Influence of second phase precipitates on mechanical and in-vitro corrosion behaviour of Mg-4Zn-0.5Ca-0.8Mn alloy in optimum homogenized conditions

The present study investigates the mechanical and in-vitro corrosion behavior of Mg-4Zn-0.5Ca-0.8Mn alloy in optimum homogenized conditions. The optimization of the homogenization parameters has been carried out employing thermodynamic calculations and kinetic modeling. The model utilizes the inter-diffusivity of the solute elements and predicts that ～6–24 h of homogenization at 633 K effectively redistributes the elements in the Mg matrix. Based on the insights obtained from the simulations, the as-cast Mg-4Zn-0.5Ca-0.8Mn alloy was subjected to homogenization heat treatment process for 6–24h. The microstructural study through optical microscopy and scanning electron microscopy(SEM) revealed that the interconnected network of second phase precipitates substantially dissolve within 24 h, implying adequate homogenization. Moreover, fine Mg-Zn based precipitates with varied morphology and phase fractions also evolved during homogenization treatment, as confirmed through SEM and transmission electron microscopy. In the 12 h homogenized specimen, the highest fraction of uniformly dispersed fine precipitates resulted in the highest strength(～225 MPa). On the other hand, a substantial disruption in coarse precipitate network and lower aspect ratio of fine Mg-Zn precipitates led to the highest ductility(～8%) in this specimen. In the 24 h homogenized specimen, the ductility reduced marginally owing to higher aspect ratio of fine precipitates. The immersion and electrochemical tests(viz.,potentiodynamic polarization and electrochemical impedance spectroscopy) carried out in Hank’s solution revealed that the 24 h homogenized specimen exhibits the best corrosion properties. The least fraction of Ca2Mg6Zn3phase with maximum disruption in interconnectivity, in combination with a small fraction of fine equilibrium MgZn2precipitates, resulted in suppression of localized corrosion in this specimen.This promotes the formation of the most stable and compact product layer over the specimen, resulting in the highest corrosion resistance.