Microstructural evolution of penetrated Zn and its role in adhesive strength of galvannealed steels

• Zn penetration enhances adhesive strength in galvannealed steel coatings. • Morphological evolution of Zn was clarified, from droplet formation to Γ / Γ 1 phase development • Adhesion is governed by dual mechanisms: mechanical interlocking in GB-free regions and anchoring via Γ phase formation at Zn-penetrated GBs. • Thermodynamic analysis revealed that Γ -phase formation causes significant volumetric expansion; this expansion is inferred to generate compressive stress and thereby reinforce anchoring at GBs. This study investigates how distinct interfacial microstructures influence the adhesive bonding strength of galvannealed (GA) steel. Two steels with different bonding strengths, measured by stud pin testing, were analyzed using field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and thermodynamic modeling. In grain boundary (GB)-containing regions, Zn penetration and the formation of Zn-rich Fe–Zn intermetallic compounds (IMCs), primarily the Γ phase, were frequently observed. Thermodynamic analysis revealed that the Γ phase possesses a ∼ 19–20 % larger molar volume than α-Fe, resulting in significant volumetric expansion during phase transformation and cooling. This expansion is interpreted to generate compressive stress along the penetrated GBs, thereby anchoring the coating to the substrate. While the Γ phase is conventionally regarded as detrimental due to interface flattening and brittleness, our findings reveal that its effect is spatially dependent – weakening adhesion in GB-free regions but enhancing it in areas where Zn penetration occurs along GBs. Overall, adhesive strength is governed by two distinct mechanisms: mechanical interlocking at rough features in GB-free areas and volume-expansion-driven anchoring at GB-containing regions. These insights highlight the dual role of interfacial phase formation and suggest new approaches for optimizing adhesion in GA coatings.