Investigation On Deformation Mechanism And Properties Of Magnesium Alloy During Cold Drawing

As the lightest metallic structural materials,magnesium and its alloys have been widely applied in many industrial and technological fields.With the rapid development of biomedical technology in recent years,the human implant materials have become an important branch of material science frontiers.Owing to its good biocompatibility and biodegradability,Mg alloys show considerable application potential in the field of implant materials.Although the development and application of Mg alloys have gone through over a hundred years and the most of industrial countries have established various grades of commercial Mg alloys and corresponding technical standards,however,systematic investigations concerning the processing technologies,deformation behavior and the relationship between the chemical composition and properties of Mg alloy wires for biomedical application have not been reported.Therefore,the research on Mg alloy wires has become the focus of attention in the area of materials science for recent years.In the present work,the microstructures and mechanical properties of pure Mg,three Mg-Zn binary alloys(Mg-2 wt%Zn(Z2),Mg-4 wt%Zn(Z4),Mg-6 wt%Zn(Z6))and Mg-4.7 wt%Gd(G4.7)under various processing conditions were systematically studied by a variety of macro and micro analytical methods,and the effects of heat treatment on the microstructure and properties of these alloys were also investigated.Moreover,the texture evolution of the wires studied during cold drawing as well as the mechanism of the static recrystallization during high temperature heat treatment were emphatically discussed.The results obtained will be of great significance to the deep understanding of the deformation behavior of Mg alloys and the expansion of Mg alloy applications.The experimental results found that the cold deformation behavior of pure Mg was different from that of common metallic materials.The dynamic recrystallization occurred during cold drawing of extruded pure Mg when the accumulative true stain(ATS)reached about34%and the equiaxed small grains were observed in its microstructure,resulting in work-softening(reduction of both yield strength and microhardness).With the increase of ATS,the strength and hardness of the wires finally reached a steady-state level.Zn additions into pure Mg caused the microstructure refinement and the strengthening of the alloy,while the ductility of the alloys decreased with the increase of Zn content.Different form pure Mg,dynamic recrystallization did not occur in Mg-Zn binary alloys during cold drawing.Only a small amount of subgrains were observed in the Z2 alloy with low Zn content(2 wt%)after drawing with ATS up to 91%.The microstructural observation conducted on the wire of the Z4 drawn with larger ATS showed fibrous morphology,and the grains were elongated along the DD but the grain boundaries could still be distinguished in wires of the Z6 alloy.With the increase of ATS,the dislocation density of all three Mg-Zn alloys studied increased,and the typical work hardening was also detected.The addition of Gd resulted in significant improvement on the deformation behavior of Mg alloys.The maximum ATS of G4.7 alloy reached as high as 165%,much higher than that of pure Mg and Mg-Zn binary alloys under the same processing conditions.A small amount of grains with Nano-sizes were observed by TEM in the microstructure of the G4.7 alloy wires after drawing with large ATS,but dynamic recrystallization did not occur.The fiber texture with<10(?)0>//DD formed in all the wires of pure Mg and both Mg-Zn and Mg-Gd binary alloys after cold drawing with large ATS.However,the texture evolution was different for these alloys during cold drawing.The texture evolution of pure Mg was divided into three stages.With ATS rising up to~28%,the texture component transformed from as-extruded basal texture to typical<10(?)0>fiber texture with<10(?)0>//DD.Subsequently it rapidly returned to the basal texture({0002}//DD)accompanied by the weakening of fiber texture at larger ATS of 34%owing to the occurrence of DRX during drawing.With further increase of ATS,the weak fiber texture(<10(?)0>//DD)gradually appeared again.The obvious fiber texture with<10(?)0>//DD gradually formed in all three Mg-Zn binary alloys studied at later stages of cold drawing.The difference was that the basal texture({0002}//DD)was first strengthened and then weakened in Z2 wires with a low Zn content as ATS increased,while besides the formation of a main basal texture,a weaker non-basal texture also appeared in as-extruded Z6 alloy with a higher Zn content.The addition of Zn promotes the formation of non-basal texture.Different from pure Mg and Mg-Zn binary alloys,the weak texture components with~45~o to TD?DD instead of the basal texture formed in as-extruded G4.7 alloy,which was beneficial to the subsequent cold drawing.As ATS increased,grains rotate around the DD and gradually approach the TD direction.Finally,an obvious<10(?)0>//DD fiber texture formed.Results of systematic heat treatment tests showed that the recrystallization temperature of drawn wires of pure Mg,Z2 and G4.7 alloys were 250?,200?,350?,respectively,and the values of activation energy of recrystallization was 91 kJ/mol,77 kJ/mol,136 kJ/mol,respectively.After annealing at 300?,225?,and 375?,pure Mg,Z2 and G4.7 alloys,all showed good drawing deformability due to their fine and uniform microstructure.Moreover,the deformability of pure Mg and Z2 wire was further improved when the wires was annealed at higher temperatures,although the grains was coarsened after annealing.The different recrystallization textures appeared in pure Mg,Z2 and G4.7 alloy wires after annealing treatment.After annealing at low temperature,a weaker basal texture formed in pure Mg wires and its intensity increased with the increase of annealing temperature or time.For wires of the Z2 alloy,the main texture component formed was<11 2?0>//DD after low temperature annealing.With the increase of annealing temperature and time,the texture component,<11(?)0>//DD,was weakened and a weaker texture component,<10(?)0>//DD,appeared.Moreover,the texture intensity of basal plane became larger after annealing at higher temperature.For wires of the G4.7 alloy,the main texture component was<10(?)0>//DD after annealing at low temperature.With the increase of temperature,the basal texture was gradually strengthened.The inverse pole figure showed that the texture component transformed from<10(?)0>//DD to a recrystallized texture based on<11(?)0>//DD.