Direct observation of antiphase boundaries in the AuCu3superlattice

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Abstract
Antiphase domains have been revealed for the first time in fully and partially ordered AuCu3 by transmission electron microscopy of foils prepared by electro-thinning bulk samples. The domains are observed to form a network of rectangular blocks, the boundaries of which lie on cube planes. In addition, it is concluded that alloys annealed for long periods just below the critical ordering temperature consist of imperfectly ordered domains within which the order is homogeneous. as been shown both theoretically and experimentally that in order to observe the domain structure, the ordered crystal must be oriented for diffraction contrast by a strong superlattice reflection. All of the various types of domain boundary contrast seen in the electron micrographs can be explained by application of the dynamical theory developed by Whelan and Hirsch for the contrast of stacking faults when the appropriate phase angle and extinction distances for superlattice reflection are introduced. The agreement between theory and experiment is very good. The extinction distances of the superlattice reflection involved in the domain boundary contrast are found to be about four times that of normal reflections causing stacking-fault contrast. The maximum deviation from the Bragg angle at which the domain boundaries can still be observed is only about one-fourth that for stacking faults. Another consequence of the present theory is that not all of the domain boundaries present can be observed for any one strong superlattice reflection, and this effect causes an apparent intricate maze pattern of antiphase domains in many of the micrographs.

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