Preliminary characterization of the intermetallic `YNi' obtained by both mechanical grinding and alloying

Article abstract of DOI:10.1016/S0925-8388(99)00805-1

The intermetallic compound YNi and the (Y+Ni) mixture were submitted to a high-energy milling process under argon atmosphere. X-ray powder diffraction and microprobe analysis revealed in both cases the formation of a new phase `YNi'. Its structural properties were different from those of the YNi as-cast sample (orthorhombic FeB type) and presented some similarities with GdNi (orthorhombic CrB type) milled under the same conditions. The application of isostatic pressure on the YNi as-cast sample gives a comparable result. This non-equilibrium material `YNi' still exists even after annealing treatments performed up to 873 K. The presence of air during milling leads to the formation of a mixture of (oxy)nitride Y(O,N) and Ni metal and the new phase `YNi' is not synthesized under such conditions. Also, the influence of the milling process on the magnetic properties of similar compounds such as GdNi and ErNi is discussed.

Full text of DOI:10.1016/S0925-8388(99)00805-1

Journal of Alloys and Compounds 299 (2000) 94–100
L
www.elsevier.com/locate/jallcom
Preliminary characterization of the intermetallic ‘YNi’ obtained by both
mechanical grinding and alloying
*
M. Nakhl, B. Chevalier , J-L. Bobet, B. Darriet
Institut de Chimie de la Mati e` re Condens e´ e de Bordeaux (ICMCB), CNRS [UPR 9048], Universit e´ Bordeaux I, Avenue du Docteur A. Schweitzer,
3
3608 Pessac, France
Received 2 November 1999; accepted 29 November 1999
Abstract
The intermetallic compound YNi and the (Y1Ni) mixture were submitted to a high-energy milling process under argon atmosphere.
X-ray powder diffraction and microprobe analysis revealed in both cases the formation of a new phase ‘YNi’. Its structural properties
were different from those of the YNi as-cast sample (orthorhombic FeB type) and presented some similarities with GdNi (orthorhombic
CrB type) milled under the same conditions. The application of isostatic pressure on the YNi as-cast sample gives a comparable result.
This non-equilibrium material ‘YNi’ still exists even after annealing treatments performed up to 873 K. The presence of air during milling
leads to the formation of a mixture of (oxy)nitride Y(O,N) and Ni metal and the new phase ‘YNi’ is not synthesized under such
conditions. Also, the influence of the milling process on the magnetic properties of similar compounds such as GdNi and ErNi is
discussed.
 2000 Elsevier Science S.A. All rights reserved.
Keywords: Mechanical alloying and grinding; Yttrium–nickel system; Structural transition; SEM examination; Magnetic properties
1
. Introduction
using X-ray powder diffraction, microprobe analysis,
scanning electron microscopy and magnetization measure-
ments. The results are discussed considering those ob-
tained on similar compounds such as GdNi and ErNi.
The milling performed at high-energy allows mainly: (i)
the amorphisation of crystalline compounds prepared
conventionally by solid state reaction, by melting of pure
elements. (ii) The synthesis of compounds from the
mixture of the elemental constituents. These applications
can be illustrated considering for instance the numerous
studies devoted to the intermetallic ZrNi. This compound
which crystallizes in the orthorhombic CrB type, is ob-
tained as amorphous phase after mechanical grinding
2. Experimental
The intermetallics YNi, GdNi and ErNi (as-cast samples)
were prepared by arc melting of stoichiometric amounts of
the constituent elements in purified argon atmosphere. The
purity of the starting materials was as follows: Y, Gd and
Er 99.9% and Ni 99.99%. The alloy ingots were turned and
remelted several times to ensure homogeneity. The weight
loss of ingots during melting was negligible.
(
MG) under an argon atmosphere [1–3]. A similar amor-
phous phase is obtained after mechanical alloying (MA) of
a (Zr1Ni) mixture of equiatomic composition [1]. Amor-
phous ZrNi reacts easily with nitrogen or hydrogen
producing ZrN or ZrNiH and ZrH .
In order to determine the hydrogen absorption properties
of YNi, we have performed MG and MA experiments. We
present here the characterization of the resulting products
Y and Ni powders with, respectively, initial grain size of
420 and 4 mm were used as starting materials for MA
experiments. A 4-g amount of the (Y1Ni) mixture was
sealed under 0.1 MPa of purified argon gas, in a stainless
3
2
3
steel container (80 cm in volume) with 48 g of steel balls
(
10 mm in diameter). Milling was performed using a
*
Corresponding author. Tel.: 133-5-5684-6336; fax: 133-05-5684-
761.
E-mail address: chevalie@icmcb.u-bordeaux.fr (B. Chevalier)
Fritsch pulverisette 5 high-energy instrument. The rotation
speed of the plateau was fixed at 200 rpm for various
2
0
925-8388/00/$ – see front matter
 2000 Elsevier Science S.A. All rights reserved.
PII: S0925-8388(99)00805-1

M. Nakhl et al. / Journal of Alloys and Compounds 299 (2000) 94–100
95
Fig. 1. X-ray powder patterns of the initial (Y1Ni) mixture alloyed for various times (1 indicates the peak position relative to the new phase ‘YNi’).
duration times. Similar experimental conditions were used
for MG experiments performed on YNi, GdNi and ErNi
as-cast samples.
Microprobe examination was carried out in a Cameca
SX-100 scanning electron microscope. The analysis of the
samples containing Y and Ni was based on the measure-
ments of Y–La1 and Ni–Ka1 X-ray radiations, which were
compared to those obtained for YNi used as reference. The
particle size and morphology were investigated using a
Jeol 840 microscope.
The samples were checked by conventional X-ray
powder diffraction using a Philips 1050-diffractometer
(Cu-Ka radiation). Typical crystallite sizes L for various
samples were calculated before and after grinding from the
Scherrer formula L 5 0.91lCu /bcos u where lCu is the
Cu-Ka radiation, b the peak width (in radian) at half-
maximum peak and u is the diffraction angle. Magneti-
zation measurements were carried out between 2 and 300
K using a SQUID (superconducting quantum interference
device) magnetometer.
Fig. 2. Comparison between the X-ray powder patterns of YNi obtained by: (a) melting (Miller indices hkl are given), (b) alloying of the (Y1Ni) mixture
for 48 h and (c) grinding of as-cast sample for 48 h.

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M. Nakhl et al. / Journal of Alloys and Compounds 299 (2000) 94–100
3
. Results and discussion
It is well known that the equiatomic binary compounds
RNi (R5rare earth) crystallize with two different ortho-
rhombic structures types depending on the size of R
element: (i) For R5La→Gd the intermetallics adopt
preferentially the CrB structure type. (ii) For R5Dy→Lu
the FeB type is observed [6]. In both structures the Ni
The X-ray powder patterns of the (Y1Ni) mixture
mechanically alloyed for various times are presented in
Fig. 1. The pattern of the unmilled sample (bottom curve),
clearly shows the peaks characteristics of Y and Ni metals
(
initial product). The influence of MA is: (i) the peak
atoms are inside characteristic trigonal prisms [R ]. These
6
characteristic of Ni metal (2u(44.58) is reduced as the
milling duration increases but is still always detected. (ii)
On the contrary, the peaks of the Y metal disappear rapidly
with increasing MA duration; its presence cannot be
detected by X-ray powder analysis. (iii) Moreover, the
peaks at 2u(29.88, 33.48, 35.08, 41.48 and 57.28 appear
with this treatment. (iv) Finally, a very broad and diffuse
peak is observed in the 2u(25–458 range suggesting a
lack of structural order. The microprobe analysis of the
product alloyed for 72 h indicates that the main phase
prisms are stacked differently in the two structures. The
structural transition between CrB and FeB types was
explained considering unit-cell twining [6,7]. Under these
conditions, it is interesting to study the influence of MG
treatment on RNi compounds that adopt either one of these
two structure types.
Fig. 4 presents the X-ray powder pattern of an GdNi
(
5
(96% in volume) has the chemical composition Y:
0.460.3%, Ni: 49.660.3% (in atomic percent) and it
reveals the presence of amounts of Ni metal.
Fig. 2 compares the X-ray powder patterns of the YNi
as-cast sample (a) after grinding for 48 h (c). For the
discussion, the pattern of the product resulting from MA of
the (Y1Ni) mixture is also presented (b). As logically
expected, the pattern of the as-cast sample can be com-
pletely indexed on the basis of an orthorhombic cell having
the FeB-type structure [4]. Drastic changes can be ob-
served under MG (c). All the peaks have decreased
significantly and even some of them as (0 1 1), (1 1 1) and
(
2 1 1) have completely or almost disappeared. Moreover,
an important broadening of the peaks is visible. These
observations suggest a reduction of the crystalline size.
The corresponding values of L derived from the broaden-
ing of diffraction peaks were calculated using the Scherrer
equation. They are equal to L 5 5065 nm before grinding
and L 5 1063 nm after grinding and the development of
microstrain in the ground YNi sample. We note that this
last product has an X-ray powder pattern similar to those
observed for the product obtained by MA (b). The
complete or almost complete disappearance of the (0 1 1),
(
1 1 1) and (2 1 1) peaks after milling suggests the
occurrence of a structural transition.
Fig. 3 shows SEM micrographs for the ‘YNi’ phase
obtained by MA and MG. The morphology and the size of
the particles resulting from the two treatments, presents
obvious differences. After 48 h of MA (Fig. 3a) the
particles are basically spherical in shape and appear to be
made up of layers of material and have an average size of
about 10 mm. On the contrary, the particles prepared by
MG (Fig. 3b) are much smaller in size ((5 mm) and seem
to be fragmentary and flattened. This reflects the dominat-
ing effect of fracturing of the material. The appearance of
significant amounts of debris on the particles in the first
case is due to the interdiffusion between Y and Ni during
the MA treatment and also due to the cold-welding
process. A similar observation was previously made during
Fig. 3. SEM micrographs of the new phase ‘YNi’ obtained by: (a) MA of
(
Y1Ni) mixture and (b) MG of YNi as-cast sample (magnification,
the synthesis of Mg Ni [5].
32000).
2

M. Nakhl et al. / Journal of Alloys and Compounds 299 (2000) 94–100
97
Fig. 4. Comparison between the X-ray powder patterns of GdNi obtained by melting (a) (Miller indices hkl are given) and GdNi (b), ErNi (c) and YNi (d)
ground for 48 h.
as-cast sample (CrB type) after grinding for 48 h. After
MG, only a decrease of the peak intensity is observed. The
crystallite size L decreases strongly from 6065 to 1563
nm. The grinding performed on GdNi and YNi leads to
products having X-ray powder patterns showing many
similarities. The same observation can be made for an ErNi
as-cast sample (FeB type) ground for 48 h (Fig. 4c). This
result suggests that the compounds GdNi, YNi and ErNi
present after grinding the same structural properties.
We have investigated at room temperature the effect of
the pressure on the structural properties of the YNi as-cast
sample. Fig. 5 exhibits the X-ray powder pattern of this
intermetallic at two pressures: 0.1 MPa (room pressure)
and 8.4 GPa. A decrease of some peak intensities is
observed under pressure. For instance, the Bragg reflec-
tions (1 1 1) and (2 1 1) are less visible. This behaviour is
similar to that observed at the time of the milling of YNi
(Fig. 2). These two effects (pressure or grinding) seem to
produce the same change in the structural properties of
YNi.
An investigation by high temperature X-ray powder
diffraction was performed up to 873 K under vacuum, in
order to study the thermal stability of the new phase ‘YNi’.
The temperature was increased from 323 to 873 K in
gradual steps of 50 K, the time required for acquiring data
being around 3 h. The X-ray powder patterns obtained,
respectively at room temperature, 573 and 873 K are given
in Fig. 6. No drastic change appears at this last tempera-
ture, only a decrease of the peak width is observed. No
reflection centered around 2u537.18 (corresponding to the
Fig. 5. X-ray powder patterns of YNi as-cast sample under a pressure of 0.1 MPa and 8.4 GPa.

9
8
M. Nakhl et al. / Journal of Alloys and Compounds 299 (2000) 94–100
Fig. 6. X-ray powder patterns of the new phase ‘YNi’ at three temperatures 300, 573 and 873 K.
(
2 1 1) peak of the as-cast sample; see Fig. 2) is noted.
occurrence of yttrium (oxy)nitride during the MG process
under air (YN crystallizes with the cubic NaCl type with
a50.4877 nm [8]). In this MG experiment, YNi is
decomposed into a mixture of Y(O, N)1Ni.
The new phase ‘YNi’ is thermally stable at least up to 873
K. An annealing of this phase under vacuum at 1173 K for
1
day is necessary in order to restore YNi having the FeB
type but some traces of oxide Y O and intermetallic YNi
As high-energy ball milling changes the structural
properties of the well-crystallized compounds, modifica-
tions of their magnetic properties are expected. Therefore,
magnetization measurements of ground GdNi and ErNi are
of prime interest.
The temperature dependence of the magnetization of the
GdNi as-cast sample is characteristic of ferromagnetic
behaviour (Fig. 8). The Curie temperature TC estimated
from the inflection point of the magnetization curve is
equal to 71.560.5 K. This value agrees with that reported
2
3
2
are observed after this thermal treatment. The occurrence
of these impurities is certainly linked to the contamination
by oxygen during the milling process.
The synthesis by MA or MG of the new phase ‘YNi’
should be performed under argon atmosphere. The X-ray
analysis of the resulting product obtained by MG under air
reveals the coexistence of two phases (Fig. 7): (i) Ni metal
giving a broad peak around 2u541–448. (ii) The second
phase is characterized by the reflections appearing at 2u5
3
1–328, 36–378 and 52–538. They were indexed with a fcc
previously T 573 K [9]. After 48 h of grinding, GdNi
C
cubic unit cell having as parameter a50.4962 nm. The
remains a ferromagnet but its Curie temperature T 5
C
structural properties of this second phase indicate the
77.560.5 K is observed to shift to higher temperature (Fig.
Fig. 7. X-ray powder pattern of YNi as-cast sample ground for 48 h under air (Miller indices are relative to the fcc phase).

M. Nakhl et al. / Journal of Alloys and Compounds 299 (2000) 94–100
99
Fig. 8. Temperature dependence, measured in an applied field of 0.1 T, of the magnetization of GdNi before and after grinding for 48 h.
8
). Similar effects were claimed recently during the milling
Fig. 9 presents the temperature dependence of mag-
of GdPt , GdIr and GdRh [10] and this was ascribed to
netization for ErNi before and after the grinding process. It
is to be noted that the ErNi as-cast sample orders fer-
2
2
2
the decrease of the unit cell volume which increases the
ferromagnetic RKKY interactions. We also note that
amorphous a GdNi film sample prepared by sputtering has
romagnetically below T 510.760.5 K. This temperature
C
agrees well with the literature value of T 510 K [9,12].
C
a Curie temperature T 578 K, close to that determined for
ground GdNi [11]. Finally, the examination of the mag-
netization curves of GdNi unmilled and milled for 48 h
On the contrary, no clear anomaly can be distinguished in
the magnetization curve relative to the ground sample. In
order to obtain more information on this sample, we have
performed ac-magnetic susceptibility measurements with-
out external magnetic field. Fig. 10 shows for ground ErNi
the thermal dependence of the real x9 and imaginary x0
parts of the ac-susceptibility at different frequencies n in
the temperature range 2–15 K. The x9 5 f(T) curve for
n51.25 Hz exhibits two anomalies: an inflection point at
T 511.060.5 K and a peak around T 5 4.460.5 K.
C
(
Fig. 8) shows that the second sample presents the greater
magnetization at low temperature. This result can be
ascribed to the change of particle size during the grinding
process. A SEM experiment shows a reduction of the
average size from 70–60 to 10 mm. The small particles
orientate more easily in the applied low magnetic field of
0
.1T inducing an increase of the measured magnetization.
C
1
Fig. 9. Temperature dependence, measured in an applied field of 0.1T, of the magnetization of ErNi before and after grinding for 48 h.

1
00
M. Nakhl et al. / Journal of Alloys and Compounds 299 (2000) 94–100
Fig. 10. Frequency dependence of the real (x9) and imaginary (x99) part of the ac-magnetic susceptibility of ErNi ground for 48 h (ac-field of 0.1 mT,
dc-field of 0T and n51.25, 12.5 and 125 Hz).
These two transitions are associated with similar anomalies
on the x05f(T) curve. This reflects important energy
losses in the magnetically ordered state, presumably con-
nected with domain effects appearing for instance in
ferromagnetic, ferrimagnetic, canted or spin-glass systems.
In Fig. 10, we can observe that both anomalies occurring at
concerning the structure of this new phase ‘YNi’ are now
in progress.
Acknowledgements
TC and T are frequency dependent. They shift to higher
1
The authors express their gratitude to Eric Lebraud for
his assistance during X-ray powder diffraction versus
temperature experiment.
temperatures with increasing n. For instance, for n5125
Hz the values of T and T are 11.460.5 K and 4.860.5
C
1
K, respectively. This behaviour is typical for a metallic
spin-glass [13]. We note that the transition temperature TC
of the ground sample is close to that observed for the
as-cast one (Fig. 9). Our study indicates that the grinding
process has a drastic effect on the magnetic properties of
ErNi. The measurements suggest a strong disorder (very
broad magnetic transition, spin-glass behaviour) appearing
in the ground sample. We recall that this process induces a
structural change for this intermetallic and perhaps this
change is not complete.
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8
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73 K under vacuum, exhibits an X-ray powder pattern
[
[
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