I.A. Santos et al. / Journal of Alloys and Compounds 325 (2001) 194–200
195
Thus,
a
metastable eutectic reaction L1R→(R1
Samples of typically 50 mm in length and 8 mm in
diameter were obtained employing this procedure. The DS
technique makes microstructures that change along sam-
ples length according to the cooling sequence expected
from the phase diagrams (no radial changes were ob-
served). Thus, the directionally solidified samples were
divided in two parts: the inferior part (the first that
solidifies) and the upper part (the last one to be solidified).
This division is valid for all the characterization techniques
employed in this work. Our calorimetric experiments were
made in a Calvet type calorimeter; model HT1000D, from
Setaram, run in the scanning mode. The scanning rate used
for most of our experiments was 0.058C/min in the
temperature range of interest. All samples directionally
solidified and run in the calorimeter were characterized by
metallography. Some of these samples were analyzed in an
electron microprobe in order to determine the chemical
composition of the formed phases. In order to determine
the Curie temperature of the phases contained in the
samples, we did AC susceptibility measurements in a
homemade Thermomagnetic Analyzer (TMA) as previous-
ly described [15].
P1)eutectic1(R), where R5Pr or Nd, occurs simultaneously
with a second eutectic L1R→(R1Fe R )eutectic1(R).
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7
2
The first eutectic would give origin to the feathered
morphology and the second to the globular one. It was
clear that the study of the phase relations in this region of
the phase diagrams would require experimental techniques
able to cope with phenomena occurring in a small tempera-
ture interval and able to produce morphologies coarser
than the ones obtained in the usual melting procedures.
The objective of this work is to report results obtained in
directional solidification (DS) of the alloys with com-
positions near to the Fe–Nd and Fe–Pr eutectic com-
positions. The samples were obtained and studied using
DS and very slow Calvet-type calorimetric measurements.
The microstructure characteristics of the phases observed
in the eutectic region, as well as some of their magnetic
properties, were determined through metallography and
thermomagnetic analysis.
2
. Experimental procedure
The samples were obtained by arc-melting appropriate
proportions of 99.99% pure iron, 99.9% pure
praseodymium and 99.9% pure neodymium (Aldrich). The
metals, as well as the synthesized alloys, are always kept
stored under controlled argon atmosphere to avoid any
oxidation. The arc-melting procedure consists in fusing the
precursor metals directly into the arc-furnace. During this
process, the samples have their position reversed many
times in the crucible (between successive fusion) to
guarantee that the metals components had been mixed
homogeneously. For the DS experiments, samples were
placed in cylindrical CaF2 crucibles obtained by the
methods previously reported in the literature [14]. The
choice of these crucibles is justified because there are no
contamination of the ingots by calcium fluoride. Direction-
al solidification was made for compositions with 15 at. %
Fe and 85 at. % Pr (hereinafter denominated 15Fe–85Pr)
and 20 at. % Fe and 80 at. % Nd (hereinafter denominated
3. Results and discussion
The microstructure of as-cast 30Fe–70Pr and 25Fe–
75Nd samples, obtained by arc-melting, are shown in Fig.
1. Fig. 1a shows the microstructure of the 30Fe–70Pr
sample and Fig. 1b shows the microstructure of the 25Fe–
75Nd sample. These alloys composition are situated in the
hypoeutectic region of the corresponding phase diagrams
for both systems [8]. In these figures, it can be observed
two typical eutectic morphologies previously cited, the
feathered (1) and globular (2) structures, and probably the
Fe Pr or Fe Nd phases (3). This is the typical micro-
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17
2
structure of the samples of these systems solidified at high
cooling rates, as the ones proportioned by arc-melting. The
microstructural morphologies for both materials are very
similar, however the TMA signals are very different. For
the binary Fe–Pr system case, it can be observed a strong
signal around 2048C corresponding to the so-called P1
phase [6,7] together a similarly strong signal corre-
sponding to the Fe Pr phase, as shown in Fig. 2a
2
0Fe–80Nd) in a radio frequency (RF) furnace. For doing
the DS experiments a vertical Bridgman crystal growth
unit was used. A furnace with a variable coil, which
promotes a convenient temperature gradient, composes this
crystal growth unit. As a susceptor, we used a tantalum
tube held in place by a niobium holder and quartz tubes.
For all experiments, the furnace was evacuated down to
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(30Fe–Pr70 as-cast sample). As it can be seen in Fig. 2b,
the amount of this phase decreases as the 15Fe–Pr85
sample is DS at 1 mm/h (superior part), and a phase with a
Tc of 448C starts to appear in the TMA signal. This phase
2
6
1
0
Torr and then filled with high purity argon up to 4
was characterized as being the Fe Pr phase and will be
2
atm. This procedure is necessary to prevent the contamina-
tion of the samples due to their high reactivity, once they
are rich in the rare earth elements. During the DS runs the
temperature was monitored using a Leeds and Northrup
optical pyrometer. Cooling rates, measured as the speed of
the vertical movement of the crucible, were 1, 10 and 20
mm/h. In this way, the DS duration was 50, 5 and 2.5 h.
discussed below regarding the DS samples characteristics.
In the binary Fe–Nd system case, a strong signal corre-
sponding to the Fe Nd , and two other signals that
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correspond to the g (at 1668C) stable phase and the P1 (at
2428C) metastable phase [16], can be observed (Fig. 3a,
as-cast 25Fe–75Nd sample). When the 20Fe–80Nd sample
is DS at 1 mm/h (superior part), the signals of the Fe Nd
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Santos
