The atomic arrangement and microsegregation in the vicinity of thermally induced antiphase boundaries in a Ni50Mn20In30 Heusler alloy are investigated through multislice simulations, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray diffraction pattern Rietveld analysis, energy-dispersive X-ray spectroscopy, and modified Bragg–Williams–Gorsky calculations. Intensity analysis of atomic-resolution HAADF-STEM images revealed that a 180° geometrical phase shift occurred across the antiphase boundaries, accompanied by a decay in the degree of atomic order and an enrichment of In. Energy-dispersive (X-ray) spectroscopy analysis revealed an In enrichment as high as 2.7 at.% at the center of the antiphase boundaries, comparable to that (1.0–1.8 at.%) suggested by the parallel tangent law for the Gibbs energy curves of the L21 and B2 phases using the modified Bragg–Williams–Gorsky calculation. This suggests that microsegregation occurs at the antiphase boundaries and can be understood basically within the framework of the local thermodynamic equilibrium.
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