可降解超細(xì)晶鎂合金在生物醫(yī)學(xué)中的應(yīng)用_英文_

1、Rare Metal Materials and EngineeringVolume 43, Issue 11, November 2014Online English edition of the Chinese language journalCite this article as: Rare Metal Materials and Engineering, 2014, 43(11): 2561-2566.ARTICLEUltra-Fine Grained Degradable Magnesium for Biomedical ApplicationsGe Qiang1,Ehsan Mo

2、staed1,Caterina Zanella2,Yu Zhentao3,Maurizio Vedani11Politecnico di Milano, Milan 20156, Italy; 2Università di Trento, Trento 38123, Italy; 3 Northwest Institute for Nonferrous Metal Research, Xian 710016, ChinaAbstract: Properties of commercially available purity magnesium and wrought ZM21 Mg

3、 alloy were investigated in view of their biodegradable applications. In particular, the opportunities offered by grain size refinement down to the ultra-fine scale achieved by equal channel angular pressing (ECAP) and warm extrusion were discussed and material properties were analyzed. Results show

4、 that the grain refinement will lead to a significant improvement in compression strength. The tension strength of the coarse grained alloy is always significantly higher than that measured in compression due to the sharp texture of the starting wrought alloy. ECAP also causes an attenuation of the

5、above texture effects, promoting marked changes in plastic flow behavior. The corrosion behavior of the investigated materials are affected by a combination of microstructural effects such as chemistry, grain size and the extent of lattice distortion inherited from previous processing stages. ECAP l

6、eads to refinement of grain size and to increased lattice defect density which apparently produce counterbalancing effects on corrosion performance. The improved dispersion of second-phase particles brings positive effects on development of pitting.Key words: Mg alloy; ECAP; ultrafine grained alloy;

7、 plastic anisotropy; texture; corrosionBiodegradable metals have been attracting significant re- search efforts owing to their promising performance in medi- cal applications where a temporary device with structural functions is required. Cardiovascular stents and os- teo-synthesis prostheses are ty

8、pical applications where Mg-based alloys or Fe-Mn alloys can potentially be used for tailored service periods and then dissolve into the human tis- sues without toxic or allergenic effects 1-5. The development of biodegradable alloys and the design of suitable devices should match a number of requir

9、ements that can be summa- rized in: (i) definition of alloy composition considering toler- ance and toxicity of alloying elements 6-8; (ii) tuning of the corrosion behavior based on the expected service time 9, 10; (iii) selection of the alloy strength for the structural design loads 10. Given these

10、 requirements, the number of known alloys suitable for applications in this field is rather limited, especially when considering commercially available materials.New opportunities are offered by the use of ultra-fine grained metals featuring a submicrometric or nanometricgrain-size scale.Wrought mag

11、nesium alloys feature a strong directionality of the mechanical strength caused by the particular deforma- tion behavior of the hexagonal close-packed crystal structure. Several investigations have already demonstrated that, due to the texture developed during extrusion, basal slip and pyra- midal 1

12、0-12 twinning are inhibited when the load is applied in tension, along the extrusion direction, thus resulting in higher yield strength. In contrast, other tensile directions and any compressive loading are able to activate slip and twinning, resulting in significantly lower yield strength 11-13. Th

13、is effect has a strong impact on the design of Mg structures and on the devices that are loaded in service under complex modes 14. Of relevance in this context is the evidence that grain refining by some SPD-based methods can also fully destroy the above fiber texture 15-17.Finally, it is to conside

14、r that grain refining also leads to mod- ification of corrosion behavior in metallic alloys. In general it can be stated that in fine-grained microstructures an increase inReceived date: March 25, 2014Corresponding author: Ge Qiang, Ph. D., Dipartimento di Meccanica, Politecnico di Milano, Milan 201

15、56, Italy, E-mail: maurizio.vedanipolimi.it Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.repassivation tendency and the break-up of second-phase par- ticles (leading to more homogeneous microstructures) improve corrosion resi

16、stance. However, the increased chemical activity at grain boundary regions as well as the higher density of bulk crystalline defects could even negatively affect the dissolution rate of fine grained materials 16, 18-21.1 ExperimentThe materials investigated were commercially available wrought ZM21 (

17、Mg-1.78Zn-0.89Mn) alloy as well as 99.94 wt% pure Mg. Grain refinement was achieved by the SPD tech- nique, adopting warm-temperature ECAP. The ECAP of the starting materials were described in details elsewhere 22, 23.For ZM21 alloy, a first set of ECAP cycles was carried out at temperature of 200 &

18、#176;C up to 8 passes followed by a second set of pressings conducted at a lower temperature of 150 °C for other additional 8 passes. For pure magnesium, to avoid specimen failure, the first set of ECAP could only be carried out at the temperature of 275 °C for 2 passes and the second set

19、at 250 °C for 4 passes. Starting from the ECAPed billets, minitubes with outer and inner diameter of 4 mm and 2 mm, respectively, to be considered as precursors of vascular stents were manufactured by hot extrusion using a laboratory devicealready adopted for coarse grained alloys and described

20、 in de- tails elsewhere24. Minitubes with optimal microstructure could be produced at temperatures as low as 150 °C for the ZM21 alloy.The microstructures were specified by optical microscopy (OM) and field-emission gun scanning electron microscopy (FEG-SEM) on samples both after ECAP or extrus

21、ion. Me- chanical properties after ECAP were measured on tensile (gage length of 12 mm and diameter of 4 mm) and compres- sion (cylindrical specimens of diameter 10 mm and length 15 mm) test specimens. Due to size limitations, mechanical properties of the extruded tubes were characterized by Vick- e

22、rs micro-hardness tests with a load of 1N on tube sections taken along extrusion direction. Micro-hardness was also evaluated on samples subjected to interrupted extrusion tests thereby containing both a residual billet region (only sub- jected to holding at processing temperature) and the extruded

23、tube region (subjected to both plastic strain and thermal ef- fects) 22.Finally, to assess the corrosion behaviors of ECAPed ZM21 alloy and pure magnesium as a function of grain size, open circuit potential (OCP) and potentiodynamic polarization tests were carried out. All the corrosion measurements

24、 were per- formed in phosphate buffered solution (PBS) consisting of an isotonic saline electrolyte with buffered pH of 7.4. Corrosion tests were carried out with a standard three-electrode cell, thermostatically controlled at 37 °C using an Ag/AgCl 3 mol/L electrode as the reference electrode,

25、 a platinum ring as the counter electrode and the corrosion sample as the working electrode. Samples were cut, embedded in resin and polishedto a mirror-like finish. Duplicate tests were carried out for each condition to confirm the repeatability of the results. For the OCP tests, samples were dippe

26、d in the PBS solution and their equilibrium potential with the electrolyte was recorded vs. Ag/AgCl 3 mol/L for 24 h. Potentiodynamic polarizations were performed with a scan rate of 0.166 mV/s.2 Results and Discussion2.1 Microstructure and hardnessThe optical micrographs of the materials investigat

27、ed shown in Fig.1 reveal that the alloy structure is composed of equiaxed grains with homogeneous size. The pure Mg sample features coarse polygonal grains with large amounts of twins at their interior.To achieve a significant grain refinement of Mg and Mg al- loy, the ECAP has to be performed at te

28、mperatures as low as possible. However, the processing of the coarse-grained start- ing billets directly at room or moderately high temperatures will lead to extensive cracking due to lack of ductility. A suc- cessful strategy for the Mg alloys reveals to be the initial processing of the billets at

29、higher temperatures. Once the alloy structure is refined and its formability improved, the billets can be successfully deformed even at lower temperatures without damage, thus further improving the refinement effect.In Fig.2, the grain structure obtained in ZM21 after the above described two-stage w

30、arm ECAP is depicted. A homo- geneous and equiaxed ultrafine structure with grain size of 500 nm was achieved after processing.In Fig.3, representative optical micrograph of the ECAPed pure Mg sample is depicted. In this case only a moderate graina20 µmb100 µmFig.1 Representative optical m

31、icrographs of the materials investi- gated: (a) ZM21 alloy and (b) pure Mg 1 µm 20 µmFig.2 Microstructure of the alloy after 2 steps of ECAPFig.4 Microstructure of the ZM21alloy after minitube extrusion50 µmVickers Microhardness (HVn)/×10 MPa90ZM21, 150 ºC 4P, ECAPZM21, ECAP

32、+extrusion 150 ºC8070Billet60Transition regionExtruded tubeFig.3 Microstructure of the pure Mg investigated after 2 passes at 275 °C followed by additional 4 passes at 250 °Crefinement could be achieved since pure Mg revealed to beless prone to SPD, showing frequent cracking of the bi

33、llets and rapid grain coarsening during warm ECAP. This behavior50051015202530Distance along Sample Axis/mmFig.5 Micro-hardness of ZM21 alloy after interrupted extrusion 400is supposed to be related to the original coarser microstructure of the samples, with a grain size exceeding 100 mm and to thei

34、r intrinsic properties. Magnesium has a poor formability owing to its hexagonal close-packed structure with extremely350Stress/N·mm-2300250200CG, tensionZM21 alloylimited slip systems 25. Moreover, the multi-step warm ECAPdoes not revealed to be fully appropriate owing to the limited thermal st

35、ability of pure Mg that leads to the rapid growth of the grains even on the short heating stages before any follow-150100500200 ºC 8P, tension 150 ºC 8P, tension CG, compression200 ºC 8P, compression 150 ºC 8P, compressioning ECAP pass.The microstructure and the micro-hardness of

36、 the minitube extruded at 150 °C starting from the ultrafine-grained ZM21 are shown in Fig.4 and 5, respectively. From the hardness curves of Fig. 5 it is observed that even warm-temperature holding induces a limited hardness drop that is promptly re- covered by extrusion, reaching again values

37、 close to the initial ECAPed billets. These data are consistent with the observed microstructure featuring an ultra-fine and homogeneous grain size in the warm-extruded minitube.2.2 Mechanical properties and texture effectsA comparison of the compression and the tension curves of the alloy is summar

38、ized in Fig.6. Upon considering the com- pression yield strength values, the positive effects of grain re- finement induced by the SPD processing is clearly confirmed. The coarse grained (CG) samples are softer than ECAPed00.10.20.30.4StrainFig.6 Engineering tension-compression curves of the ZM21 al

39、loy as a function of ECAP conditionsamples and the strength is improved by warm ECAP and by decreasing the ECAP temperature. As expected, the tension strength of the coarse-grained alloy is always significantly higher than that measured in compression (considering both yield and ultimate strength da

40、ta) due to expected texture ef- fects of the starting wrought alloys. Furthermore, it can addi- tionally be observed that ECAP leads to a considerable im- provement in compression behavior.The asymmetric tensile-compression behavior of the coarse-grained alloy is in close agreement with previous dat

41、aon ZM21 and AZ31 alloys 14 and with earlier literature re- search works 26, 27. Moreover, the attenuation of the texture effects and the improvement of ductility brought about by ECAP are positively evaluated since they promote a more re- liable material behavior for the design of biomedical device

42、s. It can be speculated that crystallographic anisotropy (withinMg, ECAPedZM21, ECAPedMg, CGZM21, CG-1.4Potential/V-1.5-1.6Fig.7 shows a SEM image of the strut of a full scale stent. The typical load-bearing structure of a human vascular stent is made up of tiny beams of cross section of the order o

43、f 150 µm×200mm. It can be argued that for a coarse grained alloy, such sectionis occupied by a limited number of grains (for instance about 2µm×3 µm rather than 15 µm×20 µm crystals when using pure Mg or one of the alloys of the present investigation, respecti

44、vely). The presence of a limited number of anisotropic crystals in small devices lends itself to unpredictable effects related to random orientation of grains close to geometrical notches and at most severely stresses regions. On the contrary, refining of the grain structure down to the sub-micromet

45、ric scale allows occupying the same volume with a greatly enhanced number of crystals, leading to a material behavior that can be considered isotropic even at such small size scale.A similar observation could also be raised when considering tension-compression asymmetry induced by texture. Referring

46、 to the typical geometry of a stent, it can be understood that during stent deployment and recoiling against the vessel wall, tensile and compressive stresses are developed on bending of the stent strut. A reduction of this effect would imply wider possibilities of accurate describing the material b

47、ehavior by constitutive models in order to design and predict the service performance of the device with improved accuracy 14.2.3 Corrosion behaviorIn order to draw information about the effect of grain refin- ing on corrosion behavior, comparative tests were carried out on pure Mg and on ZM21 alloy

48、 samples. Fig.8 depicts the100 µmFig.7 SEM image of the strut of a full-size stent prototype (after la- ser cutting, before final electro-polishing)510152025the scale of a single grain) as well as texture-induced aniso-tropy (related to a preferred orientation of crystals on the ma-1.7croscopic

49、 scale size) are considered as subtle effects in designof miniaturized devices, as it is the case of the biodegradablestents here considered.-1.80Time/hFig.8 OCP curves of pure Mg and ZM21 alloy with coarse grained structure (CG) and after ECAP processingevolution of the OCP curves up to testing tim

50、es of 24 h. In the coarse grained condition, a nobler electrochemical behavior of the ZM21 alloy can be observed over pure Mg. After ECAP, a shift of about 0.2 V toward more positive potentials could be measured in both samples. This implies a nobler behavior of the refined samples in contact with t

51、he corrosive environment. In agreement with the electrochemical measurements, mor- phological studies on the samples surfaces of the OCP test af- ter 24 h show a finer and more homogeneous corrosion pattern in the ECAPed samples, with significantly lower amount of localized corrosion pits (Fig.9).Th

52、e free corrosion potential of the ZM21 alloy is nobler than that of pure Mg, with a value of about -1.5 V. However, the potential value after ECAP (ultrafine grained structure) does not vary significantly from that before ECAP (coarse grained structure) for the alloy.Representative polarization curv

53、es of the materials investi- gated are depicted in Fig. 10. Pure Mg, both before and after ECAP, featured a very low free corrosion potential (around-1.8 V vs. Ag/AgCl 3 mol/L), with a slight gap value of 70 mV between the two conditions. These materials also show the same corrosion current at value

54、s of about 1×10-5 A/cm2. A first oxidation phase is evident up to -1.5 V from the free cor- rosion potential curves at higher resistance. The corrosion rate increases above -1.5 V when the corrosion behavior becomes localized. For ZM21 alloy the current densities are quite close, with values of

55、 2.8×10-5 for the ECAPed sample and 3.5×10-5 for the coarse grained sample, respectively. The anodic branch of the curve features a low Tafel slope which is comparable to the second part of the anodic branch of pure Mg at higher current densities.The general data about corrosion here prese

56、nted, albeit not fully exhaustive, suggest that the corrosion behaviors of Mg and of Mg alloys are a complex combination of effects, also related to microstructural parameters such as chemical com- position, grain size and extent of lattice distortion inherited from previous processing stages. ECAP

57、leads to grain refine-abcd 200 µm Fig.9 Views of the ZM21 and pure Mg sample surfaces after 24 h of OCP testing: (a) coarse grained Mg, (b) ECAPed Mg, (c) coarse grained ZM21, and (d) ECAPed ZM21-1.0E vs. Ag/AgCl/V-1.2ment and the increase of lattice defects that apparently pro- duce counterbal

58、ancing effects on corrosion performance. It can be stated that the refinement of the structure with im- proved dispersion of second phase particles surely induces a beneficial effect in terms of development of pitting (Fig.9), while full exploitation and comprehension of the grain-size related phenomena would deserve further research activities to better control microstructural features of the materials.3 Conclusions1) ECAP can achieve a significant grain refining of the Mg alloys in the submicrometer range.