Enhanced in-vitro degradation resistance and cytocompatibility of a thermomechanically processed novel Mg alloy: Insights into the role of microstructural attributes

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.