study of decagonal approximant and -brass-type … · intermetallic phases that are all linked with...

Study of decagonal approximant and -brass-typecompounds in AlndashCrndashFe thin films

V Demangea)

Laboratoire de Science et Geacutenie des Mateacuteriaux et de Meacutetallurgie UMR CNRS-INPL-UHP 7584Ecole des Mines Parc de Saurupt F-54042 Nancy France

J GhanbajaService Commun de Microscopie Electronique agrave Transmission UHP Nancy IF-54042 Vandoeuvre-les-Nancy Cedex France

C BeeliLaboratory of Solid State Physics ETH Zuumlrich CH-8093 Zurich Switzerland

F Machizaud and JM DuboisLaboratoire de Science et Geacutenie des Mateacuteriaux et de Meacutetallurgie UMR CNRS-INPL-UHP 7584Ecole des Mines Parc de Saurupt F-54042 Nancy France

(Received 20 February 2004 accepted 6 April 2004)

This paper reports the preparation conditions and structure characteristics of AlndashCrndashFevery thin films (10ndash30 nm) obtained by the flash evaporation technique The films areeither amorphous or crystallized depending on the thickness of the sample andtemperature of the substrate Annealing of amorphous films leads to crystallization ofintermetallic phases that are all linked with quasicrystals In particular we haveidentified by transmission electron microscopy the following structuresbody-centered-cubic (bcc) -brass phase monoclinic ndashAl13(CrFe)4 phase andorthorhombic O1-phase all of them already observed in this system together with fournew structures ie a face-centered-cubic (fcc) -brass phase (superstructure of the bccphase) monoclinic -phase (related to the -phase) and two orthorhombic phases(11 11) and (10 21) approximants of the decagonal phase) In this study we pointout the occurrence of twin defects of the ndashAl13(CrFe)4 phase Films prepared directlyin the crystalline state comprise the O1 approximant Electron energy loss spectroscopymeasurements show that all films are not oxidized except for the presence of a nativeoxide layer that forms in ambient atmosphere with a thickness that cannot exceed03 nm Optical properties were investigated and show that films need to be largeenough (gt30 nm) to reproduce the properties of bulk alloys Finally contact anglewetting measurements reveal that the presence of such films on a substrate even atvery low thickness considerably decreases the wetting behavior by water

I INTRODUCTION

In the last decade intensive experimental effort wasdevoted to the preparation of quasicrystalline and ap-proximant thin films Indeed these materials present arestricted interest in view of applications when used inbulk form due to their brittleness Our aim in the present

study was therefore focused at the preparation of verythin films for optical applications Recently we havereported results of ellipsometry measurements of AlndashCrndashFe films on a glass substrate1 In this paper we will focuson the different preparation procedures used to prepareAlndashCrndashFe thin films In a second part we will present theresults of a structure investigation on these films usingtransmission electron microscopy (TEM) In a final sec-tion we will discuss the optical and wetting properties onthe films

II EXPERIMENTAL DETAILS

All films were prepared by flash evaporation tech-nique The AlndashCrndashFe ingots were obtained by melting

a)Address all correspondence to this authorPresent address Laboratoire de Science et Geacutenie des SurfacesUMR CNRS 7570 Ecole des Mines Parc de Saurupt F-54042Nancy Francee-mail valeriedemangeminesinpl-nancyfrDOI 101557JMR20040311

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the pure constituents in a levitation induction furnaceCompositions (obtained by microprobe analysis) thatlead reproducibly to the formation of films of the wantedphases are respectively A Al676Cr233Fe91 B Al70Cr22Fe8 C Al725Cr195Fe8 D Al755Cr158Fe87

and E Al775Cr145Fe8 (in at) The so-obtained bulkalloys were characterized by transmission electronmicroscopy (TEM) and will be the subject of a forthcom-ing article2 Some details are nevertheless already avail-able in Refs 3 and 4

Small fractions of about 25 mg of the chosen alloywere evaporated with the help of a hot tungsten filamentunder secondary vacuum (10minus7 mbar) The films weredeposited simultaneously on a glass substrate and on acarbon coated nickel microscopy grids This device wasable to operate at room temperature but also to pre-heatthe substrate up to 584 degC The films obtained at roomtemperature were systematically found amorphous (to-tally or partially depending on the thickness of the as-deposited coating) Subsequently some of them wereannealed in situ in the electron microscope Conventionalelectron microscopy was performed on Philips CM 12(120 kV) and Philips (Eindhoven Netherlands) CM 200Double-twin instruments whereas high resolutionmicroscopy was performed on a Philips CM200 Super-twin microscope

In addition to structural characterization we per-formed electron dispersive spectroscopy (EDS) measure-ments on each film to compare nominal composition ofbulk alloys and composition of corresponding films Tothis end a calibration bulk sample was primarily char-acterized by microprobe analysis

III RESULTS

A Thickness and microstructure induced by thepreparation conditions

Table I gives data on films thickness deposition tem-perature and microstructure of TEM samples togetherwith composition of the ingot and films composition Thethickness was obtained by TEM and by calibration of aquartz balance used during the preparation process

Crystallization of films is strongly dependent on thepreparation parameters Films are partially crystallized

even when deposited at room temperature if the thicknessis important enough Otherwise films remain completelyamorphous In addition heating the substrate prior toevaporation is favorable to crystallization as expectedFinally a rather thick film (30 nm) is completely crys-tallized for a substrate heated up to 300 degC whereascomplete crystallization of a thinner film (10 nm) re-quires heating the substrate at 584 degC

Except for the F2 sample (see Table I for the coding ofthe specimens) the films composition is systematicallyricher in aluminum compared to the bulk alloy Theamount of aluminum in the films varies from 726 to856 at and plays an important role in the resultingstructure as we show in the next section

B In situ annealed amorphous films

To know if it was possible to obtain crystallized filmsby heating as deposited amorphous samples we per-formed postannealing in situ in the electron microscopeusing F1a F1b and F1c samples (Table I) The annealingtreatment occurs at 460 degC during 24 h for F1a sampleand at 600 degC for F1b and F1c films for 38 and 20 hrespectively

The first step of crystallization occurs after 2 h ofannealing it is followed by the growth of crystallites butthe major part of the layer remains amorphous even afterseveral hours of annealing

A bright-field image of the F1a sample is shown inFig 1(a) This micrograph points out the growth of smallcrystals in an amorphous matrix Most crystals possess asquare shape showing a preferential growth texturenamely the lang100 rang zone axis of a cubic phase with a latticeparameter a 912 Aring This phase corresponds preciselyto the cubic -brass ndashAl9(CrFe)4 isostructural with thebinary Al9Cr4 compound The phase composition meas-ured by EDS is precisely close this stœchiometric com-position (Table II) with a poor amount of iron Thisphase which was identified in the binary system by Brad-ley and Lu5 and in the ternary one by Palm6 crystallizesin the I43m space group Its complete structure was de-scribed by Brandon et al7 The -brass phases were ob-served in several binary alloys Their structure is built upby atomic clusters of 26 atoms The cluster is a sequence

TABLE I Preparation parameters (thickness and temperature) of Al-Cr-Fe thin films and obtained microstructures and compositions

Thin filmsamples

Estimated thickness(nm)

Preparationtemperature Microstructure

Nominal composition ofthe precursor bulk alloy (at)

Thin film compositionmeasured by EDS (at)

F1a F1b F1c 10 Room temperature Amorphous C Al725Cr195Fe8 Al856Cr116Fe28

F2 20 Room temperature Amorphous + crystallized D Al755Cr158Fe87 Al726Cr217Fe57

F3 10 200 degC Amorphous + crystallized E Al775Cr145Fe8 Al791Cr138Fe31

F4 30 325 degC Crystallized A Al676Cr233Fe91 Al777Cr180Fe4


F6 10 325 degC Amorphous + crystallized B Al70Cr22Fe8 Al733Cr206Fe61

F7 11 584 degC Crystallized B Al70Cr22Fe8 Al744Cr20Fe56

V Demange et al Study of decagonal approximant and -brass-type compounds in AlndashCrndashFe thin films

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of atomic shells ie an inner tetrahedron of 4 atomssurrounded by an outer tetrahedron then by an octahe-dron of 6 atoms and finally by a cuboctahedron of12 atoms The cubic -brass phases are either primitivewith a CsCl-type of cell (containing two clusters withdifferent atomic decoration)8 or body-centered (two

identical clusters) or face-centered (clusters with 4 dif-ferent atomic compositions)9 If the two first types ex-hibit a lattice parameter around a 9 Aring the last typecorresponds to a superstructure of the two former oneswith a larger lattice parameter (a 18 Aring)

An alternative description of these structures was pro-posed by Brandon et al10 The -brass phases are alsoapproximations of the tetrahedral stacking Each atom inthe structure is surrounded by 12 atoms forming a dis-torted icosahedron This polyhedron is convex with nointernal atoms except the central one and possesses 20triangular facets All atoms on the external shell have asurface coordination number 5 so that each atom is thecommon vertex of 20 tetrahedra The icosahedra sur-rounding the outer tetrahedron (first description scheme)are coupled into a tetrahedral arrangement by mutualface sharing11 This icosahedra arrangement forms a 38-atom cluster with an icosahedric symmetry of higherdimension12

We have already observed a rhombohedral -brassphase (distortion of the cubic lattice) in an AlndashCrndashFebulk alloy4

The [001] [110] [115] [113] [111] and [221] zoneaxes electron-diffraction patterns (EDPs) of the cubicndashAl9(CrFe)4 phase are presented in Fig 1 The -brassphases are considered approximants of both decagonaland icosahedral phases In particular the [113] and [110]directions possess orientation relationships with the 10-fold and the 5-fold axes of quasicrystalline phases re-spectively Indeed these directions correspond to twodifferent 5-fold axes of the icosahedra building up thestructure For example particular plane angles can bemeasured on EDPs like 36deg 5 [Figs 1(c) and 1(e)]

The second sample F1b is a mixture of two phasesThe first one is the orthorhombic O1 approximant pri-mary discovered in the AlndashCu-CrndashFe system13 and alsoobserved in the AlndashCrndashFe system3 Figure 2 shows abright-field image of the sample and the EDPs along the[011] [101] [101] and [001] zone axes of this approxi-mant This phase is a (32 21) approximant of the de-cagonal phase with rather large lattice parameters a 323 Aring b 124 Aring c 236 Aring The measured com-position (Al784Cr176Fe4) for this phase is close to thenominal composition of bulk alloy (ie Al775Cr165Fe6)where the O1 structure was primarly observed3

The second phase we observed in this sample is againa cubic -brass phase which could be indexed in threedifferent ways by considering either a primitive cell witha 912 Aring or a body-centered cell with a doubledparameter (1824 Aring) or a face-centered cell with adoubled parameter as well We believe that this phase isface-centered because of the occurrence of a similarstructure in the AlndashCundashCr system usually called ndashAl-CuCr14ndash18 In addition no primitive -brass phase wasever quoted in the two systems under consideration

FIG 1 (a) Dark-field micrograph of F1a thin film (b) electron dif-fraction pattern (EDP) along the [001] zone axis of the cubicndashAl9(CrFe)4 -brass phase (c) same for the [110] zone axis(d) same for the [115] zone axis (e) same for the [113] zone axis(f) same for the [111] zone axis and (g) same for the [221] zone axis


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(AlndashCr and AlndashCundashCr) Concerning the second possibil-ity no body-centered cell with a lattice parameter around18 Aring was discovered to our knowledge in any systemknown to contain -brass phases Moreover all the cubicsuperstructures already characterized in several systems(like AlndashCundashCr Pt-Zn Rh-Mg CundashSn FendashNindashZn andGandashFendashCundashSi) are face-centered9111218ndash22 Figure 3presents the EDPs along the [001] [111] and [110] zoneaxes of the cubic superstructure Comparison betweenFigs 1 and 2 shows strong structural relationships be-tween the two cubic -brass phases observed in thiswork In particular 330 and 600 spots of the body-centered-cubic (bcc) phase fall at the same distance than660 and 12 00 planes of the face-centered-cubic(fcc) superstructure This phase is poor in iron as the bccstructure but is richer in aluminum and is then closer tothe composition of the O1 approximant

In the last annealed sample (F1c) we have identified 3phases namely O1ndashAlCrFe phase ndashAl9(CrFe)4 phaseand the monoclinic ndashAl13(CrFe)4 phase ThendashAl13Fe4 phase is a well-known approximant of bothdecagonal and icosahedral phases23 This structure waspreviously observed in the AlndashCrndashFe system by Palm6

Figure 4 presents a few EDPs of this phase taken alongthe [100] [301] [101] [010] and [110] zone axes In themean time we have observed a structure that is clearlyderived from that of the of ndashAl13(CrFe)4 phase TheEDPs of this new phase (called hereafter ) can be in-dexed by considering a primitive monoclinic cell withthe same parameters as in In other terms the extinc-

tions due to the C-based cell are not visible anymore inthe EDPs taken along the [100] [101] and [110] zoneaxes (Fig 5) Compositions of these two phases are close(Table II) and richer in aluminum than the stoichiometricAl13(CrFe)4 phase

Both EDPs along the [010] zone axes ofndashAl13(CrFe)4 show twins on (100) planes in Fig 4(d)the two exciting crystal lattices in twin relationship areunderlined in the pattern (spots marked A and B) togetherwith the trace of the orthorhombic cell of this phase (C)In Fig 4(e) the number of twins is too large to distin-guish the two lattices Another kind of twins is visible inthe EDP along [110] [Fig 4(f)] ie the twin plane is(001) The EDP along the [101] direction [Fig 4(c)]shows twins along a 10-fold symmetry This can be un-derstood by considering a composite EDP of twins Fi-nally the elongated shape of the spots in Fig 4(b) revealsthe presence of planar defects perpendicularly to the[010] direction In this case the twins lie on either a(100) or a (001) plane

The [010] direction ndashAl13(CrFe)4 is known to be inorientation relationship with the 10-fold axis of the de-cagonal phase Actually the angle between (100) and(001) planes is 72deg (which is related to the golden mean)The occurrence of twins showing a 10-fold symmetryalong this zone axis is a well known effect as well24

Another point is that the (100) planes of the lattices intwin relationship do no have all their spots superimposedThe coincidence of spots occurs only for l 0 3 5 8hellip [Fig 4(e)] This sequence defines the beginning of the

TABLE II Designation cell parameters space group and composition of phases observed in the present work

DesignationLattice

parametersSpacegroup

Composition measuredby EDS (at) Remarks References

ndashAl9(CrFe)4 a 912 Aring Cubic I 4 3 m Al715Cr275Fe10 Approximant of icosahedral and decagonal phase 7O1ndashAlCrFe a 323 Aring Orthorhombic Bmm2 Al784Cr176Fe40 (32 21) approximant of decagonal phase 4

b 124 Aringc 236 Aring

ndashAlCrFe a 1824 Aring Cubic Fm3m Al771Cr212Fe17 Superstructure of ndashAl9(CrFe)4 This workndashAl13(CrFe)4 a 1549 Aring Monoclinic C2m Al814Cr33Fe153 Approximant of icosahedral and decagonal phases 8

b 809 Aringc 1235 Aring 1077deg

ndashAl13(CrFe)4 a 1549 Aring Monoclinic P Al809Cr41Fe150 This workb 809 Aringc 1235 Aring 1077deg

XndashAlCrFe a 106 Aring Monoclinic C b lattice parameter could not be determined This workb Aringc 270 Aring 112deg

OEndashAlCrFe a 125 Aring Orthorhombic P Al750Cr65Fe185 (11 11) approximant of decagonal phase This workb 124 Aringc 143 Aring

OFndashAlCrFe a 77 Aring Orthorhombic P Al812Cr126Fe62 (10 21) approximant of decagonal phase This workb 124 Aringc 237 Aring



Finonacci series known to be related to approximants ofquasicrystals This effect was also observed for twins inthe monoclinic 2-Al13Co4 phase25 This phenomenoncan be explained via the occurrence of angles related tothe golden mean Indeed when the two lattices are incoincidence the following relationship takes place

cos 72deg lda2hdc

where da and dc are the distances between two spotsalong the [100] direction and the [001] direction respec-tively and h and l are the Miller indices This relation-ship leads to the following one l (2 minus )h For h 12 3 hellip the two lattices are therefore in coincidence forl asymp 3 5 8 hellip

Figures 6(a) and 6(c) show high-resolution electronmicroscopy micrographs of the ndashAl13(CrFe)4 phase re-corded along the [010] and [110] direction respectivelyOn both pictures twin defects on planes (100) and (001)

are clearly visible From Fig 6(b) we are able to deducethe presence of two kinds of defects on (100) planes Adefect of type 1 corresponds to a mirror operation to-gether with a glide component whereas a defect of type2 is given by a glide operation without mirror symmetryand introducing a new orthorhombic lattice between twomonoclinic cells Structural models of both kinds oftwins were deduced from the structure of ndashAl13(CrFe)4Figure 7(a) presents this structure along the [010] direc-tion according to the structure determination make byGrin et al26 If the clusters of the structure are representedby pentagons the monoclinic lattice is described by aparallel tiling of elongated hexagons The type 1 of twindefects is drawn in Fig 7(b) and shows that hexagons arelocally arranged in staggered rows The glide vector isthen equal to

FIG 2 (a) Bright-field micrograph of F1b thin film (b) EDP alongthe [011] zone axis of the O1 approximant phase observed in an an-nealed AlndashCrndashFe film (c) same for the [101] zone axis (d) same forthe [101] zone axis and (e) same for the [001] zone axis FIG 3 (a) EDP along the [001] zone axis of the face-centered cubic

superstructure (b) same for the [111] zone axis and (c) same for the[110] zone axis



T = d001

with d the pentagon edge length and the golden ratioTherefore the magnitude of the glide vector is T -2c d 4768 Aring

A second type of twin defect was already observed in

this structure27ndash29 This twin mechanism introduces anorthorhombic phase with the following parameters a 1445 Aring b 8089 Aring c 12485 Aring [Fig 7(c)] Thisnew primitive structure corresponds to the Al13Co4 phaseand is a (11 11) approximant of the decagonal phaseActually we also observed this structure in our sample[Fig 8(a)]

These twins mechanisms explain the occurrence of

FIG 4 (a) EDP along the [100] zone axis of the monoclinic ndashAl13(CrFe)4 phase (b) same for the [301] zone axis (c) same for the [101] zoneaxis and (d) same for the [010] zone axis A twin defect is visible on the (100) plane Both crystals in twin relationship are drawn (A and B)the rectangular cell (C) of crystallite A is shown on the EDP as well (e) EDP along the [010] zone axis showing a large amount of twins on (100)planes and (f) EDP along the [110] zone axis showing a large amount of twins on (001) planes



10-fold symmetry in EDPs along the [010] zone axisIndeed the glide operation corresponds to a diagonallength of a pentagon and allows sharing of pentagons ofneighboring lattices through the twin25 This sharing to-gether with a rotation by 36deg of elongated hexagons attwin boundaries induces strong spots in EDPs in 10-foldsymmetry at distances in reciprocal space that corre-spond to the edge length of a pentagon in direct space

We also performed high-resolution electron micros-copy (HREM) on the other phases In particular wefocused on a part of the sample as shown in Fig 3(a)This zone corresponds to four grains in orientation

FIG 5 (a) EDP along the [100] zone axis of the monoclinic -Al13(CrFe)4 phase (b) same for the [101] zone axis and (c) same forthe [110] zone axis

FIG 6 (a) High-resolution electron microscopy (HREM) micrographof ndashAl13(CrFe)4 phase along the [010] zone axis showing twins onthe (100) plane (b) Same at higher magnification both monoclinic andcorresponding orthorhombic lattices are drawn Two different kinds oftwins are visible (note 1 and 2 on the picture) (c) HREM micrographof ndashAl13(CrFe)4 phase along the [110] zone axis showing twins on(100) planes



relationships as illustrated in Fig 3 All EDPs are repre-sented at the same scale and orientation It clearly ap-pears that a strong crystallographic link exists betweenthese phases along their respective pseudo-3-fold axissince the stronger spots forming a hexagonal pattern fallexactly at the same position These phases arendashAl13(CrFe)4 ndashAl9(CrFe)4 O1ndashAlCrFe and XndashAl-CrFe This last phase is unidentified yet and can be in-dexed by considering a monoclinic C-based lattice with

parameters a 106 Aring c 270 Aring and 112deg Thefollowing orientation relationships are then deducedfrom the EDPs

100 111 101O1 010X

010 011 909O1 102X

025 101 555O1 500X

025 110 555O1 105X

We notice the presence of twins in the ndashAl9(CrFe)4

phase as shown by the satellite spots on each EDP andon HREM micrographs These defects will be studiedmore in details in another paper2

Finally we observed two news approximants in thissystem which we named OE and OF phases (Fig 8)Both phases possess primitive orthorhombic cells withthe following lattice parameters aE 125 Aring bE 124 Aring cE 143 Aring aF 77 Aring bF 124 Aring cF 237 Aring These phases are therefore respectively the (1111) and (10 21) approximants of the decagonal phaseTheir respective compositions (OE Al750Cr65Fe185 andOF Al812Cr126Fe62) show that OF-phase leads in thesame part of the ternary diagram as O1-phase but thatOE-phase is located in the chromium rich part of thediagram which is known to contain also approximantstructures

The samples we studied include a large number ofphases some of them being necessarily metastable ac-cording to the rules of thermodynamics Table II sum-marizes the features of all the phases observed in thiswork Obviously the composition plays an essential roleduring the growth diffusion of elements allows crystal-lisation of phases rich in aluminum (like approximants)together with stable phases belonging to the equilibriumphase diagram (like -brass phases)

C Crystallized films

The F4 and F7 films were crystallized in situ duringpreparation Figures 10(a) 10(c) and 10(d) presentbright-field micrographs of both samples The grainsize is slightly different in each film (5ndash10 nm for F45ndash20 nm for F7) and the second film does not covertotally the substrate probably due to a weaker thicknessin the coating This last point is confirmed by the micro-graph in Fig 10(d) showing the film at high magnifica-tion We can also see periodic arrays of planes in somegrains The corresponding EDPs are shown in Fig 10(b)10(e) and 10(f) Indexing of the observed diffractionrings shows that both films are constituted of the O1

approximant (alternatively by a mixture of close ap-proximants or nanodomains of approximants) Concern-ing the second film the rings are dotted showing thatthe dimensions of the crystallites are larger in thissample Moreover comparison between EDPs taken in

FIG 7 (a) Structural model of the monoclinic ndashAl13(CrFe)4 phasealong the [010] zone axis White circles Al atoms full circles tran-sition metal atoms (b) Structural model of twin defects (type 1) onplane (100) (c) Same for type 2 of twins on plane (100) [seeFig 6(b)]



two different parts of the sample shows that diffractionfrom (0 0 8) planes is the most intense in the first case[Fig 10(e)] whereas the contribution of (8 0 8) planes isthe most important in the second part [Fig 10(f)]We suggest that a texture effect takes place and explainsthis phenomenon which is in agreement with the ob-servation done by brightfield diffraction [Fig 10(d)]The composition measured by EDS (Table I) is in agree-ment with this structural characterization since bothsamples possess a composition close to that of the O1

approximant3

IV PROPERTIES

A Electron energy loss spectra measurements

Figure 11 presents aluminum electron energy lossspectra (EELS) of sample F4 (thick and crystallized)F6 (thinner and partially crystallized) and F7 (thin andcrystallized after annealing) together with an aluminum

FIG 8 (a) EDP along the [010] zone axis of the OE approximantphase and (b) EDP along the [010] zone axis of the OF approximantphase

gt

FIG 9 (a) Bright-field micrograph of F1c thin film (b) EDP alongthe [100] zone axis of the monoclinic -Al13(CrFe)4 phase (c) EDPalong the [111] zone axis of the cubic ndashAl9(CrFe)4 ndashbrass phase(d) EDP along the [101] zone axis of the O1 approximant phase(e) EDP along the [010] zone axis of the monoclinic X-AlCrFe phase(see text) (f) HREM image of ndashAl13(CrFe)4 phase (g) same forAl9(CrFe)4 -brass phase (h) same for the O1 approximant phaseand (i) same for the X-AlCrFe phase



metal thin film (10 nm) obtained using the same pro-cedure These spectra are compared with those of alumi-num and alumina according to Ref 30 All the samplespresent the same features than those experienced on bulkaluminum In contrast the aluminum spectrum is rathersimilar to that of alumina in particular with the presenceof the Al L23 peaks of the oxide AlCrFe samplesare therefore less oxidized than the pure aluminumfilm even if they are very thin whatever the preparation

process used This point was also observed for bulksamples431

B Optical properties

Figures 12(a) and 12(b) show optical reflectivity andtransmission measurements of F4 F6 and F7 samplestogether with the glass substrate within a large range ofwavenumbers The spectra are similar with a differencein intensity in agreement with thickness differences

FIG 10 (a) Bright-field micrograph of F4 thin film (b) EDP of the F4 thin film indexed by considering the orthorhombic O1 approximant(c) bright-field micrograph of F7 thin film (d) same at higher magnification showing isolated grains (e) EDP of F7 thin film indexed byconsidering the orthorhombic O1 phase and (f) same in another part of the sample showing a texture effect



(thinner samples are more transparent and therefore thereflective) Nevertheless the reflectivity for the thickersample decreases weakly between 3 times 104 and 5 times104 cmminus1 This behavior is comparable to the one ob-served on bulk approximants32 The transmission of theglass substrate is almost constant except between 0 and2 times 103 cmminus1 (no transmission) and around 104 cmminus1

where an absorption depression occurs In contrast trans-mission of the AlndashCrndashFe samples decreases slowly frominfrared to visible range Due to the substrate all samplesare totally opaque in the ultraviolet range

The absorption value was deduced from these spectraafter correction for the substrate contribution [Fig 12(c)]according to the relationship A 1 ndash (R + T) Wesimply do not consider here any loss contribution Thebehavior of samples F6 and F7 is comparable with anincreasing absorption from low frequencies to higher val-ues The absorption of the thicker sample is also increas-ing between 14 times 104 and 23 times 104 cmminus1 but is weakerthan in the other films Conversely the absorption isstrongly decreasing from infrared to 14 104 cmminus1 Inconclusion there is no noticeable difference between acrystallized film (F7 sample) and an amorphous one (F6)To probe the optical properties of the material the filmmust be thick enough (at least 30 nm) as eg sample F4In this case the transmission is too weak and does notallow any applications

C Wetting properties

By means of contact-angle measurements we havestudied the wetting of our samples (F4 F6 F7) by watertogether with that of the glass substrate and the alumi-num thin film Table III presents wetting angles of purewater at the surface of these five samples The first set ofdata corresponds to measurements performed a shorttime (less than 24 h) after preparation whereas the second

set of data was obtained 8 weeks after preparation Thewetting angle is systematically larger for all AlndashCrndashFesamples than for the aluminum film The larger anglecorresponds to the thicker sample It appears that thewetting angle has considerably increased between thetwo sets of measurements In this case larger values ofwetting angles are obtained for the F4 and F7 samples

FIG 11 Comparison of Al L23 EELS spectra of F4 F6 and F7 thinfilms bulk aluminium metal aluminum thin film and alumina

FIG 12 (a) Reflectivity of F4 F6 and F7 thin films and glass sub-strate (b) same for transmission and (c) same for absorption



which are both crystallised and constituted of approxi-mate phases Therefore it seems that composition thick-ness and crystalline structure play together a role in de-termining the wetting mechanisms Whatever the coatingthat was deposited at the surface of the glass substratethe wetting angle is largely increased in comparison tothe angle measured on a free-surface substrate

V CONCLUSION

We were able to produce approximant thin films byusing the flash evaporation technique The AlndashCrndashFesystem we chose contains several intermetallic phasesknown to be approximants of both decagonal and icosa-hedral quasicrystals Composition of the bulk alloys usedfor evaporation has to be close to the one looked at formaking the films The preparation technique allows thegrowth of metastable phases that are not listed in thephase diagram (ie O1- OE- OF- - -phases) andstable phases as well (- -phases) Twin defects occurfrequently in at least two structures (ie and ) andshould be related with the preparation technique It ap-pears also that preferential growth orientation occurs de-spite the substrate is amorphous Moreover the phasespresent orientation relationships between each otherwhich suggest the possibility of epitaxial growth on well-defined substrates

Optical applications for such films can possibly beconsidered if the thickness is large enough so that filmproperties would be those of bulk approximants In allcases the wetting properties against water are rather sat-isfactory compared to those of glass substrates or alumi-num thin films

ACKNOWLEDGMENTS

The authors would like to thank gratefully M Vergnatand F Mouginet for their fruitful help to produce thinfilms and JB Ledeuil for microprobe analysis Wegratefully acknowledge the financial support offered toone of us (V Demange) by Reacutegion Lorraine and Saint-Gobain Recherche as well as by the French Ministry of

Research and New Technologies under contract ldquoERTQuasicristaux Industrielsrdquo

REFERENCES

1 L Johann A En Naciri L Broch V Demange F Machizaudand JM Dubois Spectroscopic ellipsometric characterization ofapproximant thin films of AlndashCrndashFe deposited on glass substratesPhysica E 17 552 (2003)

2 V Demange PhD Thesis Dissertation INPL (unpublished)3 V Demange JS Wu V Brien F Machizaud and JM Dubois

New approximant phases in AlCrFe Mater Sci Eng A 294-29679 (2000)

4 V Demange JW Anderegg J Ghanbaja F MachizaudDJ Sordelet M Besser PA Thiel and JM Dubois Surfaceoxidation of AlCrFe alloys characterized by x-ray photoelectronspectroscopy Appl Surf Sci 173 327 (2001)

5 AJ Bradley and SS Lu An x-ray study of the chromium-aluminium equilibrium diagram J Inst Met 60 319 (1937)

6 M Palm The AlndashCrndashFe system-phases and phase equilibria in theAlndashrich corner J Alloys Compd 252 192 (1997)

7 JK Brandon WB Pearson PW Riley PC Chieh andR Stokhuyzen -brasses with R cells Acta Crystallogr B 331088 (1977)

8 AJ Bradley and SS Lu The crystal structures of Cr2Al andCr5Al8 Z Krist 96 20 (1937)

9 MH Booth JK Brandon RY Brizard C Chieh andWB Pearson -brasses with F cells Acta Crystallogr B 33 30(1977)

10 JK Brandon PC Chieh WB Pearson and PW Riley An er-ror parameter analysis of tetrahedral configurations in brassstructures Acta Crystallogr A 31 236 (1975)

11 L Arnberg A Jonsson and S Westman The structure of the-phase in the CundashSn system A phase of -brass type with an 18Aringsuperstructure Acta Crystallogr A 31 S98 (1975)

12 SP Ge and HK Kuo Ordered -brass structures coexisting withthe decagonal quasicrystal in a Ga46Fe23Cu23Si8 alloy J MaterRes 14 2799 (1999)

13 C Dong and JM Dubois Quasicrystals and crystalline phases inAl65Cu10Fe10Cr5 alloy J Mater Sci 26 1647 (1991)

14 H Selke U Vogg and PL Ryder Approximants of the icosahe-dral phase in as-cast Al65Cu20Cr15 Philos Mag B 65 421 (1992)

15 W Liu and U Koumlster Eutectoid decomposition of the icosahedralquasicrystals in melt-spun Al65Cu20Cr15 alloys Mater Sci Eng A154 193 (1992)

16 V Khare NP Lalla RS Tiwari and ON Srivastava On thenew structural phases in Al65Cu20Cr15 quasicrystalline alloyJ Mater Res 10 1905 (1995)

17 K Sugiyama H Saito and K Hiraga On the crystal structures ofthe AlndashCundashCr alloy system J Alloy Comp 342 148 (2002)

18 YH Qi ZP Zhang ZK Hei and C Dong The microstructureanalysis of AlndashCundashCr phases in Al65Cu20Cr15 quasicrystallineparticlesAl base composites J Alloys Compd 285 221 (1999)

19 A Johansson and S Westman Determination of the structure ofcubic gamma-PtZn A phase of gamma brass type with Aring super-structure Acta Chem Scand A 24 3471 (1970)

20 L Westin and LE Edshammar On the crystal structure ofRhMg6 Acta Chem Scand A 25 1480 (1971)

21 L Arnberg A Jonsson and S Westman The structure of the-phase in the CundashSn system A phase of -brass type with an 18Aring superstructure Acta Chem Scand A 30 187 (1976)

22 S Lidin M Jacob and AK Larsson (FeNi)Zn65 a superstruc-ture of -brass Acta Crystallogr C 50 340 (1994)

23 JN Barbier N Tamura and JL Verger-Gaugry Monoclinic

TABLE III Contact angle measurements on different samples with aninterval of 8 weeks between the two sets of experiments

Sample

Contact angle (deg)

First set ofmeasurement

Second set ofmeasurement

F4 66 plusmn 1 90 plusmn 2F6 85 plusmn 2 94 plusmn 2F7 65 plusmn 1 94 plusmn 2Aluminum film 62 plusmn 1 85 plusmn 2Glass substrate 20 plusmn 05 20 plusmn 05



Al13Fe4 approximant phase A link between icosahedral and de-cagonal phases J Non-Cryst Solids 153ndash154 126 (1993)

24 XD Zou KK Fung and KH Kuo Orientation relationship ofdecagonal quasicrystal and tenfold twins in rapidly cooled AlndashFealloy Phys Rev B 35 4526 (1987)

25 XL Ma and KH Kuo Crystallographic characteristics of theAlndashCo decagonal quasicrystal and its monoclinic approximant 2-Al13Co4 Metall Mater Trans A 25 47 (1994)

26 J Grin U Burkhardt and M Ellner Refinement of the Fe4Al13

structure and its relationships to the quasihomologocal homeo-typical structures Z Kristall 209 479 (1994)

27 J Grin U Burkhardt M Ellner and K Peters Crystal structureof orthorhombic Co4Al13 J Alloys Compd 206 243 (1994)

28 K Saito K Sugiyama and K Hiraga Al13M4-type structures and

atomic models of their twins Mater Sci Eng A 294ndash296 279(2000)

29 XL Ma H Liebertz and U Koumlster Multiple twins of mono-clinic Al13Fe4 showing pseudo-orthorhombic and fivefold sym-metries Phys Status Solidi 158 359 (1996)

30 C Cahn and OL Krivanek EELS Atlas Edited by ASU HREMFacility Tempe AZ and Gatan Warrendale PA (1983)

31 V Demange JW Anderegg F Machizaud DJ SordeletPA Thiel and JM Dubois New approximants in the AlndashCrndashFesystem and their oxidation resistance J Alloys Compd 342 24(2002)

32 V Demange A Milandri MC de Weerd F MachizaudG Jeandel and JM Dubois Optical conductivity of AlndashCrndashFeapproximant compounds Phys Rev B 65 144205 (2002)



the pure constituents in a levitation induction furnaceCompositions (obtained by microprobe analysis) thatlead reproducibly to the formation of films of the wantedphases are respectively A Al676Cr233Fe91 B Al70Cr22Fe8 C Al725Cr195Fe8 D Al755Cr158Fe87

and E Al775Cr145Fe8 (in at) The so-obtained bulkalloys were characterized by transmission electronmicroscopy (TEM) and will be the subject of a forthcom-ing article2 Some details are nevertheless already avail-able in Refs 3 and 4

Small fractions of about 25 mg of the chosen alloywere evaporated with the help of a hot tungsten filamentunder secondary vacuum (10minus7 mbar) The films weredeposited simultaneously on a glass substrate and on acarbon coated nickel microscopy grids This device wasable to operate at room temperature but also to pre-heatthe substrate up to 584 degC The films obtained at roomtemperature were systematically found amorphous (to-tally or partially depending on the thickness of the as-deposited coating) Subsequently some of them wereannealed in situ in the electron microscope Conventionalelectron microscopy was performed on Philips CM 12(120 kV) and Philips (Eindhoven Netherlands) CM 200Double-twin instruments whereas high resolutionmicroscopy was performed on a Philips CM200 Super-twin microscope

In addition to structural characterization we per-formed electron dispersive spectroscopy (EDS) measure-ments on each film to compare nominal composition ofbulk alloys and composition of corresponding films Tothis end a calibration bulk sample was primarily char-acterized by microprobe analysis

III RESULTS

A Thickness and microstructure induced by thepreparation conditions

Table I gives data on films thickness deposition tem-perature and microstructure of TEM samples togetherwith composition of the ingot and films composition Thethickness was obtained by TEM and by calibration of aquartz balance used during the preparation process

Crystallization of films is strongly dependent on thepreparation parameters Films are partially crystallized

even when deposited at room temperature if the thicknessis important enough Otherwise films remain completelyamorphous In addition heating the substrate prior toevaporation is favorable to crystallization as expectedFinally a rather thick film (30 nm) is completely crys-tallized for a substrate heated up to 300 degC whereascomplete crystallization of a thinner film (10 nm) re-quires heating the substrate at 584 degC

Except for the F2 sample (see Table I for the coding ofthe specimens) the films composition is systematicallyricher in aluminum compared to the bulk alloy Theamount of aluminum in the films varies from 726 to856 at and plays an important role in the resultingstructure as we show in the next section

B In situ annealed amorphous films

To know if it was possible to obtain crystallized filmsby heating as deposited amorphous samples we per-formed postannealing in situ in the electron microscopeusing F1a F1b and F1c samples (Table I) The annealingtreatment occurs at 460 degC during 24 h for F1a sampleand at 600 degC for F1b and F1c films for 38 and 20 hrespectively

The first step of crystallization occurs after 2 h ofannealing it is followed by the growth of crystallites butthe major part of the layer remains amorphous even afterseveral hours of annealing

A bright-field image of the F1a sample is shown inFig 1(a) This micrograph points out the growth of smallcrystals in an amorphous matrix Most crystals possess asquare shape showing a preferential growth texturenamely the lang100 rang zone axis of a cubic phase with a latticeparameter a 912 Aring This phase corresponds preciselyto the cubic -brass ndashAl9(CrFe)4 isostructural with thebinary Al9Cr4 compound The phase composition meas-ured by EDS is precisely close this stœchiometric com-position (Table II) with a poor amount of iron Thisphase which was identified in the binary system by Brad-ley and Lu5 and in the ternary one by Palm6 crystallizesin the I43m space group Its complete structure was de-scribed by Brandon et al7 The -brass phases were ob-served in several binary alloys Their structure is built upby atomic clusters of 26 atoms The cluster is a sequence

TABLE I Preparation parameters (thickness and temperature) of Al-Cr-Fe thin films and obtained microstructures and compositions

Thin filmsamples

Estimated thickness(nm)

Preparationtemperature Microstructure

Nominal composition ofthe precursor bulk alloy (at)

Thin film compositionmeasured by EDS (at)

F1a F1b F1c 10 Room temperature Amorphous C Al725Cr195Fe8 Al856Cr116Fe28

F2 20 Room temperature Amorphous + crystallized D Al755Cr158Fe87 Al726Cr217Fe57


F4 30 325 degC Crystallized A Al676Cr233Fe91 Al777Cr180Fe4


F6 10 325 degC Amorphous + crystallized B Al70Cr22Fe8 Al733Cr206Fe61

F7 11 584 degC Crystallized B Al70Cr22Fe8 Al744Cr20Fe56





















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T = d001
















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010 011 909O1 102X

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IV PROPERTIES




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V CONCLUSION



ACKNOWLEDGMENTS



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Sample

Contact angle (deg)






































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IV PROPERTIES




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V CONCLUSION



ACKNOWLEDGMENTS



REFERENCES

























Sample

Contact angle (deg)





















cos 72deg lda2hdc








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100 111 101O1 010X

010 011 909O1 102X

025 101 555O1 500X

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IV PROPERTIES




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V CONCLUSION



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Contact angle (deg)




















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IV PROPERTIES




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V CONCLUSION



ACKNOWLEDGMENTS



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Sample

Contact angle (deg)




























100 111 101O1 010X

010 011 909O1 102X

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V CONCLUSION



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V CONCLUSION



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Sample

Contact angle (deg)




















study of decagonal approximant and -brass-type … · intermetallic phases that are all linked with...

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