Structure and Stoichiometry of Mg_xZn_y in Hot-Dipped Zn–Mg–Al Coating Layer on Interstitial-Free Steel

Abstract

Correlations of stoichiometry and phase structure of Mg_xZn_y in hot-dipped Zn–Mg–Al coating layer which were modified by additive element have been established on the bases of diffraction and phase transformation principles. X-ray diffraction (XRD) results showed that Mg_xZn_y in the Zn–Mg–Al coating layers consist of Mg₂Zn₁₁ and MgZn₂. The additive elements had a significant effect on the phase fraction of Mg₂Zn₁₁ while the Mg/Al ratio had a negligible effect. Transmission electron microscope (TEM) assisted selected area electron diffraction (SAED) results of small areas Mg_xZn_y were indexed dominantly as MgZn₂ which have different Mg/Zn stoichiometry between 0.10 and 0.18. It is assumed that the Mg_xZn_y have deviated stoichiometry of the phase structure with additive element. The deviated Mg₂Zn₁₁ phase structure was interpreted as base-centered orthorhombic by applying two theoretical validity: a structure factor rule explained why the base-centered orthorhombic Mg₂Zn₁₁ has less reciprocal lattice reflections in the SAED compared to hexagonal MgZn₂, and a phase transformation model elucidated its reasonable lattice point sharing of the corresponding unit cell during hexagonal MgZn₂ (a, b = 0.5252 nm, c = 0.8577 nm) transform to intermediate tetragonal and final base-centered orthorhombic Mg₂Zn₁₁ (a = 0.8575 nm, b = 0.8874 nm, c = 0.8771 nm) in the equilibrium state.