Effect Of Hot-extrusion On Microstructure And Properties Of Medical Mg-Zn-Y-Nd Alloy

Magnesium alloy has excellent biocompatibility and degradability,become the best candidate material of fully degradable human implant devices.However,the lack of mechanical properties of magnesium alloy and the rapid degradation rate in human body,preventing its clinical application.The mechanical properties and corrosion resistance of magnesium alloy can be optimized by adjusting alloy elements and adopting the multi-pass hot-extrusion process.In this thesis,the effect of multi-pass hot-extrusion on microstructure,mechanical properties and corrosion resistance of Mg-4Zn-0.8Y-0.5Nd and Mg-4Zn-0.5Y-0.5Nd alloys were systematically studied.The microstructure evolution and corrosion behavior of Mg-Zn-Y-Nd alloy under different hot-extrusion passes were explained.The as-cast Mg-Zn-Y-Nd alloy consists of?-Mg matrix and continuous skeleton W-phase distributed along the grain boundary.After one-pass extrusion,Mg-Zn-Y-Nd alloy consists of fine equiaxed dynamic recrystallized grains,strip-like original grains and W-phase distributed linearly along the extrusion direction with incomplete dynamic recrystallization occurs.After two-pass extrusion,the microstructure of Mg-Zn-Y-Nd alloy is further refined,composed of fine equiaxed dynamic recrystallized grains and dispersed W-phase with complete dynamic recrystallization occurs.For mechanical properties,the yield strength and tensile strength of the two-pass extruded Mg-4Zn-0.8Y-0.5Nd alloy increases to 293.7MPa and 309.4MPa,Mg-4Zn-0.5Y-0.5Nd alloy increases to 285.3MPa and 304.7MPa,respectively.With the increase of hot-extrusion passes,the microhardness of Mg-Zn-Y-Nd alloys also increase.For corrosion resistance,Mg-Zn-Y-Nd alloy in Hank's solution and high-sugar medium solution(DMEM)is mainly galvanic corrosion between?-Mg matrix and W-phase,Mg(OH)2 is dense and continuous,formed by reaction,which can effectively isolate the contact between the alloy and the corrosive medium,reduce the corrosion rate to 0.8 mm/year(Hank's solution)?0.01 mm/year(DMEM).However,the Cl^-in the solution will react with Mg(OH)2 to form soluble MgCl2,which further deepens the corrosion behavior.Compared with the two solutions,the Cl^-content in the DMEM is less than Hank's solution,so the protective effect of Mg(OH)2 film on the surface of the alloy is long-lasting in DMEM.In the later stage of corrosion,due to the presence of H₂PO₄^-,HPO₄^2-and Ca²⁺in both solutions,the corrosion products(Ca,Mg)3(PO4)2 and Ca10(PO4)6(OH)2 were further formed by the reaction with MgCl2 and attached to the cracked Mg(OH)2 film.With the increase of hot-extrusion passes,the grains and W-phases in the alloy are refined continuously,improving the strength and hardness of the alloy and alleviate galvanic corrosion.With the increase of dynamic recrystallization rate,the crystal defects are reduced,which improves the mechanical properties and corrosion resistance of the alloy.