(7.2)--Influence of ageing treatment on机械工程材料机械工程材料.pdf

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1、journal homepage: Available online at Research PaperInfluence of ageing treatment on microstructure,mechanical and bio-corrosion properties of MgDy alloysLei Yangn,Yuanding Huang,Frank Feyerabend,Regine Willumeit,Karl Ulrich Kainer,Norbert HortHelmholtz Zentrum Geesthacht,Institute of Materials Rese

2、arch,Max-Planck-Str.1,D-21502 Geesthacht,Germanya r t i c l e i n f oArticle history:Received 20 January 2012Received in revised form6 April 2012Accepted 9 April 2012Available online 24 April 2012Keywords:MgDy alloysHeat treatmentMechanical propertiesBio-corrosion propertiesa b s t r a c tMgDy alloy

3、s have shown to be promising for medical applications.In order to investigatethe influence of ageing treatment on their mechanical and corrosion properties,threeMgxDy alloys(x10,15,20 wt%)were prepared.Their microstructure,mechanical andcorrosion behavior were investigated.The results indicate that

4、ageing at 250 1C has littleinfluence on the mechanical and corrosion properties.In contrast,ageing at 200 1Csignificantly increases the yield strength,and reduces the ductility.After ageing at200 1C,the corrosion rate of Mg20Dy alloy increases largely in 0.9 wt%NaCl solution,but remains unchanged in

5、 cell culture medium.&2012 Elsevier Ltd.All rights reserved.1.IntroductionA number of recent works have emphasized the possibilitiesof Mg alloys as a new class of degradable and bio-resorbablebiomaterials for orthopedic applications(Witte et al.,2008;Gu and Zheng,2010).As degradable materials,Mg all

6、oys donot stay as permanent implants in the body and do notrequire a second surgery after the tissue get healed.TheYoungs modulus of Mg alloys(4045 GPa)is closer to that ofhumanbones(523 GPa)thanthatofstainlesssteels(193 GPa)or titanium alloys(114 GPa)(Witte et al.,2008).Additionally,Mg ion is the f

7、ourth most prevalent constituentof human serum,and the metabolism of Mg and excretionvia the kidneys is natural physiological process.Thus thedegradation products of Mg are expected to be non-toxic(Staiger et al.,2006).However,some problems still remain to be solved before Mgmight be used as a new c

8、lass of biomaterials.One of the keyissues is that Mg alloys do not undergo a moderate andhomogeneous degradation to maintain the mechanical integ-rity of the implant during the healing process.This is accom-paniedbyinsufficientmechanicalproperties,toorapidcorrosion or localized corrosion of Mg alloy

9、s(Hermawanet al.,2010;Witte,2010).In response to this problem,manynew Mg alloys have been investigated to develop an alloy whichcan fulfill the requirement as implant(Zhang and Yang,2008;Hort et al.,2010;Zhang et al.,2010).However,this is not onlydependent on the elemental composition and processing

10、 his-tory,but also dependent on the subsequent treatment such assurface coatings and heat treatment.Ageing treatment generally is an effective way to adjustboth the mechanical and corrosion behavior of Mg alloys by1751-6161/$-see front matter&2012 Elsevier Ltd.All rights reserved.http:/dx.doi.org/10

11、.1016/j.jmbbm.2012.04.007nCorrespondence to:Helmholtz Zentrum Geesthacht,MagIC-Magnesium Innovation Centre,Institute of Materials Research,Max-Planck-Str.1,D-21502 Geesthacht,Germany.Tel.:49 4152 87 1923;fax:49 4152 87 1909.E-mail address:lei.yanghzg.de(L.Yang).j o u r n a l o f t h e m e c h a n i

12、c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 1 3(2 0 1 2)3 6 4 4changing their microstructures and redistributing alloyingelements.MgRE(rare earths)alloys,such as LAE442(Gruhlet al.,2009;Witte et al.,2010)and WE43(Witte et al.,2005,2006),have been widely tested for medical applica

13、tion due totheir good combination of mechanical and corrosion proper-ties.However,the influence of ageing treatment on theproperties of MgRE alloys,especially the bio-corrosion beha-vior,is not well understood.From a practical point of view,itis essential to know the effects of the suitable ageing t

14、reat-ment on the mechanical properties and degradation rate inan in-vitro environment.In the present work,the influence ofageing treatment on the mechanical and corrosion propertiesof MgDy alloys,which have the potential as an implant(Yang et al.,2011),is investigated.2.Experimental procedures2.1.Ma

15、terial preparationPermanent mold direct chill casting(Peng et al.,2010)was usedto prepare MgDy alloys.High-purity Mg(MEL,UK,99.94%)wasmolten in a mild steel crucible under a protective atmosphere(Ar2%SF6).Pure Dy(99.5%,Grirem,Beijing,China)was addedat a melt temperature of 720 1C in the amount of 10

16、,15 and20 wt%.Then the melt was stirred for 30 min at 200 rpm.Thesize of ingot was 20 cm?12 cm?6 cm.Solution treatment(T4)was carried out at 520 1C for 24 h followed by water quenching.Ageing treatment(T6)was performed at 250 1C and 200 1C fordifferent time followed by air cooling.All specimens were

17、 takenfrom the same position of the ingots.The chemical compositions of the alloys were analyzed usingX-ray fluorescence(XRF)analyzer(Bruker AXS S4 Explorer,Karlsruhe,Germany)for Dy and Ni,and spark optical emissionspectroscopy(Spectrolab M9,Kleve,Germany)for Fe and Cu.Inthe measurement of impuritie

18、s with spark optical emissionspectroscopy,the content of Ni could not be determined due tointerference of strong Dy peaks.Therefore,the Ni content isdetermined from the XRF results.2.2.Microstructure analysisSpecimens for transmission electron microscopy(TEM)wereprepared by grinding them mechanicall

19、y to a thickness ofabout 120 mm and then thinned by electropolishing.Thiselectropolishing was carried out in a twin jet system usinga solution of 2.5%HClO4and 97.5%ethanol at?45 1C and avoltage of 40 V.The TEM examinations were carried out on aPhilips CM 200 instrument operated at 200 kV.For the pha

20、se analysis,X-ray diffraction(XRD)measure-ments were performed using a diffractometer(SiemensD5000,Germany)withCuK-a1radiation(wavelengthk0.15406 nm)and a secondary monochromator.The dif-fraction patterns were measured from 151 to 901(2y)for eachspecimen.The step size was 0.021 and the step time is

21、3 s.CaRIne crystallography software(version 3.1)was used tosimulate the diffraction peak of b0phase based on its crystalstructure(orthorhombic,a0.64 nm,b0.22 nm,c0.52 nm)(Vostry et al.,1999;Honma et al.,2005).2.3.Mechanical testsThe Vickers hardness measurement was carried out using aHMV 2000 machin

22、e with a load of 5 kg and a dwelling time of10 s.Ten points were averaged for each specimen.Tension andcompression tests were performed at room temperature using aZwick 050 testing machine(Zwick GmbH&Co.,KG,Ulm,Germany)according to DIN EN 10002 and DIN 50106(DIN,2001;DIN,1978),respectively.Tensile s

23、pecimens with a gaugelength of 30 mm,a diameter of 6 mm and threaded heads of10 mm were used.The compression specimens were cylinderswith a height of 16.5 mm and a diameter of 11 mm.Both tensionand compression tests were done at a strain rate of 1?10?3s?1.At least three specimens were tested for eac

24、h condition.2.4.CorrosionWeight loss tests were performed in 0.9 wt%NaCl solution atroom temperature as well as in cell culture medium(CCM)undercell culture conditions(37 1C,21%O2,5%CO2,95%rH).The CCMconsisted of Dulbeccos modified eagle medium(Sigma AldrichChemie,Taufkirchen,Germany)and 10%fetal bo

25、vine serum(FBSGold,PAA Laboratories,Linz,Austria).The composition ofDulbeccos modified eagle medium is listed in Table 1.Thespecimens were prepared by grinding each side with 2400 gridemery paper and degreasing the surfaces with ethanol prior tocorrosion tests.For the corrosion tests in CCM,the spec

26、imenswere sterilized in 70%ethanol for 15 min before the corrosiontests and then all procedures were carried out under sterileconditions.As the corrosion rate of the alloys in NaCl solution ismuch higher than that in CCM,the specimens were immersed in0.9 wt%NaCl solution for 3 day and immersed in CC

27、M for 14 day,respectively.Corrosion products were removed via immersingthe specimens into chromic acid(180 g/l)for 20 min at roomtemperature.The average corrosion rate was calculated inmillimeter per year(mm/y)by using the following equation:CR8:76?104?DgAUtUrwhere Dg is weight change in g,A is surf

28、ace area in cm2,t isimmersion time in hours(h)and r is density of the alloy ing cm?3.At least three specimens were tested for each condition.In order to understand the corrosion mechanism,thecorrosion morphologies of specimens after 1 day immersionin 0.9 wt%NaCl solution and 7 day immersion in CCM w

29、ereTable 1 Compositions of DMEM(mg/l).InorganicsaltsCalcium chloride(CaCl2?2 H2O)264Ferric nitrate(Fe(NO3)3?9 H2O)0.1Magnesium sulfate(MgSO4?7 H2O)200Potassium chloride(KCl)400Sodium bicarbonate(NaHCO3)3700Sodium chloride(NaCl)6400Sodium phosphatemonobasic(NaH2PO4?2H2O)141OrganicsaltsVitamins35.6Ami

30、no acids1852D-Glucose(dextrose)4500Phenol red15Sodium pyruvate110j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 1 3(2 0 1 2)3 6 4 437observed with a Zeiss Ultra 55(Carl Zeiss GmbH,Oberkochen,Germany)scanning electron microscopy(SEM)equipped with

31、energy dispersive X-ray analysis(EDX).3.Results3.1.Chemical analysisTable 2 shows the actual composition of experiment alloys,which is close to their nominal content.No Ni is detected byXRF in the alloys,but the detection limit of Ni for the usedXRF instrument is 0.004 wt%.Thus it is known that thec

32、ontent of Ni is less than 0.004 wt%in these alloys.3.2.Ageing behaviorThe effect of ageing time on the Vickers hardness of MgDyalloys aged at different temperatures is shown in Fig.1.At250 1C,the ageing effect is less noticeable for all alloys(Fig.1(a).In contrast,the effect of ageing at 200 1C on h

33、ardness issignificant for Mg20Dy alloy while still very weak for bothMg15Dy and Mg10Dy alloys(Fig.1(b).For Mg20Dy alloy agedat 200 1C,there is little change of the hardness in the initialperiod(stage I).And then,the hardness increases apparentlywith the increase of ageing time up to 168 h(stage II).

34、When theageing time exceeds 168 h(stage III),the increasing rate ofhardness becomes very slow.However,the peak hardness is notobserved even after ageing for 480 h.3.3.MicrostructureOptical and scanning electron microscopy(SEM)observationsof microstructure for the as cast and T4 conditions weredescri

35、bed and published elsewhere(Yang et al.,2011).For thephase analysis,XRD was performed for Mg20Dy alloy in T4and T6 conditions.However,due to the small amount ofphases in T4 and in 250 1C ageing conditions,only peaks ofMg were detected and they are not shown here.For Mg20Dyalloy aged at 200 1C for 16

36、8 h,b0phase is identified in the XRDpattern as shown in Fig.2.Fig.3 presents the precipitates distribution and correspond-ing diffraction pattern of Mg20Dy alloy.After aged at 250 1Cfor 16 h(T6-1),only a few precipitates are formed anddistributed unevenly in the matrix(Fig.3(a).After ageing at250 1C

37、 for 72 h,these precipitates further grow.In the mean-time,some new precipitates are formed,as indicated byarrows(Fig.3(b).Due to the small size and small amount ofthe precipitates,selected area diffraction for them was verydifficult to obtain.When aged at 200 1C,based on our previouswork on MgRE al

38、loys and present study the precipitationsequence can be divided into three stages until peak agedcondition as shown in Fig.1.The b00phase forms at the stage Iwhere has little change in hardness.A mixture of b00and b0phases are the precipitates at the stage II where the hardnessTable 2 Actual chemica

39、l composition of experimentalloys(wt%).AlloyDyFeNiCuMgMg10Dy9.20.005o0.0040.007BalanceMg15Dy13.00.007o0.0040.008BalanceMg20Dy18.60.009o0.0040.01Balance15060708090100110120130140stage stage Vickers hardness,HvAging time at 200C,h Mg-10Dy Mg-15Dy Mg-20Dy168hstage 10100Fig.1 Vickers hardness as a funct

40、ion of ageing time forMgDy alloys aged at:(a)250 1C and(b)200 1C.Fig.2 XRD patterns of Mg20Dy alloys after ageing at200 1C for 168 h.j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 1 3(2 0 1 2)3 6 4 438increase remarkably.In the third stage,b0is

41、the dominantphase.Fig.3(c)shows the TEM microstructure for the speci-men aged at 200 1C for 168 h(T6-2),which is close to the peakageing.The microstructure of the specimen was observed withbeam direction parallel to 1 0 10a.Very fine plate-like shapeprecipitates are homogeneously distributed all ove

42、r the a-Mgmatrix,which have a size of 1030 nm in length and o10 nmin width(Fig.3(c).The selected area diffraction(Fig.3(d)indicates that these fine precipitates are the metastable phaseb0,which is consistent with the XRD result.Similar precipitatesand diffraction pattern were also reported in WE54 a

43、ndMgGdY alloys(Nie and Muddle,2000;Honma et al.,2005).3.4.Mechanical propertiesFig.4 shows the tensile and compressive properties of alloysin different conditions.In comparison with T4 condition,inT6-1 condition the tensile yield strength(TYS)remainsunchanged for Mg10Dy and Mg15Dy alloys and increas

44、esslightly for Mg20Dy alloy(Fig.4(a).In T6-2 condition,theTYS of Mg10Dy alloy is slightly increased.With the increaseof Dy amount,the increment of TYS increases.A significantimprovement from 110 to 167 MPa is obtained for Mg20Dyalloy(Fig.4(a).The elongation remains at the same level inT4,T6-1 and T6

45、-2 conditions for Mg10Dy alloy,while itreduces by around 35%and 50%for Mg15Dy and Mg20Dyalloys after T6-2(Fig.4(b).After T6-2,the ultimate tensilestrength(UTS)is significantly increased from 147 to 219 MPafor Mg20Dy alloy(Fig.4(c),while it remains at the samelevel for Mg10Dy and Mg15Dy alloys.The re

46、sults obtained in compression follow a similar trendto that obtained in tension.T6-1 has no effects on thecompressive properties for Mg10Dy and Mg15Dy alloys,while it contributes to a marginal improvement in compres-sive yield strength(CYS)for Mg20Dy alloy(Fig.4(d).In T6-2condition,an increase in Dy

47、 content contributes to moreincrement in CYS and UCS,while it reduces the compressiveductility.For example,after T6-2 the CYS and UCS ofMg20Dy alloy are increased dramatically from 116 and175 MPa to 229 and 360 MPa,respectively(Fig.4(d,f).But atthe same time,its compressive ductility is reduced from

48、 11%to 5%(Fig.4(e).Fig.3 Precipitates distribution and corresponding diffraction pattern of Mg-20Dy alloy:(a)low magnification in T6-1 condition;(b)high magnification in T6-1 condition;(c)T6-2 condition;(d)corresponding diffraction pattern to(c),the zone axis is 1 0 10a.j o u r n a l o f t h e m e c

49、 h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 1 3(2 0 1 2)3 6 4 4393.5.CorrosionFig.5 shows the corrosion rate of MgDy alloys after immer-sion for 3 day in 0.9 wt%NaCl solution at room temperature.Compared with T4 condition,the corrosion rate slightlychanges for all alloys

50、 after T6-1.It increases by around 32%,from 3.13 to 4.16 mm/year,for Mg20Dy alloy.However,itdecreases by around 28%and 25%,from 1.34 to 0.97 and from1.37 to 1.03 mm/year,for Mg10Dy and Mg15Dy alloys,respectively.After T6-2,the corrosion rate of Mg10Dy alloyremains at the same level in comparison wit