Researches On Design,Preparation And Properties Of Biomedical Degradable Mg-RE Alloys

Biomedical degradable magnesium alloys have advantages of non-toxicity,avoiding second operation and no stress shielding.However,quick corrosion process and mismatch between the mechanical properties and the healing cycles restrict the practical application of the magnesium alloys as implant materials.Based on the prospect of biomedical materials,this dissertation designed and prepared Mg-Nd-Sr and Mg-Gd/Nd-Zn-Zr-Mn Rare Earth magnesium alloys?Mg-RE?.Microstructure evolutions of the Mg-RE alloys as well as the effects of heat treatment and hot extrusion on the mechanical properties of the Mg-RE were systematically studied.The corrosion properties and corrosion modes of the Mg-RE alloys with different microstructures in Hank's solution were explored.The stability of the mechanical properties of typical Mg-Gd/Nd-Zn-Zr-Mn alloys in the Hank's solution was investigated.Compatibility and cytotoxicity of the Mg-Gd/Nd-Zn-Zr-Mn alloys were evaluated.Main results are as follows:?1??phase on the grain boundaries of Mg-1.76Nd-2.24Sr?MNS1?alloy with low Nd/Sr atomic ratio was consisting of Mg₁₂Nd and Mg₁₇Sr₂.Solid solution and aging treatment improved the mechanical properties of the MNS1 alloy.The concentration and distribution of the?phase affected the polarization properties of the MNS1 alloy.The?phase of the Mg-2.07Nd-0.24Sr?MSN3?alloy,with high Nd/Sr atomic ratio,obtained by the semi-continuous casting was Mg₄₁Nd₅.Yield strength?YS?and elongation?EL?of the MNS3-E5 alloy achieved about 238.7 MPa and 21.6%,respectively.?2?Microstructure of the as-cast Mg-Gd-Zn-Zr-Mn?GZKM?alloy showed a dendritic morphology,consisting of?I-Mg matrix and??+?-?Mg Zn?3Gd phases,among which the?compounds distributing continuously along the grain boundary.The precipitates of the heat treated GZKM alloy showed a petaloid distribution due to the segregation of the components.The precipitating phases were mainly Zn-Zr phase,including a long nano needle-like Zn₂Zr₃phase and a regular rectangular Zn₂Zr phase.Dynamic recrystallization process occurred during hot extrusion,and the grains were obviously refined.Microstructure was more uniform with extrusion ration was 17.4 as compared to 8.4.The long needle-like precipitates in the pretreatment stage were broken to short needles during hot extrusion.An appropriate heat treatment process improved the mechanical properties of the GZKM alloy.The extrusion GZKM alloys possessed good mechanical properties with YS over 200 MPa and EL above 15%.The main reasons are fine grain strengthening and precipitation strengthening.The fracture modes and fracture characteristics changed with microstructures.The GZKM-AC alloy showed an intergranular cleavage fracture,and the GZKM-T4-2 alloy existed both intergranular and transgranular fracture,and GZKM-E2 alloy showed transcrystalline plastic fracture.The microstructure of GZKM alloy was more uniform after second extrusion,which the YS was around 363 MPa and EL was 22%.?3?In Hank's solution,?phase with continuous network acted as a cathode and?-Mg acts as an anode,constituting micro corrosion galvanic.?-Mg around the?phase was preferentially corrode and degrade,and exacerbating the corrosion process after shedding.When the solution comes across a discontinuous?phase,the corrosion propagation path tends to longitudinally expanding to a new corrosion galvanic,resulting in a new local pitting.The discontinuously?phase failed to protect the underlying matrix,and making corrode expanding to deep matrix,which resulting in pitting.It comes to expanding transversely when meeting continuous dense?phase,results in a tendency to form uniform corrosion morphologies at the macro level.The broken granular?phase of the extruded GZKM alloys act as cathode phase during the corrosion process.While the area of the cathode relative to the anode was smaller,making the galvanic corrosion process slowed down.The?-Mg around the?phase or precipitates gives priority to corrode.The granular cathode phases failed to protect the?-Mg matrix,and fell into solution with the surrounding?-Mg corroded.So that the corrosion original battery was destroyed,and the corrosion process was delayed to some extent.Corrosion expansion path was along the gradient concentration of the granular?phase and precipitates.After corroding process,the cathode phases fell off and gradually formed shell-like morphology.A suitable extruded GZKM-E2 alloy achieves a corrosion rate of less than 0.5 mm/yr.?4?Microstructure morphology has no obvious influence when changed the RE element from Gd to Nd for the Mg-Nd-Zn-Zr-Mn?NZKM?.The strength of NZKM-AC alloy was slightly lower while the elongation increased a bit after the Rare Earth element Nd instead of Gd.The extrusion NZKM alloy launched the twin mechanism,further improved the plastic deformation ability during the tensile test.The corrosion degradation models of NZKM alloys were similar to the GZKM alloys.Compared to the GZKM alloys,the oxide film during corrosion process of the NZKM alloy was more integrity and stability,moreover,the?phase of the NZKM alloy was denser without layer plate gap,which can effectively protect the internal substrate.So the corrosion rate of the NZKM alloys in Hank's solution was slower than that of the GZKM alloys.The corrosion rates of the heat treated and hot extruded NZKM alloys were less than 0.5 mm/yr.?5?GZKM alloys and NZKM alloys tend to be brittle fracture after immersed in Hank's solution for 24 h,and the strength was slightly decreased,while the plasticity was significantly decreased.After immersed for 120 h,the strength of the immersed alloys decreased significantly,and the plastic deformation ability was further deteriorated.As the solution ions erode with Mg,the continuity and mechanical stability of the magnesium alloys were reduced.The corrosion product becomes a brittle crack source during the tensile stress,and the crack propagated along the corrosion path.The biocompatibility of the alloy was characterized by the hemolytic rate and cytotoxicity of the typical GZKM and NZKM alloys.The results showed that the hemolysis rates of the NZKM alloys were lower than that of the GZKM alloys,and the hemolysis rate was close to 5%.The cytotoxicity of GZKM-2E alloy was the lowest among the testing Mg-RE alloys.