High Strength Biodegradable Mg-based Materials for Medical Application

With aging populations and the rise of musculoskeletal disorders, the demand for biodegradable orthopedic implants is increasing, with millions of bone fixations performed annually. Concurrently, the need for lightweight structural materials is growing to enhance energy efficiency and to reduce carbon emissions. Magnesium (Mg) is highly attractive for both, structural applications and bioresorbable medical implants, due to its low density and excellent biocompatibility. However, its low strength, limited ductility, and poor corrosion resistance hinder a widespread use, especially in biomedical alloys. While rare-earth element (REE) based Mg-alloys provide sufficient strength and corrosion resistance, they are unsuitable for biomedical applications due to the potential toxicity of REE. REE-free approaches rely on using biosafe alloying elements (e.g., Ca, Zn), but strength rarely reaches >400 MPa, needed for load-bearing applications. Moreover, conventional alloying most often compromises corrosion resistance, as due to the inherently low solubility for most elements, micro-galvanic couples (i.e., precipitates) are easily formed within the Mg-matrix. Mg-based metallic nanocomposites offer exceptional multifunctional properties, including high strength, thermal stability, and resistance to shock and radiation, but they have not been a focus, as Mg forms galvanic couples with almost every metal. However, a recent study suggests that this issue can be mitigated by reducing the Mg phase spacing to the submicron or nanoscale. Nevertheless, to be used in engineering applications, such composites need to be processed at bulk scale, which has remained a challenge. Attempts to fabricate bulk Mg-based nanocomposites (e.g., by accumulative roll bonding (ARB)) failed, as strain immediately localized within the Mg phase, leading to layer break-up and preventing refinement of the phases. High-pressure torsion (HPT) offers a promising alternative, as it enables much higher shear strains, allowing even for the consolidation and refinement of arbitrary metallic powder mixtures. Additionally, its adjustable processing parameters make it ideal for exploring optimal processing conditions. Goal of this thesis was to find processing strategies to overcome the aforementioned strain localization problems within the Mg phase, to obtain homogeneous Mg-Fe nanocomposites and to analyze their mechanical properties and degradation rates. Three compositions—Mg50Fe50, Mg30Fe70, and Mg70Fe30 (vol.%)—were deformed by HPT using different process parameters, particularly deformation temperature (293 K to 773 K) and strain (up to 2200), to understand their influence on microstructural refinement, mechanical properties, and corrosion behavior. The findings indicate that increasing the HPT deformation temperature from 293 K to 673 K enhances phase refinement and is essential to achieve homogeneous Mg-Fe nanostructures. In contrast, HPT deformation below 673 K leads to strain localization and inhomogeneous composite structures, likely due to the softening from rapid basal texture development within the Mg-phase. Mg50Fe50 composites processed at 673 K, result in phase spacings below 100 nm, and exhibit outstanding mechanical properties, achieving 1 GPa strength (hardness of 2.2 GPa) at ambient temperature. This strength surpasses that of the strongest Mg-REE alloys (<550 MPa) and falls within the range of medium-strength steels (>700 MPa). Additionally, the nanoscale phase spacing (<100 nm) significantly suppresses micro-galvanic corrosion in the Mg50Fe50 nanocomposite, resulting in degradation rates comparable to ultra-high-purity Mg. Hence, the composite architecture provides a huge lever to overcome Mg’s strength-corrosion trade-off. Since strain localization limits homogeneous nanocomposite formation for HPT deformation below 673 K, another focus of the thesis was to assess strategies for overcoming it, and to even further refine the composite structure. The strategies involved avoiding basal texture formation or enhancing strain hardening by reducing the initial phase spacing, altering phase fractions, or applying intermediate heat treatments. Techniques such as two-step HPT (i.e., HPT at 673 K followed by HPT at room temperature), inter-stage annealing, and powder ball milling were hence investigated. However, microstructural analysis revealed that, although these strategies accelerated microstructural refinement in Mg-Fe composites, strain localization could not be completely suppressed below 673 K. The findings indicate that only high deformation temperatures (≥673 K) reliably enable the transformation of a coarse into a homogeneous Mg-Fe nanostructure with exceptional strength and slow degradation. Nevertheless, combining 1 GPa strength with exceptionally slow degradation in an Mg-based composite opens new avenues for novel structural materials and load-bearing biodegradable implants.