Advanced phase characterization of galvannealed steel coatings – Overcoming challenges with EBSD/TKD and 4D-STEM

Zn-based coatings are widely utilized for corrosion protection in steel applications, with galvannealed (GA) coatings being particularly valued for their hardness and weldability. This study explores the application of advanced electron microscopy diffraction techniques to characterize Fe–Zn intermetallic phases in GA coatings, addressing critical challenges in their identification and differentiation. In the current investigation, a key accomplishment for high-accuracy identification of the contradictory phases was achieved by implementing a pattern-matching-based indexing method in electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD), which successfully overcame the limitations of traditional Hough transform-based indexing. This approach reliably differentiated closely related cubic Fe–Zn phases, such as Γ and Γ1, and its reliability was validated through correlative analysis with transmission electron microscopy (TEM) techniques, including selected area electron diffraction (SAED), precession electron diffraction (PED), and high-resolution (HR) TEM. The study also highlighted the potential of 4D-scanning (S) TEM as an alternative for phase characterization. However, its applicability is limited to site-specific TEM lamellae and requires dedicated PED/STEM hardware. While both EBSD/TKD and 4D-STEM approaches demonstrated high accuracy in phase identification, structural deviations, such as high stacking fault densities, require additional refinement through HRTEM. Thus, this work establishes a robust framework for detailed phase characterization. Future efforts should focus on optimized sample preparation methods and refining indexing algorithms. Such advancements are essential for improving microstructural characterization, paving the way for optimizing GA coatings for industrial applications.