GRAIN SIZE DISTRIBUTION EFFECTS ON THE MECHANICAL AND CORROSION BEHAVIOUR OF BIODEGRADABLE METALLIC BIOMATERIALS

Abstract

Maintaining a balance between mechanical and corrosion behaviours is a big challenge in the design and development of metallic biomaterials for biodegradable medical implant application. A current strategy to overcome this challenge is through grain refinement techniques as corrosion rate and mechanical properties of polycrystalline metals are significantly affected by average grain size. Recent studies suggest that grain size distribution (gsd) can also influence dissolution and mechanical behaviours of polycrystalline metals. This work investigated the effect of grain size deviation on the corrosion and mechanical behaviours of pure iron, one of the most studied biometallic materials for biodegradable medical implant application with a view to utilizing the outcome in controlling and adjusting these properties. Thermomechanical processing route of cold rolling and annealing was used to induce different average grain sizes and grain size distributions in pure iron. The grain sizes and gsd were measured with optical microscope. The mechanical properties of yield strength and ductility were evaluated using tensile testing while the corrosion rates were determined in Hank’s solution, a near-neutral (pH~7.4) simulated physiological fluid, using weight loss and potentiodynamic polarization methods. The results demonstrate that the corrosion rate decreases and the yield strength increases as the grain size distribution decreases and vice versa. An increase in gsd is accompanied by an increase in residual stress, which lowers strength, but increases corrosion rate due to more active sites of low activation energy. This finding could enable the optimization of mechanical and corrosion properties of biodegradable metallic biomaterials.