Microstructures, mechanical properties, corrosion, and biocompatibility of extruded Mg-Zr-Sr-Ho alloys for biodegradable implant applications

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.