Analyzing the polytwin microstructure in L10+L12 Fe–Pd alloys: Theory and experiment

Document Type

Article

Publication Date

8-15-2026

Department

Department of Materials Science and Engineering

Abstract

Bulk Fe–Pd within the L10+L12 eutectoid region forms a two-phase microstructure consisting of L10 polytwin plates embedded in an L12 matrix. Analyzing this microstructure via transmission electron microscopy (TEM) is challenging due to its complex three-dimensional (3D) nature, which involves multiple L10 orientation variants, local lattice rotations, and numerous twin boundaries and habit planes oriented along diverse crystallographic directions. To address these challenges, this study utilizes Invariant Plane Strain (IPS) theory to guide TEM experiments and provide a robust framework for data interpretation. By integrating this theoretical framework with dark-field TEM imaging, specific zone axes and superlattice reflections are strategically selected to quantitatively reveal the 3D geometrical features of the polytwin microstructure. This methodology enables precise identification of L10 orientation variants, measurement of twin variant volume fractions, and determination of the orientations of both twin boundaries and habit planes. Furthermore, the influence of sample thickness variations on the observed microstructure is analyzed, explaining the emergence of wedge-shaped boundary shadows and the progressive narrowing of visible internal twin areas within the plates in electropolished TEM specimens. The excellent agreement between theoretical predictions and experimental observations unveils the intricate microstructural details of the L10+L12 Fe–Pd system with unprecedented clarity. Ultimately, the established theoretical framework is generally applicable to other alloy systems forming polytwin microstructures.

Publication Title

Acta Materialia

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