Microstructural refinement and intermetallic strengthening in Zn–Mn alloys

The influence of Mn addition on the microstructure, defect structure, intermetallic phase evolution, and mechanical performance of as-cast Zn-Mn alloys was systematically investigated across Mn compositions from 0 to 2.4 wt%. Field emission scanning electron microscopy (FESEM) and electron backscatter diffraction (EBSD) analyses revealed a monotonic reduction in average grain size from 626 ± 15 µm to 282 ± 7 µm at Mn2.4, accompanied by an increase in high-angle boundary (HAB) fraction from 58 to 75%. At low-Mn contents (0.4–0.8 wt%), refinement was governed primarily by solute drag and constitutional undercooling, with fine, discrete MnZn₁₃ precipitates providing moderate Zener pinning. When Mn addition above 1.0 wt%, lamellar polygonal MnZn₁₃ networks became prevalent, imposing stronger boundary mobility constraints and increasing lattice strain. Mechanical testing showed that Vickers hardness increased from 38.42 ± 4.33 HV to 51.24 ± 6.31 HV with Mn addition, corresponding to estimated yield strength and Young’s modulus increases from ~ 125 MPa and ~ 37.7 GPa to ~ 168 MPa and ~ 50.5 GPa, respectively. Hall–Petch analysis confirmed grain boundary strengthening as the primary hardening mechanism, supplemented by dislocation-based and particle strengthening at higher Mn levels. The results establish a clear processing microstructure property relationship in Zn-Mn alloys, highlighting low to moderate Mn additions as optimal for balancing strength, ductility, and corrosion resistance, while higher Mn levels maximize strength at the expense of potential galvanic stability.