Prospects and strategies for magnesium alloys as biodegradable implants from crystalline to bulk metallic glasses and composites-A review

As a biodegradable metal (BM), alloys of magnesium (Mg) offer great potential as an alternative to the permanent metallic implants currently being used for fracture repairs and tissue-healing processes. These alloys exhibit superior biocompatibility and appropriate mechanical strength and dissolution behavior in the physiological environment, essential prerequisites for a BM. However, rapid and generally non-uniform corrosion has been the major drawback of Mg alloys. Abrupt deterioration in mechanical strength is experienced due to the inhomogeneous corrosion, which is also considered detrimental to the surface passivation process. This review has analyzed a variety of strategies that can be adopted to address the core challenges with Mg alloy biomaterials. In addition, the review provides fundamental understanding of the mechanisms associated with these challenging problems, including discussion of crystalline and bulk metallic glasses (BMGs) and composites. Comparison among the properties and mechanisms observed in other metal alloy systems, including zinc (Zn) and iron (Fe) alloys and prominent BMGs, are also presented for analysis in order to provide new approaches to resolving the critical issues of Mg alloys. Statement of Significance The effects of alloying elements, microstructure, heat treatment and deformation on the mechanical and corrosion properties of biodegradable metals such as Mg-based alloys and bulk metal glasses (BMGs) are identified. Theoretical models and experimental findings are comprehensively analyzed to corroborate the actual corrosion and deformation mechanisms observed in biodegradable metals (BMs). This work also provides an in-depth comparison of mechanical and corrosion properties among the prominent biodegradable metal alloy systems, illustrating a clear outlook on their potentials. The proposed strategies to address the current challenges in BMs are substantiated with fundamental theories and experimental evidence. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.