Microstructures And Room Temperature Mechanical Properties Of Biodegradable Zn-Mg(-Mn) Alloys

Pure zinc(Zn)and Zn-based alloys have attracted much attention as a new kind of biodegradable materials due to their proper biodegradation rate and good biocompatibility.However,the mechanical properties of the biodegradable Zn-based alloys can hardly satisfy the requirements of medical applications,especially for stents application.At present,most studies have not focused on the microstructural evolution and the relationship between the microstructure and mechanical properties,especially for the texture.On the other hand,extrusion and drawing process plays an important role in preparing metallic bars,wires and tubes.But it is not yet well understood the evolution of the microstructure and mechanical properties during the extrusion and drawing process.Therefore,in present work,the Zn-Mg(-Mn)alloy bars and wires were prepared by hot indirect extrusion and drawing process.The microstructure and mechanical properties were characterized by scanning electron microscope(SEM),X-ray diffraction(XRD),eletron backscattered diffraction(EBSD),transmission electron microscope(TEM),tensile and compression tests.The microstructural evolution with different strains was investigated during the tensile and compression tests of as-extruded Zn-Mg-Mn alloys,as well the drawing process of Zn-Mg wires.The correlations among the alloying content,preparation process,microstructure and mechanical properties of the Zn-Mg(-Mn)alloys were discussed to provide valuable information to optimize the preparation process and mechanical properties of biodegradable Zn-based alloys.The main results are as follows:With increasing the Mg content in the as-extruded Zn-(0～1)Mg alloys,both the amount and size of the second phase Mg2Zn11 increase,and they distribute along the extrusion direction(ED).Meanwhile,the equiaxed grains are significantly refined,and the average grain size decreases from 100～200 μm in Zn to 10 μm in Zn-1Mg alloy.In addition,a lower extrusion temperature could refine the grain size in the Zn-(0～0.2)Mg alloys.As the Mg content exceeds 0.02 wt.%,the texture of Zn-Mg alloys transforms from ED‖<1100>basal fiber texture to non-basal texture with basal planes tilted away from ED.With increasing the Mg content,the tensile yield strength(TYS)and the ultimate tensile strength(UTS)of the as-extruded Zn-(0～1)Mg alloys increase from 64～70 MPa and 105～110 MPa for Zn to 250～260 MPa and 320～330 MPa for Zn-1Mg alloy,respectively.And a lower extrusion temperature is beneficial to improve both the strength and elongation.The grain refinement strengthening and the second phase strengthening contribute to the improvement of TYS,and the former one plays a dominant role.On the other hand,the alloys with a small amount of Mg(less than 0.05 wt.%)extruded at lower temperature exhibit ductile fracture corresponding to high elongation up to 25%.With increasing the Mg content,the alloys exhibits cleavage fracture corresponding to elongation less than 15%.The as-extruded Zn-(0～1)Mg alloys exhibt good compression resistance.All the specimens do not break with a strain up to 60%during compression tests.With increasing Mg content,the compression yield strength(CYS)increases from 110～120 MPa for pure Zn to 285～290 MPa for Zn～1Mg alloy.More interestingly,the yield asymmetry has been significantly reduced with increasing the Mg content.Zn-0.02Mg alloy wires were prepared by multi-pass drawing at room temperature with cumulative area reduction up to 97%(wire with a diameter of 0.8 mm).Both TYS and UTS increase from 136 and 167 MPa for the as-extruded alloy to 388 and 455 MPa for the as-drawn wire with a diameter of 0.8 mm,while the elongation decreases from 27%to 5.4%.During the drawing process,the deformation mechanism includes dislocations and {1012} twinning.The equiaxed grains are elongated along the drawing direction(DD).As the cumulative area reduction reaches 45%,the DRX occurs at room temperature.Finally fully DRXed microstructure with equiaxed grains in size of 1 μm forms in the wire with a diameter of 0.8 mm.With increasing the cumulative area reduction,the increase of the microhardness of the wires exhibits three stages due to the work hardening,DRX softening and grain refinement strengthening.The as-extruded Zn-0.02Mg alloy bar exhibits ED‖<1100>basal fiber texture.At the early stage of the drawing process,the deformation texture DD‖<0001>forms.As the cumulative area reduction exceeds 45%,the room-temperature DRX results in the texture with DD deviating 70° away from<0001>.The as-extruded Zn-0.2Mg-(0.1～0.8)Mn alloys contain second phases Mg2Zn11 and MnZn13 distributed along ED.The equiaxed grains with 2～5 μm in size are formed,which are less than the average grain size of 16 μm in the Zn-0.2Mg alloy.The as-extruded Zn-Mg-Mn alloys exhibit non-basal texture with basal planes tilted away from ED.With increasing the Mn content,the TYS and UTS increase from 178 and 233 MPa for the Zn-0.2Mg alloy up to 348 and 383 MPa for the Zn-0.2Mg-0.8Mn alloy,while the elongation increases from 7%for the Zn-0.2Mg alloy to 30%for the Zn-0.2Mg-0.5Mn alloy,then reduced to 21%for the Zn-0.2Mg-0.8Mn alloy.The as-extruded Zn-0.2Mg-(0.1～0.8)Mn alloys do not break with strain up to 40%～60%during compression tests.The CYS increases from 183 MPa for the Zn-0.2Mg alloy to 393 MPa for the Zn-0.2Mg-0.8Mn alloy.Moreover,the yield asymmetry has been slightly increased with increasing the Mn content.Therefore,the grain refinement,MnZn13 phase and non-basal texture contribute to the improvement in both the strength and ductility of the Zn-Mg-Mn alloys.Zn-0.2Mg-0.8Mn alloys extruded with temperature between 300℃ and 150℃contain equiaxed grains with an average size of 2 μm.With decreasing the extrusion temperature,the size of the second phases Mg2Zn11 and MnZn13 are refined,and the texture of the alloys transforms from ED‖<1100>basal texture to the non-basal texture with basal plane tilted away from ED about 10～25°.The extrusion temperature does not cause significant change in both TYS and UTS which reach up to 330～350 MPa and 360～385 MPa,respectively,but it results in the increase of elongation from 12%to 33%.The specimens do not break with strain up to 60%during the compression tests.The CYS stays between 350 MPa and 393 MPa,and it reaches maximum for the alloy extruded at 200℃.Then the Zn-0.2Mg-0.8Mn alloys extruded at 200 and 150℃ exhibit high strength and ductility.For the tensile deformation,the Zn-0.2Mg-0.8Mn alloy with the non-basal texture could accumulate more LAGBs than the alloy with basal texture,resulting in a larger elongation.The tensile deformation does not cause transformation of the initial texture.In addition,near the fracture surface of the alloys,a certain amount of {1012}twins are activated and result in the formation of texture component TD‖<0001>(Tensile direction TD).The compression deformation causes the transformation from the initial texture to CD‖<1210>texture(Compression direction CD).As the strain exceeds 40%,DRX occurs in the alloys resulting in further refined grains with size less than 1 μm and DRXed texture with the CD deviating 15～70° away from<0001>.