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Intermetallics 40 (2013) 1e9

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Intermetallics

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Oxidation and crystallographic features of the new prototypestructure Ti4NiSi4

N. Chaia a, M. François a, S. Mathieu a,*, E. Elkaïm b, F. Rouillard c, M. Vilasi a

aUniversité de Lorraine, Institut Jean Lamour e UMR7198, BP 70239, 54506 Vandoeuvre lès Nancy, Franceb Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, FrancecCEA, DEN, DPC, SCCME, Laboratoire d’Etude de la Corrosion Non Aqueuse, F-91191 Gif-sur-Yvette, France

a r t i c l e i n f o

Article history:Received 15 February 2013Received in revised form19 March 2013Accepted 20 March 2013Available online 27 April 2013

Keywords:A. Silicides, variousB. Crystal chemistry of intermetallicsB. OxidationB. Phase transformationC. CoatingsG. Corrosion- and erosion-resistantapplications

* Corresponding author. Tel.: þ33 3 83 68 46 70.E-mail address: stephane.mathieu@univ-lorraine.f

0966-9795/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.intermet.2013.03.014

a b s t r a c t

The oxidation resistance and the crystal structure of the new Ti4NiSi4 compound were investigated. Thiscompound was manufactured both as a single-phase and as the outer layer of a protective coating forvanadium alloys. The recorded oxidation rates were very low at 650 �C and 750 �C in air, which rep-resents a more severe environment than those envisaged for the targeted application: fuel cladding forsodium-cooled fast reactors. The powder XRD measurements performed at the synchrotron SOLEILallowed for the crystallographic structure determination of Ti4NiSi4 (S.G. Pnma; a ¼ 15.63931;b ¼ 5.08321; c ¼ 12.75151) with high confidence factors (Rwp ¼ 0.07; RBragg ¼ 0.067). Its structureconsists of atomic planes stacking along the b-axis with planes at coordinates y ¼ 0, ¼, ½ and 3/4 ; it cantherefore be considered as a pseudolamellar structure. It has been suggested that this peculiar featureinduces a low activation energy for the phase transformation of Ti4NiSi4 into Ti4Ni4Si7, which thusrenders the transformation possible at moderate temperature. This structural rearrangement is accom-panied by the liberation of Ti and Si, which leads to the formation of both TiO2 and SiO2 oxidationproducts, which are responsible for the high environmental resistance observed at 650 �C in air.

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1. Introduction

Research in coating solutions for structuralmaterials used for fuelcladding in sodium-cooled fast reactors (SFR) in generation IV nu-clear systems has led to the investigation of silicide compoundswithhigh oxidation resistance at temperatures of 550e750 �C. We havepreviously shown [1,2] that MSi2 compounds (with M ¼ V, Cr, Ti),deposited on the surface of vanadium alloys using the halide-activated pack-cementation (HAPC) technique have high oxidationperformances, both in air and in impure helium, and have highcompatibilitywith liquid sodiumat 550 �C. However,manufacturingthese coatingswith chloride species as the halide activator led to theformationof cavitiesat the substrate-coating interface. These cavitiescontained vanadium dichloride VCl2, which has high stability asevidenced by its Gibbs enthalpy of formation (Eq. (1)).

VðsÞ þ Cl2ðgÞ ¼ VCl2ðsÞ (1)

DGrð1200�CÞ ¼ �262:8 kJ:mol�1 ½3�Several strategies can be followed to limit the formation of this

compound during deposition and to avoid the presence of defects in

r (S. Mathieu).

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the coating. One strategy consists of reducing the formation of VCl2through modification of the gaseous phase composition. Thesemodifications, based on thermodynamic assessment of the pack-cementation process, were presented in [1] and [2]. A secondmethod consists of the formation of a diffusion barrier to vanadiumprior to the pack-cementation process.

In this regard, Ni is an efficient diffusion barrier and is easy tomanufacture on the vanadium surface using well-establishedelectrolytic or electroless processes. Moreover, because Ni pos-sesses a low activation energy, i.e., it generates no or few radioac-tive elements, we chose thismetal to coat Ve4Cre4Ti alloys prior tothe pack-cementation step.

However, the choice of nickel has some consequences on thepack-cementation parameters. Because of the low temperature ofthe NiSieNiSi2 eutectic (960 �C) [4], it is impossible to use a Si-richmaster alloy (with high Si activity) to deposit a protective coating.Indeed, because pack-cementation is controlled by solid-statediffusion, this process has to be done at high temperatures typicallyabove 1000 �C, which would lead to the melting of the superficialpart of the alloy and to a poor quality coating.

Based on our experience with silicide coatings [5e8], ternaryand quaternary silicide compounds containing only 45e46 at.% Simay have also high oxidation resistance. Their manufacture re-quires the use of a master alloy with a lower silicon activity than for

N. Chaia et al. / Intermetallics 40 (2013) 1e92

the manufacture of MSi2-type coatings to avoid superficial melting.For the present study, a TiSi þ TiSi2 mixture (aSi ¼ 0.12, valuecalculated at 1100 �C using ThermoCalc software [9] and Seifertdata [10]) was chosen as the master alloy to perform the coating.

Therefore, the first part of this paper deals with the metallo-graphic characterisation and oxidation resistance both ofthe manufactured NieTieSi coating (considered here for theprotection of vanadium substrates from oxidation in future gen-eration IV reactors) and of the Ti4NiSi4 outer phase of this coating.The oxidation tests were mainly conducted in air at 650 �Cand 750 �C, which corresponds to more harshly oxidative condi-tions than those expected in SFR applications (550 �Ce650 �C,w5e10 ppm O2).

The second part of the present paper describes the new crys-tallographic structure of the outer phase Ti4NiSi4. To accomplishthis, the Ti4NiSi4 compound was synthesised as a pure single-phase. The crystallographic data were obtained using the SOLEILsynchrotron facilities.

2. Experimental methods and materials

2.1. Manufacturing of NieTieSi coating

Ve4Cre4Ti substrate was supplied by CEA Saclay. This alloy wasprovided by GfE Metalle und Materialien GmbH, Nuremberg, Ger-many; the manufacturing process is detailed elsewhere [11]. Thesamples were cut from a rolled plate and recrystallised at 1000 �C;their dimensions were approximately 10 mm � 10 mm � 1 mm.Surface preparation consisted of polishing down to 1200 grid, andthe corners were also rounded with SiC paper. Then, the sampleswere cleaned ultrasonically in ethanol and dried.

The as prepared plates were coated with 30 mm of nickel using aNi electroplating technique (Watts nickel plating bath, 50 �C, cur-rent density 20 mA.cm�2). Then, the Ni-coated Ve4Cre4Ti, themaster alloy (TiSi þ TiSi2), the activator salt (CrCl3) and inert filler(SiO2) were placed in a closed silica tube under vacuum to carry outhalide-activated pack-cementation (HAPC). The process was con-ducted isothermally at 1100 �C for a period of 9 h. The master alloyused for HAPC (TiSiþ TiSi2) was synthesised by arcmelting Ti and Siunder argon using a non-consumable tungsten electrode on awater-cooled copper crucible.

Fig. 1. Cross section of multilayer NieTieSi coating manufactured on Ve4Cre4Tisubstrate (Step 1: RT Electrolytic Ni deposition, Step 2: Halide-activated pack-cementation with the master alloy TiSi þ TiSi2, 9 h, 1100 �C).

2.2. Synthesis of the Ti4NiSi4 compound

The Ti4NiSi4 compound was manufactured in two steps: thecomposition Ti4NiSi4 was prepared by inductive melting of puremetals and silicon (high purity Ti 99.7%, Ni 99.9% and Si 99.9999%)on a water-cooled copper crucible. Melting was performed threetimes to obtain homogeneous ingots. The ingots were hand-crushed into a powder until a grain size less than 80 mm wasobtained. A sample of the powder was retained for X-ray charac-terisation. The remainder was densified by uniaxial hot pressingunder argon to produce single-phase samples of Ti4NiSi4. Foruniaxial hot pressing, the powder was introduced onto a graphitedie that had previously been coated with boron nitride (BN) spray.BN works as a diffusion barrier between graphite and metallicpowders, and thus limits the formation of carbide. The die washeated to 1200 �C for 4 h under a pressure of 27 MPa, after whichthe load was removed and heating was stopped. The cooling timeto room temperature was approximately 4 h. Specimen di-mensions were approximate (Ø 25 mm, h 12 mm). To characterisethe microstructure and study oxidation, coupons10 mm � 10 mm � 2 mm in dimension were cut from the hot-pressed samples.

2.3. Crystal structure determination

High-resolution X-ray powder diffraction (XRPD) data werecollected at ambient temperature with synchrotron radiation atSOLEIL (Saint Aubin, France) on a CRISTAL beamline [12]. Amonochromatic beamwas extracted from the U20 undulator beamby means of a Si(111) double monochromator. Its wavelength(l ¼ 0.78912 �A) was refined from a LaB6 (NIST Standard ReferenceMaterial 660, cubic Pm3m) powder diagram recorded just prior tothe experiment. The high angular resolution was obtained using atwo circle diffractometer (SMP, Lyon based company) equippedwith a multi-analyser containing 21 perfect Si(111) crystals. Thesample was enclosed in a capillary tube (r ¼ 0.3 mm) and mountedon a spinner rotating at 5e10 Hz to improve powder averaging.Patterns were recorded for one hour in the angular range2�<�2q < 75� with an interval of 0.002�. Transmission of the filledcapillary was measured using an X-ray camera, allowing a reliablemr value (where m is the absorption coefficient and r is the capillaryradius) to be fixed at 1.90 during Rietveld refinement.

2.4. Metallographic characterisation

Metallographic observations were performed using a JEOL JSM-7600F equipped with an SDD-type EDX detector coupled with anOxford INCAWAVEWDS spectrometer. Chemical point analysis wasperformed using a Cameca X100 microprobe. Metal disilicides(MSi2 with M ¼ V, Cr or Ti) and pure nickel were used as standardsfor the quantitative analysis.

The oxidised samples were covered by a protective nickel elec-troplating layer, embedded with a cold epoxy resin, grinded from240 to 2400 grid with SiC paper, and finished using a colloidal SiO2suspension to characterise the cross sections after oxidation.

2.5. Oxidation tests

For oxidation tests, the samples were grinded to 2400 gradeusing SiC paper and then cleaned in ethanol with ultrasonic waves.

For the single-phase Ti4NiSi4, isothermal oxidation tests wereconducted at 650 �C and 750 �C in air using both thermogravimetryand an open furnace (for long exposure times). During thermog-ravimetry, specimens were suspended in the hot zone of a ther-mobalance furnace using a platinum wire covered with thin

Fig. 2. Microstructure of hot-pressed Ti4NiSi4 silicide compound.

N. Chaia et al. / Intermetallics 40 (2013) 1e9 3

alumina tubes. Air was introduced into the reaction chamber at aflow rate of 1.5 L h�1.

On the coated samples, cyclic oxidation tests were performed ina tubular furnace in air for a 1-h cycle at 650 �C and at 1100 �C. Thespecimens were then removed from the furnace, cooled for 10 minat room temperature and thenweighed by hand using an analyticalbalance with a precision of 0.1 mg.

3. Experimental results

3.1. Characterisation of manufactured coatings and hot-pressedTi4NiSi4

The result of cementation of theNi-coatedVe4Cre4Ti alloy underthe conditions defined is given in Fig.1, where the cross section of thesample and the chemical compositions of each layer are presented.Co-depositionof Ti andSiby theHAPCprocessofNi-coatedvanadiumalloy resulted in the growth of a multilayer silicide coating. The

a)

b)

0 100 200 300 400 500 600 700 800 900 1000-0.025

0.000

0.025

0.050

0.075

wei

ght c

hang

e (m

g.cm

-2)

Number of 1h cycles

0 100 200 300 400 500 600 700 800 9001000-0.025

0.000

0.025

0.050

0.075

wei

ght c

hang

e (m

g.cm

-2)

Number of 1h cycles0 50 100 150 200 250 300 350

0

1

2

3

4

5

6

7

8

Fig. 3. Mass change versus number of cycles (1-h cycle þ 100 at room temperature) forVe4Cre4Ti coated with multilayer NieTieSi in air a) at 650 �C and b) at 1100 �C.

interfacial vanadium substrate-coating did not present cavities likethose observed when applying the chloride-activated pack-cemen-tation technique directly to the vanadium alloy [1]. The outer fourlayers corresponded to ternary NieTieSi silicides, from the outer toinner layer: Ti4NiSi4, Ni4Ti4Si7, NiTiSi and Ni49Ti14Si37 [13]. The moreinternal phases contained a lower amount of Ti and corresponded tophases in the VeNieSi system [14]. Almost all of the nickel initiallyelectroplated at the surface of Ve4Cre4Ti was incorporated into themultilayer coating after the pack-cementation step. The presence of athin layer of s0 phase (VeNi) showed that a very small amount ofnickel did not transform during the HAPC diffusion treatment.

Fig. 2 displays the microstructure of hot-pressed Ti4NiSi4. Thesamples were well densified and free of porosity. Some cracks wereobserved, but they did not affect the integrity of the sample, as dis-cussed in the section presenting the results of oxidation. Thecomposition of the compound was determined using EPMA. Quan-titative analysis indicated that the stoichiometry of Ti4NiSi4 wasmaintained. However, it appeared clearly that the very narrowcomposition range of this compound made obtaining a pure sampledifficult. Many authors [13,15] have mentioned the Ti4NiSi4 com-pound, also called “H” phase or “s8” phase, which is described in theNieTieSi phase diagram system as a point phase. Thus, the specimenalsocontaineda small amountof twootherphaseswithcompositionsof TiSi and Ti5Si3.

3.2. Oxidation behaviour of the NieTieSi coating in air

Cyclic oxidation was performed both at 650 �C, which corre-sponds to the highest temperature envisaged for SFR applications,and at 1100 �C, which would be representative of an incidentalincrease in temperature. Cyclic oxidation conditions allowed for theevaluation of, in a single experiment, both the oxidation resistanceof the coating and the effect of thermo-mechanical stresses appliedto the coating-substrate system. Fig. 3a and b display the resultsobtained at 650 �C and 1100 �C, respectively. The test was stoppedafter 800 1-h cycles at 650 �C because the weight of the sample hadnot changed (it is important to note the small change in weight inFig. 3a). Indeed, themeasuredweight change remained in the rangeof the balance sensitivity. At 650 �C, corrosion products were notdetectable after 800 h of exposure using conventional XRD and SEMtechniques.

At 1100 �C, the weight gain evolved slowly for the first 50 h ofexposure and then evolved exponentially up to 300 h. The test wasstopped at 311 cycles because a trace of V2O5 liquid oxide(Tf ¼ 680 �C) was visible on the surface of the sample. The presenceof V2O5 was evidence that oxygen had reached the vanadiumsubstrate and that the local integrity of the coating had been lost.Fig. 4 shows the cross section of the sample exposed for 311 cyclesat 1100 �C. Cracks were observed in the coating, but they generallystopped before reaching the substrate. A thick and dense scale ofcorrosion products at the top of the remaining coating was alsoobserved. XRD (Fig. 5) revealed the formation of mainly rutile TiO2and cristobalite SiO2; amorphous SiO2 should also be present in thecorrosion products. A small amount of Ni-containing oxides(NiTiO3) was also observed. Under the oxide scale, the rich Ni-containing compound G00 (Ni49Ti14Si37) [13] was still present.

3.3. Oxidation behaviour of the Ti4NiSi4 compound in air

Because Ti4NiSi4 was the outer phase of the coating, it repre-sented the first barrier to oxidation. Thus, the oxidation behaviourof this new compound was also studied. Isothermal oxidation testswere carried out using thermogravimetry at 650 �C and 750 �C(Fig. 6a). The evolution of weight gain as a function of time for 50 hwas somewhat parabolic for both sets of data (Fig. 6a). However the

Fig. 4. Cross-sectional morphology of Ve4Cre4Ti coated with multilayered NieTieSiafter 311 1-h cycles at 1100 �C in air.

a)

b)

0 5 10 15 20 25 30 35 40 45 500.000

0.005

0.010

0.015 65O°C 750°C

Wei

ght c

hang

e (m

g.cm

-2)

Time (h)

0 10 20 30 40 50 60 70 80

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-0.2

0.0

0.2

0.4750°C

650°C

Wei

ght c

hang

e (m

g.cm

-2)

Time (days)

Fig. 6. Mass change versus time of a single-phase Ti4NiSi4 sample oxidised at 650 �Cand 750 �C in air (samples were removed daily from the furnaces to be weighed).

N. Chaia et al. / Intermetallics 40 (2013) 1e94

weight gains remained very low because of the very high oxidationresistance of Ti4NiSi4 in this temperature range. Thus, oxidationtests were performed for long times (Fig. 6b) at both temperaturesto validate the high stability of the Ti4NiSi4. The results definitivelydemonstrated the high stability of this compound in air at 650 �Cand 750 �C because the amount of oxidation products remainedvery low after 203 days of testing (the samples were not weighedafter 80 days). The samples exposed at 650 �C and 750 �C gained0.16 and 0.42 mg.cm�2, respectively.

Fig. 7 shows the cross section of the Ti4NiSi4 sample after 203days of exposure at 650 �C. A thin oxide layer (2e3 mm on average)was observed, consisting of small particles of TiO2 embedded in asilica layer. Below this oxide layer, a very thin layer of Ti4Ni4Si7was identified using data not shown here and obtained at 850 �C.At this temperature, the amount of Ti4Ni4Si7 that formed washigher; thus, quantification of the local composition using WDSwas possible.

3.4. Structural determination of the Ti4NiSi4 compound

Because the crystal structure of Ti4NiSi4 has not previously beeninvestigated, Ti4NiSi4 powder was characterised using the SOLEILsynchrotron facilities. Fig. 8 shows the XRD patterns obtained.

Fig. 5. X-ray diffraction pattern performed on the surface of Ve4Cre4Ti coated withmultilayer NieTieSi after 311 1-h cycles at 1100 �C in air.

Indexing of the 20 first Bragg peaks and determination of thespace group was performed using the ‘XCell’ application imple-mented in theMStudio package software [16]. A solutionwas foundin the orthorhombic space group Pnma with a ¼ 15.639 �A,b ¼ 5.083 �A, c ¼ 12.751 �A. The space group Pnma was confirmedusing the Fullprof program running in the ‘profile matching’ mode(Le Bail decomposition) [17].

An initial model was found by direct-space methods using theFOX program [18] based on knowledge of i) the EPMA compositionTi4NiSi4, ii) the multiplicity of the space group and iii) densityconsiderations. The following scatters have been considered in theunit cell, all initially placed in random position: eight scatters (Ti1-Ti8) for titanium, seven scatters (Si1-Si7) for silicon and two

Fig. 7. Cross-sectional morphology of a single-phase Ti4NiSi4 sample after 208 days ofisothermal exposure at 650 �C in air.

Fig. 8. Observed (red), Rietveld calculated (black) and difference (green) synchrotronXRPD patterns (l ¼ 0.77752 �A). The vertical bars indicate the Bragg positions. (Forinterpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)

N. Chaia et al. / Intermetallics 40 (2013) 1e9 5

scatters (Ni1 and Ni2) for nickel. The global optimisation procedureused was a parallel tempering algorithm. A satisfactory solution(Rwp ¼ 15%) led to the retention of 15 independent crystallographicsites for the structure, 12 being in 4c and the other three in generalpositions. At this stage of resolution, allocation of the Ti and Nielements (which differ by 6 electrons) on the various sites was notcompletely satisfactory and had to be made through Rietveldrefinement. The unit cell contains 72 atoms.

3.5. Rietveld refinement

The pattern (Fig. 8) was first used in the ‘profile matching’modeof Fullprof in order to refine the cell and profile parameters. LaB6(NIST, SRM 660a) was used as a standard reference material todefine the instrumental resolution function (IRF) file. This IRF fileallowed for a description of the peak shape profile with theThomson-Cox-Hasting pseudo-Voigt function for the sample

Table 1Crystal data and structural refinement parameters.

Compound Ti4NiSi4

Fw (g.mol�1) 366.091System OrthorhombicSpace group P n m aa (�A) 15.63931(4)b (�A) 5.08321(1)c (�A) 12.75151(3)V (�A3) 1013.718(5)Z 8Dx (g cm�3) 4.751Wave length (�A) 0.789122q range (deg) 2.0e75.0Nobs of points 14200Nref 2345Rp 0.0407Rwp 0.0704RBragg 0.067RF 0.079N profile parameters 20N intensity dependent parameters 48

contribution. In addition, anisotropic peak broadening was neces-sary to model these high-resolution data, and a successful solutionwas found by refining 6 crystallite sizes and 6 strain effect pa-rameters. The 6 size parameters corresponded to the sphericalharmonic coefficients in the phenomenological model. Theyinclude, in the Scherrer formula, anisotropic size broadening as alinear combination of spherical harmonics. A Ti5Si3 impurity(SG ¼ P63/mcm), which is not visible in standard data from XRD,has been detected in the high-resolution data and was included inthe refinement. The impurity represents only 0.59 wt % of thesample, with the most intense impurity line appearing at 20.7� 2qin the pattern.

The 20 parameters refined over the course of the Le Baildecomposition were then fixed to perform Rietveld refinement of48 intensity-dependent parameters.

Coordinates and isotropic temperature factors were refined forseveral sites: Ti1-6, Si1-6 (4c) and Ti7, Si7 and Ni1 (8d). The normalvalues of the temperature factors indicated that the elements werecorrectly attributed to the sites. Moreover, the model led to theexpected composition of Ti4NiSi4. The final Rietveld refinement of69 parameters led to satisfactory confidence factors: Rwp¼ 0.07 andRBragg ¼ 0.067. The resulting crystallographic and refinement pa-rameters are given in Table 1, and the final Rietveld plot is pre-sented in Fig. 8. Atomic coordinates can be found in Table 2. Theaccuracy of these synchrotron powder diffraction results (see thestandard deviations) was comparable to that of data obtained froma single crystal.

3.6. Structural description

Projections of the new structural type Ti4NiSi4 along the a- andb-axis are given in Fig. 9a and b, respectively. The interatomicdistances are reported in Table 3; the shortest distance 2.304(4) �Awas found for the Ni1-Si1 bond. This value can be compared withthe short distances of NieSi previously reported in known struc-tures of the TieNieSi system (Pearson’s crystal data): 2.342 �A inNi3SiTi2 (Mg2Cu3Si type, P63/mmc), 2.305 �A in Ni16.7Si7Ti6((Ti0.42Al0.58)15Pt7Al8 type, Fm3m), 2.342 �A in Ni3SiTi2 (Mg2Cu3Sitype, P63/mmc), 2.307 �A in TiNiSi (TiNiSi type, Pnma), and 2.230 �Ain Ni4Si7Ti4 (Zr4Co4Ge7 type, I4/mmm).

The coordination polyhedra surrounding the 15 crystallographicsites are presented in Fig. 10. Their geometrical descriptionsand compositions are given in Table 3: Ti2eTi4, Ti6 centred a17-vertex pseudo-Frank Kasper polyhedron and Ti1, Ti5 andTi7 centred a 17-, 16-, and 14-vertex Frank Kasper polyhedron,respectively; Si1-Si4 centred a monocapped square antiprism,Si5-Si6centreda tricapped trigonal prism, andSi7centredabicapped

Table 2Fractional atomic coordinates and temperature factors for Ti4NiSi4.

Sites Site x y z B(�A2)

Ni1 8d 0.68454(8) 0.9971(3) 0.15816(10) 1.13(2)Ti1 4c 0.94569(16) 0.25000 0.5293(2) 1.19(5)Ti2 4c 0.53699(16) 0.75000 0.6504(2) 1.28(5)Ti3 4c 0.20964(16) 0.75000 0.6793(2) 1.10(5)Ti4 4c 0.35171(17) 0.75000 0.3177(2) 1.46(6)Ti5 4c 0.55384(17) 0.75000 0.26304(20) 1.09(5)Ti6 4c 0.17851(17) 0.25000 0.5427(2) 1.19(5)Ti7 8d 0.86604(10) 0.4988(4) 0.95817(13) 1.22(3)Si1 4c 0.9317(3) 0.75000 0.6117(3) 1.48(9)Si2 4c 0.2685(2) 0.25000 0.9830(3) 1.23(9)Si3 4c 0.1216(3) 0.25000 0.2099(3) 1.32(9)Si4 4c 0.8027(3) 0.25000 0.1150(3) 1.47(10)Si5 4c 0.4849(3) 0.75000 0.4455(3) 1.29(8)Si6 4c 0.7828(3) 0.25000 0.8092(3) 1.10(8)Si7 8d 0.56411(16) 0.4965(7) 0.8397(2) 1.26(5)

Fig. 9. Projections of the Ti4NiSi4 crystal structure along the a- and b-axis.

N. Chaia et al. / Intermetallics 40 (2013) 1e96

square antiprism. The only Ni site was located at the centre of anirregular icosahedron.

4. Discussion

The results obtained showed that an electrolytic deposit ofnickel prior to performing the HAPC process can be used to avoidboth vanadium dichloride formation and the formation of cavitiesat the substrate-coating interface.

However, the use of nickel renders the manufacturing of aprotective silicon-rich coating impossible using a Si-rich masteralloy (with a Si activity around 1), such as those previously used[1,2], because of the low temperature of the NiSieNiSi2 eutectic(954 �C). Based on thermodynamic data of the NieSi system [4], theuse of Si or TiSi2þSi would be conducive to melting of the surface ofthe Ni-coated sample.

The choice of the master alloy TiSi þ TiSi2 was firstly governedby the fact that its silicon activity was strongly decreased (aSi¼ 0.12,calculated value at 1100 �C using ThermoCalc [9] and Seifert data[10]). As a consequence, NiSi and NiSi2 cannot form during theHAPC process. Secondly, the ternary compounds described in theNieTieSi thermodynamic database [13,15] exhibit high tempera-ture capabilities that are essential in case of an unforeseen increasein operating conditions (e.g., temporary failure of the cooling sys-tem of the reactor). Thirdly, ternary NieTieSi compounds likeNi4Ti4Si7 and Ti4NiSi4 contain a sufficient amount of silicon (45e46 at.%) to form silica-rich protective oxide scales [5,19].

The synthesised coating consisted of a succession of ten layersthat were consistent both with the NieTieSi phase diagram [13] forthe four outer layers (Ti4NiSi4, Ni4Ti4Si7, NiTiSi and Ni49Ti14Si37) andwith the VeNieSi phase diagram [14] for the inner layers. Forma-tion of the coating occurred by solid-state diffusion of elements

Table 3Coordination, neighbourhood and interatomic distances (�A) in Ti4NiSi4.

Atoms Atom d(�A) Atom Atom d(�A) Atom Atom d(�A)

Ni1CN ¼ 12

Si1 2.304(4) Ti4CN ¼ 17

Si5 2.645(5) Si2CN ¼ 9/11

Ni1 2 � 2.315(3)Si2 2.315(3) Si6 2.654(5) Ti3 2.526(5)Si4 2.317(4) Si7 2 � 2.708(4) Ti7 2 � 2.573(3)Si3 2.335(4) Si4 2 � 2.790(2) Ti1 2.776(4)Si6 2.355(4) Si2 2.824(4) Ti6 2 � 2.780(2)Ni1 2.512(2) Ni1 2 � 2.917(3) Ti4 2.824(4)Ni1 2.571(2) Ti2 2 � 3.107(2) Si4 2 � 3.043(3)

Ti5 2.747(3) Ti7 2 � 3.138(3) Si3CN ¼ 9/11

Ni1 2 � 2.335(4)Ti6 2.867(3) Ti5 3.237(4) Si1 2.423(6)Ti1 2.903(3) Ti3 2 � 3.240(2) Ti7 2 � 2.502(4)Ti4 2.917(3) Ti6 3.538(4) Ti2 2.594(5)

Ti3 2.947(3) Ti5CN ¼ 16

Si5 2.564(5) Ti3 2.668(5)

Ti1CN ¼ 17

Si1 2.629(5) Si7 2 � 2.586(4) Ti5 2 � 2.775(2)Si7 2 � 2.744(4) Si6 2.622(5) Si6 2 � 2.959(3)

Si1 2 � 2.7589(18) Ni1 2 � 2.747(3) Si4CN ¼ 9/11

Ni1 2 � 2.317(4)Si2 2.776(4) Si3 2 � 2.775(2) Ti2 2.547(5)Si7 2 � 2.791(4) Ti1 2.981(4) Ti7 2 � 2.565(4)Ni1 2 � 2.903(3) Ti7 2 � 3.060(3) Ti3 2.630(5)Ti5 2.981(4) Ti2 2 � 3.114(2) Ti4 2 � 2.790(2)Ti1 2 � 3.147(2) Si1 2 � 3.199(3) Ti6 2.796(5)Ti6 2 � 3.328(2) Ti4 3.237(4) Si2 2 � 3.043(3)

Ti3 3.602(4) Ti6CN ¼ 17

Si6 2.495(5) Si5CN ¼ 9/11

Ti5 2.564(5)Ti6 3.645(4) Si1 2.617(5) Ti7 2 � 2.568(4)

Ti2CN ¼ 17

Si4 2.547(5) Si7 2 � 2.649(4) Ti4 2.645(5)Si3 2.594(5) Si2 2 � 2.780(2) Ti7 2 � 2.657(4)Si5 2.737(5) Si4 2.796(5) Ti2 2.737(5)Si7 2 � 2.769(4) Ni1 2 � 2.867(3) Ti2 2 � 2.841(2)Si5 2 � 2.841(2) Ti3 2 � 3.119(2) Si5 2 � 2.935(3)

Ti4 2 � 3.107(2) Ti7 2 � 3.194(3) Si6CN ¼ 9/11

Ni1 2 � 2.355(4)Ti5 2 � 3.114(2) Ti1 2 � 3.328(2) Ti6 2.495(5)Ti7 2 � 3.148(3) Ti4 3.538(4) Ti5 2.622(5)Ti7 2 � 3.270(3) Ti1 3.645(4) Ti7 2 � 2.627(4)

Ti3 3.465(4) Ti7CN ¼ 14

Si3 2.502(4) Ti4 2.654(5)Si1 3.452(5) Ti7 2.529(3) Ti3 2 � 2.791(2)

Ti3CN ¼ 17

Si2 2.526(5) Ti7 2.554(3) Si3 2 � 2.959(3)

Si7 2 � 2.627(4) Si4 2.565(4) Si7CN ¼ 10

Si7 2.506(5)Si4 2.630(5) Si5 2.568(4) Si1 2.516(5)Si3 2.668(5) Si2 2.573(3) Si7 2.577(5)Si6 2 � 2.791(2) Si6 2.627(4) Ti5 2.586(4)Ni1 2 � 2.947(3) Si5 2.657(4) Ti3 2.627(4)Ti6 2 � 3.119(2) Ti5 3.060(3) Ti6 2.649(4)Ti4 2 � 3.240(2) Ti4 3.138(3) Ti4 2.708(4)Ti7 2 � 3.269(3) Ti2 3.148(3) Ti1 2.744(4)Ti2 3.465(4) Ti6 3.194(3) Ti2 2.769(4)Ti1 3.602(4) Ti3 3.269(3) Ti1 2.791(4)

Ti2 3.270(3)

Si1CN ¼ 9/12

Ni1 2 � 2.304(4)Si3 2.423(6)Si7 2 � 2.516(5)Ti6 2.617(5)Ti1 2.629(5)Ti1 2 � 2.759(2)Ti5 2 � 3.199(3)Ti2 3.452(5)

N. Chaia et al. / Intermetallics 40 (2013) 1e9 7

both from the pack and from the substrate. Silicon and titaniumdiffused from the surface towards the Ni-coated substrate. Theseelements (Ti and Si) formed gaseous species with the chlorideactivator (SiClx and TiClx, respectively, with x ¼ 1e4) during thehigh temperature process that were easily brought to the substratesurface, where chemical reactions allowed for the co-deposition ofTi and Si [for more information, see also [1,20,21,22]]. Nickel andvanadium diffused from the Ni-coated substrate towards the outerlayers to react mainly with silicon, which has a larger diffusioncoefficient than Ti. This particular method of forming a coating,solid-state diffusion, resulted in excellent adherence between thecoating and substrate.

With the experimental parameters employed (1100 �C, 9 h),almost all of the nickel initially deposited electrolytically at thesurface of Ve4Cre4Ti was included in the silicide compounds afterthe cementation step with the TiSi þ TiSi2 master alloy. The pres-ence of a thin layer of the s0 phase (VeNi) showed that a very smallamount of nickel did not transform during the HAPC diffusiontreatment. This brittle phase can be easily eliminated by thermaltreatment of the coated sample (after HAPC) or by slightlyincreasing the duration of HAPC.

The coating contained the compound Ti4NiSi4 as the outer layer.This compound should control the oxidation behaviour underoperating conditions. Several authors [23,24] mentioned the

Fig. 10. The 15 independent crystallographic sites for the structure of Ti4NiSi4 (12 arein 4c and the 3 others are in general positions).

Table 4Stacking periods in various pseudolamellar parent phases in the TieNieSi system.

Pseudo lamellar phases Ti4Ni4Si7 Ti4Ni1Si4 Ti5Si3 (hexagonal)

Stacking period 4.9343 5.0832 5.171Inter layer composition Ni0.80Si0.20 Ti0.33Ni0.33Si0.33 Ti1.00Average metallic radius

in the interlayer1.2598 1.3289 1.462

Layer composition Ti0.40Si0.60 Ti0.50Si0.50 Ti0.50Si0.50

N. Chaia et al. / Intermetallics 40 (2013) 1e98

existence of a compound named H having the composition Ti4NiSi4.Weitzer [24] indicated that this compound is stable up to 900 �C.Hu et al. [15], as well as Tokunage [13], included this compound in adescription of the isothermal section of the NieTieSi ternary phasediagram at 1100 �C, also mentioning its narrow range ofcomposition.

In the present paper, the crystallographic structure of thiscompound was determined. From the XRD pattern obtained at thesynchrotron SOLEIL, the crystal data (see Table 1) of this compound,which crystallises in the orthorhombic system (S.G. Pnma), wasdeduced. Possible isotypes with Ti4NiSi4 were searched in Pearson’scrystal database using lattice parameter considerations. No resultswere found, thereby confirming that Ti4NiSi4 is a new structuraltype.

The structure of Ti4NiSi4 consists of atomic planes that stackalong the b-axis with planes at coordinates y ¼ 0, ¼, ½ and 3/4(Fig. 8a). Therefore, it can be considered to be a pseudolamellarstructure. The layers at y ¼ ¼ and 3/4 and the interlayer at y ¼ ½have compositions of Ti3Si3 and TiNiSi, respectively. The distancebetween the atomic planes of the related pseudolamellar structuresis reported in Table 4 and can be discussed as a function of the

composition of these interlayers. In the three phases of the TieNieSi system, the stacking period increased with increasing titaniumconcentration in the ‘interlayers’: 4.93 �A for 0% Ti in Ni4Ti4Si7 and5.171�A for 100% Ti in Ti5Si3; the period of 5.0832�A for Ti4NiSi4 wasintermediate with a Ti concentration of 33%. On the other hand, thestacking period appears to be less sensitive to the composition ofthe layers at y ¼ 0 and ½ because for the three cases, the compo-sition of these layers is close or even equal for Ti4NiSi4 and Ti5Si3(Ti0.5Si0.5). These observations consolidate the points of view thatthese phases have a pseudolamellar character and that the struc-ture of Ti4NiSi4 possesses a lot of similarity to that of Ni4Ti4Si7 andTi5Si3.

The very low oxidation rate of the Ti4NiSi4 compound in aircould be finally attributed to these structural characteristics.Indeed, it was observed that the oxidation of Ti4NiSi4 at 650 �C ledto a very low oxidation rate. The oxidation products were identifiedwith difficulty using conventional characterisation devices (SEM-FEG; XRD) after 203 days at this temperature, which would be theupper operating temperature in an SFR reactor.

Such behaviour was not encountered with other silicides con-taining transition metals like TiSi2, VSi2, CrSi2, MoSi2 [2,26,27] inthis temperature range although they all contained more siliconthan Ti4NiSi4. These MSi2 intermetallic compounds developed“thick (>500 nm)” and observable oxide scales in this temperaturerange after a few hundred of hours of exposure. In our opinion, thevery high chemical resistance of Ti4NiSi4 in air at 650 �C and 750 �Cresulted in easy access of Si and Ti to the surface, where theyformed a very thin SiO2 þ TiO2 protective film. Such a film can format these moderate temperatures because the phase transformationof Ti4NiSi4 into Ni4Ti4Si7 requires a low activation energy, which isnot the case with refractory silicides such as TiSi2, VSi2, CrSi2, MoSi2because the phase transformation, e.g., of MoSi2 into Mo5Si3, re-quires a higher activation energy. Indeed, it was previously [27]observed that the phase transformation related to oxidation reac-tion, e.g., of MoSi2 into Mo5Si3, took place at a temperature above800 �C. Below this temperature, thermal agitation was not suffi-cient to activate silicon diffusion in MoSi2 [28] and to transformMoSi2 into Mo5Si3. Consequently, the simultaneous oxidation in airof Mo and Si led to the formation of a ‘thick’ oxide scale that was notas protective at moderate temperatures as at the higher tempera-tures (>1000 �C) at which the transformation can occur.

In the present system, the phase transformation of Ti4NiSi4 inNi4Ti4Si7 was observed after long exposure times (Eq. (2)). Thus,this reaction is possible at moderate temperature (650 �C) becausethere is little energy expenditure to achieve structural rearrange-ment during phase transformation.

4Ti4NiSi4/Ti4Ni4Si7 þ 12Tiþ 9Si (2)

As presented above, both structures can be described by apseudolamellar structure in which the coordinates of the stackedplanes along the b-axis are y ¼ 0, ¼, ½ and 3/4 . During the phasetransformation, the composition of the interlayers (y ¼ ¼ and 3/4 )changed drastically from Ni0.33Ti0.33Si0.33 to Ni0.8Si0.2, therebyindicating that both Ti and Si had left the interlayer; however, the

N. Chaia et al. / Intermetallics 40 (2013) 1e9 9

composition of the layers at y ¼ 0 and ½ did not change to a largeextent (Ti0.5Si0.5 > Ti0.4Si0.6). As a consequence, the activation en-ergy required for the phase transformation is low. Therefore, thediffusion of Ti and Si from the interlayer occurs readily and couldexplain the formation of a thin protective oxide film even atmoderate temperatures.

When considering the oxidation resistance, the low oxidationrates obtained here are considered low enough to have applicationsin future generation IV nuclear reactors. We showed that, even inan oxidative environment (in air), the NieTieSi coating possessedvery high oxidation resistance at 650 �C. This temperature corre-sponds to those envisaged for fast reactors cooled with liquid so-dium (SFR), with PO2

w10�6atm. Another interesting feature of thissystem (vanadium substrate þ Ni–Ti–Si coating) is the high resis-tance in air at a temperature which could represent incidentalconditions. Indeed, the system was able to withstand more than300 cycles between 1100 �C and room temperature in air, i.e., morethan 300 h.

Finally, considering its pseudolamellar characteristics, thisoriginal Ti4NiSi4 compound possesses the potential for being a hoststructure like clay, zeolite or Chevrel phases, for active species (e.g.,H2, cations, CO, NOx). In the future, electrochemical measurementswill be performed to investigate the reversibility of reactionsengaging the atoms present in the interlamellar space.

5. Conclusions

An electrolytic deposition of nickel onto a vanadium substrate,prior to performing a silicidation step using the HAPC process, canbe used to avoid the formation of vanadium dichloride as well asthe formation of cavities at the substrate-coating interface.

- This process led to a multilayer coating with the Ti4NiSi4compound as the outer layer. This compound possessed a verylow oxidation rate in air at 650 �C, which corresponds to amoresevere environment than those envisaged for the targetapplication (fuel cladding for gas and sodium fast reactors).

- The crystal structure of the new Ti4NiSi4 compound wasdetermined from the XRD pattern obtained at the synchrotronSOLEIL with Ti4NiSi4 powder. Its crystal data were deduced(S.G. Pnma; a ¼ 15.63931; b ¼ 5.08321; c ¼ 12.75151) with avery high confidence ratio (Rwp ¼ 0.07; RBragg ¼ 0.067). Thestructure of Ti4NiSi4 consists of atomic planes stacking alongthe b-axis with planes at coordinates y ¼ 0, ¼, ½ and 3/4 .Ti4NiSi4 can therefore be considered to be a pseudolamellarstructure.

- The structure of Ti4NiSi4 possesses strong similarities to that ofNi4Ti4Si7. It is suggested that this peculiar feature led to a loweractivation energy for the phase transformation of Ti4NiSi4 intoNi4Ti4Si7 and that it is responsible for the high oxidationresistance observed at 650 �C in air.

With such characteristics, this system (vanadium substrate plusNieTieSi coating) could potentially be employed in future fuelcladding applications.

Acknowledgements

The authors wish to thank GNR GEDEPEON, PF-MATERIAUX deNEEDS and CEA for their funding and O. Rouer and S. Mathieu of theCommon Service of Microscopy andMicroanalysis of the Universityof Lorraine.

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