Atomic-resolution evaluation of microsegregation and degree of atomic order at antiphase boundaries in Ni50Mn20In30 Heusler alloy

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 degrees 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 L2(1) 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. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY-NC ND license.