Characterization of microstructure, mechanical properties, and degradation behavior of Mg–Zn–Y alloys: A comparative study of as-cast and heat-treated states

Mg–Zn–Y alloys containing less than 4 wt% of alloying elements show desirable mechanical properties and corrosion resistance. However, limited systematic research has been conducted on alloys with lower alloying contents and different Zn/Y ratios, and the effects of heat treatment on their properties remain poorly understood, requiring further investigation. This study investigates the effects of varying Zn/Y ratio and solution heat treatment on the microstructure, mechanical properties, and corrosion behavior of Mg– x Zn– y Y alloys, where ( x , y ): (1,1), (2,1), (2,2), (3,1). The microstructure of alloys was examined by optical microscopy, field-emission electron microscopy and X -ray diffraction. Hydrogen evolution, polarization, and electrochemical impedance spectroscopy tests were also used to evaluate the corrosion behavior of the alloys. Furthermore, the mechanical properties of the samples were evaluated by the shear punch test. Microstructural analysis of all as-cast alloys revealed a dendritic structure containing W and LPSO precipitates. The as-cast Mg–1Zn–1Y and Mg–2Zn–2Y alloys exhibited a ternary eutectic structure of the LPSO and W phases. However, the as-cast Mg–2Zn–1Y and Mg–3Zn–1Y alloys with larger Zn/Y ratios showed the discrete W and I phases, along with a reduced LPSO phase content. Heat treatment across all alloys led to grain coarsening and partial dissolution of precipitates. The as-cast Mg–1Zn–1Y and Mg–2Zn–2Y alloys demonstrated superior corrosion resistance, with polarization current densities of 3.65 μA/cm 2 and 3.18 μA/cm 2 , respectively, compared to Mg–2Zn–1Y (5.12 μA/cm 2 ) and Mg–3Zn–1Y (4.18 μA/cm 2 ), due to their continuous LPSO that form a uniform and pseudo-passive layer on the sample and shield the W phase from corrosion in the ternary eutectic structure. Furthermore, heat treatment improved the corrosion resistance of all alloys by reducing microstructural defects, homogenizing second-phase distribution, and partially dissolving galvanically active W and I precipitates. Mechanical analysis demonstrated that the as-cast Mg–2Zn–2Y alloy achieved superior mechanical properties with an ultimate shear strength of 123.8 MPa, attributable to its refined structure and high volume fraction of second phases.