Effect Of Hot Extrusion And Cyclic Extrusion Compression Progress On The Microstructure And Property Of Mg-Zn-Y-Nd Alloy For Vascular Stent Application

In this study, hot extrusion and cyclic extrusion compression (CEC) were used to treat cast Mg-Zn-Y-Nd alloy, the effect of hot extrusion temperature, extrusion ratio and CEC passes on the microstructure, mechanical properties and corrosion properties of alloy were researched by OM SEM&EDS XRD、electrochemical test and tensile test. At the same time, the influence mechanism was studied, which provided theoretical ground for Mg-Zn-Y-Nd alloy as vascular stent application.The results show that:The microstructure of hot extrusion alloy was heterogeneous because of incompletely recrystallized alloys.The second phase distributed along grain boundaries with rod shape and some particles gathered at the junction of three grains. The microstructure characteristics of alloy varied with extrusion process parameters.The grain size of alloy grew up gradually with the increase of extrusion temperature at the same extrusion ratio condition. When the extrusion temperature is270℃, grain size of alloy is about5～8μm,10～13μm at320℃and15～20μm at370℃. The second phase particles size grew up slightly and volume fraction decreased gradually.Different extrusion ratio corresponding to the grain size is different at the same extrusion temperature conditions. The grain size decreases slightly with increasing extrusion ratio, but the change is not obvious when the extrusion ratio increases to a certain value.The microstructure of CEC treated Mg-Zn-Y-Nd alloy became finer and more homogenous due to the dynamic recrystallization occurred during the deformation with the increase of CEC pass and some nano-particles uniformly precipitated in grains. After CEC1pass treatment, microstructure of alloy was heterogeneous. Coarse grains with4～7μm in size were surrounded by fine recrystallized grains with1～3μm in size. But the grain size was refined to about1μm after CEC2passes and4passes treated alloy and the microstructure was uniform equiaxed. The second phase distributed at grain boundaries with big grid shape. Some particles were brittle and formed into a cluster. At the same time some nano-particles which came from the cluster phase uniformly precipitated in grains. The nano-particles’ size was about200nm.The mechanical property results show that:The extruded alloy obtained good comprehensive mechanical properties when the extrusion temperature was320℃and extrusion ratio was36. The ultimate tensile strength(UTS), yield strength(YS), elongation(δ) of alloy were268MPa,212MPa and34.5%respectively. After CEC2passes treatment, Mg-Zn-Y-Nd alloy had good mechanical properties with UTS316MPa, YS238MPa and830.7%. As a result, the hot extruded and CEC treated alloy improved mechanical property compared with cast alloy(UTS209MPa, YS105MPa and δ8%). Therefore, the mechanical poperties of extruded alloy meets the performance requirements as vascular stent materials.The electrochemical measurements results show that:In the same extrusion ratio conditions, the corrosion potential of alloy increased and corrosion current density decreased with the increase of extrusion temperarure.The corrosion potential and corrosion current density values of extruded alloy were-1.705V and1.375e-5A·cm-2respectively when the extrusion temperature was370℃and extrusion ratio was56. The corrosion potential values of CEC treated alloy increased with the increase of CEC pass. After CEC4passes treatment, the corrosion potential values of alloy was-1.723V and corrosion current density decreased to5.775e-5A·cm-2. The corrosion potention of CEC treated alloy is lower but corrosion current density dereased a magnitude than hot extruded alloy. Therefore, the corrosion resistance of CEC treated alloy is better than hot extrusion alloy.The immersion test results show that:Mg-Zn-Y-Nd alloy with different conditions were immersed in SBF solution at37℃for24h. The macrography of the CEC treated samples exhibited uniform corrosion due to the grain refinement, grid second phase distributed at grain boundaries and nano-sized particles distribution in grains. However, the as cast and extruded alloy suffered from pitting corrosion. So the CEC processing will be a promise candidate process for the vascular stent.