The peculiarities of the structure formation in directionally crystallized eutectics EuB6-MeB2

Article abstract of DOI:10.1016/S0925-8388(00)01440-7

The possibility of producing fiber-strengthened eutectic composites in situ in the quasibinary EuB₆-MeB₂ (Me-Zr, Hf, Sc) alloys is shown. By directional crystallization of the eutectic composition perfect real structures on the base

Full text of DOI:10.1016/S0925-8388(00)01440-7

Journal of Alloys and Compounds 317–318 (2001) 367–371
L
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The peculiarities of the structure formation in directionally crystallized
eutectics EuB –MeB
6
2
*
Yu. Paderno , V. Paderno, N. Shitsevalova, V. Filippov
I. Frantsevich Institute for Problems of Materials Science NASU, 3, Krzhyzhanovsky Str., 03142 Kiev, Ukraine
Abstract
The possibility of producing fiber-strengthened eutectic composites in situ in the quasibinary EuB –MeB (Me–Zr, Hf, Sc) alloys is
6
2
shown. By directional crystallization of the eutectic composition perfect real structures on the base of uniformly distributed MeB fibers
2
in the EuB matrix may be formed.
 2001 Elsevier Science B .V . All rights reserved.
6
Keywords: Boride eutectics; Direction crystallization; Europium hexaboride
1
. Introduction
Europium hexaboride has attracted the attention of many
scientists due to its interesting properties. Its nuclear
properties present a special interest due to the very high
neutron absorption of both boron and europium atoms for
the thermal and especially for the fast neutrons [1–3].
Unlike other isomorphous rare earth hexaborides
europium hexaboride dissolves a small amount of carbon
[4] which reduces the possibility of the formation of
unstable europium carboborides in moisture media which
in turn makes this compound more stable under operational
conditions. Furthermore it was shown that the EuB6
neutron absorbing characteristics exceed the similar prop-
erties of other boron and europium-containing materials
(
B C, Eu O ) [3].
4 2 3
The compact pieces of europium hexaboride are usually
produced by sintering, hot pressing or melting of source
powders. The sintering process results in a high porosity of
pellets, while the hot pressing due to strong contamination
of EuB6 with carbon and formation of europium car-
boborides results in the destruction of pellets on exposure
to a moisture media. The melting processes (a crucibleless
induction heating or the arc melting in a cooled cup) make
it possible to obtain the EuB pieces both in polycrystal
6
and single crystal forms without the essential contamina-
tion with carbon. However, the decomposition of europium
hexaboride owing to the europium high vapor pressure at
its melting temperature ($25808C) may take place [5].
*
Corresponding author.
Fig. 1. Structure of the EuB –ZrB alloy (32 mol% ZrB ) after arc
6 2 2
E-mail address: paderno@ipms.kiev.ua (Yu. Paderno).
melting (a) and after drop quenching (b) (etched).
0
925-8388/01/$ – see front matter
 2001 Elsevier Science B .V . All rights reserved.
PII: S0925-8388(00)01440-7

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Yu. Paderno et al. / Journal of Alloys and Compounds 317–318 (2001) 367–371
Apart from this, a very important drawback to the use of
The reducing of europium oxide with boron produced
europium hexaboride items, especially as possible func-
tional elements of different devices, is their high brittleness
the source EuB powder. The arc melting of the scandium
6
and boron mixtures produced ScB . The ZrB and HfB
2
2
2
[
6]. Attempts to strengthen them by using powder metal-
powders were furnished from Chemreaktiv (Donetsk,
Ukraine).
lurgy technology methods, including different composites
based on EuB have been unsuccessful [7,8].
The formation of a fiber eutectic structure in the above
mentioned boride mixtures was checked using samples
prepared by arc melting in an argon atmosphere, with
quenched drops obtained by throwing the melt onto a
cooled copper cup and with the directionally crystallized
rods.
6
This work presents the results of the investigations of
the possibility of creating fiber-strengthened europium
hexaboride composites, as was done for isomorphous
lanthanum hexaboride [9–13].
The directional crystallization was achieved by means of
an induction zone melting process using Crystal-111
equipment. With the high evaporation rate of europium in
mind, the argon pressure in the chamber was set up at 2.5
MPa.
In previous works devoted to the lanthanum hexaboride-
based eutectics it was shown that their strength properties
correlate with the real structure; the more perfect the real
structure is, the more strength, fracture toughness and
thermal shock resistance of the materials are achieved [11].
Thus in this work, the main emphasis was on the research
of the real structure of composites.
2
. Materials and methods
By analogy with the systems based on lanthanum
hexaboride the possibility of eutectic formation in the
EuB –MeB systems was supposed and ZrB , HfB and
ScB were selected as d-transition metal diborides.
Zirconium diboride was chosen, as it is known that this
diboride forms perfect single crystal whiskers in the
eutectic LaB –ZrB mixture. We also used mixtures with
hafnium diboride in order to find possible additions, which
should increase the neutron absorption ability and with
scandium diboride, which may reduce the density of a
composite material.
6
2
2
2
2
6
2
The structure of the samples was investigated using
scanning electron microscopes Stereoscan 54-10 and
Fig. 2. Structure of the cross (a–d) and longitudinal (f) sections of the EuB –ZrB alloy samples containing 32 mol% ZrB (a, b) and 37 mol% ZrB (c–f)
6
2
2
2
(etched). (e) X-ray map of the microstructure, presented in (d) in X-ray ZrLa radiation.

Yu. Paderno et al. / Journal of Alloys and Compounds 317–318 (2001) 367–371
369
Camebax SX-50. The phase composition and the single
crystal perfection were studied using X-ray diffractometer
HZG-4A.
3
. Results and discussion
For the investigation of alloys in the EuB –ZrB
6
2
system, compositions in the limits of 32–42 mol% of ZrB2
were chosen. The lower limit of the ZrB2 content was
taken as similar to the eutectic composition in the LaB6 –
ZrB system [9]. The samples were produced by all the
2
above mentioned methods, i.e. arc melting, quenched drops
and directional crystallization.
The arc melting method showed the existence of eutectic
blended plate-like and fibrous structures (Fig. 1a). The
quenched drop method confirmed the formation of a
eutectic whisker structure (Fig. 1b), similar to that ob-
served in the LaB –ZrB system [10].
6
2
The directional crystallization of alloys having 32 mol%
of ZrB2 results in the formation of a regular fibrous
(whiskers) eutectic structure (Fig. 2a). Some excess of the
matrix EuB6 phase distributed in interlayers between
eutectic columns has been observed (Fig. 2b).
The increasing of the content of the ZrB phase up to 37
2
mol% causes a considerable lowering of the amount and
size of such interlayers (Fig. 2d and f). The amount and
size of such interlayers in this case is negligible, which
testifies to their proximity to the eutectic composition. The
composite eutectic structure having a single crystal matrix
of EuB6 and, uniformly distributed in it, the practically
equithickness single crystal whiskers of the ZrB2 phase
with a diameter near 1.0 mm and length up to 900 mm is
formed (Fig. 2c and f). According to the X-ray map of the
microstructure of this sample in X-ray ZrLa radiation (Fig.
2
e) the surplus phase that is separated on boundaries of
eutectic columns is the europium-based phase.
A further addition of the diboride phase up to 42 mol%
proves to be in excess. The X-ray map analyses of the
non-etched surface of such samples in X-ray ZrLa and
EuLa radiation confirmed this conclusion (Fig. 3a–c).
The results which we obtained, permitted us to conclude
that the eutectic composition for the EuB –ZrB system
6
2
exists in the range of 37–40 mol% ZrB . The determi-
2
Fig. 3. Microstructure (a) and X-ray maps of the not etched surface of the
nation of the eutectic composition in the systems on the
base of the europium boride is complicated owing to the
uncontrolled shift of the melted zone composition, in
comparison with the source mixture due to the volatility of
europium.
EuB –ZrB alloy with 42 mol% ZrB2 in ZrLa (b) and in EuLa (c)
6
2
radiation.
for the individual EuB . Moreover, for compositions that
6
Earlier it was shown that the quasibinary LaB –TiB
are close to the eutectic point the melting process is the
most stable.
6
2
[
14], LaB –ZrB [15], LaB –HfB [16] and LaB –CrB
6
2
6
2
6
2
[
1
17] systems have a eutectic temperature lower in limits
00–2008 compared with that for individual LaB . Similar-
The fracture surface of such materials presents a wood-
like character (Fig. 4a). We can also observe some areas
where the whiskers are drawn off from the matrix channel
and ruptured near the crack surface (Fig. 4b). Such a
branching fracture surface, characterized by cleaving and
6
ly the lower melting temperature of the eutectic mixtures
on the EuB base causes a reduction of the evaporation
rate of the europium from the melt in comparison with that
6

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Yu. Paderno et al. / Journal of Alloys and Compounds 317–318 (2001) 367–371
Fig. 6. Structure of the cross section of EuB –ScB eutectic alloy
6
2
(etched). (Contrary to the ZrB2 and HfB2 systems the ScB2 phase due to
much less chemical stability is pitted from the EuB6 matrix).
whiskers structure (Fig. 5), similar to those obtained in the
EuB –ZrB system.
6
2
EPMA analyses have not shown remarkable (or any)
mutual solubility of both borides. Similar results were
obtained earlier for the studied LaB –MeB (Me: Ti, Zr,
6
2
Hf) systems [14–16] and for the EuB – Hf system [19].
6
The alloys of the EuB –ScB system were only pro-
6
2
duced by the arc melting method, as in this case it was
possible to employ small amounts of scandium. For this
reason the results obtained only have a preliminary charac-
ter.
Fig. 4. Fracture surface of the eutectic EuB –ZrB alloy: (a) the general
view, (b) the regions with drawn-off whiskers.
6
2
Compositions containing 22, 33 and 45 mol% of ScB2
were studied and in all cases structures presenting eutectic
columns and an excess of the matrix EuB6 phase were
formed. The eutectic areas showed a mixed plate and fiber
character. A minor quantity of excess matrix phase and a
more regular fiber eutectic structure (Fig. 6) was obtained
in the composite having 45 mol% of ScB2.
shearing off, favor the rising of the mechanical properties
of the material [18].
The alloys in the EuB –HfB system were produced
6
2
using the arc melting and the directional crystallization
methods. Compositions having 16, 21 and 32 mol% of
HfB were studied and similarly to the EuB –ZrB system
2
6
2
three kinds of structures, hipoeutectic, eutectic and
hipereutectic were obtained.
The composition closest to the eutectic point was
4
. Conclusion
obtained at 21 mol% of HfB . In this case the directional
crystallization also resulted in the formation of regular
2
It is shown that EuB , similarly to LaB , in some
6
6
compositions with typical d-metals borides can form a
fiber-strengthened structure, consisting of the diboride
single crystal whiskers distributed in a single crystal matrix
of hexaboride.
The regular structures may be obtained in situ by
directional crystallization of the eutectic compositions of
the studied systems. Such a structure should ensure
mechanical properties higher those obtained for the in-
dividual EuB6 pieces, as was shown for LaB –MeB
6
2
directionally crystallized eutectic systems [9].
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2
(etched).
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