The Investigation Of The Fatigue Properties Of The Biodegradable Magnesium Alloy

Magnesium alloys is the lightest metal structural material.It has been widely used in automotive,3C and aerospace industry.At the same time,as the magnesium alloy has the biodegradable properties,good biological compatible and excellent mechanical capacity,magnesium alloys gradually get into our vision as biodegradable implant materials.Magnesium alloy is subjected to the interaction of corrosion and cyclic stress in vivo,so the corrosion fatigue failure is the main failure mode.In recent years,the study of the corrosion and fatigue properties of the magnesium alloy has a good effect on the application of magnesium alloys in clinical practice.In this paper,LA91 magnesium alloy was used to immersed in PBS simulated body fluid for different period.Microstructural observations reveal pitting and filament corrosion defects,which impair the load-bearing capacity of the alloy and cause stress concentration,thus leading to an accelerated accumulation of ratcheting strain and shortened fatigue life under the same nominal loading conditions.Comparing Smith model,Smith-Watson-Topper model and Paul-Sivaprasad-Dhar model,a ratcheting fatigue life prediction model based on the Broberg damage rule and the PaulSivaprasad-Dhar model was proposed,and the model yielded a superior prediction for the studied magnesium alloy.Twinning is an important plastic deformation mechanism during the service of magnesium alloy structure,and it will have an important influence on the fatigue behavior of magnesium alloy.In this paper,the effects of the low cycle fatigue properties of AZ31 extruded bars under constant strain amplitude,and the effects of pre-compression and pre-torsion on the low cycle fatigue properties of the material are studied,and the results are obtained as follows:(1)the original AZ31 Mg alloy during cyclic deformation appeared in the course of cyclic hardening phenomenon,in every cycle there are twining,de-twining changes.Therefore,the hysteresis loop curves exhibit asymmetry properties.AZ31 in the fatigue test has tensile mean stress and it will accelerate the fatigue crack expansion and reduce the fatigue life of the material.(2)the pre-compression in the ED direction will introduce twining in the material.As a result,the c-axis of partial crystal rotated 90 degrees.The number of twins varied with the degree of pre-compression strain.Then the asymmetry of the hysteresis curve of the pre-compression material is weakened,the average stress is changed to the compression direction,so the fatigue life of the pre compression magnesium alloy is improved than the original material.(3)in the cross section of the extruded rod magnesium alloy will show twins for the pre-torsion sample.Most of the twins are in the plane perpendicular to the ED direction,and the c-axis is still perpendicular to the ED direction.The dislocation motion of ED in the direction of tension is blocked by twins,so the stress of the pre twisted material in the cyclic process is larger,and the other mechanical responses are basically the same as that of the original state.