Direct Observation of Sub-boundaries in Aluminium Single Crystals Grown from the Melt

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In order to clarify the fine structure and the origin of the lineage boundaries, aluminium single crystals were grown from the melt under various conditions and investigated by transmission electron microscopy, the selected area electron diffraction method, and the X-ray Laue and pseud-Kossel diffraction methods. Three types of the sub-boundaries were found in accordance with the changes of the purities and the growth rate; they were of type (1) which was composed of a regular array of dislocations, type (2) of the dislocation networks and type (3) of the wall of fine tangling dislocation segments, respectively. While the sub-boundaries of types (1) and (2) do not lie on any peculiar crystallographic plane, the type (3) sub-boundaries lie mostly on {311} or {211} planes. From a sudden disappearance of all the dislocation contrasts at a certain tilting angle even in the case of type (3), it was concluded that strains around the fine tangling dislocations may be described by a certain vector B which is a sort of resultant of the Burgers vectors of the dislocations in problem and satisfies the condition g·B=O, where g is the reciprocal lattice vector of the reflecting plane. On the basis of the above observations, the origin of the lineage boundaries was discussed, and it was suggested that the interactions between edge dislocations, screw dislocations and impurities may play an important role in their formation.

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