Study On Stress Corrosion And Corrosion Fatigue Of Biodegradable Mg-Zn-Y-Nd Alloy

Biodegradable magnesium alloy stents will occur the decrease of mechanical properties and fatigue failure after implanted into the human body due to the pulsating load of cyclic systolic,diastolic blood pressure and the long-term influence of body temperature at 37?.These could result in the deformation,collapse,displacement and even fracture of stents.In addition,the stents play a supporting role and suffer from corrosion of ions in the blood at the same time.Under the combined action of stress and corrosion environment,the corrosion degradation behavior of stents is complicated,and the mechanical properties of stents are seriously decreased.Therefore,it is necessary to study the stress corrosion cracking?SCC?and corrosion fatigue?CF?behavior of alloys and their stents,which will provide a theoretical basis for the structural design and clinical application of degradable magnesium alloy stents.In this paper,the slow strain rate tension?SSRT?and corrosion fatigue test were performed to study the SCC and CF of the extruded Mg-Zn-Y-Nd alloy in dynamic cycling simulated body fluid?SBF?environment.In order to study the effect of different corrosion behaviors on corrosion fatigue tests,reciprocating extrusion was used to prepare samples with fine grains and uniformly distributed second phase.A dynamic corrosion device was designed to analyze the dynamic corrosion process of two extruded alloys.And the corrosion fatigue tests of two extruded alloys under 60 MPa stress were carried out.In the tests of stress corrosion,the SCC susceptibility of this alloy was mainly investigated at the strain rate of 10^-6s^-1,And combined with the experimeital results of different strain rate,the extruded Mg-Zn-Y-Nd alloy has higher SCC susceptibility.Fractographic features showed that the SCC occurred due to anodic dissolution and hydrogen embrittlement.As for corrosion fatigue tests,a dynamic cyclic corrosion fatigue environment was built.The fatigue and corrosion fatigue S-N curves showed that the fatigue limit of extruded Mg-Zn-Y-Nd alloy in air was 65 MPa at 107 cycles.However,it had no obvious fatigue life limit in SBF and showed a linear relationship between the fatigue life and stress amplitudes.Fractographic features showed that the stress concentration caused by the slip and defection were the initiation sites for fatigue source in air,while crack caused by stress corrosion and hydrogen embrittlement play an important role in the corrosion fatigue of extruded Mg-Zn-Y-Nd alloy in SBF.Corrosion experiment showed that corrosion environment had a great influence on the corrosion behavior of materials.Reciprocating extruded alloys had lower corrosion rate and uniform corrosion morphology in static corrosion environment,while the extruded alloys showed higher corrosion rate and appeared large corrosion pits.However,in dynamic corrosion environment,the corrosion morphology of reciprocating extruded alloys was relatively uniform but had a higher corrosion rate,while the corrosion rate of ordinary extruded alloys decreased and the corrosion morphology also had improved.In corrosion fatigue tests of two extruded alloys under 60 MPa stress,the results showed that the corrosion fatigue life of reciprocating extruded alloys was slightly longer.However,its fatigue crack originated from minor corrosion pitting.In other words,the corrosion rate has little effect on the corrosion fatigue life,while the corrosion forms affect the formation of corrosion crack source.In a word,the SCC and CF are significant for clinical safety.The analysis of SCC and CF behavior is helpful to improve the performance of magnesium alloy stents.