Magnesium (Mg)-based bone implants degrade rapidly in the physiological environment of the human body which affects their structural integrity and biocompatibility before adequate bone repair. Rare earth elements (REEs) have demonstrated their effectiveness in tailoring the corrosion and mechanical behavior of Mg alloys. This study methodically investigated the impacts of scandium (Sc) and terbium (Tb) in tailoring the corrosion resistance, mechanical properties, and biocompatibility of Mg–0.5Zn–0.35Zr–0.15Mn (MZZM) alloys fabricated via casting and hot extrusion. Results indicate that addition of Sc and Tb improved the strength of MZZM alloys via grain size reduction and solid solution strengthening mechanisms. The extruded MZZM–(1–2)Sc–(1–2)Tb (wt.%) alloys exhibit compressive strengths within the range of 336–405 MPa, surpassing the minimum required strength of 200 MPa for bone implants by a significant margin. Potentiodynamic polarization tests revealed low corrosion rates of as–cast MZZM (0.25 mm/y), MZZM–2Tb (0.45 mm/y), MZZM–1Sc–1Tb (0.18 mm/y), and MZZM–1Sc–2Tb (0.64 mm/y), and extruded MZZM (0.17 mm/y), MZZM–1Sc (0.15 mm/y), MZZM-2Sc (0.45 mm/y), MZZM-1Tb (0.17 mm/y), MZZM-2Tb (0.10 mm/y), MZZM–1Sc-1Tb (0.14 mm/y), MZZM-1Sc-2Tb (0.40 mm/y), and MZZM–2Sc–2Tb (0.51 mm/y) alloys, which were found lower compared to corrosion rate of high-purity Mg (~1.0 mm/y) reported in the literature. Furthermore, addition of Sc, or Tb, or Sc and Tb to MZZM alloys did not adversely affect the viability of SaOS2 cells, but enhanced their initial cell attachment, proliferation, and spreading shown via polygonal shapes and filipodia. This study emphasizes the benefits of incorporating Sc and Tb elements in MZZM alloys, as they effectively enhance corrosion resistance, mechanical properties, and biocompatibility simultaneously.

In this study, the microstructures, mechanical properties, corrosion behaviors, and biocompatibility of extruded magnesium-zirconium-strontium-holmium (Mg-Zr-Sr-Ho) alloys were comprehensively investigated. The effect of different concentrations of Ho on the microstructural characteristics, tensile and compressive properties, corrosion resistance, and biocompatibility were investigated. The microstructures of the extruded Mg-1Zr-0.5Sr-xHo (x = 0.5, 1.5, and 4 wt.%) alloys consisted of α-Mg matrix, fine α-Zr particles, and intermetallic phase particles of Mg₁₇Sr₂ and Ho₂Mg mainly distributed at the grain boundaries. Extensive {1012} tensile twins were observed in the partially recrystallized samples of Mg-1Zr-0.5Sr-0.5Ho and Mg-1Zr-0.5Sr-1.5Ho. Further addition of Ho to 4 wt.% resulted in a complete recrystallization due to activation of the particle stimulated nucleation around the Mg₁₇Sr₂ particles. The evolution of a rare earth (RE) texture was observed with the Ho addition, which resulted in the weakened basal and prismatic textures. Furthermore, a drastic increase of 200% in tensile elongation and 89% in compressive strain was observed with Ho addition increased from 0.5 to 4 wt%, respectively. The tension-compression yield asymmetry was significantly decreased from 0.62 for Mg-1Zr-0.5Sr-0.5Ho to 0.98 for Mg-1Zr-0.5Sr-4Ho due to the weakening of textures. Corrosion analysis of the extruded Mg-Zr-Sr-Ho alloys revealed the presence of pitting corrosion. A minimum corrosion rate of 4.98 mm y⁻¹ was observed in Mg-1Zr-0.5Sr-0.5Ho alloy. The enhanced corrosion resistance is observed due to the presence of Ho₂O₃ in the surface film which reduced galvanic effect. The formation of a stabilized surface film due to the Ho₂O₃ was confirmed through the electrical impedance spectroscopy and XPS analysis. An in vitro cytotoxicity assessment revealed good biocompatibility and cell adhesion in relation to SaOS2 cells.