Extrusion of novel Mg‒Zr‒Sr and Mg‒Zr‒Sr‒RE alloys for improving mechanical and corrosion properties in biodegradable implants

Biodegradable metals (BMs) have drawn growing attention in medical applications due to their suitability for being used as temporary implants, as they are capable of in vivo absorption after having achieved their required function. To date, magnesium (Mg) is considered the most suitable candidate as the base-metal for biodegradable implants due to its excellent biocompatibility, low density, high specific strength, and comparable elastic modulus to that of natural bone, which is considered essential to prevent stress-shielding effect. Based on the effects on the toxicity, mechanical and corrosion properties, strontium (Sr), calcium (Ca), and zirconium (Zr) are considered the most suitable alloying elements in Mg for biodegradable implant applications, while zinc (Zn), manganese (Mn), yttrium (Y), and gadolinium (Gd) among some other rare earth elements (REEs) have good potential to be used in Mg but further research is needed due to the availability of limited data. In this thesis, the microstructures, mechanical, corrosion, and biocompatibility properties of extruded Mg‒Zr‒Sr, Mg‒Zr‒Sr‒Dy, and Mg‒Zr‒Sr‒Ho alloys were comprehensively assessed for bone-implant application. First, the extruded Mg‒Zr‒Sr alloys displayed intermetallic Mg17Sr2 phases at the grain boundaries, which triggered particle-stimulated nucleation (PSN) during extrusion, resulting in a reduction in grain size (GS) and deformation twining. Increasing Zr content, i.e., Mg-xZr-1Sr (x = 0.5 to 3 wt.%) resulted in an increase of 25.4% in elongation and 5.9% in ultimate tensile strength (σUTS). On the other hand, increasing Sr content i.e, Mg-0.5Zr-xSr (x = 1 to 3 wt.%) led to an improved σUTS by 19.3%. The maximum σUTS of 302 MPa was observed in extruded Mg-0.5Zr-3Sr. The tensile yield strength (σTYS) of Mg-xZr- ySr (x = 0.035–3 and y = 0.2–3 wt.%) ranged from 210 to 275 MPa. The fracture analysis revealed a transition from intergranular fracture (IGF) to transgranular fracture (TGF) with an increase in Sr content due to the presence of brittle Mg17Sr2 particles at the GBs. A corrosion mechanism for the low CR in the extruded Mg‒Zr‒Sr was proposed which involved the accumulation of a large number of Mg17Sr2 particles at the GBs, effective protection from an early deposition of a dense layer of Mg(OH)2 and Sr2P2O7 in the corrosion product layer in simulated body fluid (SBF). In comparison to their as-cast counterparts, the extruded Mg‒Zr‒ Sr alloys showed a significant increase of 22.6% in σUCS, a drastic increase of 100% in σTYS, and a significant decrease in corrosion rate (CR). Second, in extruded Mg‒Zr‒Sr‒Dy alloys, the microstructure was composed of an α–Mg matrix containing {10-12} extension twins and secondary phases of intermetallic compounds Mg17Sr2 and Mg2Dy mainly distributed at the grain boundaries. EBSD analysis revealed the evolution of basal and rare earth (RE) textures with an increase in Dy content to 2 wt.% in Mg-1Zr-0.5Sr-2Dy, resulting in texture randomization and strengthening of the RE component due to PSN and a change from discontinuous to continuous dynamic recrystallization. This texture randomization led to an improved tension–compression yield asymmetry of 0.87. Extrusion of the Mg‒Zr‒Sr‒Dy alloys significantly enhanced their tensile and compressive properties due to improved distribution of alloying elements and RE texture evolution. An addition of 2.0 wt.% Dy to extruded Mg-1Zr-0.5Sr caused a drastic increase in ductility of 181.2% with a moderate decrease in σTYS of 12.5% and σUTS of 8.3%. Also, a significant increase of 74.2% in compressive strain, 15.5% in σCYS, and 34.2% in σUCS were observed during compression testing. The CR of the Mg‒Zr‒Sr‒Dy alloys determined by hydrogen evolution (HE) testing, potentiodynamic polarization (PDP), and electrical impedance spectroscopy (EIS) showed similar trends for each composition and the lowest CR of 3.37 mmy⁻¹ was observed in Mg-1Zr- 0.5Sr-1Dy. Improved surface protection was observed due to the presence of Dy2O3 in the inner layers of the Mg(OH)2 surface film. A comparison among the as-cast and extruded Mg‒Zr‒Sr‒Dy and Mg‒Zr‒Sr alloys demonstrated that both extrusion and addition of Dy in Mg‒Zr‒Sr improved the CR, cell viability, and cell adhesion in relation to human osteoblast–like SaOS2 cells. Third, the microstructures of 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 {10-12} tensile twins were observed in the partially recrystallized samples of Mg-1Zr-0.5Sr-0.5Ho and Mg-1Zr-0.5Sr-1.5Ho. Further increase in Ho addition to 4 wt.% resulted in complete recrystallization due to PSN around Mg17Sr2 particles and the evolution of RE texture, leading to weakened basal and prismatic textures. Furthermore, an increase in Ho addition from 0.5 to 4 wt.% resulted in a drastic increase of 200% in tensile elongation, 89% in compressive strain, 58% in σCYS, and 32.3% in σUCS. 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. Compared to extruded Mg-1Zr-0.5Sr, extruded Mg-1Zr-0.5Sr-4Ho showed an increase of 22.5% in σCYS, 46.5% in σUCS, 147.4 % in compressive strain, 292.9% in tensile elongation, with only a marginal decrease in σUTS of 10.7%. Addition of Ho in extruded Mg-Zr-Sr alloys resulted in an improved corrosion resistance due to the formation of Ho2O3 in the surface film and the extruded Mg-1Zr-0.5Sr-0.5Ho showed a decrease of 53.4% in CR than the extruded Mg-1Zr-0.5Sr. Also, extruded Mg–Zr–Sr–Ho alloys showed a decrease of 19.5% in CR compared with their as-cast counterparts. Comparison of the mechanical and corrosion properties of the Mg‒Zr‒Sr‒Ho alloys indicated that both extrusion and Ho addition are very beneficial. An in vitro cytotoxicity assessment on the extruded of Mg‒Zr‒Sr‒Ho alloys revealed good cell viability and cell adhesion in relation to osteoblast-like SaOS2 cells.