Diffraction contrast at planar interfaces of large coherent precipitates

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Abstract
Fringe contrast has been observed by transmission electron microscopy at the interfaces of large, coherent γ’ precipitates in an aged Ni-6·71 wt.% Al alloy. The fringes display characteristic δ-fringe symmetry; they are anti-symmetrical in bright field and symmetrical in dark field. In addition, specific extinction conditions are satisfied; whenever the diffraction vector, g, lies in the plane of the interface, fringe contrast vanishes at that interface. A model describing the lattice distortions near the γ′-matrix interface is considered in detail. This model is consistent with the extinction conditions and accounts for the difference between the deviation parameters of the precipitate and matrix which produces δ-fringe contrast. Also, the model correctly predicts the nature of the fringes (i.e. bright or dark) at the top and bottom of the foil.

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