Study On Torsional Properties And Biodegradability Of Biodegradable WE43 Mg Alloy

Bone screws are commonly used implants for fracture treatment in clinic.Mg alloys are expected to be ideal materials for bone screw applications due to their biodegradable properties and mechanical properties that match the bones.The researches on mechanical properties of Mg alloys mainly focus on tensile,compression,bending and fatigue properties etc.,while bone screws are subject to high torsional shear stress during implantation,and the torsional properties of metallic bone screws are specified in standards.However,there is lack of systematic research on the torsional properties of Mg alloys.Besides,rapid corrosion also prevents the application of Mg alloy implants.Thus,effects of microstructure on the torsional and biodegradable properties of Mg alloys were studied in this thesis,and methods to improve both the torsional properties and the corrosion resistance of Mg alloys were explored.In this thesis,the as-extrude WE43 Mg alloy(Mg-Y-Nd-Zr alloy)was used as an initial material and it was subjected to different heat treatments(Solution heat treated(T4),Artificially aged(T5),Solution heat treated and artificially aged(T6)).The effects of microstructure on torsional properties of WE43 after heat treatments were stuided,which were compared with that on tensile properties.The results showed that the effects of microstructure on torsional and tensile properties were not exactly the same.Grain refinement improved the torsional strength and plasticity,and the second phases improved the torsional strength but reduced the plasticity.The effects of grain size and second phases on tensile properties were similar to that on torsional properties.The texture should be a significant factor that resulted in the inconsistent between torsional and tensile properties of WE43 after heat treatments.T5 treated WE43 had the highest torsional strength and higher torsional plasticity,as well as the lowest corrosion rate,which was much lower than that of T4 treated WE43,and the corrosion rate of T4 treated WE43 decreased after subsequent aging,which indicated that the rare-earth-rich phases in WE43 did not cause severe galvanic corrosion.The grain refinement and moderate amount of second phases would improve the corrosion resistance of WE43.Equal channel angular pressing(ECAP)was performed on the extruded WE43 at high temperature(390?).After ECAP,there were no significant refinement in grain size and some changes in the amount and distribution of second phases,but the texture greatly changed.After ECAP,the changes in torsional and tensile properties were completely different,the torsional properties reduced to varying degrees,but the tensile properties increased,especially for the 2-pass and 4-pass ECAP processed samples,the changes in strength and plasticity showed opposite trends in torsional and tensile tests.It indicated that the different effects of texture on torsional and tensile deformation led to the different changes in torsional and tensile properties after ECAP,the basal fiber texture improved the torsional plasticity and reduced the torsional strength,but increased the tensile strength and reduced the tensile plasticity.The uniformity of grain size also had significant effects on the torsional plasticity.The corrosion rate of as-extrude WE43 decreased first and then increased with increasing EACP passes,but the corrosion rate of ECAP processed samples was lower than that of as-extrude WE43.Heat treated(T4,T5 and T6)WE43 were subjected to ECAP at 390? to further study the effects of microstructure on torsional and biodegradable properties of WE43,as well as to explore methods for obtaining WE43 with both high torsional properties and corrosion resistance.The results showed that the tensile properties of heat treated WE43 improved after ECAP,but the torsional properties increased a little or even decreased,and the corrosion resistance also decreased to varying degrees.Besides,it further indicated that the uneven grain size significantly reduced the torsional plasticity of WE43 Mg alloy.The second phases greatly affected the corrosion resistance of WE43,the moderate amount of second phases improved the corrosion resistance,but too densely distributed second phases significantly deteriorated the corrosion resistance.The extruded WE43 was also subjected to low-temperature(250?)ECAP and subsequent aging treatment.The low-temperature ECAP resulted in significant grain refinement.The torsional and tensile strength increased as ECAP pass increased,but the plasticity decreased.After aging,the torsional and tensile strength of single-pass ECAP processed sample increased without reduction in plasticity,the torsional strength,plasticity and corrosion resistance were higher than that of multi-pass ECAP processed samples.Therefore,low-temperature single-pass ECAP and subsequent aging could significantly improve the torsional properties and corrosion resistance of WE43 at the same time.