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Research paper
Microstructure,mechanical properties,biocorrosion behavior,and cytotoxicity of as-extruded Mg–Nd–Zn–Zr alloy with different extrusion ratios
Xiaobo Zhang a ,Guangyin Yuan a ,b ,?,Jialin Niu a ,Penghuai Fu a ,Wenjiang Ding a ,b
a National Engineering Research Center of Light Alloy Net Forming,Shanghai Jiao Tong University,Shanghai 200240,China
b Key State Laboratory of Metal Matrix Composite,Shanghai Jiao Tong University,Shanghai 200240,China
A R T I C L E I N F O Article history:
Received 16November 2011Received in revised form 4February 2012
Accepted 5February 2012
Published online 13February 2012Keywords:Magnesium alloy Mechanical properties Biocorrosion behavior Cytotoxicity Extrusion ratio
A B S T R A C T
Recently,commercial magnesium (Mg)alloys containing Al (such as AZ31and AZ91)or Y (such as WE43)have been studied extensively for biomedical applications.However,these Mg alloys were developed as structural materials,not as biomaterials.In this study,a patented Mg–Nd–Zn–Zr (denoted as JDBM)alloy was investigated as a biomedical material.The microstructure,mechanical properties,biocorrosion behavior,and cytotoxicity of the alloy extruded at 320?C with extrusion ratios of 8and 25were studied.The results show that the lower extrusion ratio results in fi ner grains and higher strength,but lower elongation,while the higher extrusion ratio results in coarser grains and lower strength,but higher elongation.The biocorrosion behavior of the alloy was investigated by hydrogen evolution and mass loss tests in simulated body fl uid (SBF).The results show that the alloy extruded with lower extrusion ratio exhibits better corrosion resistance.The corrosion mode of the alloy is uniform corrosion,which is favorable for biomedical applications.Aging treatment on the as-extruded alloy improves the strength and decreases the elongation at room temperature,and has a small positive in fl uence on the corrosion resistance in SBF .The cytotoxicity test indicates that the as-extruded JDBM alloy meets the requirement of cell toxicity.
c ?
2012Elsevier Ltd.All rights reserved.1.Introduction
Magnesium (Mg)alloys have become an interesting candidate
as a bioactive substance in the fi eld of biodegradable implant materials such as stents,bone fi xtures,plates,and screws.However,the magnesium alloys currently under investigation as implant materials are mostly commercial alloys such as the AZ series (Kirkland et al.,2010;Liu et al.,2007;Witte
Corresponding author at:National Engineering Research Center of Light Alloy Net Forming,Shanghai Jiao Tong University,Shanghai 200240,China.Tel.:+8602134203051;fax:+8602134202794.
E-mail address:gyyuan@https://www.doczj.com/doc/5e17501385.html, (G.Yuan).
et al.,2006;Alvarez-Lopez et al.,2010;Scharnagl et al.,2009;Witte et al.,2005)and WE series (Castellani et al.,2011;Walter and Bobby,2011;Gu et al.,2010;Hanzi et al.,2009;Erbel et al.,2007;Waksman et al.,2006)magnesium alloys,which were not originally developed as biodegradable materials.Aluminum (Al)is a risk factor for Alzheimer’s disease (Ferreira et al.,2008),and it can cause muscle fi ber damage (Shingde et al.,2005).For use in humans,Witte et al.(2008)
1751-6161/$-see front matter c ?
2012Elsevier Ltd.All rights reserved.doi:10.1016/j.jmbbm.2012.02.002
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recommended using Al-free magnesium alloy systems.The results of evaluations of short-term effects of rare earth(RE) elements used in magnesium alloys on primary cells and cell lines show that the highly soluble Dy and Gd seem to be more suitable than Y;suitable elements with low solid solubility could be Eu,Nd,and Pr(Feyerabend et al.,2010).The following three issues should be considered for a biodegradable Mg alloy.(1)It must have acceptable biocompatibility.(2)It requires suf fi cient yield strength and ductility to support the lesion site.(3)It also must have a slow corrosion rate, especially a uniform corrosion mode,ensuring a functionality time of3–6months or even longer.In the literature on Mg alloys designed for biomedical applications,many of them can meet the biocompatibility condition(Erdmann et al.,2011; Zheng et al.,2010;Zhang et al.,2010b,2009),but few of them can meet the mechanical and slow uniform corrosion requirements simultaneously.
The desired biomaterial can be designed by considering the following factors.Actually,some RE elements,such as Nd,Dy,and Gd,are bene fi cial in terms of mechanical and corrosion properties and have acceptable toxicity(Feyerabend et al.,2010).Mg–Nd binary alloys have already shown signi fi cant strengthening effects.Zn is also one of the abundant nutritionally essential elements in the human body.A small amount of Zn contributes to strength due to solid solution strengthening.Zirconium(Zr)is an effective grain-re fi ning agent in Al-free magnesium alloys both in as-cast and as-extruded conditions,and it contributes to strengthening due to the formation of fi ne grains.The biocompatibility of a small amount of zirconium in a magnesium alloy has been veri fi ed(Ye et al.,2010).As a result,we have recently developed a patented biodegradable Mg alloy Mg–Nd–Zn–Zr(hereafter,denoted as JDBM,an abbreviation of Shanghai Jiao Da Bio-Magnesium)with good mechanical properties,biocompatibility,and uniform corrosion resistance for medical applications(Zhang et al., 2012).
Grain re fi nement,achieved through thermo-mechanical processing or microalloying,improves the mechanical properties and corrosion resistance(Alvarez-Lopez et al., 2010;Ralston and Birbilis,2010;Hamu et al.,2009;Ben-Haroush et al.,2008).The extrusion process of Mg alloy is a thermo-mechanical process,and it could be a grain re fi nement technique,as the extrusion conditions are controlled.It is believed that the most important parameters in the extrusion process in fl uencing grain re fi nement are the working temperature and the extrusion ratio.There is an agreement that the grain sizes increase with increasing extrusion temperature(Zhang et al.,2010a,b;Uematsu et al., 2006;Murai et al.,2003;Yuan et al.,2008;Fu et al.,2009;Li et al.,2010).However,for the extrusion ratio,contradictory results were found:some studies reported that the grain size of the Mg alloys decreased with increasing extrusion ratio, such as AZ31(Chen et al.,2007;Murai et al.,2003),and AZ61A and AZ80(Uematsu et al.,2006),while others reported that the grain size of the Mg alloys increased with increasing extrusion ratio,such as Mg97Zn1Y2alloy(Hirano et al.,2010). As for the JDBM alloy,previous work has demonstrated that a lower extrusion temperature resulted in fi ner grains, higher strength at room temperature,and better corrosion resistance in simulated body fl uid(SBF)(Zhang et al.,2012). However,the effects of the extrusion ratio on the grain re fi nement,mechanical properties,and corrosion resistance are still unclear.Moreover,since it is a new biodegradable alloy,its cytotoxicity needs to be evaluated.In the present study,therefore,the JDBM alloy was extruded under two extrusion ratios.The microstructure,mechanical properties, biocorrosion behavior,and cytotoxicity of as-extruded JDBM alloy were studied.An aging treatment on the as-extruded JDBM alloy was also carried out to improve the mechanical properties and biocorrosion properties.
2.Experimental
2.1.Alloy preparation
A JDBM alloy ingot(diameter108mm×4500mm)was cast by semi-continuous casting with high-purity Mg (≥99.99%),Zn(≥99.995%),Mg–25%Nd(impurities≤0.1%)and Mg–30%Zr(impurities≤0.5%)used as raw materials.The semi-continuous casting process is as follows.The melt was transferred to a semi-continuous casting machine at680?C, and cast into billet with a diameter of108mm at a velocity 200mm/min.The billet was pre-chilled by fl owing water in the mould and then direct chilled by water.The chemical composition of the alloy was analyzed by inductively coupled plasma atomic emission spectrometry(ICP-AES).The cast bars(diameter108mm×300mm)cut from the ingot were solution treated at540?C for10h and quenched into water at room temperature(T4treatment).The cast alloy was extruded directly into cylindrical rods at320?C with extrusion ratios of8and25(hereafter,denoted as R8and R25,respectively).The extrusion speed was2mm/s,and the extruded rods were cooled at room temperature.Some extruded rods were aged at200?C for8h in an oil bath (hereafter,denoted as R8+Aging and R25+Aging).
2.2.Microstructure observation and mechanical proper-ties test
The specimens for microstructure observation were cut parallel to the extrusion direction and polished with320 grit waterproof abrasive paper,600grit metallographic paper, 5μm and1μm diamond grinding paste,cleaned in distilled water and ethanol,and then dried in warm fl owing air.The polished specimens were etched with acid solution(10ml acetic acid,4.2g picric acid,70ml ethanol,and10ml distilled water).The microstructure of the specimens at different conditions was observed using optical microscopy(OM) and scanning electron microscopy(SEM)in back-scattered electron(BSE)mode,in conjunction with energy-dispersive spectrometry(EDS).The grain size of the alloy was measured by the linear intercept method according to GB/T6394-2002. Tensile test samples with dimensions of10mm width,2mm thickness,and30mm gauge length were cut by an electric-sparking wire-cutting machine according to GB/T228-2002 and then successively polished with papers and diamond pastes to a mirror surface.Three specimens were tested in each group.The tensile tests were carried out on a material test machine at room temperature with an initial strain rate of1.7×10?3s?1.The fracture surfaces were observed by SEM.
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155 Fig.1–Microstructure of the as-cast JDBM alloy.(a)OM image(b)XRD pattern.
2.3.Biocorrosion behavior test
The biocorrosion behavior of the alloy was measured using hydrogen evolution and mass loss tests.Cylindrical speci-mens with a diameter of12mm and thickness of5mm(total surface area:4.1±0.1cm2)were cut by an electric-sparking wire-cutting machine.The specimens were polished follow-ing the same procedure as that for microstructure observa-tion.Simulated body fl uid(SBF),composed of8.0g/L NaCl, 0.4g/L KCl,0.35g/L NaHCO3,0.2g/L MgSO4.7H2O,0.14g/L CaCl2,0.06g/L Na2HPO4,and0.06g/L KH2PO4,was used as the test solution.The pH value was adjusted to7.4with NaOH or HCl solution before the experiments,and the temperature was kept at37±0.5?C during the experiments.The ratio of SBF volume to the specimen surface area is30ml/cm2ac-cording to ASTM G31-72.The SBF was renewed every24h in order to keep a relative stable pH value,and the immer-sion test lasted for240h.The hydrogen volume was recorded before renewing the SBF.After the immersion test,the cor-rosion products were removed in a chromic acid solution (200g/L Cr2O3+10g/L AgNO3).Then the samples were rinsed with distilled water and ethanol,and then dried in warm fl owing air.The dried samples were weighed and the corro-sion rate was calculated.Five specimens were measured for each alloy condition.The corrosion rate of the alloy was cal-culated by the following formula according to ASTM-G31-72:
Corrosion rate=(K×W)(A×T×D),
where K is a constant,W is the mass loss in g,A is the area in cm2,T is the time of exposure in hours,and D is the density in g/cm3.
2.4.Cytotoxicity evaluation
The methylthiazolyldiphenyl-tetrazolium bromide(MTT) test,carried out by an indirect contact method,was used to evaluate the cell toxicity of the as-extruded JDBM alloy. L-929cells were cultured in Dulbecco’s Modi fi ed Eagle’s Medium(DMEM)containing10%fetal bovine serum(FBS)at 37?C with5%CO2in a humidi fi ed incubator.Extracts were prepared for72h according to ISO10993-5:1999,and were stored at4?C before the experiments.Cells were seeded in 96-well plates at5×104cells/ml,100μl for each well,and incubated for24h to allow attachment.Then,the medium was replaced by100μl of extracts.After1,3,and5days of incubation,20μl MTT(5mg/ml in phosphate buffered saline,PBS)was added to each well,and the medium was replaced by150μl dimethyl sulfoxide(DMSO)4h later.The optical density(OD)was measured at a wavelength of490nm. The morphologies of L-929cells were observed by a inverted fl uorescence microscope.The cell viability was expressed as the relative growth rate(RGR).
3.Results
3.1.Chemical composition and microstructure
The chemical composition of the JDBM alloy analyzed by ICP-AES is as follows:Nd3.130,Zn0.164,Zr0.413,Fe0.003,Ni 0.001,Cu0.001,Si0.003,Mn0.001,and Mg balance(mass fraction).
Fig.1(a)shows the as-cast microstructure of JDBM alloy. The microstructure of the as-cast alloy consists of an α-Mg matrix and eutectic compounds distributed along the grain boundary areas.From X-ray diffraction(XRD)analysis (Fig.1(b)),the eutectic compounds are Mg12Nd,as indicated in Fig.1(a).The average grain size of the as-cast alloy is about40μm.Fig.2shows microstructures of the extruded JDBM alloy parallel to the extrusion direction.In comparison with the as-cast alloy,the average grain size of the cast alloy in T4condition is about45μm;all the eutectic phase was dissolved into the Mg matrix,and some Zr-containing particles(Fu et al.,2008)can be observed at grain interiors (Fig.2(a)).Remarkable grain re fi nement is introduced by a hot extrusion process with different extrusion ratios.Generally, in the R8alloy(lower extrusion ratio),the average grain size of the fi ner equiaxed grains is about2μm,while the long elongated grains are hundreds of micrometers in length and tens of micrometers in width(Fig.2(b)and(c)).Long elongated grains were also reported in AZ21(Azeem et al., 2010)and Mg–1.0Zn–0.5Ca alloys(Zhang et al.,2010a,b);they have been suggested to arise from previous unextruded structures that have survived dynamic recrystallization. Since these grains are favorably oriented to accommodate extrusion strains,in the basal slip system,they can undergo deformation without twinning and latent hardening.Hence, these grains do not have large enough stored plastic energy to trigger recrystallization(Azeem et al.,2010).However,when the extrusion ratio increases to25,these long elongated
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Fig.2–Microstructure of the as-extruded JDBM alloys parallel to extrusion direction.(a)T4,(b)R8,(c)R8+Aging,(d)R25, and(e)R25+Aging.
grains disappear,and a more homogeneous microstructure is obtained,as shown in Fig.2(d)and(e).The average grain size of the R25alloy is about5μm,indicating that grain coarsening occurs.In addition,some fi ne particles can be observed among the recrystallized grains,and they are Mg12Nd compounds according to a previous study(Fu et al.,2009).Furthermore,no obvious difference of the microstructure can be observed by OM after aging treatment on the as-extruded alloy.According to the previous study of the cast alloy(Fu et al.,2008),some nano precipitates formed during the subsequent aging treatment.
In order to distinguish the difference between the precipitation phases in the as-extruded R8and R25alloys, SEM-BSE micrographs of them are shown in Fig.3.In R25 alloy,some white particles,1or2μm in size,can be observed clearly(Fig.3(b)).And these particles are rich in Nd according to the EDS result,as shown in Fig.4.Therefore,they are bright in the SEM-BSE micrographs.The atom ratio of Mg and Nd of the white particle is close to12:1;together with the XRD results(Fig.1(b))and previous study(Fu et al., 2008,2009),it is presumed to be Mg12Nd.Lots of tiny white particles can be noted in R8alloy;some of them are about 0.5μm in size,and others are less than0.1μm(Fig.3(a)).The precipitated particles are too fi ne to be analyzed by EDS accurately.These tiny particles are also Mg12Nd compounds, which has been con fi rmed by TEM analysis(Fu et al., 2008).The extrusion ratio not only in fl uences the grain size of recrystallized grains but also the size and density of precipitated particles during hot extrusion.A lower extrusion ratio leads to fi ner precipitated particles with higher density, which will presumably improve the strength of the alloy.
3.2.Mechanical properties
The mechanical properties of JDBM alloys are listed in Table1. The strength of the alloy is signi fi cantly improved after hot extrusion.Both the ultimate tensile strength(UTS)and the yield strength(YS)of the R8alloy are over300MPa.The strength of the R25alloy is lower than that of R8but much higher than that of the T4alloy.Furthermore,the elongation of the R25alloy increases remarkably up to26%.After aging treatment on the as-extruded alloy,the yield strength is improved more obviously than the ultimate tensile strength, while the elongation reduces.
In order to clarify the obvious elongation difference between the as-extruded JDBM alloys with different extrusion
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Fig.3–Back-scattered SEM micrographs of JDBM alloys extruded with different extrusion ratios.(a)R8and (b)R25.
ratios,the fracture morphologies of the R8and R25alloys are shown in Fig.5.It is apparent that the fracture patterns of R8are cleavage fracture of long elongated grains and intergranular fracture of fi ne equiaxed grains.However,a large number of uniformly distributed dimples can be observed in the fracture morphology of R25alloy,which indicate ductile fracture.As a consequence,the R25alloy exhibits much better elongation.
3.3.Biocorrosion behavior
Fig.6shows the hydrogen evolution curves of the JDBM alloys immersed in SBF at 37±0.5?C for 240h.It is seen that the H 2evolution volume of the as-extruded JDBM alloys is much less than that of the as-cast T4alloy.As for as-extruded alloys,the H 2evolution volume of R8is less than that of R25.Additionally,the H 2evolution volume reduces slightly after aging treatment on R8and R25alloys.
Fig.7shows the mass loss test results of the JDBM alloys.It demonstrates that the corrosion rate of the T4alloy is the highest;those of the as-extruded alloys are much lower.As for as-extruded alloys,the corrosion rate of the R8alloy is much lower than that of R25alloy.After aging treatment on the as-extruded alloys,the corrosion rates of the as-extruded alloys show a slight reduction.These results correspond well with the results of the hydrogen evolution test.Generally,the extrusion process on JDBM alloys can enhance the corrosion resistance in SBF ,and lower extrusion ratio at 320?C can enhance corrosion resistance more remarkably.
Fig.8shows the corroded morphologies of the JDBM alloys after removing the corrosion products from the surface of the specimens.It is seen that the polished surfaces of the T4alloy are corroded completely while there are still some polished surfaces of the as-extruded JDBM alloys which have not undergone corrosion.This demonstrates that the T4alloy underwent more severe corrosion than the as-extruded JDBM alloys.This result is consistent with the results of the hydrogen evolution and mass loss tests.Moreover,the as-extruded JDBM alloys exhibit a much more uniform corrosion mode than T4alloy.The corroded pits of the as-extruded JDBM alloys are distributed uniformly,which indicates a uniform corrosion mode.
3.4.Cytotoxicity
An indirect assay was performed to evaluate the L-929cell response to as-extruded JDBM alloys with different extrusion ratios (R8and R25).Fig.9shows the cell viability cultured in 100%R8and R25extraction medium for 1,3,and 5days compared with the negative control.The cell viabilities for both R8and R25extracts during 1,3,and 5days of culture compared with the negative control are over 80%,which indicates that the R8and R25alloys induce only slight toxicity to L-929cells and meet the requirement of cell toxicity according to ISO 10993-5:1999.Fig.10shows the morphologies of the L-929cells cultured in negative control,R8,and R25alloy extracts for 1,3,and 5days.Most cells show a healthy morphology with a fl attened spindle shape.As the culture time increases,some cells exhibit a round shape due to the proliferation and concentration of the cells,which is also healthy.
4.Discussion
The dominating parameters for grain re fi nement in the extrusion process are considered to be the working temperature and the extrusion ratio.In this study,the extrusion temperature is constant.Hence,the different grain sizes of the alloy are caused by the extrusion ratio.However,previous reports have reported that the grain size of the AZ series of Mg alloys decreased with increasing extrusion ratio,such as AZ31(Chen et al.,2007),and AZ61A and AZ80(Uematsu et al.,2006),which is inconsistent with the results of the present study.The growth of recrystallized grains and precipitates with the increase of extrusion ratio can be explained by the rise in temperature during hot extrusion.During hot extrusion,the alloy is forced to
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Fig.4–EDS result of the bright particle of the R25JDBM alloy.(a)SEM micrograph and (b)spectrum of bright particle.
deform from a larger cross-section into a smaller https://www.doczj.com/doc/5e17501385.html,rger extrusion ratio leads to larger total strain and reduces the temperature of recrystallization.When extrusion is carried out at the same speed,larger total strain leads to higher strain rate,hence leading to higher deformation stress,higher deformation energy,and lower recrystallization temperature.Higher deformation energy implies higher temperature increase during the hot extrusion process.Therefore,the grains and precipitates grow at a higher temperature.As a consequence,R8alloy exhibits partial dynamic recrystallization while R25alloy undergoes complete dynamic recrystallization and grain growth.Similar results were reported in rare-earth-containing Mg 97Zn 1Y 2alloy (Hirano et al.,2010).
It is inspiring that a yield strength of 308MPa for the alloy under an extrusion ratio of 8can be achieved,which is an increase of 242%compared with that of the as-cast T4alloy.It is mentioned that the grain size of the R8alloy is smaller than that of R25alloy and the lower extrusion ratio leads to fi ner precipitated particles with higher density.Therefore,the remarkable strength improvement of the R8alloy is mostly attributed to the grain re fi nement and precipitation of Mg 12Nd phases.In addition,as with other magnesium alloys processed by hot extrusion,the as-extruded JDBM alloys also show fi ber texture (Fu et al.,2009),which may also improve the strength of the alloys.However,the elongation of the R8alloy is not signi fi cantly improved,despite it having fi ne grains.The main reason is that the alloy exhibits an inhomogeneous microstructure.It is well known that there are deformed structures in partially recrystallized alloys.The presence of deformed structures induces stronger basal texture and higher dislocation density,which will be detrimental to elongation (Zhang et al.,2010a ,b ).In addition,the yield strength and ultimate tensile strength of the R8alloy are very close,exhibiting higher yield ratio (yield strength/ultimate tensile strength)than that of the R25alloy.The reason may be as follows.Generally,a basal slip system can be easily activated,which accelerates the accumulation of dislocations during plastic deformation,which is the origin of work hardening.The highest dislocation density of the R8alloy has almost reached the critical level,which restricts further dislocation accumulation and work hardening.However,the complete dynamic recrystallization of the R25alloy decreases the dislocation density.The larger grain size and the lower dislocation density in grains are bene fi cial to the dislocation accumulation,which generates work hardening during tensile deformation.Therefore,remarkable work hardening occurs in the R25alloy.
Although the grains of the R25alloy are coarser than those of the R8alloy,higher elongation was still obtained,possibly due to the homogeneous microstructure.As mentioned above,the R8alloy shows a very inhomogeneous microstructure (fi ne equiaxed grains and long elongated grains),and this structure is detrimental to elongation.In addition,the strength of the alloys is further improved
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Fig.5–Secondary-electron SEM fracture morphologies of the as-extruded JDBM alloys extruded with different extrusion ratios.(a)R8and (b)
R25.
Fig.6–Hydrogen evolution volume of the JDBM alloys immersed in SBF for 240h.
after aging treatment,but the elongation decreases.During the aging treatment,tiny particles precipitate from the Mg matrix.Precipitated particles are likely to bring about additional barriers to the movement of dislocations,resulting in the enhancement of tensile strength.However,the precipitate may act as crack sources and in turn decrease the tensile elongation (Fu et al.,2008
).
Fig.7–Corrosion rate of the JDBM alloys immersed in SBF for 240h.
Song et al.(1999)studied the effects of the secondary phase (β-phase Mg 17Al 12)on the corrosion of diecast AZ91D.The study shows that the β-phase can act either as a barrier or as a galvanic cathode which serves a dual purpose in corrosion and depends on the microstructure.The β-phase is expected to act as a barrier when the β-phase fraction is not too low and nearly continuous over the small grain size Mg matrix.In Mg–Nd–Zn–Zr alloy,Mg 12Nd phase is corrosion resistant,and its corrosion potential is only a little more positive than that of pure Mg (Chang et al.,2007),so its negative in fl uence on corrosion caused by galvanic corrosion is very slight.Previous reports (Ben-Haroush et al.,2008;Hamu et al.,2009;Ralston and Birbilis,2010;Alvarez-Lopez et al.,2010)have indicated that fi ne grains can enhance the corrosion properties of magnesium alloys.The fi ne grains of the Mg matrix and the second phase are obtained by extrusion.The uniformly distributed tiny Mg 12Nd particles after extrusion can reduce the activity of galvanic corrosion.Moreover,if the grain boundary density dictates the oxide fi lm conduction rate on the surface with low to passive corrosion rates,then fi ne grain structures are expected to be corrosion resistant.The grain re fi nement has retarded cathodic kinetics,helping sti fl e corrosion overall.Consequently,the signi fi cantly improved corrosion resistance of the as-extruded JDBM alloy compared with the as-cast T4alloy,and particularly the R8alloy,is mainly ascribed to the grain re fi nement and probably somewhat to the Mg 12Nd phase.After aging treatment,the corrosion rate decreases slightly.This is probably due to the precipitation of tiny particles of Mg 12Nd.
The MTT test is a widely used cytotoxicity test,but a recent report by Fischer et al.(2010)shows that the use of the MTT test may lead to false positive or false negative results for Mg materials.It suggested that the MTT test should be used with caution,and that the test should be avoided when investigating the cytotoxicity of different Mg materials in static in vitro assays.In the present study,the MTT test is used for just a kind of Mg alloy with different extrusion ratios.Furthermore,the cells cultured in the extracts for 1,3,and 5days show healthy morphology.Therefore,the result of
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Fig.8–Secondary-electron SEM images of JDBM alloys immersed in SBF for240h after removing corrosion products.(a)
T4,(b)R8,(c)R8+Aging,(d)R25,and(e)R25+
Aging.
Fig.9–Cell viability expressed as a percentage of the
viability of L-929cells after1,3,and5days of culture in
100%as-extruded JDBM alloy extraction media.
the cytotoxicity of the JDBM alloy to L-929cell is reliable,and
JDBM alloy showed acceptable cytocompatibility.
5.Conclusion
The microstructure,mechanical properties,biocorrosion
behavior,and cytotoxicity of Mg–Nd–Zn–Zr alloy extruded
at the same working temperature(320?C)but at different
extrusion ratios(8and25)were studied in this work.
The results show that the alloy with the extrusion ratio
of8has fi ner but more inhomogeneous microstructure
than the alloy with the extrusion ratio of25.The R8
alloy underwent partially dynamic recrystallization while
the R25alloy underwent complete dynamic recrystallization
and grain growth.The R8alloy exhibits good mechanical
properties with high strength(over300MPa)and moderate
elongation.The R25alloy exhibits high elongation(over20%)
and moderate strength.Therefore,the optimal mechanical
properties of the JDBM alloy may be obtained when the
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Fig.10–Optical morphologies of L-929cells that were cultured in the control(a,d,and g),100%R8(b,e,and h),and100% R25(c,f,and i)alloy extraction media for1(a,b,and c),3(d,e,and f)and5(g,h,and i)days.
alloy undergoes just complete dynamic recrystallization and the grains do not grow when the extrusion parameters are proper.The corrosion rate of the as-extruded alloy in SBF is much lower than that of the as-cast T4alloy.The R8alloy has better corrosion resistance than the R25alloy.The as-extruded alloys show a uniform corrosion mode in SBF.Aging treatment on the as-extruded alloy can improve the strength and corrosion resistance.The cytotoxicity evaluation using L-929cells reveals that the as-extruded JDBM alloy meets the requirement of cell toxicity for biomaterials in a short-term study.
Acknowledgments
This project was supported by the Science and Technol-ogy Commission of Shanghai Municipality(11DJ1400300, 10JC1407400),the National Key Technology R&D Program of China(2012BAI18B01)and China Postdoctoral Science Foun-dation funded project(20100470030).The authors thank Dr.Bing Ye for valuable discussions and language polish.
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