Chemical and thermodynamic properties of several Al–Ni–Rsystems

G. Borzone*, R. Raggio, S. Delsante, R. Ferro

Dipartimento di Chimica e Chimica Industriale, Universita di Genova, I-16146 Genoa, Italy

Abstract

The standard enthalpies of formation at 300 K of the RNiAl phases (R=rare earth) have been obtained by using a high tem-perature direct reaction drop calorimeter and an aneroid isoperibol calorimeter. State and composition of the samples were checkedby X-ray diffraction analysis. Metallographic examination was performed and the phases were further identified by electron

microscopy and electron probe microanalysis. The results obtained are discussed and compared with those available for the binaryRNi2 and RAl2 compounds.# 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Nickel aluminides, based on NiAl; A. rare-earth intermetallics; B. Thermodynamic and thermochemical properties; F. Calorimetry

1. Introduction

The rare earth–nickel–aluminium alloys and com-pounds have received much attention due to a variety ofspecial properties they possess.We may mention that R-Ni (R=rare earth) alloys of

general composition AB5 have been found to be primecandidates for a variety of energy applications becauseof their good hydrogen absorption/desorption kineticsand large hydrogen storage capacity. Substitution eitherof La or Ni with other elements such as an Al compo-nent was found to modify their desorption pressure [1].The remarkable mechanical properties useful for

industrial applications have been highlighted for the Al-based amorphous alloys combined with a late transitionmetal (M) and a rare earth (R) element. Addition of Alor Ga to R-M alloys was found to be very effective inincreasing the glass-forming ability and expanding theglass formation range. Bulk amorphous alloys withdiameters of several millimetres have been obtained inR-Al-M (M=Co,Ni,Cu) by the copper-casting method.Ductile (R-rich) or brittle (Al-rich) mechanical beha-

viour, depending on the alloy composition, have beenreported [2].With regard to the ability of these elements to form

compounds, we may observe that, generally, isothermalsections in the solid state with a rare earth content up to30–40 at.% have been studied and data on these systemsassessed [3]. As an example, Fig. 1 shows the isothermalsection of the Pr–Ni–Al system at 800 �C as assessed in[3]. Notice the complexity of these alloys, the highnumber of ternary intermediate phases present and theirdifferent stoichiometries.We may mention that the crystallochemistry of the R–

Ni–Al phases is still incomplete because not all thestructures have been determined and some uncertaintiesfor several phases still remain.The objective of modern materials science is to tailor

a material, starting with its chemical composition,constituent phases and microstructure, in order toobtain a desired set of properties suitable for a givenapplication.In the investigation of complex alloy systems, sub-

stantial progress could be made by an integratedapproach where key experiments and thermodynamicmodelling are combined in a recursive procedure. As aresult, a more detailed and consistent description of theinvestigated systems should be obtained.The knowledge of their thermodynamic properties

may therefore be useful in gaining information on stableand metastable phases.

0966-9795/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0966-9795(03)00161-4

Intermetallics 11 (2003) 1217–1222

www.elsevier.com/locate/intermet

* Corresponding author. Tel.: +39-010-353-6153; fax: +39-010-

362-5051.

E-mail address: borzone@chimica.unige.it (G. Borzone).

Selected Al–Ni–R ternary systems are now beingconsidered in our laboratory in order to study differentthermodynamic properties.Experimental investigations have been performed by

using different analytical and preparative techniques: X-ray diffraction, metallographic, EPMA analyses anddirect calorimetry. Besides calorimetric measurementson selected R–Ni–Al alloy compostions, several R–Ni–Al systems have also been investigated mainly by usingdifferent metallographic techniques, in order to definecomplete or partial isothermal sections. Refinement ofthe ternary tie-triangles in the Al-rich region have beenstudied and are still in progress [4, 5].In this paper the results of the enthalpy of formation

recently obtained for the RNiAl phases are presented.

2. Literature data on RNiAl phases and R–Ni–Al

formation functions

The equiatomic stoichiometry is typical for the seriesof R–T–Al (T=transition metal) alloys with the differ-ent rare earths.The RNiAl phases crystallize in the hexagonal hP9-

ZrNiAl type [6, 7], with the exception of the LaNiAl, forwhich a oP24-LaNiAl type structure was reported [8].Much information is available for the standard

enthalpies of formation: Colinet and co-workers by Alsolution calorimetry measured for LaAlNi4 a value of�40.2 or �40.9 kJ/mol at [9]. With the same techniqueTimofeev et al. determined for the YNiAl4 phase a value

of �fH=�60.2�2 kJ/mol at [10] at 298 K. The enthal-pies of the ternary Y–Ni–Al alloys have recently beendetermined by high temperature reaction calorimetry[11]. For several Ce–Ni–Al ternary composition mea-surements on the enthalpies of formation have beenperformed by our group [12].

3. Experimental

The thermochemical investigation was carried outby using two different kinds of calorimeters built inour laboratory: an aneroid isoperibol calorimeter[13,14] and a high temperature direct reaction calori-meter [15].In the first one, a few calorimetric samples were syn-

thesized in the calorimeter maintained in a water ultra-thermostat at 27�0.01 �C. The formation heats wereconsidered measured at 300 K because the sample in thecalorimeter cooled down to this temperature during themeasurement. The sample (about 10–15 g) is a mixtureof fine metal powders enclosed in a gas-tight inox cru-cible which is then inserted in the calorimeter and, afterthermal equilibration, heated until the reaction starts.Two small electric furnaces are used for starting thereaction in the sample and for electric calibrationrespectively. The temperature of the calorimeter isfollowed by a multiple-junction thermopile (differen-tially connected to the similar thermopile of anothercalorimeter which is identical to the first and used as areference).

Fig. 1. Pr-Ni-Al system. Partial isothermal section at 800 �C.

1218 G. Borzone et al. / Intermetallics 11 (2003) 1217–1222

For the measurements carried out using the hightemperature direct reaction calorimeter, the three com-ponents were carefully mixed in the appropriate molarratio, pressed into pellets and sealed under argon. Acompact of about 0.7 g sealed in tantalum crucible wasdropped from a room temperature thermostat into thehigh temperature direct reaction calorimeter. Theworking calorimeter temperature was set at a valueincluded between 700 and 820 �C, according to the rareearth component of the alloy. Each sample was droppedtwice: in the first run the alloy synthesis takes place, andin the second run the synthesized alloy is used for areference measurement. The enthalpy of formation atroom temperature of the alloy is obtained as the differ-

ence of the thermal effects observed in the reaction andreference runs.In each run (reaction and reference runs) the calori-

meter is calibrated with a procedure consisting of drop-ping four weighed silver spheres, two before and twoafter the sample.EPMA and XRD analyses were used to characterize

all the samples to make sure that the reaction has beencompleted. The X-ray analysis of the various alloys wascarried out by the Debye method and by powder dif-fractometry using Cu and Fe Ka radiation and pure Sias an internal standard. Powder photographs were usedboth for phase analysis and identification and for latticeparameter measurements.The microstructure was investigated by optical and

scanning electron microscopy (SEM); micrographswere taken with an OXFORD INCA Energy 300which allowed a chemical analysis by energy-dispersive-spectrometry (EDS).From the microprobe analysis we obtained the indi-

cation that there is no reaction of the starting materialswith crucibles.

4. Results and discussion

The crystal structure data of the RNiAl ternary pha-ses have been confirmed. The observed diffractionintensities were compared with the values calculated by

Table 1

Standard enthalpies of formation of solid RNiAl alloys at 300 K

Phase

�fH� (kJ /.mol at) References

YNiAl

�59.0�2 This work

LaNiAl

�49.0�2 This work

CeNiAl

�54.0�2 [11]

PrNiAl

�56.0�3 This work

NdNiAl

�55.0�2 This work

SmNiAl

�54.0�2 This work

GdNiAl

�57.5�2 This work

DyNiAl

�62.0�2 This work

HoNiAl

�61.0�2 This work

ErNiAl

�61.0�2 This work

YbNiAl

�45.0�2 This work

Fig. 2. RNiAl solid alloys. Standard enthalpies of formation at 300 K vs the atomic number of the rare earths.

Fig. 3. RNiAl phases. Average atomic volume vs the atomic number of the rare earth metals.

G. Borzone et al. / Intermetallics 11 (2003) 1217–1222 1219

1220

Fig. 4. R–Ni–Al alloys (R=Sm, Gd, Yb). The trends of the �fH (kJ/mol at.) of solid alloys at 300 K are shown along the xAl/xNi = 33.3 at.%R isopleth sections.

G.Borzoneetal./Interm

etallics

11(2003)1217–1222

1 and

means of the Lazy-Pulverix program [16] and a goodagreement with the literature data was generallyobtained. As for the LaNiAl phase, the calculatedvalues were obtained by using the same atom positionsreported for the isostructural CeRhAl phase [17]. Thefollowing lattice parameters were obtained: a=719.2,b=419.8, c=1608.8 pm, which are close to thosereported by [8] (a=719.9, b=420.3, c=1608.5 pm).The enthalpies of formation of the intermetallic

RNiAl phases referred to the solid components at 300 Kare listed in Table 1.Micrographic examination showed that the samples

were mainly single phase. In several cases a small quan-tity of another phase was observed and the heat evolvedwas considered only as a limiting value of the �H offormation. The corresponding heat values were cor-rected accordingly.For the TbNiAl phase, however, it was not possible to

obtain reliable data, because the different samples syn-thesized in the high temperature calorimeter, even atdifferent temperatures, resulted in a combination ofequal ratio of RNiAl and RNi2Al3 phases.As for the YNiAl it may be noticed a fair agreement

between the enthalpy of formation value �54.1�0.9 kJ/mol at. reported by [11] and the value �59.0�2.0 kJ/mol at. now obtained.Fig. 2 provides the trend of the standard enthalpies of

formation for the RNiAl alloys determined in this work,as a function of the atomic number of the rare earths.We may notice the systematic higher exothermicity ofthese phases on passing from the light to the heavy rareearths.The peculiar behaviour of the YbNiAl alloy may be

noteworthy. As observed for the binary YbNi2 [18] andYbAl2 [19,20] alloys, the lower absolute value of theenthalpy of formation may be related to a change in thevalence state of Yb, from its divalent state in the puremetal, to a trivalent value in the alloy. The presence ofthe quasi-trivalent Yb valence state in the YbNiAlphase may also be confirmed by its average atomicvalue (see Fig. 3). For a discussion on the role thatthermochemical measurements may play in the defini-tion of the valence state of Eu and Yb, see Gschneidner[21,22], Miedema [23], Johansson and Martensson [24]and Borzone et al. [18,25,26].A comparison between the RNiAl enthalpies of for-

mation and those available for a number of RNi2(R=Sm [27], Gd [28], Yb [18]) and RAl2 (R=Sm [29],Gd [19], Yb [20]) binary phases is shown in Fig. 4. Wenotice a moderate increase in formation exothermicitycompared with the linear trend which can be obtainedfor the mechanical mixture of the binary phases.In the same figure, considering the isopleth R–AlNi

section, we observe the increasing exothermicity ongoing from the pure rare earth towards the AlNi binaryphase (the value of �67.0 kJ/mol at. for the enthalpy of

formation proposed in different assessments [30–32] hasbeen accepted) with a discontinuity in the trend corre-sponding to the RNiAl phase.

5. Conclusions

The standard enthalpies of formation for the RNiAlalloys have been determined by calorimetry and a par-ticularly high relative stability of these phases can bepresumed with the increase in the rare earth atomicnumber.During this investigation, a higher exothermicity was

also observed for the RNi2Al3 phases and work is inprogress on their thermodynamic characterization.

Acknowledgements

The Italian Ministero dell’Istruzione dell’Universita edella Ricerca is acknowledged with thanks for thefinancial support given in the framework of a Nationalresearch project entitled ‘‘Leghe e composti inter-metallici: stabilita termodinamica, proprieta fisiche ereattivita’’.

References

[1] Buschow KHJ. In: Gschneider Jr KA, Eyring L, editors. Hand-

book on the physics and chemistry of rare earths, vol. 6.

Amsterdam (The Netherlands): Elsevier; 1984. p. 83.

[2] Inoue A. In: Gschneider Jr KA, Eyring L, editors. Handbook on

the physics and chemistry of rare earths, vol. 24. Amsterdam

(The Netherlands): Elsevier; 1997. p. 83.

[3] Ferro R, Zanicchi G, Marazza R. In: Petzow G, Effenberg G,

editors. Ternary alloys. A comprehensive compendium of eval-

uated constitutional data and phase diagrams, vol. 8. Weinheim

(Germany): VCH; 1993. p. 58.

[4] Raggio R, Borzone G, Ferro R. Intermetallics 2000;8:247.

[5] Raggio R, Borzone G, Ferro R. Journal of Alloys and Com-

pounds in press.

[6] Dwight AE, Mueller MH, Conner RA, Downey JW, Knott H.

Transactions of the Metallurgical Society of Aime 1968;242:2075.

[7] Rossi D, Mazzone D, Marazza R, Ferro RZ. Anorg Allg Chem

1983;507:235.

[8] Cordier G, Dorsam G, Kniep R. Journal of Magnetism and

Magnetic Materials 1988;76–77:653.

[9] Pasturel A, Chatillon-Colinet C, Percheron-Guegan A. Journal

of the Less-Common Metal 1982;84:73.

[10] Timofeev VS, Turchanin AA, Zubkov AA, Tomilin IA. Ther-

mochimica Acta 1997;299:37.

[11] Nash P, Su HN, Kleppa O. Trans Nonferrous Met Soc China

2002;12:754.

[12] Borzone G, Parodi N, Ferro R. In: Chang YA, Sommer F,

editors. Thermodynamics of alloy formation. Orlando, (FL):

TMS; 1997. p. 59.

[13] Capelli R, Ferro R, Borsese A. Thermochimica Acta 1974;10:13.

[14] Ferro R, Borzone G, Parodi N, Cacciamani G. Journal of Phase

Equilibria 1994;15:317.

[15] Cacciamani G, Borzone G, Ferro R. Journal of Alloys and

Compounds 1995;220:106.

G. Borzone et al. / Intermetallics 11 (2003) 1217–1222 1221

[16] Yvon K, Jeitschko W. Parthe E. Journal of Applied Crystal-

lography 1977;10:73.

[17] Schwer H, Hulliger F. Journal of Alloys and Compounds 1997;

259:249.

[18] Brutti S, Ciccioli A, Balducci G, Gigli G, Borzone G, Raggio R,

Ferro R. Journal of Phase Equilibria 2002;23:51.

[19] Colinet C, Paturel A, Buschow KHJ. Journal of Chemical Ther-

modynamics 1985;17:1133.

[20] Borzone G, Cardinale AM, Parodi N, Cacciamani G. Journal of

Alloys and Compounds 1997;247:141.

[21] Gschneidner Jr KA. Journal of the Less-Common Metals 1969;

17:13.

[22] Gschneidner Jr KA. Metals Materials and Processes 1990;1:241.

[23] Niessen AK, de Boer FR, Boom R, de Chatel PF, Mattens

WCM, Miedema AR. CALPHAD 1983;7:51.

[24] Johansson B. Martensson. In: Gschneidner Jr KA, Eyring L,

Hufner S, editors. Handbook on the physics and chemistry of

rare earths, vol. 10. Amsterdam (The Netherlands): Elsevier;

1987. p. 241.

[25] Sommer F, Predel B, Borzone G, Parodi N, Ferro R. Inter-

metallics 1994;3:1.

[26] Borzone G, Ferro R, Parodi N, Saccone A. Gazzetta Chimica

Italiana 1995;125:263.

[27] Borzone G, Parodi N, Raggio R, Ferro R. Journal of Alloys and

Compounds 2001;317–318:532.

[28] Colinet C, Pasturel A, Buschow KHJ. Journal of Applied Physics

1987;62(9):3712.

[29] Borzone G, Cardinale AM, Saccone A, Ferro R. Journal of

Alloys and Compounds 1995;220:122.

[30] Du Y, Clavaguera N. Journal of Alloys and Compounds 1996;

237:20.

[31] Ansara I, Dupin N, Lukas HL, Sundman B. Journal of Alloys

and Compounds 1997;247:20.

[32] Huang W, Chang YA. Intermetallics 1998;6:487.

1222 G. Borzone et al. / Intermetallics 11 (2003) 1217–1222