Structure and anisotropy of antiphase boundaries in rapidly solidified Ni3Al

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Abstract
Antiphase boundaries (APBs) were studied by transmission electron microscopy in rapidly solidified Ni3Al compounds. It is shown that the antiphase vector can be decomposed into R = R o + r. The main term R o = ½ [110] gives rise to a strong fringe pattern, of the π type, when superlattice reflections are present. A small relaxation term r, giving a faint fringe contrast when only fundamental reflections (i.e. 111) are acting, was found to be maximum when lying in the boundary plane. For a given R o, the modulus of r varies with the boundary orientation. Comparison between experimental and simulated images indicated that r is at least one order of magnitude smaller than R o. Statistical determinations of the APB orientations evinced no anisotropy in their distribution. It is concluded that r restores the isotropy of the APB energy, which is unexpected if R = R o.