Research On Microstructure And Corrosion Properties Of Mg-Zn-Y-Ca-Zr Alloys For Degradable Cardiovascular Stent Application

With the development of new technology to treat cardiovascular disease,many researchers had gradually focused on the design and application of biodegradable magnesium alloys cardiovascular stent.Magnesium alloys had good biocompatibility degradation ability,as compared with traditional biological metal materials.However,magnesium alloys had poor corrosion resistance,which greatly restricts it from wide application in the field of biological materials.For improving the corrosion resistance of degradable magnesium alloys in this paper,based on a quaternary Mg-Zn-Y-Ca alloys are treated in modification,Zr was selected for the alloy as alloy element,then,Mg-Zn-Y-Ca-Zr biological magnesium alloys are designed.It was investigated that Zr element,solution treatment and hot extrusion influence on the microstructure and corrosion resistance of the biodegradable magnesium alloys.The experimental results are summarized as follows:The as-cast Mg-3.0Zn-0.6Y-0.3Ca-xZr alloys mainly consisted of an?-Mg phase,Ca₂Mg₆Zn₃ phase and I-phase?Mg₃YZn₆?.All the as-cast alloys mainly display a dendrite configuration,and as the Zr content increases from 0 wt.%to1.0 wt.%,the trend is to a more equiaxed structure.Moreover,With the increasing Zr content,the grain size decreases and the isolated particles of the Zr-free alloy are gradually replaced by the strip-like phases precipitated at the grain boundaries.The results indicated that the alloy with a Zr content of 0.5wt.%has the best corrosion resistance of these alloys.The corrosion process of the Mg-3.0Zn-0.6Y-0.3Ca-0.5Zr alloy can be divided into three stages,corresponding to corrosion features with different times.At the initial stage,the solute depleted zones formed corrosion grooves about several microns in width;at the middle stage,the regions with high concentration of Zr formed many corrosion pits in the?-Mg matrix;at the final stage,the corrosion grooves and corrosion pits extended toward the?-Mg matrix.The dendrites disappeared,the grain size increased and the phase composition did not altered of as-cast Mg-3.0Zn-0.6Y-0.3Ca-0.5Zr alloy after solution treatment.As the solution temperature?360?,420?and 450??increases,the precipitated phases is gradually dissolved into the matrix while a new Zr-rich phase is formed.The immersion experiment and the electrochemical measurement results indicate that the solution treated alloy at 420?for 24 h exhibits an optimal corrosion resistance,the corrosion rate?mass loss?,the corrosion potential and the corrosion current density are 0.32 mm/year,-1.468 V and 8.943?A/cm²,respectively.The grains refined apparently of the Mg-3.0Zn-0.6Y-0.3Ca-0.5Zr alloy after hot extrusion,which is the consequence of dynamic recrystallization.Besides the majority of precipitated phases are broken into small granules which evenly distributed along the extrusion direction.Furthermore,the?0001?basal plane of most grains of the extruded alloy is parallel to the extrusion direction,formed a typical basal texture.The volume of rerecrystallization increases and grain size increases with the increasing extrusion temperature.Results indicate that the extruded alloy at 300?exhibits excellent corrosion resistance,the corrosion rate?mass loss?and the corrosion current density are 0.24 mm/year and 4.772?A/cm²,respectively.