The role of microstructural features on in-vitro degradation and surface film development of a thermomechanically processed Mg-4Zn-0.5Ca-0.8Mn alloy has been investigated employing electrochemical studies, scanning electron microscopy and X-ray photoelectron spectroscopy. The specimen forged at 523 K temperature developed a coarse unimodal microstructure consisting of basal oriented grains, whereas the specimens forged at 623 K and 723 K temperatures exhibited bimodal microstructures containing randomly oriented fine grains and basal oriented coarse grains. The bimodal microstructures exerted higher resistance to corrosion compared to the unimodal microstructure in presence of a protective surface film. The optimum size distribution of fine and coarse grains as well as the prevalence of basal oriented grains led to the lowest anodic current density in the specimen forged at 623 K. The morphology of Ca₂Mg₆Zn₃ precipitates governed the cathodic kinetics by controlling the anode to cathode surface area ratio. Despite the specimen forged at 723 K comprised comparatively lower fraction of precipitates than at 623 K, the mesh-like precipitate morphology increased the effective cathodic surface area, leading to enhanced localised corrosion in the former specimen. Optimal microstructural features developed at 623 K forging temperature formed a well-protective surface film with lower Mg(OH)₂ to MgO ratio, exhibiting distinctly high polarization resistance and superior cytocompatibility in terms of cell-proliferation and cell-differentiation.

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 Ca₂Mg₆Zn₃ phase with maximum disruption in interconnectivity, in combination with a small fraction of fine equilibrium MgZn₂ precipitates, 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.