Crystallization of Al-based Amorphous Alloys in Good Conductivity SolutionOriginal

Crystallization of Al-based Amorphous Alloys in GoodConductivity SolutionYonggang Wang1),Yan Liu1),Yingjie Li1),Bang An1),Guanghui Cao2),Shifeng Jin3),Yimin Sun1), Weimin Wang1)*1)Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials,Ministry of Education,Shandong University,Jinan250061,China2)Institute of Materials Science and Engineering,Shanghai University,Shanghai200072,China3)Institute of Physics,Chinese Academy of Sciences,Beijing100190,China[Manuscript received January10,2014,in revised form May18,2014,Available online16October2014]The corrosion-induced crystallization of Al94e x Ni x Gd6(x¼6and10,in at.%)metallic glasses as well as phase separation,oxidation and cracking in good conductivity solution has been investigated by various techniques.The transmission electronic microscopy(TEM)result reveals that crystalline intermetallics and oxides present on the electrochemically thinned hole edge,and the phase separation occurs in the matrix of the as-spun ribbons with the circumferential speed R c of29.3m/s.In addition,the bending and cracking of the samples occur after corrosion.The influence of Ni content on the phase separation,bending and cracking can be explained by the fact that the percolation of the backbone clusters in the amorphous alloy melts and glasses is enhanced by increasing the composition of Ni.KEY WORDS:Crystallization;Oxidation;Percolation of backbone cluster;Al-based glassy alloy1.IntroductionLightweight Al e rare-earth e transition metal(Al e RE e TM) metallic glasses with Al concentrations above80at.%typically have good corrosion resistance,high yield strength and other key properties that make them interesting for potential structural applications[1e3].Gao et al.found that when the temperature is approaching the glass transition temperature under annealing,the Al-based metallic glasses will crystallize to reduce the total free energy for the system with the presence of medium-range Al clusters in the as-spun samples[4].The tensile strength can be further enhanced when the alloys are partially crystallized to precipitate nanometer-sized Al crystals[5].Although people have reported that the stability of the Al-based amorphous phase as well as the pathway of crystallization is sensitive to the composition of these alloys[6],i.e.the addition of Ni[7,8]and Si[9] can improve the stability of the Al-based glassy alloys;the substitution for Ni by Co can worsen the glass forming ability (GFA)[9],there is still lacking of an understanding of the underlying processes.It is reported that if the annealing process leads to a phase separation into Al-rich and solute-rich amor-phous regions,crystallization appears to be preceded for Al88Gd6La2Ni4[10].It is known that the earlier investigations have focused on the phase separation and crystallization during the annealing process of the amorphous alloys[11e13].There is little work devoted to uncovering the corrosion-induced crys-tallization of Al-based amorphous alloys.The desirable properties of metallic glasses,such as superior specific strength,large ductility in bending,low coefficient of friction,high hardness and high resistance to corrosion,oxida-tion and wear are closely associated with the internal stress[14,15]. By dealloying the amorphous alloys,the selective dissolution of elements can introduce stress to the matrix and trigger the cracks[16,17].Meantime,internal stress can lead to the elastic deformation of the samples,which will definitely decrease the service life of the products[18].Cai et al.have reported that the stretched ribbons of Cu50Zr40Ti10metallic glass generate the corrosion cracks more easily than the as-quenched one[19]. Therefore,it is valuable to explore the effect of internal stress on the cracking or embrittlement of the metallic glasses.In our earlier works,we have found that a shoulder peak in X-ray diffraction(XRD)pattern of as-spun Al e Ni e Gd glassy alloys is associated with the strong Al e Ni(Gd)bonds[8].The predominant atomic pair plays a governing role in the frame of amorphous structure[20],i.e.the backbone clusters in Al e Ni e Gd*Corresponding author.Prof.,Ph.D.;Tel.:þ8653188392749;Fax:þ86 53188395011;E-mail address:weiminw@(W.Wang). 1005-0302/$e see front matter CopyrightÓ2014,The editorial office of Journal of Materials Science&Technology.Published by Elsevier Limited.All rights reserved./10.1016/j.jmst.2014.10.003Available online at ScienceDirect ScienceDirectJ.Mater.Sci.Technol.,2014,30(12),1262e1270glassy alloys[10].The microstructure of a glassy alloy shoulddetermine its corrosion behavior[21],which is closely related tothe forming of the passivationfilm[22]and the segregation ofbeneficial alloying elements due to partial crystallization[23].Meantime,it is reported that the minor addition of Ni can in-crease the strength and corrosion resistance of the Fe-basedmetallic glass[24],which indicates that the corrosion of metallicalloys is strongly influenced by the alloying elements as well asby their microstructure.The nanoscale pit initiation events aresensitive to the heterogeneities of the Al-based metallicglasses[25].Hence,it is valuable to study the relationship be-tween the backbone clusters and the corrosion behavior of theAl-based glassy alloys.Accordingly,this work intends to investigate the mechanismof corrosion-induced crystallization,phase separation,oxidation,cracking and structure character of Al e Ni e Gd glassy alloys ingood conductivity solution by electropolishing,electrochemicaltest and immersion corrosion.We try to explain the involvingresults with the backbone clusters argument in the present glassyalloys.2.ExperimentalThe ingots of Al94e x Ni x Gd6(x¼6and10)glassy alloy usedin this work were obtained by induction-melting the mixture ofpure Al,Ni and Gd ingots(purity>99.5wt%)in the air.TheAl94e x Ni x Gd6(x¼6and10)ribbons were prepared by a single-roller melt-spinning technique in argon atmosphere.The diam-eter of the copper roller is35cm,and the circumferential speedR c is29.3and25.6m/s.The ribbons are about20e40m m in thickness and2e3mm in width.The glassy structure of the specimens was investigated by highenergy powder X-ray diffraction(PXRD)at room temperatureusing a PANalytical X’Pert PRO diffractometer equipped with agraphite monochromator in a reflection mode(2q:10 e80 ,step: 0.017 and scan speed:5s/step)utilizing Cu K a radiation(40kV, 40mA).Transmission electron microscopy(TEM,JEM-2100)wasused to study the structure of the Al94e x Ni x Gd6(x¼6and10)glassy alloy.Samples were investigated for TEM by electro-chemical thinning only.A26vol.%nitric acid and74vol.%methanol electrolyte were used at approximatelyÀ25 C until ahole was formed.All the samples were checked at least threetimes to ensure the position of the phases wasobtained by energy dispersive X-ray spectrometry(EDS),whichis available in conjunction with TEM.Electrochemical measurements were carried out by using atypical three-electrode system:working electrode,platinumcounter electrode and Hg j Hg2Cl2(SCE)reference electrode.Aswe all know,the solution chosen for the electropolishing iscommonly regarded as the good conductivity solution and has aheavier corrosion behavior for the sample,so the electrolytes forthe electrochemical test were chosen as7wt%HNO3methanolsolution.LK2005A advanced electrochemical workstation wasused for measuring the potentiodynamic polarization curves witha scanning rate of5mV/s at room temperature.Corroded ribbonswere cleaned with ethanol and deionized water.All the electro-chemical measurements were repeated at least three times,whichshowed a good reproducibility.For immersion experiments,theas-spun ribbons were immersed in27wt%methanol solutionopen to air at room temperature(about298K)for120h.Thesurface morphologies of the ribbons after polarization experi-ments and immersion experiments were examined by scanning electron microscopy(SEM,Hitachi S-570).The compositions ofthe corresponding regions were analyzed by energy dispersivespectroscopy(EDS).3.Results3.1.XRD patterns and TEM images of as-spun Al94e x Ni x Gd6(x¼6and10)glassy alloysPrior to the corrosion studies,the microstructure of the ribbonpieces prepared from the specimens by single-roller melt-spin-ning has been checked in detail.Fig.1shows the XRD patternsof the as-spun Al94e x Ni x Gd6(x¼6and10,in at.%)sampleswith R c of29.3and25.6m/s.For the ribbons spun with R c of29.3m/s,a typical broad diffraction peak can be observed in theXRD patterns,which suggests that fully amorphous structure isformed under this circumferential speed.In the XRD patterns ofsamples with R c of25.6m/s,several crystalline peaks can befound in the ribbon with the composition of Ni,c Ni equal to6at.%,which are identified as a-Al phase,but are absent in the ribbon with c Ni of10at.%.Apparently,those results confirm theargument that increasing c Ni is helpful to improve the GFA ofAl-based alloys[9]and the high cooling rate is beneficial to theincrease in the amount of amorphous matrix.In addition,ashoulder diffraction peak is observed at about2q¼44 ,which is often ascribed to the combined effects of strong interaction of Al e Ni bond[1,8]and pre-existed a-Al nuclei[26]in the glassy matrix.The TEM images and the corresponding selected area electrondiffraction(SAED)patterns of the as-spun amorphous sampleswith R c of29.3m/s are shown in Fig.2.For the sample with c Niof6and10at.%just after electrochemical thinning,a layer ofblack precipitates is formed on the hole edge,and the image ofthe sample with c Ni of10at%can be found in ourearlierFig.1XRD patterns of the as-spun Al94e x Ni x Gd6(x¼6and10) samples with R c of:(a)29.3m/s,(b)25.6m/s.Y.Wang et al.:J.Mater.Sci.Technol.,2014,30(12),1262e12701263paper [8].The compositions of the hole edge and internal matrix are also shown in the Fig.2.For the sample with c Ni of 10at.%,the composition of the edge crystallites includes:c Al ¼67.7at.%,c Ni ¼11.2at.%,c Gd ¼5.5at.%and c O ¼15.6at.%,and the composition of the internal matrix is c Al ¼77.5at.%,c Ni ¼11.9at.%,c Gd ¼7.9at.%and c O ¼2.7at.%.The high ratios of c Ni /c Al and c Gd /c Al as well as high c O on the edge indicate that the crystalline phases are composed of in-termetallics and oxides.The patterns in the corresponding SAED of Fig.2(b)include Al 4Ni 3and NiO (the others are not given in this paper).The types and percentages of the precipitates iden-ti fied from 104sets of measured SAED patterns are listed in Table 1.The results show that the oxides fraction decreases and the content of intermetallics increases with the addition of nickel,which indicates that increasing nickel is bene ficial to improvingthe inoxidizability for the amorphous alloys and increasing the cohesion of the atoms.In addition,the crack of the sample with c Ni ¼6at.%fabri-cated after electrochemical thinning has a bending behavior (Fig.2(c)),which is seldom found near the crack with c Ni ¼10at.%sample (Fig.2(d)),indicating that the low nickel content ribbon has a higher stress difference between the free and wheel side.The cracks are initiated by stress concentrated regions,i.e.starting from the thinned transparent edge of the electropolished hole.Meantime,there exist some round bright zones embedded in the matrix (denoted by the arrows)which are proven to be amorphous structure in our earlier works [8],and the amount of the bright zone increases with increasing c Ni (Fig.2(c,d)).In several Al e RE e TM glasses,the contrast in the bright field image suggests the existence of phase separation [10,13].3.2.Corrosion behavior of Al 94-x Ni x Gd 6(x ¼6and 10)amorphous alloysFig.3gives the potentiodynamic polarization curves of Al 94e x Ni x Gd 6(x ¼6and 10)samples in 7wt%HNO 3methanol solution with R c of 29.3and 25.6m/s.The Al e Ni e Gd glasses exhibit an excellent spontaneous passivation behavior in good conductivity solution upon anodic polarization with a wide passive region.The involved electrochemical parameters such as corrosion potential (E corr ),passive current density (i pass )and onset pitting potential (E spit )as well as their differences between samples with R c of 29.3and 25.6m/s are listed in Table 2.InFig.2(a)Bright field TEM images of Al 94e x Ni x Gd 6(x ¼6)ribbons with R c of 29.3m/s;(b)the corresponding selected area electron diffraction(SAED)patterns and the composition informations of the dashed boxes;(c)and (d)bright field TEM images of ribbons (R c ¼29.3m/s)with x of 6and 10,respectively.Table 1Types and percentages of oxides and intermetallic phasesidenti fied from the SAED patterns for Al 94e x Ni x Gd 6(x ¼6and 10)ribbons with R c of 29.3m/s SampleIntermetallics Oxides Al x 1Gd y 1Al x 2Ni y 2Gd x 3Ni y 3Al 2O 3Gd 2O 3Ni (1,2)O x ¼622.7%11.4% 2.3%13.6%10.0%29.5%x ¼1031.7%21.7%11.6%25.0%20.5%x 1¼2,y 1¼1;x 1¼2,y 1¼3;x 1¼3,y 1¼1;x 2¼1,y 2¼1;x 2¼1,y 2¼3;x 2¼3,y 2¼1;x 2¼3,y 2¼5;x 2¼4,y 2¼3;x 3¼1,y 3¼5;x 3¼2,y 3¼15.1264Y .Wang et al.:J.Mater.Sci.Technol.,2014,30(12),1262e 1270general,the more negative the corrosion potential,the lower the corrosion resistance.As shown in Fig.3,ribbons with c Ni of 6at.%get a more negative corrosion potential than ribbons with c Ni of 10at.%,indicating that addition of Ni can defer the anodic reaction and improve the corrosion resistance for the present glassy alloys.As shown in Table 2,the change of the i pass and E spit for sample with c Ni of 10at.%spun with different cooling rates is small,suggesting that the addition of Ni can decrease the corrosion sensibility of the cooling rates and increase the struc-ture homogeneity.Furthermore,SEM was employed to investigate the morphol-ogies of the Al 94e x Ni x Gd 6(x ¼6and 10)glassy ribbons after polarization tests (Fig.4).After electrochemical corrosion some corrosion pitting holes appear on the surfaces of all four ribbons.For the sample with c Ni of 6at.%,the pit size of the sample with R c of 25.6m/s is much larger than that with R c of 29.3m/s.The pit size difference is more drastic than the sample with c Ni of 10at.%,indicating that the R c sensitivity of the samples is weakened with increasing c Ni .To understand the difference between the heavier corrosion pitting region and the film matrix,the EDS results are shown in Table 3.Here,the matrix film is denoted with F and the particles are denoted with P.The ratio of c Ni /c Al is higher in the particle (near the pit region),which in-dicates that Ni atoms are more noble and have a better corrosion resistance.Meantime,the oxygen content of the film decreases with increasing c Ni ,which is consistent with the improvement of the inoxidizability of sample during the electrochemical thinning (Fig.2).However,the c O of particles in the ribbons with c Ni of 10at.%is slightly higher than that with c Ni of 6at.%.To further investigate the corrosion behavior of Al 94e x Ni x Gd 6(x ¼6and 10)glassy alloys in the good conductivity solution without electrochemical potential,immersion experiment was performed.Fig.5shows the pictures and SEM images of the as-spun and immersed Al 94e x Ni x Gd 6(x ¼6and 10)glassy alloy specimens.The samples are immersed in 27wt%HNO 3meth-anol solution for 120h at room temperature,Fig.5(a,c,e)shows the images of samples with R c of 29.3m/s and Fig.5(b,d,f)displays the ones with R c of 25.6m/s.In the optical pictures (Fig.5(a,b)),the upper and lower ribbons are as-spun and immersed samples,respectively.In Fig.5(a),c Ni of left ribbons (1and 2)and right ribbons (3and 4)are 6and 10at.%,respectively.The bending degree of immersed ribbon with c Ni of 6at.%(ribbon 2)is severer than that with c Ni of 10at.%(ribbon 4).In Fig.5(b),c Ni of left ribbons (5and 6)and right ones (7and 8)are 6and 10at.%,respectively.Similarly,immersed ribbon 6(c Ni ¼6at.%)has a larger bending degree than ribbon 8(c Ni ¼10at.%).The bending behavior of ribbons may be explained by the different contraction degree induced by the internal corrosion stress between two sides of samples immersed in 27wt%HNO 3methanol solution.In SEM images,the morphologies of the immersed samples with c Ni of 6and 10at.%are investigated (Fig.5(c e f)).With c Ni of 6at.%,cracks propagate preferentially along with the line di-rections A and B in Fig.5(c,d),respectively.However,with c Ni of 10at.%,cracks propagate isotropically,indicating that increasing c Ni can improve the homogeneity of the present glass alloys.To understand the effect of the heavier local corrosion,the EDS re-sults of the fragment area and film are shown in Table 4,in which the local heavy corrosion region is labeled as “Fragment ”and the matrix film is labeled as “Film ”.Apparently,the oxygen content c O of fragment area of ribbons is higher than that of the matrix film,which is consistent with the electrochemical test (Fig.4and Table 3).Moreover,the c O of the fragment area in the ribbon with c Ni ¼10at.%is higher than that with c Ni of 6at.%,which is also consistent with the electrochemical test (Table 3).After being immersed in the nitric acid methanol solution for 120h,the XRD patterns of the immersed samples are shown in Fig.6.In contrast with the patterns of Fig.1,the a -Al phase is corroded into the solution and all four samples appear in a fully amorphous structure.The broad main peak,generally corre-sponding to Al clusters,of the immersed ribbons is lower,and the shoulder peak,corresponding to Ni/Gd-rich clusters,ishigherFig.3Potentiometric polarization curves of Al 94e x Ni x Gd 6(x ¼6and10)amorphous ribbons with different R c in 7wt%HNO 3methanol solution.Table 2Corrosion potential (E corr ),passive current density (i pass ),onset pitting potential (E spit )and corresponding standard deviation of theseparameters of the Al 94e x Ni x Gd 6(x ¼6and 10)samples after electrochemistry corroded in 7wt%HNO 3methanol solutionR c (m/s)c Ni (at.%)E corr (V)s E corr (V)(E RS corr ÀE SScorr )(V)log(i pass )(A/cm 2)s i pass (A/cm 2)log ði RS pass ÞÀlog ði SS pass Þ(A/cm 2)E spit (V)sE pit(V)E RS spit ÀE SSspit (V)29.3x ¼6À0.1250.0040.035À3.970.0350.25 1.760.022À0.14x ¼10À0.0980.032À0.029À4.190.067À0.01 1.790.0350.0325.6x ¼6À0.1600.030e À4.220.035e 1.900.033e x ¼10À0.0690.009e À4.180.048e 1.760.005eY .Wang et al.:J.Mater.Sci.Technol.,2014,30(12),1262e 12701265compared with the as-spun ribbons,implying that the aluminum clusters dissolve more quickly than Ni/Gd-rich clusters during the immersion test.The selective dissolution of elements/clusters can introduce the local stress to the sample and has a relationship with the cracking and bending in Fig.5.Meanwhile,with increasing c Ni ,the shoulder peak increases after immersion test,showing a more typical phase separation phenomenon.4.Discussion4.1.Structure characters of as-spun Al 94e x Ni x Gd 6(x ¼6and 10)glassy alloysKelton et al.[10]and Poon et al.[27]have proposed the back-bone argument that the Ni/Gd-rich backbone clusters as well asAl inclusion clusters exist in the Al e Ni e Gd glassy alloys.They suggested that the bulk metallic glasses (BMGs)can be divided into MSL (majority atom-small atom-large atom)class and LS (large atom e small atom)class [27].The atomic sizes of Al,Ni and Gd in the present Al e Ni e Gd alloy are 0.143,0.124and 0.180nm,considered as M (majority),S (small)and L (large)size,respectively [28].The interaction of the stress fields can lead to the formation of stable short-ranged ordered clusters of small (Ni)and large (Gd)atoms.The structure-reinforced network is called the backbone clusters.The small inclusion clusters are separated by the network structure [29].It should be mentioned that the basic structural units of backbone clusters are solute centered quasi-equivalent clusters surrounded by Al atoms [30].The backbone clusters are more stable and stronger than Al inclusion clusters [31].Accordingly,it is expected that ahigherFig.4SEM micrographs of free surfaces of the Al 94e x Ni x Gd 6(x ¼6and 10)glassy ribbons by electrochemical corrosion in 7wt%HNO 3methanolsolution:(a)R c ¼29.3m/s and x ¼6;(b)R c ¼29.3m/s and x ¼10;(c)R c ¼25.6m/s and x ¼6;(d)R c ¼25.6m/s and x ¼10.The film matrix is labeled as F and the particles are labeled as P for the EDS region.Table 3Atomic fraction of elements on the surface of Al 94e x Ni x Gd 6(x ¼6and 10)glassy ribbons by EDS after electrochemistry corrosion R c (m/s)Composition Position c Al (at.%)c Ni (at.%)c Ni /c Al c Gd (at.%)c Gd /c Al c O (at.%)29.3x ¼6Film 85.1 5.50.06 5.80.07 2.6Particle 28.1 2.90.100.90.0368.9x ¼10Film 88.8 5.70.06 5.50.06e Particle 23.9 3.40.14 1.30.0571.825.6x ¼6Film 86.9 5.30.06 5.30.06 2.5Particle 34.2 1.90.050.60.0163.3x ¼10Film 88.0 6.40.07 5.60.06e Particle26.82.60.090.80.0269.81266Y .Wang et al.:J.Mater.Sci.Technol.,2014,30(12),1262e 1270Fig.5Optical pictures and SEM images of immersed Al 94e x Ni x Gd 6(x ¼6and 10)glassy ribbons in 27wt%HNO 3methanol solution for 120h.Thecooling rates for the samples in left and right columns are 29.3and 25.6m/s,respectively.In optical picture with R c of 29.3m/s,ribbon 1,as-spun sample with x of 6;ribbon 2,immersed sample with x of 6;ribbon 3,as-spun sample with x of 10;and ribbon 4,immersed sample with x of 10.In optical picture with R c of 25.6m/s,ribbon 5,as-spun sample with x ¼6;ribbon 6,immersed sample with x of 6;ribbon 7,as-spun sample with x of 10;and ribbon 8,immersed sample with x of 10.(c)and (d)SEM images of immersed samples with x of 6,(e)and (f)SEM images of samples with x of 10.Film denotes the film matrix,fragment denotes the heavy corrosion region.Table 4Atomic fraction of elements on the surface of Al 94e x Ni x Gd 6(x ¼6and 10)with R c of 29.3m/s glassy ribbons by EDS after immersioncorrosion Composition Position c Al (at.%)c Ni (at.%)c Ni /c Al c Gd (at.%)c Gd /c Al c O (at.%)x ¼6Film 74.57.20.09 6.90.0911.4Fragment 59.0 5.80.09 5.80.0929.4x ¼10Film 76.39.60.12 5.70.078.4Fragment41.16.20.153.80.0948.9Y .Wang et al.:J.Mater.Sci.Technol.,2014,30(12),1262e 12701267c Ni /c Al present in the severe corrosion zone after electrochemical thinning,polarization and immersion tests (Fig.2and Table 3).The percolation theory has been adopted to describe the me-chanical and structural properties of several metallic glasses [31,32].It is inferred that the mechanical properties may be changed discontinuously when the microstructural percolation is ach-ieved [32].Introducing particular solute species may promote the percolating backbone of short range orders in some glassy systems [31].It is understood that,with R c of 25.6m/s,the Al precipitates appear in the sample with c Ni of 6at.%,but are absent in the sample with c Ni of 10at.%(Fig.1(b)).The sensitivity to the composition of the alloys could be dramatic due to the phase transition nature of a percolation,which has possibly mixed the discontinuous and continuous characters [33].Increasing the cooling rate is another method to promote the percolation of the backbone clusters in the glass alloys [34].It is expected that,with c Ni of 6at.%,the Al precipitates appear in the sample with R c of 25.6m/s,but absent in the sample with R c of 29.3m/s (Fig.1).4.2.Crystallization,phase separation and oxidation behavior in Al 94e x Ni x Gd 6(x ¼6and 10)glassy alloys after corrosion When a glassy alloy is electrochemically decomposed in the electrolyte,the noble metal atoms diffuse through the alloy sur-face to form new nuclei and grow to crystals [35].During deal-loying Al e Cu e Mg amorphous precursor alloys the selective dissolution of elements occurs and nanoporous crystal Cu is synthesized [36].Our earlier ab initio molecular dynamic simula-tion on Al 94e x Ni x Gd 6(x ¼6e 12)amorphous alloys indicated that the Al e Ni and Al e Gd bonds are strong bonds [8].Hence,it is expected that crystalline intermetallics Al 4Ni 3and Al 2Gd 3have remained in the sample after electrochemical thinning (Fig.2and Table 1).The solvent aluminum is active in the Al e Ni e Gd glassyalloys in the annealing process and Al clusters in the melt have a lower stability than the Ni/Gd-rich clusters in the cooling pro-cess [4].Therefore,it is expected that Al phase precipitates from the melt with c Ni of 6at.%at a low cooling rate of 25.6m/s (Fig.1(b)).According to the resemblance between the glass state and the liquid state [37]and the fact that the aluminum is easier to be dissolved into the solution [8],it is understood that the a -Al peaks in XRD pattern of as-spun sample with c Ni of 6at.%and R c of 25.6m/s disappear after immersion test (Figs.1(b)and 6(b)).The phase separation prior to crystallization,forming Al-rich and solute-rich regions,has been found in Al 88Gd 6La 2Ni 4[10],Al 85Ni 5Y 6Fe 2Co 2[13],Al e Fe e Y [3]and Al 89Ni 6La 5[38]amor-phous alloys.3D e atomic probing technology (APT)investiga-tion demonstrates the phase separation occurring in the Al e Fe e Y and Al e Ni e La metallic glasses,which have negative mixing enthalpies among their three binary subsystems [3,38].As sug-gested first by Hume-Rothery and Anderson [39],in a region of composition bene ficial to the development of short-and medium-range order involving five-fold symmetry,the G-curve dips,which makes the occurrence of eutectic be more likely.In metallic glasses,the eutectic composition is usually considered as the ideal composition to form the glass.According to Hume-Rothery and Anderson ’s hypothesis [39],a narrow miscibility gap may open near the composition corresponding to maximal atomic packing ef ficiency due to rapid dip in the Gibbs free energy curve [40].The hypothetical form of the free energy curve and a metastable miscibility gap,including a spinodal region of amorphous phase separation,in Al e RE binary amorphous alloys can be presented by solid lines in Fig.7[41].Here,the second derivative of the free energy of mixing de fines the spinodal points (C and D),which locate between the binodal points (A and B,which is de fined as the first derivative of the Gibbs free energy).Above the binodal line,two liquids are miscible and the system is stable(s);below this line there is a metastable region (m),in which the system is stable to small fluctuations but is unstable to large fluctuations;below the spinodal line,the system is unstable [42].In our earlier work [8],the electropolished as-spun Al 84Ni 10Gd 6amorphous ribbon formed white regions,which were still amorphous struc-ture in the matrix;the corresponding SAED patterns showed an asymmetric halo of the amorphous structure,which may corre-spond to two overlapped diffraction peaks.The phase separation phenomenon is also con firmed by the XRD patterns of sample with c Ni ¼10at.%after the immersion corrosion (Fig.6).Increasing the content of nickel can widen the composition range of phase separation,which is shown schematically by the dashed line in Fig.7.Considering the similar RE content of both alloys as well as Hume-Rothery and Anderson ’s hypothesis [39],we suppose that the binodal curve for x equal to 10is overlapped by that with x of 6.Here,with increasing c Ni ,the backbone clusters are strengthened,leading to a heavier dip in the Gibbs free energy [41,43].Accordingly,the spinodal region is wider and it is easier to perform the spinodal decomposition with a higher content of nickel.Hence,it is expected that the sample with c Ni of 10at.%undergoes a heavier phase separation after corrosion than that with c Ni ¼6at.%(Figs.2and 6).The composition range of phase separation locates always near/inside the opti-mized glass formation ranges,i.e.the more easily the phase separation occurs in a glass,the more dif ficultly the nucleation and growth of crystallites occur [41].Hence,it is understood that the sample with c Ni of 10at.%has a heavier phase separation and a higher crystallization temperature than the counterpart with c Ni of 6at.%(Figs.2and 6and Table 3in Ref.[8]).Fig.6XRD patterns of the immersed Al 94e x Ni x Gd 6(x ¼6and 10)sample with:(a)R c ¼29.3m/s,(b)25.6m/s.1268Y .Wang et al.:J.Mater.Sci.Technol.,2014,30(12),1262e 1270。