Second phase effect on corrosion of nanostructured Mg-Zn-Ca dual-phase metallic glasses

Chng Liu,Qing Wng,Bin Hn,Junhu Lun,Ji-Jung Ki,Chin-Tsun Liu,Ge Wu,*,Jin Lu,d,e,f,*

a Department of Mechanical Engineering,City University of Hong Kong,Hong Kong,China

b Max-Planck-Institut für Eisenforschung,Max-Planck-Stra?e 1,Düsseldorf,40237,Germany

c Laboratory for Microstructures,Institute of Materials Science,Shanghai University,Shanghai 200072,China

d Hong Kong Branch of National Precious Metals Material Engineering Research Centre,Department of Material Science and Engineering,City University of Hong Kong,Hong Kong,China

e Centre for Advanced Structural Materials,City University of Hong Kong Shenzhen Research Institute,Greater Bay Joint Division,Shenyang National Laboratory for Materials Science,Shenzhen 518057,China

fCityU-Shenzhen Futian Research Institute,Shenzhen 518045,China

Abstract Dual-phase metallic glasses(DP-MGs),a special member of the MGs family,often reveal unusual strength and ductility,yet,their corrosion behaviors are not understood.Here,we developed a nanostructured Mg57Zn36Ca7(at.%)DP-MG and uncovered its corrosion mechanism in simulated body flui(SBF)at the near-atomic scale utilizing transmission electron microscope(TEM)and atom probe tomography(APT).The 10-nm-wide Ca-rich amorphous phases allow oxygen propagation into the DP-MG,resulting in a micrometer thick hydroxides/oxides layer.This dense corrosion layer protects the DP-MG from further corrosion,enabling a corrosion rate that is 77% lower than that of Mg(99.99% purity).? 2021 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Magnesium alloys;Metallic glass;Corrosion;Sputtered films TEM;APT.

1.Introduction

Magnesium(Mg)alloys have the highest specifi strength among structural materials and similar Young’s modulus to that of bone,making them promising materials in the fiel of orthopedic and cardiovascular surgery[1-3].A major advantage of Mg alloys over conventional metallic biomaterials is their biodegradability[4].Mg-based implants can provide sufficien mechanical support for tissue healing in the early stage,while they degrade graduallyviacorrosion by body flui in the later stage,avoiding a secondary surgery to remove the implants[5].Nevertheless,the practical applications of Mg alloys are hindered by their undesirable fast degradation[6],which deteriorates the mechanical integrity of implants and results in premature failure[7].

Metallic glasses(MGs)are free from grain boundaries and dislocations,and often possess better corrosion resistance[8]and higher strength[9]than their crystalline counterparts.Therefore,various Mg-based MGs such as Mg-Cu-Y[10],Mg-Cu-Gd[11],and Mg-Zn-Ca[12]have been developed in the form of bulk metallic glasses(BMGs)[13,14]or metallic glass film(MGFs)[15].Among them,ternary Mg-Zn-Ca MGs are attractive as implants,due to their excellent biocompatibility and corrosion resistance[12,16,17].Minor doping of Ca(1wt.%)is known to enhance the general and pitting corrosion resistance of Mg alloys[18].Besides,there exists a positive correlation between the corrosion resistance of Mg-Zn-Ca alloys and the Zn content[19].When the Zn content exceeds 28 at.%,a passive layer enriched with Zn and O can be formed on the Mg-Zn-Ca BMG upon corrosion in body fluid resulting in a notable reduction in corrosion rate and H2gas generation[20].In addition to the beneficia effect of a higher corrosion potential of Zn(compared to Mg)and its relatively insoluble corrosion products,the single-phase amorphous structure is essential for the corrosion resistance of Mg-Zn-Ca MGs.Schlüteret al.[15]tuned the structure of Mg68Zn28Ca5(at.%)alloy from amorphous to polycrystalline,while keeping the composition unchanged,and observed an increased tendency to localized corrosion and a deteriorated corrosion resistance in phosphate buffered saline(PBS).

Although extensive efforts have been paid to design Mgbased MGs for biomedical implants,the previous studies mostly focused on monolithic(single-phase)MGs.Dualphase MG(DP-MG)containing nanometric-sized amorphous phases connected by interfaces is a special member of the MGs family[21].DP-MGs can be prepared by inert gas condensation(IGC)[22],electrodeposition[23],and magnetron sputtering[23].Compared to the monolithic MGs,the two amorphous phases of DP-MGs have different hardness and free volume content.Upon deformation,shear bands usually nucleate from the softer phase and are obstructed by the harder phase,leading to a change of shear band thickness and distribution[23].Therefore,DP-MGs reveal mechanical properties and deformation mechanisms different from those of their monolithic counterparts.For instance,DP-MGs often exhibit higher plastic deformability with multiple shear bands,distinct from the catastrophic failureviasingle shear band in the monolithic MGs[24].Although detailed researches about the mechanical properties of DP-MGs have been carried out,little is known about their corrosion behaviors.Yet,DP-MGs with excellent anti-corrosion properties have great potential for biomedical implants,and unveiling their corrosion mechanisms are of high interest[25].The presence of two nanometric amorphous phases in the DP-MGs naturally raises the expectation that nano-galvanic cells will be formed during corrosion.Now the questions arise:How will the dual-phase structure of the DP-MGs and the possible formation of nanogalvanic cells affect their corrosion resistance?And how will the structure evolution during corrosion change their corrosion performance?

We aim to address these questionsviaperforming a nearatomic scale investigation on the structure evolution of a Mgbased DP-MG upon corrosion.We developed a nanostructured Mg57Zn36Ca7(at.%)DP-MG comprising 10-nm-wide Ca-rich amorphous phases using magnetron sputtering.The compositions and structures of the Mg57Zn36Ca7DP-MG after corrosion in simulated body flui(SBF)for varying durations were examined by utilizing transmission electron microscope(TEM),TEM-electron diffraction spectroscopy(TEM-EDS),and atom probe tomography(APT).Uncovering the corrosion mechanism of the Mg57Zn36Ca7DP-MG will provide insights for the future design of corrosion-resistant DP-MGs.

2.Materials and methods

2.1.Materials

The 2.8-μm-thick Mg57Zn36Ca7(at.%)alloy fil was deposited on a Si(001)substrate by magnetron sputtering using a Mg60Zn35Ca5(at.%)alloy target(99.9% purity,Beijing Core Source Technology Co.,Ltd).The deposition rate was controlled to be 3.7nm/min.During sputtering,the substrate bias voltage was maintained at?50V and the substrate temperature was 100 °C.The cast Mg metal(99.99% purity)was prepared as reference materials.

2.2.TEM and apt observations

The sample structure was analyzed using a transmission electron microscope(TEM,JEOL JEM 2100F FEG)operated at 200kV.The micro-sized lamella for TEM observation was lifted out,transferred to a Cu grid,and gradually thinned to～20nm thick inside a focused ion beam(FIB,FEI SCIOS)workstation.Needle specimens for atom probe tomography(APT)analysis were fabricated in the FIB machineviaAPT lift-out procedures[26].The TEM lamellae and APT needles were cleaned using 2kV,28 pA to minimize ion beam damage.APT measurements were performed by using a LEAP5000 XR instrument(CAMECA)under an ultrahigh vacuum of 2×10?11Torr.The as-deposited sample was tested in laser mode with a pulse repetition rate of 200kHz,a pulse energy of 15 pJ,a target evaporation rate of 3 ions for 1000 pulses,and a specimen temperature of 40K.The passive layer was analyzed in voltage mode with a pulse repetition rate of 125kHz,a pulse fraction of 20%,and a specimen temperature of 70K.The Integrated Visualization and Analysis Software(IVAS)3.8.4 protocol was employed to reconstruct the atomic maps[27].

2.3.Corrosion properties evaluation

Electrochemical tests were conducted at room temperature in simulated body flui(SBF,pH 7.4)using Zahner Zennium electrochemical workstation.The SBF was composed of 142.0 Na+,5.0 K+,1.5 Mg2+,2.5 Ca2+,147.8 Cl?,4.2 HCO3?,1.0 HPO42?,and 0.5mM SO42-,which is similar to those in human blood plasma[28].Reference electrode was an Ag/AgCl electrode,counter electrode was a platinum plate,and the working electrode was the sample being tested.Electrochemical impedance spectroscopy(EIS)was performed with an AC excitation of 5mV in the frequency range from 100kHz to 100 mHz.Polarization tests were carried out at a scan rate of 1mV/s from?300mV to 600mV with respect to the open circuit potential.The corrosion current density(icorr)was acquired by Tafel extrapolation of polarization curves and was used as an indicator for corrosion rate.The hydrogen generation process in SBF was monitoredin-situusing a stereomicroscope.

Fig.1.Structure of the Mg57Zn36Ca7(at.%)alloy.(a)Three-dimensional TEM images of the sample,showing that the～10-nm-wide amorphous phases(bright contrast)are embedded in the amorphous matrix(dark contrast).The x-y plane denotes the surface of the alloy and the y-z plane indicates the depth from the surface.(b and c)HRTEM image indicating the amorphous atomic structure of the 10-nm-wide lamellar-like phases(b)and the matrix(c).The inset in Fig.1(a)is the SAED pattern.The inset in Fig.1(b)and(c)is the corresponding fast Fourier Transformation(FFT)pattern.The FFT image shows a diffuse pattern,verifying the amorphous structure of the alloy.

3.Results and discussions

3.1.Structure and elemental distribution of Mg57Zn36Ca7(at.%)DP-MG

Fig.1 displays the transmission electron microscopy(TEM)images of the as-deposited Mg-Zn-Ca alloy.The alloy comprises around 10-nm-wide amorphous phases hierarchically distributed in the amorphous matrix.The selected area electron diffraction(SAED)pattern reveals a broad halo ring(Fig.1a)and the high-resolution TEM(HRTEM)images show maze-like patterns(Fig.1b,c),both indicate an amorphous structure.The 10-nm-wide lamellar-like phases exhibit a brighter contrast in TEM observation.The amorphous alloy is free from diffraction contrast in TEM investigation[29],thus the contrast is originated from a mass-thickness variation.The brighter contrast of the 10-nm-wide amorphous phases indicates a higher percentage of lightweight elements than that of the matrix.We utilized atom probe tomography(APT)with near-atomic spatial and mass resolution[27]to quantitatively investigate the chemical information of the Mg-Zn-Ca alloy at the nanoscale.APT investigation suggests that the alloy has an average composition of Mg57Zn36Ca7(at.%),which agrees with the TEM-electron diffraction spectroscopy(TEM-EDS)measurement.There is prominent enrichment of Ca at the 10-nm-wide amorphous phases(Fig.2a).The Ca reaches 19 at.%in concentration,which is 4 times higher than that in the matrix(Fig.2b).The Mg,Zn,and Ca distribute randomly within the matrix with an average composition of Mg59Zn37Ca4(at.%)(Fig.2c).By applying appropriate ion bombardment and heating to the substrate during the sputtering process,the nanostructured Mg57Zn36Ca7dual-phase metallic glasses(DP-MG)is developed,which is different from the homogeneous composition distribution of the ordinary Mg-Zn-Ca bulk metallic glasses(BMGs)[14,20]and metallic glass film(MGFs)[30].

Fig.2.Atom probe tomography(APT)characterization of the Mg57Zn36Ca7(at.%)DP-MG.(a)Three-dimensional reconstruction of the APT sample.(b)1-D compositional profil measured across the Ca-rich amorphous phase,as indicated by the red arrow in(a).(c)1-D compositional profil along the yellow arrow marked in(a),showing the composition of the matrix.(For interpretation of the references to colour in this figur legend,the reader is referred to the web version of this article.)

3.2.Corrosion behavior of Mg57Zn36Ca7 DP-MG in simulated body flui

Electrochemical impedance spectroscopy(EIS)test is known to be a non-destructive approach due to a very small perturbing signal applied to the system,making it useful to monitor the corrosion process of alloy[6].Fig.3 presents the EIS results of the as-deposited DP-MG and the DP-MG after corrosion in simulated body flui(SBF)for 1-hour and 3-hour.The corrosion behavior of Mg metal(99.99% purity)was included as a reference.The Nyquist plots of the four samples reveal similar features,in which a depressed capacitive semi-circle covers the high-frequency region and a depressed capacitive loop dominates the low-frequency region(Fig.3a).The capacitive loops of the DP-MG after 3-hour corrosion in SBF reveals the largest diameter,followed by that of the 1-hour corroded DP-MG,as-deposited DP-MG,and Mg metal.The larger diameter capacitive loop corresponds to an enhanced corrosion resistance[31].This findin indicates that the DP-MG reveals higher corrosion resistance than the ultra-high purity Mg,and a protective corrosion product layer can be formed on its surface upon corrosion in SBF.Additionally,the DP-MG after 3-hour corrosion in SBF exhibits the highest impedance modulus(|Z|)within the range of our investigation(Fig.3b).The values of|Z|at 0.1Hz usually indicate the polarization resistance of the alloy in the solution[32].The|Z|values of the Mg,as-deposited DP-MG,DP-MG upon 1-hour and 3-hour in SBF at 0.1Hz is 213.1Ω·cm2,1179.8Ω·cm2,1905.9Ω·cm2,and 3223.2Ω·cm2,respectively.Mg has a maximum phase angle of 20.2,while that of the as-deposited DP-MG,1-hour corroded,and 3-hour corroded DP-MG increases to 30.9,34.9,and 50.3,respectively(Fig.3c).The higher phase angle maxima can be attributed to a more capacitive behavior[6],indicating that the surface of DP-MG and corroded DP-MG reveals a good protection ability against corrosion in SBF.

Fig.3.Corrosion behavior of the Mg57Zn36Ca7 DP-MG in SBF.(a)Nyquist plots of the Mg metal,Mg57Zn36Ca7 DP-MG,and DP-MG after 1-hour and 3-hour immersion in SBF.The inset is the Nyquist plot of the Mg metal.(b)Bode amplitude plots of the samples,the inset is the equivalent circuit utilized to model the EIS results.(c)Bode phase angle plots and(d)polarization curves of the Mg,as-deposited DP-MG,and corroded DP-MG.

To provide a more quantitative evaluation of the corrosion resistance of the alloys,an equivalent circuit model was used to fi the experimental EIS results.The presence of two capacitive semi-circles in Nyquist plots reveals the two-timeconstants characteristics of the samples.Usually,the highfrequency time constant is associated with the charge transfer at the sample/electrolyte interface,and the one at the lowfrequency region corresponds to the mass exchange in the corrosion product layer[33].Zhaoet al.studied the corrosion behavior of Mg-Ca and Mg-Sr alloys in various corrosive mediums including SBF[34].They suggested a parallel combination of a constant phase element(CPEdl)and a resistance(Rt)to describe the double layer capacitance and charge transfer resistance,and another parallel circuit(CPEfRf)to represent the surface fil capacitance and the corresponding resistance.In the present study,the equivalent circuit containing two time-constantsRs(R1CPE1)(R2CPE2)(inset of Fig.3b)was utilized for fittin the experimental EIS results of the Mg metal and Mg57Zn36Ca7DP-MG.Here,theRsdenotes solution resistance,the parallel circuit(R1CPE1)represents the charge transfer process at the alloy/flui interface,and the parallel circuit(R2CPE2)describes the corrosion product layer formation and dissolution on the alloy surface.The Nyquist result of the DP-MG after 3-hour corrosion in SBF shows scattering at the lower frequency range,suggesting the possible presence of an inductive loop.The inductive loop is usually associated with the desorption of absorbed species and pit formation in the corrosion product layer[35].Nevertheless,the proof for the inductive loop is not suffi cient here.Therefore,we use the simple equivalent circuitRs(R1CPE1)(R2CPE2)to model the experimental EIS results of the 3-hour corroded DP-MG.

The fitte data are displayed in Table 1.The value of polarization resistanceRp,a sum ofR1andR2,is proposed to describe the corrosion resistance of the alloys.TheRpof the as-deposited DP-MG(1130.0Ω·cm2)is approx.7 times higher than that of the Mg metal(145.7Ω·cm2).Moreover,the 3-hour corroded DP-MG has an even higherRpof 3106.7Ω·cm2,which is one order of magnitude higher than that of Mg.Rpis a critical parameter to evaluate the anti-corrosion property of an alloy,and a higherRpvalue generally corresponds to a lower corrosion rate[6].The highRpof the corroded DP-MG suggests that its corrosion product layer has an excellent protective ability,i.e.,a pseudo-passive layer has been formed on the DP-MG upon corrosion in SBF.The protective ability is related to the stable hydroxides/oxides(most likely Zn(OH)2/ZnO),which is profoundly different from the soluble corrosion products such as Mg(OH)2/MgO formed on conventional Mg alloys.The structure and composition of the corrosion product layer,which are essential for its passivating performance,will be discussed in the following context.Fig.3d presents the polarization curves of the Mg,DP-MG,and 3-hour corroded DP-MG.A higher corrosion resistance is manifested as a lower corrosion current density(icorr)and a higher corrosion potential(Ecorr)[35].The polarization curves corroborate further the EIS results,verifying the passivation behavior of the nanostructured Mg57Zn36Ca7DP-MG and its extraordinary corrosion resistance.

Table 1Fitting results of the EIS data acquired on Mg,Mg57Zn36Ca7 DP-MG,DP-MG upon 1-hour and 3-hour corrosion in SBF.

Fig.4.SEM images of the(a)Mg57Zn36Ca7 DP-MG and(b)Mg metal after 3-hour immersion followed with polarization tests in SBF.

Fig.4 presents SEM observation of the Mg57Zn36Ca7DPMG and Mg metal after 3-hour immersion followed with polarization tests in SBF.The Mg57Zn36Ca7DP-MG reveals a fairly fla and intact surface after corrosion in SBF.By contrast,the Mg metal shows abundant cracks on the corroded surface,indicating severe corrosion.The crackled surface is a typical feature that is frequently observed on conventional Mg alloys after corrosion[6,36].Such cracks have been attributed to a low Pilling-Bedworth coefficien between MgO and Mg and/or dehydration of the aqueous corrosion product layer[37].

Corrosion of Mg alloys in human body flui usually generates hydrogen and leads to gas pockets,which have to be removed by surgery[20].Suppressing hydrogen evolution in Mg alloys upon corrosion is critical for their biomedical applications.We monitored the hydrogen generation process of the Mg57Zn36Ca7DP-MG and Mg metal in SBF using a stereomicroscope.Upon immersing the Mg57Zn36Ca7DPMG into SBF,small bubbles are generated on the surface(Fig.5a1).These bubbles disappear after 4 min without regeneration(Fig.5a2 and a3).The EIS and polarization results show that the nanostructured Mg57Zn36Ca7DP-MG reveals superior passivation ability,which facilitates a low corrosion rate.Therefore,hydrogen production is effectively suppressed in this alloy.By contrast,abundant bubbles are formed on the surface of the high-purity Mg metal,and their size increases with increasing the immersion duration in SBF(Fig.5b1-b3).These bubbles are believed to be hydrogen gas as a result of the corrosion reaction Mg+2H2O→Mg2++2OH?+H2.The continuous production of bubbles suggests a vigorous corrosion process,which is often observed on Mg metal[37].

3.3.Characterization of the passive layer

Fig.6a-d presents cross-sectional SEM images of the asdeposited and passivated Mg57Zn36Ca7DP-MG films The thickness of the alloy fil increases from 2.78μm to 4.51μm after 3-hour immersion in SBF,and does not change with increasing corrosion duration from 3 h to 7 h.The 4.51-μmthick fil is composed of a 3.38-μm-thick top layer with a 1.13-μm-thick layer beneath.EDS measurement shows that O is mainly present in the top layer,where Mg is nearly depleted(Fig.6e).This is expected since Mg has high chemical activity and its corrosion product Mg(OH)2/MgO can be dissolved in SBF[38].Zn signal distributes uniformly in the cross-section of the corroded alloy,indicating that Zn is not dissolved,instead,it forms an effective passive layer together with the other elements like O and Ca.Moreover,the 1.1-μm-thick under-layer reveals a rather low O content.This unambiguously proves that the top 3.4-μm-thick layer has blocked O and protected the base material from further corrosion.Hence,the 3.4-μm-thick layer on the top is identifie as the passive layer and the 1.1-μm-thick layer beneath is the base material that is free from corrosion.

To understand the near-atomic scale structural and elemental information of the passive layer,a combined TEM,TEM-EDS,and APT was performed.We selected the passive layer formed after 3-hour corrosion in SBF for detailed investigation since the passive layer thickness remains unchanged after this critical time point.Fig.7a and b compare the structure of the as-deposited alloy and passive layer.The passive layer shows significantl wider lamellar-like structural units(bright contrast areas,approximately 70nm wide)than the as-deposited alloy.These bright-contrast regions reveal a crystalline-amorphous structure,in which nanocrystals(diameters range from 2nm to 4nm)are uniformly dispersed in the amorphous matrix(Fig.7c).The crystalline-amorphous nanocomposite has an average composition of 7.9±1.5 at.%Zn,56.3±2.5 at.% O,29.5±1.5 at.% Mg,and 6.3±1.2 at.% Ca.The atomic ratio of oxygen to metals is 1.29:1,suggesting that the nanocomposite is composed of hydroxides and oxides.The darker region in Fig.7b is composed of nanocrystals with diameters ranging from 2nm to 19nm(Fig.7d)and has a composition of 77.9±1.7 at.% Zn,14.0±2.1 at.% O,3.5±0.4 at.% Mg,and 4.6±0.8 at.% Ca.The FFT pattern inserted in Fig.7d indicates that the darkest region in the TEM image corresponds to a Zn nanocrystal.APT investigation reveals the existence of Zn that is free from O,which corroborates with the FFT analysis and verifie the formation of Zn nanocrystals in the passive layer(Fig.7e).TEM image(Fig.7d)shows that the Zn crystal is surrounded by nanocrystals,other than amorphous phases.It is thus reasonable to assume that the ZnO,Ca,and MgO in APT investigation(Fig.7e)adopt crystalline structures,i.e.,the Zn nanocrystals are surrounded by nanocrystals with compositions of ZnO,Ca,and MgO.Therefore,the passive layer can be regarded as hierarchical structured oxides/metal.Unlike monolithic amorphous alloy that forms a single phase amorphous oxide after corrosion[8],the corrosion of the Mg57Zn36Ca7DP-MG results in a complex structured passive layer.

Fig.5.Stereomicroscopic images of the(a)Mg57Zn36Ca7 DP-MG and(b)pure Mg samples after immersed in SBF for varying duration of time.

Fig.6.SEM cross-sectional view of the as-deposited and passivated alloy films(a)Image of the as-deposited Mg57Zn36Ca7 DP-MG.(b-d)Images of the alloy after being immersed in SBF for(b)1-hour,(c)3-hour,and(d)7-hour.(e)EDS mapping results of the Mg57Zn36Ca7 DP-MG after 3-hour passivation in SBF.The yellow dashed lines highlight the corroded alloy film(For interpretation of the references to colour in this figur legend,the reader is referred to the web version of this article.)

Fig.7.Structure and composition of the passive layer.(a)TEM image of the as-deposited Mg57Zn36Ca7 DP-MG as a reference.(b)TEM image of the passive layer formed on the Mg57Zn36Ca7 DP-MG after 3-hour immersion in SBF.(c)HRTEM image of the left rectangle region in(b),showing a crystalline-amorphous structure.The white dashed circles outline the nanocrystals.The bottom right inset is the FFT pattern.(d)HRTEM image of the right rectangle region in(b).The white dashed rectangle indicates a Zn nanocrystal.The inset is the FFT pattern of the rectangle region.(e)Top-down view of the APT atom maps.

Fig.8.TEM and STEM-EDS investigations of the Mg57Zn36Ca7 DP-MG after being immersed in SBF for(a,b)5min and(c,d)30min.The white arrows in(a)and(b)indicate the O-rich lamellae in the passive layer.The dashed curves in(c)and(d)highlight the interface between the passive layer and the base material.

3.4.TEM observations on the interrupted formation process of the passive layer

We analyzed the cross-sectional structure of the Mg57Zn36Ca7DP-MG after immersing in SBF for varying durations using TEM and TEM-EDS.A～1-μm-thick passive layer was formed after immersing the alloy in SBF for 5 min(Fig.8a and b).This passive layer has much narrower hydroxides/oxides nanocomposite lamellae compared to the one formed after 3-hour corrosion,which is due to an insufficien reaction time.The white arrows highlight a typical feature at the interface between the passive layer and the base material.The O-rich lamellae gradually narrowed with getting close to the base material,implying the corrosion propagation pathway.Upon corrosion,O preferentially permeates through the Ca-rich amorphous phases since Ca has a lower standard reduction potential(?2.76V,versus SHE)than Zn(?0.76V)and Mg(?2.37V)[39].As the lamellae are gradually enriched with O,the different potential between the lamellae and surrounding materials serves as the driving force for Mg atoms to diffuse from the original amorphous matrix towards the O-rich lamellae,which broadens the lamellae.The EDS mapping results confir that the O-rich lamellae are also enriched with Mg and the matrix is depleted with Mg(Fig.8b).The depletion of Mg in the matrix causes structural reorganization and promotes an increment of Zn and Ca concentrations.When the sample was removed from SBF,the propagation and broadening process of the lamellae were terminated.Consequently,the O-rich lamellae are broader at the passive layer side and narrower near the base material.

Fig.8c and d display the structure and composition of the alloy after corroded in SBF for 30 min.The dashed curve in Fig.8c indicates the interface between the passive layer and the base material.An approximately 100-nm-thick layer with bright contrast is present above the interface.STEMEDS measurement reveals that this layer is enriched with Mg and O,and depleted with Zn and Ca,which is similar to the nanocomposite lamellae(the bright region)in the passive layer.The presence of O-rich layer at the interface between the passive layer and the base material indicates that O atoms can transport through the passive layer at the early stage of passivation due to their small atomic size.At the interface,metal atoms react with O and form oxides,which is similar to the corrosion reactions at the alloy surface.Consequently,the passivation process propagates into the interior of the alloy through an evolving interface.As corrosion reaction proceeds,a large amount of Mg dissolves into solution in the form of Mg(OH)2/MgO.The Zn concentration in the passive layer gradually increases,rendering an enhanced protection effect and improved corrosion resistance.The growing passive layer then effectively obstructs O movement and protects the alloy from further corrosion,which is confirme by the absence of O in the base material(Fig.6).

3.5.Discussion on the corrosion mechanism of the Mg57Zn36Ca7 DP-MG

In general,the DP-MG share certain corrosion characteristics with those of single-phase Mg-Zn-Ca MGs[12,20,31].For instance,they all reveal more noble corrosion potential and significantl suppressed H2gas generation compared to high-purity Mg.In the DP-MG,however,the Ca-rich amorphous phases act as anodes and form nanoscale galvanic couples with the amorphous matrix,which leads to rapid corrosion of the Ca-rich phases.Mg and Ca atoms dissolve with the production of metallic ions[40],and then trap the nearby anions(mainly O2?ions)to form oxides(Fig.9a1 and a2).With the permeation of the anions through the sample,the original Ca-rich amorphous phases transform into the crystallineamorphous oxide nanocomposite and the corrosion process propagates into the interior of the alloy.Meanwhile,the Mg atoms in the matrix are attracted to the O-rich lamellae.The depletion of Mg atoms in the matrix causes structural reorganization,and the less chemically active Zn atoms in the matrix experience a gradual increase in concentration(Fig.9b1 and b2).With the progress of corrosion process,the depth of the affected layer increases.Inside this layer,the O-rich lamellae are gradually enlarged with corrosion,and the original amorphous matrix reorganizes to be mainly composed of relatively insoluble Zn,ZnO,and Ca(Fig.9c1 and c2).It should be noted that the passivation process is non-uniform at the nanometer scale since the Ca-rich amorphous phases act as preferential corrosion sites.Nevertheless,the passivation process is uniform at the micrometer scale(see Fig.6).

Fig.9.Schematic drawing illustrates the passivation mechanism of the Mg57Zn36Ca7 DP-MG.The white color corresponds to the Ca-rich amorphous phases,and the gray color represents the amorphous matrix.(a1)and(a2)shows the structure of the as-deposited DP-MG,in which the Ca-rich amorphous phases are embedded in the amorphous matrix.(b1)and(b2),O propagates into the alloy through the Ca-rich phases.Mg and Ca atoms trap nearby O to form oxides,resulting in the formation of the oxide laminates.The loss of Mg and Ca atoms in the matrix causes structural reorganization,and facilitates the formation of Zn nanocrystals.(c1)and(c2),the thickness of the passive layer increases as O penetrates deeper into the alloy.The passive layer comprises crystalline-amorphous oxide nanocomposite,which is surrounded by nanocrystalline Zn/ZnO.

The corrosion reaction in metals is generally caused by two factors.The firs factor is the chemical activity,rendering metallic atoms,as opposed to carbon or polymer,easier to lose electrons through anodic partial reactions and become metallic ions.The second factor is the compositional and/or structural heterogeneity,which results in the existence of preferential corrosion sites(for instance,second phases,grain boundaries,or dislocations).The preferential corrosion sites form galvanic corrosion cells with the surrounding materials,resulting in a high corrosion rate[41].In the present study,the passive layer formed on the DP-MG is composed of crystalline-amorphous oxide nanocomposite and nanocrystalline Zn/ZnO.The abundant oxides and Zn deliver a high resistivity and protective ability[42].Furthermore,the diameters of the nanocrystalline and amorphous phases are less than 20nm,which contributes to an electrochemical homogeneity[43].Additionally,this passive layer is compact,which suffi ciently blocks the solution from penetrating.The nanostructured DP-MGs possess extraordinary mechanical and corrosion properties due to their unique heterogeneous structure at the nanoscale.Therefore,the deposition of DP-MG film on the surfaces of Mg-based alloys may provide a bio-implant prototype to significantl enhance the corrosion resistance and prolong the service life of biomedical devices such as Mgbased bone screws.Moreover,since the compositions of the two amorphous phases in the DP-MG film are different,bulk DP-MGs are expected to be achieved by tailoring the interfacial structure among amorphous phases during fast-quenching e.g.inert gas consolidation[21].The bulk DP-MGs may also be developed through tuning the compositions of the established BMG classes to achieve separated amorphous phases with different compositions.

4.Conclusion

A Mg57Zn36Ca7dual-phase metallic glasses(DP-MG)was developed and it demonstrates a unique passivation performance.The passive layer formed on the Mg57Zn36Ca7DPMG in simulated body flui(SBF)is micrometer thick and compact,and its protection ability is enhanced with increasing corrosion duration.The structure,elemental information,and formation mechanism of the passive layer were investigated in detail using TEM and APT.The main finding are:

(1)The corrosion of the Mg57Zn36Ca7DP-MG in SBF results in a protective passive layer,in which crystallineamorphous oxide nanocomposite is surrounded by nanocrystalline Zn/ZnO.

(2)The superior protection ability of the passive layer is attributed to the following reasons:(i)the existence of abundant oxides leads to low chemical activity,(ii)both the nanocrystalline and amorphous phases have diameters less than 20nm,which minimizes structural/compositional heterogeneity and prevents galvanic corrosion,(iii)the passive layer is micrometer thick and compact,which sufficientl blocks the solution from penetrating and protects the base material.

Acknowledgements

This work was partially supported by Shenzhen-Hong Kong Science and Technology Innovation Cooperation Zone Shenzhen Park Project:HZQB-KCZYB-2020030,Hong Kong Research Grants Council Collaborative Research Fund(Ref.C4026-17 W),and Theme-based Research Scheme(Ref.T13-402/17-N).This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 958457.Atom probe tomography research of the passive layer was conducted at the Inter-University 3D Atom Probe Tomography Unit of City University of Hong Kong,which is supported by the CityU grant 9360161 and CRF grant C1027-14E.We thank Z.F.Zhou for assistance with magnetron sputtering at the City University of Hong Kong.The authors are grateful to U.Tezins,C.Bro?and A.Sturm for their technical support of the atom probe tomography and focused ion beam facilities at the Max-Planck-Institut für Eisenforschung.

Author Contributions

J.L.and G.W.designed the project.C.L.and G.W.designed the materials and experiments.C.L.conducted the corrosion and hydrogen evolution tests,and APT characterization of the as-deposited sample.G.W.performed TEM experiments.Q.W.conducted focused-ion-beam experiments.B.H.,J.L.,J.-J.K.,and C.-T.L.were responsible for APT characterization of the passive layer.C.L.,G.W.,and J.L.analyzed the data and wrote the paper.All authors contributed to the discussion of the results.

Data Availability Statement

The raw/processed data that support the finding of this study are available from the corresponding authors,G.Wu and J.Lu,upon reasonable request.