Effect of mechanical milling on both the structure and the first hydriding-dehydriding properties of a MmNi5-Ni mixture

WHEC 16 / 13-16 June 2006 – Lyon France


Effect of mechanical milling on both the structure and the first
hydriding-dehydriding properties of a MmNi5-Ni mixture

Marcelo R. Esquivel a,b and Gabriel Meyer a,b

a Comisión Nacional de Energía Atómica-Centro Atómico Bariloch, Av. Bustillo km 9.5, Bariloche, Río Negro,
Argentina, esquivel@cab.cnea.gov.ar
bConsejo Nacional de Investigaciones Científicas y Técnicas, gmeyer@cab.cnea.gov.ar



ABSTRACT:

The effects of mechanical milling on both the structure and hydriding properties of a MmNi5-Ni mixture
synthesized by mechanical alloying is presented. Mm and Ni were milled in Ar atmosphere to obtain the
mixture. Alloy was heated at 600 ºC during 5 days to obtain a crystalline compound which served as
reference. During the milling process, samples were withdrawn and studied by X-ray diffraction (XRD) and
scanning electron microscopy (SEM). XRD measurements were used to determine the change of the
crystallite size and strain on both MmNi5 and Ni peaks due to milling. SEM was used to observe the temporal
evolution in morphology and size of the particles. From this study, the stages occurring during the grinding of
the alloy were identified and characterized. Selected samples were analyzed and hydrogen absorption
curves were obtained using a modified Sievert’s type apparatus designed in our laboratory. The data
obtained from absorption curves was correlated with the defects and strain introduced into the material due
to milling.

KEYWORDS : MmNi5, Mechanical alloying, Mischmetal.


INTRODUCTION:

Mechanical alloying has evolved gradually as a widely used method of synthesis [1,2]. Simplicity, low
cost and easy scaling up are the main advantages attributed to this process over traditional ones such as full
equilibrium methods or chemical synthesis.
The findings obtained using this technique contributed to the development of alloys useful for diverse
applications [3,4].
This current trend is also observed in the synthesis of MmNi5 based alloys [5,6]. Most of these works
have been done using a high energy mill device [1]. Compounds were synthesized in short times of the
order of minutes or hours and no further investigation was focused on the characteristic stages occurring
during milling [5,6]. Features related to the evolution of the sample during this process can be analyzed if
synthesis is achieved in a low energy mill [7]. One of these characteristics is the relationship between
integrated milling time and the introduction of strain and defects into the material [1,2]. MmNi5 based alloys
have been synthesized from individual compounds using this technique [5,6,7]. A work regarding the
evolution of mechanical milling process of a MmNi5-Ni mixture obtained from Mm and Ni powders using a
low energy mill was published previously [7].
In this work, the temporal evolution of the milling process of a MmNi5-Ni mixture is presented. Initial
mixture is obtained by a combination of mechanical milling with low heating temperature [7].
The powder morphology is analyzed by scanning electron microscopy (SEM). From this analysis, the
stages governing the process of milling are obtained. Results are compared to those obtained for the milling
of a Mm-Ni mixture [7]. The crystallite size and strain of both Mm and Ni are studied using X-ray diffraction
(XRD). Values obtained at intermediate milling times are compared to those obtained previously for Ni [7].
Hydrogen-alloy Interaction of samples extracted at different times is studied using a Sievert´s type device.
From these results, the effect of milling time on both microstructure and hydrogen absorption process is
analyzed. These results are also compared to those obtained from a sample synthesized previously [7].
The potential applications of these findings in both research and technological fields makes worth this
study. This double objective aimed the elaboration of the present work.

EXPERIMENTAL:
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Pure Ni (3.80 µm, 99.99%) (Sigma Aldrich) and drilled lumps of Mischmetal (99.7 %) (Alpha Aesar) of
nominal composition 52.0 wt% Ce, 25.6 wt% La, 16.9 wt% Pr , 5.5 wt% Nd were mechanically milled under
Argon atmosphere using a Uni-Ball-Mill II apparatus (Australian Scientific Instruments).
Mischmetal (Mm) composition was verified by Neutron Activation Analysis (NAA) and Energy
Dispersive Spectroscopy (EDS). Powder mixture in a proportion of 20% excess of Nickel over the
stoichiometric MmNi5 composition and steel balls were set in a stainless steel chamber under Ar atmosphere
in a glove box. Particles size and morphology were observed by Scanning Electron Microscopy (SEM).
The balls to powder mass relation was 33.5/1. Representative amount of powder was withdrawn from
chamber at different milling times inside a glove box and samples were analyzed by X-ray powder diffraction
(XRD). During sample manipulation inside the glove box the oxygen level was monitored by a trace analyzer
(Series 3000, Alpha Omega) and kept under 5 ppm to avoid material oxidation.
Room temperature X-ray diffraction was achieved on a Philips PW 1710/01 Instrument with Cu K?
radiation (graphite monocromator). Diffraction patterns were analyzed by the Rietveld method [8] using
DBWS software [9].
A Sievert´s type equipment was used to measure hydrogen absorption-desorption curves. The sample
is placed in the reactor at constant temperature and a selected initial pressure. A PC-based data acquisition
system monitors and controls the experiment variables. Experimental set-up device details can be found
elsewhere [10].

RESULTS AND DISCUSSION:

Milling evolution of a MmNi5-Ni mixture

The diffractograms of samples extracted at different milling times are shown in Fig. 1. Initial sample
(Fig. 1.a) is a MmNi5-Ni mixture obtained by mechanical milling and heated at 600 ºC during 5 days to obtain
complete crystallization.


Figure 1. Diffractograms of a MmNi5-Ni mixture milled at different times. a) Initial.
b) 10 h; c) 20 h ; d) 40 h; e) 100 h. Ni diffraction lines are shown in dotted lines.


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MmNi5 and Ni crystalline parameters and mass percentage in initial mixture was determined using
Rietveld Method [9]. Mass percentages, Rietveld method fitting parameters at different milling times for these
and samples milled at other different times are summarized in Table 1.

Table 1. MmNi5-Ni mixture parameters and mass percentage values.

MmNi5
Ni
Percentage in
Rwp
Milling
Parameters (Å)
Parameters (Å)
Mixture (mass %)
value
Time
Volume cell (Å)3
a c
a MmNi5
Ni
0 h
4,9067
3,9774
82, 93
3,5304
94 ± 1
6 ± 1
20
4 h
4,9171
3,9860
83,46
3,5396
94 ± 4
6 ± 2
18
5 h
4,9114
3,9815
83,17
3,5367
94 ± 2
6 ± 2
15
10 h
4,9170
3,9898
83,54
3,5400
94 ± 1
6 ± 1
18
20 h
4,9072
3,9937
83,28
3,5371
92 ± 2
8 ± 2
18
40 h
4,8930
4,0001
82,96
3,5330
92 ± 2
8 ± 2
18
60 h
4,8882
4,0001
82,83
3,5242
94 ± 1
6 ± 2
20


MmNi5 structure was fitted assigning P6/mmm space group and Wyckoff positions corresponding to
LaNi5 [11]. Lanthanides (Ce, La, Nd, Pr) in MmNi5 alloy are supposed to be distributed randomly in La
positions in the structure. Ni parameters were fitted assigning space group and Wyckoff positions of
reference pattern [12].
It is clearly observed that milling increments the peaks width (Figs. 1b to 1e) for both Ni and MmNi5. Ni
diffraction lines are indicated in doted lines. Other diffraction lines correspond to MmNi5. No secondary
phases are detected. The increase of peaks width can be assigned to a decrease in the crystallite size, to a
decrease in the long range order or to an increase of the strain [13].
From Table 1, it is noticed that MmNi5 basal parameter increases with milling time from 0 h to 4h a
relative length of 0.21% and reduces from 5 h to 60 h a relative length of 0.47%. This reduction reaches
lower values than that of initial sample. MmNi5 c parameter increases with milling a relative length 0.57%.
Volume cell value increases up to 10 h and decreases up to 60 h finally reaching approximately the same
value before and after milling (variation of 0.12%).
Despite the differences in mill devices, the behavior of both a and cell volume parameters is similar to
that reported for the milling of LaNi5 at times shorter than 5 h [14]. A higher energy mill was used in
reference [14]. c parameter behavior increases from 0 to 4h being different from c behavior observed in [14]
where the values remain almost constant. Cell volume and a parameter changes are opposite to those
observed in other reference [15].
The expansion observed in c parameter in this work should not be attributed to the formation of
dumbbells [14,15]. Because an a and cell volume reduction should also be observed. A site interchange
between lanthanide and Ni atoms can not explain the process either [15]. Because the mill used here is a
device with lower energy than those of ref [14,15].
The increments in lattice parameters (a, c, cell volume) observed from 0 to 4h in this work could be
due to the defects accumulated during the first hours of mechanical milling. The effects are isotropic because
broadening of peaks is symmetrical and no preferential orientation is observed as shown in Figure 2. The
figure shows a reduced range of the diffractogram of samples un milled (full line), milled 2 h (dotted line) and
milled 4 h (dashed line). The displacement of the maxima of each MmNi5 peak is in agreement with data of
Table 1 showing an increment in a, c and volume cell parameters. Ni (111) peaks also shows a maxima
displacement indicating an increase of a parameter.
The further decrease on lattice parameter a and further increase on lattice parameter c at times longer
than 4 h can not be attributed to the creation of Ni dumbbells replacing a La atoms in a La site [15] since
volume cell values remains almost constant. A preferential reordering of higher radius lanthanides along the
C axis and lower atom radius lanthanides in the basal parameter could be the explanation to the cell
parameters evolution. A detailed study should be done to clarify this point.
Ni a parameter increases with milling from 0 h to 10 h. It decreases from 10 h to 60 h. Its maximum
relative change reaches 0,27% at 10 h of milling. Effects of milling on Ni can not be appreciated at milling
times longer than 10 h because of Ni diffraction lines disappearance.

Crystallite size and strain induced by milling

Table 2 shows the change in crystallite size and strain induced by milling at different milling times. As
observed, crystallite size decreases with milling and strain increases.
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MmNi5 diffraction lines (011), (110) and (111) were selected to analyze both changes in strain and size of
crystallites. (111) Ni diffraction line was selected. In each case selection obeyed to its higher relative
intensity at all milling times. As observed in Table 2, MmNi5 decreases with milling time. Anisotropy is
observed in milling because relative changes within the initial and final milling time are different for (011) and
(110) and (111). These structural changes were introduced by milling.
Strain changes are also observed in Table 2. It is deduced that milling introduces changes in the
structure. It is also noted the anisotropy of the phenomena.


Figure 2. MmNi5 peaks displacement and symmetry at different milling times.

Table 2. Changes in crystallite size and strain in MmNi5 and Ni structures


Crystallite size (Å) Strain
(%)
Milling time
MmNi5
Ni MmNi5
(h)
(0,1,1) (1,1,0) (1,1,1) (1,1,1) (0,1,1) (1,1,0)
(1,1,1)
0
h
421 727 508 270 0.048 0.026 ---------
4
h
439 466 491 -------- 0.067 0.054
0.044
5
h
391 448 491
--------- 0.076 0.055
0.044
10
h
298 355 329 -------- 0.098 0.068
0.064
20
h
180 236 134
---------- 0.160 0.100
0.150
40
h
159 214 128
--------- 0.185 0.115
0.165
60 h
113
200
94
--------- 0.200 0.123
0.223
100 h
127
184
79
--------- 0.232 0.134
0.223

Stages present during milling

Figure 3 shows a mosaic image of samples milled at different times. The particles morphology as
obtained by SEM is useful to determine the stages present during milling. Unlike the stages present during
the mechanical alloying of Mm-Ni powders to form MmNi [
5 7], the mechanical grinding of the MmNi5-Ni
mixture leads to a global increment on the size of the particles. Since particles of an average size of 5 µm of
un milled powders (Fig 3.a) transforms to agglomerates of sizes bigger than 20 µm (Fig 3 h). Since initial
milled powders are previously milled and heated powders, initial size is low enough not to allow fracture
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process to progress. On the contrary cold welding slowly dominates and global particle size is increased
(see Figures f -h). Competition between both processes is found at intermediate milling times (see figures d-
e).



Figure 3. SEM images of samples extracted at different milling times. a) Un milled powders. b) 2 h. c) 3 h. d)
10 h. e) 30 h. f) 60 h g) 100 h. h) 200 h.

Absorption curves

Figure 4 shows the first absorption curves for different as-milled non activated MmNi5-Ni mixtures. Y
axis represents the weight percent ( mH / (mMmNi5 + mH)) of absorbed H2. The initial pressure is 6000 kPa


Figure 4. a) Absorption curves at 25 ºC. b) Absorption curves at 90 ºC. Initial Pressure is 6000 kPa
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First hydriding curves of non activated samples were used instead of hydrogen cycled samples. This
procedure was preferred to analyze the effects of mechanical milling on microstructure than the hydrogen
interaction in activated samples after many hydrogen absorption-desorption cycles. As observed, the wt% at
the same absorption time is higher as milling time increases. This is a straight forward effect of the effect of
milling time. The introduction of defects and deformation on microstructure and surface produces a higher
reactivity. The introduction of bulk defects also contributes to a quicker absorption. As temperature increases
the effect of milling is decreased. This can be attributed to an enhanced superficial reactivity and diffusion of
H2 due to the combined effects of temperature and strain introduced by mechanical milling.


CONCLUSIONS:

In this work, the effect of mechanical milling on microstructure and further hydrogen absorption of a
MmNi5-Ni mixture was analyzed. MmNi5-Ni mass ratio was determined by Rietveld analysis. Changes in
MmNi5 cell parameters was analyzed and compared to bibliography. The evolution of cell parameters a and
c was attributed to preferential distribution of lanthanides in La position.
The effect of milling on crystallite size and structure strain was analyzed by XRD. Strain anisotropy
was observed in MmNi5.
Particles size during milling evolution was analyzed at short and long times by SEM observations.
Three main stages were observed. Fracture predominates in the first stage at times shorter than 10 h.
Equilibrium between fracture and cold welding is observed between 10 h and 30 h. At higher times, cold
welding predominates and a marked increase in particles size is observed.
Hydrogen absorption curves of samples milled at different integrated milling times were analyzed at 25
ºC and 90 ºC. Higher milling times were found to correlate with a higher and quicker hydrogen absorption. A
further analysis of samples treated at intermediate temperatures and intermediate integrated milling times is
the subject of an incoming work.


REFERENCES:

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ACKNOWLEDGEMENTS:

The authors wish to thank Comisión Nacional de Energía Atómica of Argentina (Project CNEA P5-PID-95-2)
, Secretaría de Ciecia, Técnica y Posgrado Universidad Nacional de Cuyo of Argentina, Consejo Nacional
de Investigaciones Científicas y Técnicas of Argentina (Project PIP-6448) and Agencia Nacional de
Promoción Científica y Tecnológica of Argentina (Project PICT 12-15065) for partial finantial support.






















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