| Magnesium and its alloys have been the most promising biomedical metal material for their degradable property, excellent biocompatibility and good mechanical properties and in physiological environment. However, two obstacles limited the clinical application of magnesium alloy as bone implantation:relatively p oor mechanical property and fast degrade rate. Mg alloys have poor deformability, and traditional plastic processing has little/no obvious transformation on their microstructure. Therefore, severe plastic deformation (SPD) could be an option. High pressure torsion (HPT) is an effective SPD technology to refine grains, which can be used continuously in room temperature (RT), and produce larger shear strain in the samples. During HPT process, the sample occurs shear deformation under hydrostatic pressure by the mould, and the sample is hardly rupture. Hence, HPT is appropriate for metal materials with close-packed hexagonal structure which are difficult to deform. In this paper, Mg-2Zn-0.22Ca (wt%) was treated by HPT in RT with the maximum number of revolutions (N) was 5, and annealed at different temperature afterwards. The microstructure evolution, mechanical properties and the degradability in simulated body fluid (SBF) were studied.The results show that Mg-Zn-Ca alloy tends to plastic deformation by twinning in the early stage of HPT, which is beneficial to the dislocation slip. The original equiaxed grain was elongated into stripy grain. There are dislocation lines near grain boundary (GB). Dislocation density increases with the shear strain accumulates. Dislocations aggregate and tangle into dislocation tangled zone. Then, the dispersed dislocation tangled zone transform to arrange orderly. Dislocations interact, and form dislocation cell and sub GBs. To conclude, dislocation segmentation plays a leading role in the process of grain refinement. After HPT, the grain orientation of Mg-Zn-Ca alloy transforms from (1010) and (1011) to (0002), and the specific texture for HPT is formed. The transformation of grain orientation provides a favorable condition for dislocation movement. After 5 revolutions, the average grain size of cast alloy, extruded alloy and solid solution alloy were 99nm,151nm and 94nm respectively. When the cast Mg-Zn-Ca alloy is processed by HPT, Ca2Mg6Zm phases distributing along GBs have fractured physically, and distribute non-uniformly. During HPT process, the fine and granular second phases in the extruded alloy and solid solution alloy precipitate dispersedly, and form Mg4Zm phases which have common lattice relationship with the matrix. In the period of nucleation, these second phases are easy to act as the nucleus, which is in favor of grain refinement. Meanwhile, the second phases mainly precipitate along GBs, they can pin the GBs and prevent the growth of grains. In solid solution alloy, the second phases also precipitated due to natural aging. The amount of second phases is the most, and they distribute densely which have the best microstructural homogeneity.After HPT, the microhardness and tensile strength of Mg-Zn-Ca alloy have increased significantly because of the combined action of dislocation strengthening, fine grain strengthening and precipitation strengthening. A huge amount of dislocations form during HPT process at RT. Dislocation movement result in a lot of jogs and sessile dislocations, which prevent dislocation movement and produce dislocation pile-up, and then the work hardening rate is high. The anti-deformability of GBs in polycrystal is large, and the enhancement of GB area after grain refinement leads to the increase of alloy strength. In addition, it is difficult for dislocations to pass through GBs, so the dislocations aggregate near GBs. Thus, it's easy to form dislocation pile-up. The second phases dispersedly precipitate and pin the GBs. Since the size of precipitation phases is small, and they have common lattice with matrix, dislocations pass precipitation phases by cutting according to their interaction mechanism. The increasing friction force of dislocations and lattice prevents the dislocation movement. With the increase of N, the toughness of HPT-treated Mg-Zn-Ca alloy decreases firstly, and then increases. The reason is that both the increasing dislocations and the second phases precipitated along GBs restrain the deformability of the alloyin the initial stage, so the toughness decreases. When the grains have been refined, the toughness recovers. After annealing treatment at 210?×30min, the precipitation strengthening of solid solution alloy is the strongest, but its dislocation strengthening and fine grain strengthening reduce. The decrease of micarohardness value and tensile strength is not obvious, the homogeneity of the radial distribution of micohardness is good, and the toughness is improved obviously at the same time. With these conditions, biological Mg-Zn-Ca alloy with good comprehensive mechanical properties is obtained. The result of nano-indenter test shows that with the increase of N, the elastic modulus of Mg-Zn-Ca alloy first decreases, then increases. Grain orientation and the degree of solid solution are the main reasons for the change of elastic modulus. The result of stress distribution test shows that the distribution of surface stress is high at the edge and low in the center. When N=5, the average surface stress of alloy decreases, which attributes to dynamic recovery and dynamic recrystallization.The results of electrochemical test and hydrogen evolution test indicate that the degradation rate of Mg-Zn-Ca alloy after HPT decreases with the increasing N, and it can further decreases with annealing treatment. When annealing temperature is 210?, the degradation rate of HPT-treated Mg-Zn-Ca alloy is the lowest. When N=5, the degradation product layer of solid solution alloy is compact, and the degradation interface is smooth. The alloy degrades uniformly, and there is no obvious corrosion pit. When Mg-Zn-Ca alloy contacts SBF, the matrix near GBs and the second phases corrode preferentially. In cast Mg-Zn-Ca alloy, due to the coarse grain and the second phases distributing along GBs, the degradation rate near GBs is faster than that in the interior of the grain. Because of the different degradation rates on the alloy surface, corrosion pits are easily to form. When solid solution alloy is processed by HPT, its grains are refined obviously. The second phases precipitate dispersedly and distribute uniformly. The preferential corrosion area enlarges greatly which almost cover the whole alloy surface. After HPT, grain orientation can enhance the alloy stability and reduce the degradation rate. In addition, the increase of GBs density can strength the binding force between matrix and degradation product layer. Therefore, the whole alloy surface degrades in low and similar rates at the same time. It is beneficial to the accumulation of the dense degradation products. The compact and stable degradation products layer with goods binding force can be formed to protect the matrix from further degradation. After annealing treatment, the surface stress of Mg-Zn-Ca alloy decreases significantly, and the stress corrosion cracking tendency reduces. The degradation resistance is further improved. When the annealing temperature is 210?, the degradation rate of Mg-Zn-Ca alloy is the lowest. MSC cell adhesion test shows that the adhesive ratios of Mg-Zn-Ca alloy are enhanced gradually with the increase of N. After annealing treatment, the adhesion is further enhanced. When the annealing temperature is 210?, the cellular compatibility of Mg-Zn-Ca alloy is the best. This is because the degradation rate of Mg-Zn-Ca alloy is relatively low. The ion concentration and pH value change slightly, which is beneficial to cell survival. Moreover, fewer evolving hydrogen bubbles have less effect on the process of cell adhesion.In conclusion, with proper pretreatment technology, HPT process and appropriate annealing treatment, ultra-fine grain biological Mg-Zn-Ca alloy can be obtained. The second phases are dispersive and uniformly. All the mechanical properties, degradation resistance and cellular compatibility are improved simultaneously. Hence, HPT-treated Mg-Zn-Ca alloy has great potential in application as degradable bone implant material. The research of this paper only gets preliminary results. It still has a long way to go to apply HPT-treated Mg-Zn-Ca alloy into bone implantation clinically successfully. |
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