Materials Characterization

Growth and microstructure evolution of the Nb2Al–Al3Nb

eutectic in situ composite

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

aState University of Campinas, P.O. Box 6122, Campinas, SP 13083-970, BrazilbCentro Universitario da FEI, C.P.85, CEP:09850-901, SP- Brasil

Abstract

In situ composite materials obtained by directional growth of eutectic alloys usually show improved properties, that make

them potential candidates for high temperature applications. The eutectic alloy found in the Al–Nb system is composed of the

two intermetallic phases Al3Nb (D022) and Nb2Al (D8b). This paper describes the directional solidification of an Al–Nb eutectic

alloy using a Bridgman type facility at growth rates varying from 1.0 to 2.9 cm/h. Longitudinal and transverse sections of grown

samples were characterized regarding the solidification microstructure by using optical and scanning electron microscopy,

energy dispersive spectroscopy (EDS) and X-ray diffraction. Despite both phases being intermetallic compounds, the eutectic

microstructure obtained was very regular. The results obtained were discussed regarding the effect of the growth rate on the

microstructure, lamellar–rod transition and variation of phase volume fraction.

D 2004 Elsevier Inc. All rights reserved.

Keywords: A. composites; A. intermetallic compounds; B. crystal growth; C. electron microscopy; D. microstructure

1. Introduction

In situ composites obtained by directional solid-

ification of eutectic alloys present interesting and

different properties when compared to their constitu-

ent phases. This material consists of alternating

lamellae of two phases or rods in a matrix. In the

Al–Nb system, the eutectic transformation leads to the

1044-5803/$ - see front matter D 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.matchar.2004.08.017

* Corresponding author. Tel./fax: +55 19 37883314.

E-mail address: rcaram@fem.unicamp.br (R. Caram).

formation of the Nb2Al–Al3Nb in situ composite,

which is a potentially interesting high temperature

structural material due to its high melting point and

elevated thermal stability. The j phase, Nb2Al,

presents an ordered tetragonal structure (D8b), similar

to jCrFe. As a result of the wide composition range

(30 to 40 at.% at 1,600 8C), the lattice parameters

depend on Al content, varying from a=9.9525 2 and

c=5.169 2 for 32 at.%. Al to a=9.898 2 and c=5.187

2 for 42 at.%. Al. Al3Nb also presents an ordered

tetragonal structure (D022) similar to TiAl3. The lattice

parameters are a=3.844 2 and c=8.605 2 [1,2].

54 (2005) 187–193

C.T. Rios et al. / Materials Characterization 54 (2005) 187–193188

In spite that some information about this eutectic

system is available in the literature, including the results

mainly provided by Ludin [3], Kaufman and Nesor [4]

and Jorda [1], there are inconsistencies related to the

eutectic temperature and composition, as well as the

composition range of the intermetallic phases. On the

other hand, a well-accepted phase diagram of the Al–

Nb system was suggested by Massalski et al. [5].

Therefore, the main objective of this paper is to

investigate the microstructure evolution of the direc-

tionally solidified Nb2Al–Al3Nb eutectic alloy.

2. Materials and methods

High-purity niobium and aluminum were weighted

to meet the eutectic (Al–42.2 at.% Nb) and off-

eutectic compositions and then melted in an arc

furnace equipped with a vacuum system combined

with a pure argon injection system.

After preparing the alloys, they were directionally

solidified in a Bridgman type furnace. The samples

50.0 mm long and 6.0 mm in diameter were inserted

in high-purity Al2O3 crucibles and further within a

niobium tube used as susceptor. Next, the sample

together with the alumina crucible and the susceptor

were coaxially positioned inside a 50.0-mm diameter

quartz tube and heated by an induction coil properly

connected to a radiofrequency generator. The experi-

ments were carried out at 1610 8C, measured by an

optical pyrometer, using a thermal gradient of

approximately 100 8C/cm and growth rates varying

from 1.0 to 2.9 cm/h.

The microstructure details were evaluated on both

longitudinal and transverse sections by using conven-

tional procedures of sample preparation, optical and

scanning electron microscopy, energy dispersive

spectroscopy (EDS) and X-ray diffraction technique

using CuKa radiation. Measurements of lamellar

spacing were carried out using the line intercept

method on transverse sections, and the reported values

are a ten reading average.

3. Results and discussion

The eutectic temperature and composition were

determined by thermal analysis and microstructure

examination. After studying a series of compositions,

it was found that a fully eutectic sample corresponds

to a composition close to Al-42.2 at.% Nb. Results of

differential thermal analysis indicated that the eutectic

transformation occurs at the temperature of 1595.2 8C.Such information is in good agreement with data

provided by Jorda [1] and Massalski [5].

Fig. 1 shows a sequence of directionally solidified

eutectic microstructures grown at rates varying from

1.0 to 2.9 cm/h. On the SEM micrographs, the dark

phase is Al3Nb, and the light one is Nb2Al, while

under optical microscopy it is the opposite, since the

darker phase consists of Nb2Al. The use of a 1.0 cm/h

growth rate resulted in large lamellar spacings and a

very regular eutectic microstructure. This fact is

associated with an intense cooperative growth and

efficient mass transport in the liquid ahead the solid/

liquid interface. As the growth rate was increased, the

microstructure became more refined and less regular.

A 2.9 cm/h growth rate produced smaller lamellar

spacings and the microstructure exhibited eutectic

colonies, which may be attributed both to constitu-

tional undercooling and growth out of the coupled

zone. Furthermore, it was confirmed that in the Al–Nb

system, the relationship between lamellar spacing and

growth rate follows the classic Jackson and Hunt

equation, k2V=K [7]. The measurement of the

lamellar spacing on the transverse sections of direc-

tionally solidified samples at different growth rates,

indicated that K is equal to 4.19�10�7 cm3/h, as

shown in Fig. 2. The influence of the growth rate on

the microstructure can be summarized from Fig. 3,

where an abrupt increase in the growth rate led to

changes in lamellar spacing. In this case, the final

growth rate was high enough to cease the regular and

aligned lamellar array. The decrease in the lamellar

spacing is obtained by increasing the amount of

lamellae, which may be dependent on the movements

of lamellar faults. Such an adjustment mechanism is

well described in the literature [8]. According to

Jackson and Hunt [7], the lamellar spacing selection is

related to the growth with a minimum undercooling.

In the case of nonfaceted phases, the spacing adjust-

ments are easily obtained. However, this behavior

should not be expected when the Al3Nb–Nb2Al

eutectic is concerned, since this system consists of

two intermetallic phases that usually present high

melting entropy. Intriguing, this eutectic structure

Fig. 1. Sections of the directionally solidified samples grown at different rates: (a) 1.0 cm/h, longitudinal; (b) 1.0 cm/h, transverse; (c) 1.3 cm/h,

longitudinal; (d) 1.3 cm/h, transverse; (e) 1.6 cm/h, longitudinal; (f) 1.6 cm/h, transverse; (g) 2.0 cm/h, longitudinal; (h) 2.0 cm/h, transverse; (i)

2.9 cm/h, longitudinal and (j) 2,9 cm/h, transverse.

C.T. Rios et al. / Materials Characterization 54 (2005) 187–193 189

Fig. 4. Transverse section of a Nb2Al–Al3Nb directionally solidified

sample exhibiting lamellar faults.

Fig. 2. Lamellar spacing evolution as a function of the growth rate

in the Al–Nb system.

C.T. Rios et al. / Materials Characterization 54 (2005) 187–193190

presented a very efficient and fast response to the

growth rate variation. Fig. 4 shows a transverse

section of a Nb2Al–Al3Nb directionally solidified

sample with lamellar faults.

One more interesting observation is the regularity

of the lamellar structure. Preliminary experiments

using a thermal gradient of 80 8C/cm led to a

microstructure with an elevated amount of defects.

As the thermal gradient was increased to 100 8C/cm,

the microstructure became more regular, exhibiting a

reduced amount of eutectic colonies. Apparently, this

fact comes from the solid/liquid interface stability, as

Fig. 3. Effect of a sudden increase in the growth rate on the

microstructure.

previously studied by Thomas [6] in the growth of an

Al–Nb–Cr eutectic alloy.

In spite of the lamellar structure being the

dominant morphology, some samples presented lamel-

lar–rod transition. Such a phenomenon is not entirely

understood, and several investigations relate it both to

the growth direction variation and to the solid/liquid

interface instability. According to Hunt [8], a eutectic

growth in which there is a low volume fraction of one

phase will produce a rod structure. On the other hand,

eutectics that present near the same volume fraction of

both phases will develop a lamellar structure. Hunt

and Chilton [9] and Cline et al. [10] suggested that the

main factor in such a transition is associated with the

growth direction of the solid phases. The lamellar–rod

transition occurs when a lamella is forced to grow in a

direction other than the preferred one. This fact can be

explained by the extremum condition principle, in

which it is assumed that the eutectic growth occurs at

the minimum undercooling. If the minimum under-

cooling for the lamellae is higher than that required

for the rods, the lamellae will not grow, and a

lamellar–rod transition will take place. Fig. 5 presents

a lamellar–rod transition in a directionally solidified

sample. While the lamellar structure grows parallel to

the heat flux, the rods tend to grow 208 off the axis.

In a eutectic microstructure, the volume fraction of

both phases can be evaluated as a first approach from

the phase diagram. However, depending on the growth

rate and the nature of each phase, the fractions may be

altered. Application of optical microscopy combined

with image analysis measurements revealed that the

volume fraction of the Al3Nb slightly increased as the

Table 1

Nominal and measured values of chemical composition for the

eutectic structure and its phases

Phase Nominal Measured

Al (at.%) Nb (at.%) Al (at.%) Nb (at.%

Al3Nb 75.0 25.0 75.5 24.5

Nb2Al 33.3 66.7 43.9 56.1

Eutectic Structure 57.8 42.2 60.2 39.8

Fig. 5. Longitudinal section of a Nb2Al–Al3Nb eutectic sample

showing a lamellar–rod transition.

C.T. Rios et al. / Materials Characterization 54 (2005) 187–193 191

growth rate was increased, as shown in Fig. 6. The

coupled eutectic growth is a function of the solute

diffusion in the liquid in front of the solid/liquid

interface, which decreases when the solid/liquid

undercooling is increased. As the growth rate is

increased, the undercooling is increased too, which

may limit the eutectic growth. Depending on the

melting entropy of each phase, an increase in the

growth rate restricts the growth of one phase, leading

to an increase in the volume fraction of the other phase.

The examination of the eutectic microstructure by

means of EDS allows one to determine the compo-

sition of each phase, while the X-ray emission

technique reveals the distribution of Nb and Al in

both phases. Table 1 presents the compositions of the

Fig. 6. Volume fraction of the Al3Nb phase as a function of the

growth rate. Fig. 7. X-ray image of Nb and Al distribution in a eutectic sample

)

eutectic structure and its constituent phases, where

one can observe the difference between the nominal

and measured values, especially for the Nb rich Nb2Al

.

Fig. 8. X-ray diffraction of a transverse section of the directionally solidified sample.

C.T. Rios et al. / Materials Characterization 54 (2005) 187–193192

phase. Several reasons may cause the discrepancy,

like the considerable difference of Al and Nb atomic

numbers, uncertainty about the acceleration voltage,

beam deviation and small lamellar width. Fig. 7

presents a longitudinal microstructure and the Al and

Nb distributions in a directionally solidified sample.

As expected, Nb2Al presents much more Nb, while

Al3Nb is Al rich.

It is well known that the phases in a eutectic array

show some preferred growth direction, and this may

be attributed to the selection of a low-energy interface

between the phases during the growth [11]. Fig. 8

depicts X-ray diffraction traces from transverse

sections of a sample. Accordingly, the preferred

growth directions are (210) and (420) for Nb2Al and

(004) for Al3Nb.

4. Conclusions

The Al3Nb–Nb2Al eutectic system was success-

fully grown by directional solidification. The analysis

of the results shows that a very regular lamellar

structure was obtained, as well as a relationship

between lamellar spacing, k, and growth rate, V. It

follows the Jackson and Hunt equation, k2V=K, with

K=4.19�10�7 cm3/h. Despite the dominant morphol-

ogy being lamellar, some samples presented the

lamellar–rod transition, which is probably due to

changes in the lamellae growth direction. Examination

of the solid phases volume fraction indicates that an

increase in the growth rate leads to an increase in the

Al3Nb fraction, which is possibly attributed to an

undercooling increase during the eutectic growth.

Finally, X-ray diffraction analysis revealed that Nb2Al

grows preferably along the (210) and (420) directions,

while Al3Nb grows along the (004) direction.

Acknowledgements

The authors acknowledge FAPESP and CNPq for

financial support.

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