Study On Microstructure, Mechanical Properties And Bio-corrosion Properties Mg-5Zn-xSr Alloys

Magnesium and its alloy have attracted much attention as a biodegradable implant material due to their good biocompatibility and biodegradability.The Mg-Zn-Sr alloy system is a promising candidate since zinc is an essential trace element for human being and strontium behaves similarly to calcium and can be commonly found as a Ca substitute in biomedical compounds such as hydroxyapatite.However,combinations of Zn and Sr in Mg alloys remain unclear.On the basis of the above considerations,Zn and Sr are chosen as alloying elements in this work to improve both the corrosion resistance and mechanical properties of magnesium alloys for the development of a biomedical magnesium alloy with low corrosion rate and high mechanical properties.In this paper,effects of minor Sr addition on the microstructure,mechanical properties and bio-corrosion properties of as-cast and high strain rate rolled Mg-5Zn based alloys are carefully studied.The main research results are given below:?1?Minor Sr can significant refine the the grains of the as-cast Mg-5Zn alloy,the 0.6wt.%Sr addition exhibits the best effect of refinement.The as-cast Mg-5Zn-xSr alloys are mainly consist of ?-Mg,Mg₇Zn₃,MgZn and Mg₁₁Zn₄Sr₃.The content of second phase increases,the Mg₁₁Zn₄Sr₃ phase establishes and second phase grows up along the grain boundaries with the inceasement of Sr addition.?2?With the increasement of the Sr content,the mechanical properties and elongation of the as-cast alloy first increase and then decrease.The as-cast alloy exhibits the best comprehensive mechanical property when the content of Sr is 0.2wt.%.The tensile srength,yield srength and elongation of as-cast Mg-5Zn-0.2Sr alloy are 233MPa.117MPa and 15%respectively.The bio-corrosion properties of as-cast Mg-5Zn-xSr alloys first increase and then decrease both in the Hank's solution and the 0.9%NaCl solution at 37? with the increase of the Sr content.The as-cast Mg-5Zn-0,2Sr alloy shows an optimal corrosion rate both in Hank's solution and in 0.9%NaCl solution,the average corrosion rates of which soaked in Hank's solution and in 0.9%NaCl solution are 1.67 mg/?cm2·d?and 2.89 mg/?cm2·d?respectively??3?The high srain rate rolling can significant refine the microstructure of the as-cast Mg-5Zn-xSr alloys due to the dynamic recrystallization?DRX?.The volume fraction of DRX and the average DRX grain size first decrease and then increase with the increasement of Sr content.The as-rolled Mg-5Zn-0.6Sr alloy exhibits the best refinement with an average DRX grain size of 1.14?m.Moreover,minor Sr addition can promote the dynamic precipitation of MgZn₂ during the HRSS process since the Sr addition reinforces the interactions between solute atoms and vacancies and increases the diffution of solute Zn atoms and nucleation core of precipitation.The grain boundaries of DRX grains are pinned by the dynamic precipitation,which hinders the growth of DRX grains.?4?Minor Sr addition can significant improve the mechanical properties and bio-corrosion properties of as-rolled Mg-5Zn-xSr sheet.The tensile strength,yiled strength and elongation of as-rolled Mg-5Zn-0.6Sr sheet are 359MPa.305MPa and 20%respectively.The main strengthening mechanism consists of grain refinement strengthening,precipitation strengthening and basal texture strengthening.The corrosion rate of Mg-5Zn-0.6Sr sheet soaked in Hank's solution for 30 days is 0.15 mg/?cm2·d?,which is much lower than that of the Mg-5Zn sheet.