Effects Of High Strain Rate Rolling On Microstructure And Bio-Corrosion Behavior Of Mg-Zn-Mn-xSr Alloy

Magnesium（Mg）and its alloys are exceedingly attractive in the biomedical implant material field because of their excellent biodegradability,biocompatibility and similar modulus to natural bone.But magnesium alloys present high chemical and electrochemical activity and fast degradation rate in human body due to its lower electrode potential.The rapidly generated hydrogen is easy to accumulate around the implants,which has an adverse effect on the tissue regeneration,and the excessive degradation rate will also affect the mechanical support of the implant.The poor corrosion resistance of magnesium alloys limits their applications in the biomedical field.Therefore,improving the corrosion resistance of magnesium alloys is of great significance for their practical application.This paper optimizes the corrosion resistance of magnesium alloys by alloying,heat treatment and high strain rate rolling process.The alloys compositions include Mg-5Zn,Mg-5Zn-1Mn-xSr（ZM51-xSr,x=0,0.2,0.6,1.0,2.0wt.%）.In addition,the correlation between the corrosion behavior and the microstructure is studied in depth,the corrosion mechanism in vitro is revealed,and the mechanism of the improved corrosion resistance of the magnesium alloys under the different process conditions is clarified.The main conclusions are as follows.（1）The microstructure of the alloys changes significantly after alloying,solution treatment and high strain rate rolling process.For the as-cast alloys,the grain size of the Mg-5Zn alloy decreases with the increasing of total amount of alloying elements,while the average grain size of ZM51-2.0Sr is 48μm.The volume fraction of the second phases increases with the increasing of alloying elements content.When the Sr content≤1.0%,the second phases in the as-cast alloys are MgZn₂ and Mg₇Zn₃phases.But when the Sr content reaches 2.0%,the Mg₂Sr phase appears and Mn element enrichment occurs.After solution treatment,the second phases basically disappear,and the grain size of the alloys grows slightly compared with the as-cast alloys.With the high strain rate rolling process,the grain size is significantly refined.The dynamic recrystallization grain size of the as-rolled alloys has been decreasing with the increase content of alloy elements.The average dynamic recrystallization grain size of ZM51-2.0Sr is about 0.7μm.The addition of Mn can inhibit the dynamic recrystallization degree of Mg-5Zn.With the Sr content increasing,the dynamic recrystallization degree firstly increases and then decreases.（2）Immersion and electrochemical tests show that the corrosion rate of the as-cast,as-solution and as-rolled alloys all decreases firstly and then increases with the increasing of total amount of alloying elements,and the ZM51-0.6Sr alloy under different process conditions all present the best corrosion resistance.The corrosion rate of as-cast,as-solution and as-rolled ZM51-0.6Sr alloys in Hanks’solution is 0.75mm·y^-1,0.54 mm·y^-1 and 0.44 mm·y^-1,respectively.Which decrease by 83.5%,50%and 29%respectively compared with Mg-5Zn alloys.EDS and XPS experimental results show that the corrosion product layer of the as-cast ZM51-0.6Sr alloy presents a good integrity due to the introduction of the Sr CO₃,Sr SO₄,Mn（OH）₂,Mn O₂compounds and high percentage phosphate.（3）The dynamic recrystallization grain size and dynamic recrystallization volume fraction of ZM51 alloy both increase with the increasing rolling strain rate.Both immersion and electrochemical tests show that the corrosion resistance of the ZM51alloy increases with the increasing rolling strain rate.Corrosion preferentially nucleates in the dynamic recrystallization region and propagates along the grain boundaries.During the corrosion process,the non-recrystallized region acts as a cathode while the recrystallized region acts as an anode.（4）The corrosion behavior of the ZM51-0.6Sr alloy in three process conditions（as-cast,as-solution,and as-rolled）is related to the soaking time.The as-rolled ZM51-0.6Sr alloy has fine grains and high grain boundary density,which is more conducive to the propagation of filiform-like corrosion in the early stage of the corrosion.But the initial corrosion propagation rate of the as-cast and as-solution ZM51-0.6Sr alloy is slower.In the early stage of corrosion,the as-cast ZM51-0.6Sr alloy is dominated by intragranular filiform-like corrosion and galvanic corrosion,while the as-solution ZM51-0.6Sr alloy is dominated by filiform-like corrosion.In the later period of corrosion,the as-cast ZM51-0.6Sr alloy mainly present intragranular corrosion,and the filiform-like corrosion is suppressed,which is mainly caused by the enrichment of Zn element along the grain boundary.The as-solution ZM51-0.6Sr alloy is still present filiform-like corrosion,but pitting corrosion become severely.But for the as-rolled ZM51-0.6Sr,the corrosion is still dominated by intragranular filiform-like corrosion,but the corrosion propagation rate is slower due to the more protective corrosion product film.（5）The electrochemical behavior of the ZM51-0.6Sr alloy with different process conditions is related to the immersion time at the open circuit potential（OCP）.The OCP of the as-rolled ZM51-0.6Sr alloy stabilizes rapidly compared with the as-cast and as-solution ZM51-0.6Sr alloy.The corrosion current density of the three ZM51-0.6Sr alloys basically shows a decreasing trend with the extension of the immersion time.The corrosion potentials of the three alloys is also increasing with the extension of immersion time.The Tafel slope of the as-cast ZM51-0.6Sr alloy has been increasing with the immersion time,while the as-rolled alloy shows an opposite trend.