were faceted in coincidence orientations such as , , , , or . Atom displacements from their coincidence positions were observed in all non- planes by the displacement fringe technique.

Pages

166

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