ARTICLE IN PRESS

0022-0248/$ - se

doi:10.1016/j.jcr

�Correspondi

E-mail addre

Journal of Crystal Growth 275 (2005) e153–e158

www.elsevier.com/locate/jcrysgro

Directional solidification, microstructure and properties of theAl3Nb–Nb2Al eutectic

C.T. Riosa, S. Milenkovica, P.L. Ferrandinib, R. Carama,�

aUniversidade Estadual de Campinas, FEM/DEMA, C.P. 6122, SP 13083-970, BrazilbCentro Universitario da FEI, C.P. 85, SP 09850-901, Brazil

Available online 9 December 2004

Abstract

The Al–Nb system exhibits a eutectic transformation at 1595:2 �C; which results in Al3Nb (D022) and Nb2Al (D8b)

phases. This paper is concerned with the processing of this eutectic by directional solidification. Alloys were prepared by

arc melting and directionally solidified in Bridgman-type equipment. The resulting samples were utilized to evaluate the

solidification microstructure and morphology regarding the growth conditions. Eutectic microstructures obtained were

regular with lamellar morphology. Variations of the growth rate showed that an increase in this parameter causes a

decrease in the lamellar spacing. With further increase in the growth rate, eutectic cells were observed as a result of

constitutional undercooling. Oxidation tests of eutectic microstructures showed that this alloy suffers severe

microstructure instability, with growth kinetics of the oxide scale of linear type. This indicates that the Al in the Nb–Al

eutectic alloy is insufficient to form protective oxide films, such as, a-Al2O3: Finally, the heat treatment in argon

atmosphere showed that the eutectic alloy presents a high degree of microstructure stability at 1200 �C:r 2004 Elsevier B.V. All rights reserved.

PACS: 81.30.Bx; 81.30.Fb

Keywords: A1. Directional solidification; A1. Eutectics; A1. Phase diagrams; A2. Bridgman technique

1. Introduction

In the past few decades, considerable effort hasbeen focused on developing advanced materialsfor high-temperature structural applications. Mostmonolithic materials that have acceptable high-

e front matter r 2004 Elsevier B.V. All rights reserve

ysgro.2004.10.100

ng author. Tel./fax: +55 19 37883314.

ss: rcaram@fem.unicamp.br (R. Caram).

temperature properties are very brittle at roomtemperature. Therefore, the aim of many investi-gations has been to design polyphase materials,including in situ or natural composites [1], thatexhibit a balance of properties and avoid many ofthe complex processing operations associated withthe fabrication of synthetic composites. The in situprocessing of many intermetallic alloys eliminatesinterface incompatibility between the reinforcing

d.

ARTICLE IN PRESS

C.T. Rios et al. / Journal of Crystal Growth 275 (2005) e153–e158e154

phase and the matrix, as well as allows growthnear the thermodynamic equilibrium. Compositematerials consisting of intermetallic compoundshave properties such as high melting point, lowdensity, high strength at high temperature and, insome cases, inadequate oxidation resistance, butalso poor fracture toughness at low temperature.However, ductility can be improved by alloying orby adding reinforcements, resulting in substantialimprovement in mechanical properties [2].

Solidification behaviour and structural charac-teristics of eutectic alloys in many systemscontinue to attract interest because of theirinfluence on the properties and performance ofmaterials containing eutectic constituents. Theeutectic alloys processed by directional solidifica-tion allow one to obtain highly anisotropicstructures of two or more phases, with regularlyarranged microstructures. In addition, it is apromising technique for production of the so-called in situ composites. The directional solidifi-cation technique allows components to be pro-duced from the melt in a single-stage process, andleads to a composite of phases that are thermo-dynamically stable, chemically compatible, wellaligned and finely dispersed.

In the case of the eutectic alloy combiningAl3Nb and Nb2Al intermetallic compounds, re-search efforts were directed to improvement of itsmechanical behaviour and making it an attractivematerial for high-temperature structural applica-tions [3]. Such studies have demonstrated thatboth phases exhibit high strength at elevatedtemperatures, but also poor fracture toughness atambient temperature. Near 900 �C; the Al3Nbphase [4] and near 1200 �C the Nb2Al phase [5]undergo a brittle-to-ductile transition that allowsmaterial to relieve stress by slip mechanisms. Otherstudies on the Al3Nb intermetallic (with a meltingpoint of 1680 �C; relative low density (4:54 g=cm3),with D022 crystal structure, which might besuitable for use at temperatures up to 1200 �C)and on the Nb2Al compound (with a melting pointof 1940 �C and D8b crystal structure, attractive forapplications requiring maximum operating tem-peratures of 1400 �C [6]) revealed that the lack ofadequate room temperature ductility and fracturetoughness limit their applicability. The intrinsic

brittleness of the intermetallic compounds is due tothe lack of active slip systems. In the case ofAl3Nb; only three slip systems were found and thedensity of dislocations in the Nb2Al was very small[7,8]. In this paper, the microstructure evolution ofdirectionally solidified Al3Nb–Nb2Al eutectic al-loy is characterized. In addition, the oxidationresistance and the microstructure stability at hightemperatures of the eutectic composite were alsostudied.

2. Experimental procedure

The eutectic transformation of interest of thiswork occurs at a temperature of 1595:2 �C andcompositions close to Al-42.2 at% Nb. Thesevalues were experimentally found by using differ-ential thermal analysis and microstructure exam-ination. Commercially pure niobium andaluminium were weighed to meet the eutecticcomposition, and then melted in an arc furnaceequipped with a vacuum system combined with aninjection system of high-purity argon.

After preparing the alloys, they were processedin a Bridgman-type solidification facility. Samples50.0 mm long and approximately 5.0 mm indiameter were inserted in high-purity Al2O3

crucibles and further within a tantalum tube usedas susceptor. Next, the sample together with thealumina crucible and tantalum susceptor werecoaxially positioned inside a 50.0 mm-diameterquartz tube and heated by an induction coilproperly connected to a radio-frequency genera-tor. All the experiments were conducted at atemperature of 1610 � 10 �C; a thermal gradient ofapproximately 100 �C=cm and growth rates of 2.9,2.0, 1.6 and 1.0 cm/h.

The study of the microstructure stability wascarried out on directionally solidified samples at agrowth rate of 2.0 cm/h. Thermo-gravimetricmeasurements of oxidation were conducted inoxygen at 1200 �C; for 24 h, and the samples of3:0 � 4:0 mm were mechanically polished. Theweight change of samples was recorded continu-ously during exposure using a microbalance withan accuracy of 1:0mg: The thermal stability wasstudied in isothermal conditions, at temperatures

ARTICLE IN PRESS

C.T. Rios et al. / Journal of Crystal Growth 275 (2005) e153–e158 e155

close to 1200 �C; in an argon atmosphere. For this,samples were placed inside a quartz tube and thenthe tube was inserted in a resistive furnaceequipped with vacuum and argon injection system.Further, for optical microscopy and image analy-sis, polished samples were etched with a solutioncontaining 1 part of HF, 3 parts of HNO3 and 6parts of lactic acid. The microstructure analysiswas carried out on both the longitudinal andtransverse sections, while the spacing measure-ments on the transverse sections. The microstruc-ture details were evaluated by using opticalmicroscopy, scanning electron microscopy (SEM)and X-ray diffraction (XRD) techniques.

3. Results and discussion

Analysis of the Al–Nb phase diagram [9] allowsone to predict the type of eutectic microstructureformed by the Al3Nb and Nb2Al intermetallic

Fig. 1. Al–Nb eutectic microstructure directionally solidified at grow

phase—Nb2Al and dark phase—Al3Nb:

compounds. According to several studies on thegrowth of eutectic alloys, when the volumefraction of the minor phase is greater than 28%,the eutectic microstructure is of the lamellar type.In the case of the eutectic alloy in the Al–Nbsystem, the volume fraction of the Al3Nb phase islarger than the mentioned value, and hence theeutectic microstructure formed by the Al3Nb andNb2Al phases is expected to present a lamellarmorphology. Experimental results showed that alamellar morphology was obtained and the micro-structure consisted of near 47% (in volume) ofAl3Nb and 53% of Nb2Al phase.

The microstructure of a directionally solidified(DS) binary Al3Nb–Nb2Al eutectic consists of awell-aligned lamellar structure. Fig. 1 presentslongitudinal and cross-section micrography oftypical microstructures obtained at growth ratesof 2.9, 2.0 and 1.0 cm/h, respectively. At growthrates of 1.0 and 2.0 cm/h (Fig. 1a and 1b), eutecticgrowth occurs near equilibrium conditions, which

th rates of (a) 1:0 cm=h; (b) 2:0 cm=h and (c) 2:9 cm=h: White

ARTICLE IN PRESS

C.T. Rios et al. / Journal of Crystal Growth 275 (2005) e153–e158e156

produces a very aligned and fault-free eutecticmicrostructure. With the increase in growth rate to2.9 cm/h, a transition from planar to cellularsolidification front occurred (Fig. 1c). Dependingon variations in the alloy composition, thesolidification rate and the temperature gradientin the melt, the solid/liquid interface can presentinstabilities, which leads to eutectic cell formation,which are associated with the occurrence ofconstitutional undercooling. In all experiments,although the growth rate and direction werestrongly controlled, the presence of eutecticcolonies increased when the growth rate wasincreased. According to Faivre [10], eutecticcolonies formation is related to the initial alloycomposition, which can be off-eutectic, as a resultof constitutional undercooling in front of thesolid/liquid interface provoked by the combinedeffects of impurities rejection from the solidifyingeutectic, the growth rate and the imposed thermalgradient at the solid/liquid interface. It wasobserved that an increase in the growth rate leadsto a decrease in the lamellar spacing, which agreeswith the classical Jackson and Hunt equation(l2V ¼ constant) [11]. The constant in such arelationship was found to be 4:19 � 10�7 cm3=h forthe Al–Nb system.

Fig. 2 shows that the eutectic alloy oxidizesaccording to linear kinetics. The weight change of

0 8000

20

40

60

80

Time (min

Wei

gh

t G

ain

(m

g /c

m2 )

Isothermal: 1200˚C

100

120

140

200 400 600

Nb-A

Fig. 2. Typical oxidation kinetics curve at 1200

the samples is assumed to be due solely to theuptake of oxygen by the sample. The scallops inthe kinetic curves appear to be related to thecracking of oxide scale and subsequent oxidationof the substrate. However, above 900 �C; a veryfast oxidation kinetic is observed. This suggeststhat 900 �C is a critical temperature, in which thephenomenon ‘‘pest’’ or catastrophic oxidation [12]can start. The oxidation morphology of theAl3Nb–Nb2Al eutectic alloy is shown in Fig. 3a.It is observed in three well-differentiated regions: athick external oxide scale, an intermediary oxida-tion zone (IOZ) and the substratum withoutoxidation presence. The formation of the inter-mediary oxidation zone can be facilitated by thevery fast flux of molecular oxygen through thefissures located in the Al3Nb/Nb2Al lamellarinterface and by the crack nucleation near theoxide scale. Fig. 3b shows the magnified interface:external oxide scale/IOZ, in which the externaloxide scale shows greater presence of fissures andcracks, with higher incidence in the Nb2Al whitephase, indicating that the Nb2Al phase suffers anintense degradation and does not present steps ofoxidation such as Al3Nb: This would lead one toassume that the Nb2Al phase oxidizes directly toNbAlO4 and NbO5 and, possibly, is a conductorof oxygen ions (electrical balance of reaction:2Nb2Al þ 7O2 ! 2NbAlO4 þ Nb2O5 þ O�2) [13].

utes)

0

Tem

per

atu

re (

˚C)

200

400

600

800

1000

1200

1000 1200 1400 1600

l: 1200˚C / O2

�C for the Al3Nb–Nb2Al eutectic alloy.

ARTICLE IN PRESS

Fig. 3. (a) Oxidation morphology of a Nb–Al eutectic alloy (directionally solidified) after 24 h at 1200 �C in oxygen. (b) Oxidation

interface external oxide scale/IOZ.

Fig. 4. Isothermal treatment of the Al3Nb–Nb2Al eutectic alloy at 1200 �C and in argon atmosphere: (a) initial sample and after (b) 24,

(c) 48 and (d) 96 h.

C.T. Rios et al. / Journal of Crystal Growth 275 (2005) e153–e158 e157

However, in the IOZ a selective oxidation exists, inwhich the dark phase (Al3Nb) suffers oxidationstep by depletion of Al to form Nb2Al: In this

study, using energy dispersive spectroscopy (EDS)it was not possible to identify the presence ofAl2O3: However, the oxidation of Al3Nb produces

ARTICLE IN PRESS

C.T. Rios et al. / Journal of Crystal Growth 275 (2005) e153–e158e158

a simultaneous growth of the Al2O3 and Nb2O5

phases (Al3Nb þ 72 O2 ! 3

2 Al2O3 þ12 Nb2O5), ob-

served at an oxide mixture (Al2O3j9Nb2O5)analysed by XRD [13].

The microstructure thermal stability was studiedin isothermal conditions near 1200 �C and in argonatmosphere. For this, the samples were tested instages of 24, 48 and 96 h. At each stage, the treatedsamples were cooled in water and characterized.Before to the microstructure characterization thetransversal sections of the samples were slightlypolished to 0:25mm in order to remove oxides atthe surface of the sample. The result of thistreatment is shown in Fig. 4, where it can beobserved that in none of the stages of 24, 48 and96 h morphological changes exist. Also, on sometreated surfaces it is observed that there is increaseor elimination of mismatch surfaces and lamellarfaults that possibly originated by instabilitiesduring the directional growth. However, it waspossible to conclude that there are no coarseningmechanisms in the lamellar eutectic structure ofthe Nb–Al system and that it has high thermalstability.

4. Conclusions

The eutectic alloy Al–42.2Nb yields in situcomposite, where the phases are aligned parallelto the growth direction. The microstructure wasregular with lamellar morphology and this eutecticmay be classified as a regular one. The variation ofgrowth rate showed that an increase in thisparameter causes a decrease in lamellar spacing.With a further increase in growth rate, eutecticcells were observed as a result of constitutional

undercooling. Further, the oxidation testing ofeutectic microstructure showed that the alloysuffers severe microstructure instability, withgrowth kinetics of the oxide scale of linear type.This indicates that the Al in the Nb–Al eutecticalloy is insufficient to form protective oxide films,such as a-Al2O3: Finally, heat treatment in argonshowed that the eutectic alloy presents a highdegree of microstructure stability at 1200 �C:

Acknowledgements

The authors acknowledge FAPESP and CNPqfor financial support.

References

[1] R.M. Aikin, JOM 49 (1997) 35.

[2] G. Frommeyer, et al., Z. Metallkde. 81 (1990) 307.

[3] P. Smith, et al., Scripta Metall. et Materialia 26 (1992)

1365.

[4] F. Ebrahimi, T.D. Hoelzer, Mat. Sci. Eng. A 171 (1993)

35.

[5] D.L. Anton, et al., JOM 41 (1989) 12.

[6] A.K. Misra, in: J.J. Stephens, I. Ahmad (Eds.), High

Temperature Niobium Alloys, TMS, Warrendale, PA,

1991, p. 135.

[7] S. Nourbakhsh, et al., Acta Metall. Mater. 40 (1992) 285.

[8] D. Shechtman, L.A. Jacobson, Metall. Trans. 6A (1975)

1325.

[9] T. Massalski, Binary Alloy Phase Diagrams, second ed.,

ASM, 1990, p. 179.

[10] G. Faivre, J. Crystal Growth 166 (1996) 29.

[11] K.A. Jackson, J.D. Hunt, Trans. Met. Soc. AIME 236

(1966) 1129.

[12] M. Steinhorst, J.H. Grabke, Mat. Sci. Eng. A 120 (1989)

55.

[13] J. Doychak, G.M. Hebsur, Oxidation Metals 36 (1991)

113.