Microstructures And Properties Of Mg And Mg-Zn Based Alloys During Cold Deformation And Static Recrystallization

Mg and its alloys have been increasingly attracted as the bio-degradable implant materials with the bio-degradation,good bio-compatibility and suitable mechanical properties matching with human skeleton.But the magnesium implants have not been presently applied,because of the unacceptably high degradation rate and non-uniform corrosion in the physiological environment.The second phase existes in the Mg matrix for most of alloying elements,acting as the local cathode coupled with the Mg matrix.Due to the very low corrosion potential of Mg,so that the internal galvanic corrosion and localized corrosion occur.The corrosion resistance has been mostly improved through the surface treatment of Mg-based alloys method.But there usually exist film uniformity,compactness,and problems with bond stength substrate.It was recently showed that the single-phase Mg alloys prepared by solid solution treatment possess better corrosion resistance.But the grain of Mg matrix grows during solid solution treatment at high temperature,resulting in the reduction of the mechanical properties.Grain refinement can not only improve its strong toughness,but also further improve the corrosion resistance.Therefore,in this study,pure Mg and Mg-Zn(-Ag)alloys were prepared by solid solution treatment and indirect extrusion at 300℃,and then cold drawn with small deformation per pass of 9%,finally,recrystallization annealing in the single phase region were carried out to control the microstructures and mechanical properties of as-extrued Mg-Zn based alloys.The microstructure and mechanical evolution,static recrystallization of Mg-Zn based Mg alloys were systematically studied by means of optical microscopy(OM),composition and phase analysis,microhardness and tentile tests.The primary conclusions were obtained as followes:After the solid solution treatment,the indirect extrusion was carried out at 300℃ with an extrusion ratio of 144,and the single-phase Mg-3Zn-(0～0.8)Ag alloy wires with the diameter of 2mm are obtained.On this basis,the single-phase Mg-Zn-based alloy wires with a diameter of 0.8mm and grain size about 3um are obtained by cold drawing and annealing for three timesThe microstructure evolution of as-extruded pure Mg and Mg-3Zn-(0～0.8)Ag alloys in the process of extrusion,drawing and annealing shows that the equiaxed grains of Mg-Zn based alloys form due to complete dynamic recrystallization during hot extrusion and average grain size is 30～45μm.After cold drawing,there are high-density deformation twins in the Mg-3Zn-(0-0.8)Ag alloys.So the deformation twinning is the main mechanism.After recrystallization annealing,the grain size of Mg-Zn-based alloys constinously is refined to less than 10μm.And it is found that the twin boundaries become the nucleation sites of recrystallized grain.The twins and twin boundaries are provided as a more recrystallization nucleation position,and thus,nucleation rate increased.The yield and tensile strength of as-extruded Mg-Zn-based alloys were about 145MPa and 235MPa,respectively,and their elongation firstly increased and then decreased with the Ag additions such as 16.7%for Mg-3Zn-0.5Ag.Besides,the Mg-Zn based alloys have significant work hardening during the cold drawing process.The ultimate yield strength,tensile strength of Mg-Zn-based alloys wire reach 400MPa,450MPa,respectively,while their elongation is lower about only 1.2%.After the higher temperature annealing,the hardness of the single phase Mg-Zn-based alloy wires with a diameter of 0.8mm were higher than 65HV.The static recrystallization behavior of Mg-Zn based alloys at 150℃～400℃ for different time shown that the annealing temperature and time are not sensitive to recrystallized grain size at the lower temperatures as the annealing temperature and time increased.But at the higher temperatures,the recrystallization grain size clearly increases with the extension of annealing time or increasing of annealing temperature.Ag element played a role in refining recystallizaation grain.And the static recrystallization of Mg-Zn-based alloys completes within 5min.