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Materials Transactions, Vol. 43, No. 3 (2002) pp. 470 to 473
c
2002 The Japan Institute of Metals
Crystal Structure and Protium Absorption Properties of Ti–Cr–X Alloys
∗1
Atsunori Kamegawa, Koji Shirasaki
∗2
, Takuya Tamura
∗2
, Takahiro Kuriiwa
∗3
,
Hitoshi Takamura and Masuo Okada
Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
This paper aims to develop the vanadium free Ti–Cr–X BCC solid solution alloys with high protium content. The effects of additional
elements such as Mo, W, Nb and Ta to the phase formation on the Ti–Cr alloys were studied. It was found that Mo-added heat-treated alloys
had the flattened plateau regions with the capacity of more than 2.2 mass% protium with maximum of 3.6 mass% protium content, which is
equal to that of Ti–Cr–V alloys. It was found that the Ti–Cr–Mo alloys with BCC structure transformed to protide phase with BCC or FCC
structure. The plateau pressure of the Ti–Cr–Mo alloys increased with increasing Mo content, but the lattice parameters decreased. The H/M
ratio of the alloy with less than 10at%Mo was 1.8 at 10MPa hydrogen pressure and was almost unchanged with Mo content.
(Received October 30, 2001; Accepted January 22, 2002)
Keywords: protium storage alloys, titanium-chromium-molybdenum alloys, body centered cubic phase, heat-treatment
1. Introduction
Recent strong demands for the hydrogen fuel cell (FC) as
the emission-free vehicles urge to develop protium absorb-
ing alloys with high capacity as hydrogen storage tank. Since
they have advantages as compactness, free-shape and safety
storage of hydrogen energy. However, the protium capacity
of the presently used MmNi
5
based alloys does not exceed
1.2mass% protium content, which is insufficient for using
as hydrogen storage tank. The strong candidate for the pur-
pose will be body centered cubic (BCC) solid solution alloys,
which is named as Laves phase related the BCC alloys.
1–4)
Tominaga et al.
5)
reported that Ti–Cr–V solid solution al-
loys with a BCC structure exhibit an effective protium capac-
ity of 2.6mass% protium. The optimum heat-treatment con-
ditions for the alloy were reported to be annealing at 1573 K
for 1min, and then quenching in water. Okada et al.
6)
reported
that the Ti–Cr–V alloys with low V content (5–7.5at%V)
yield the high capacity of nearly 3mass% protium, which
is the highest value at 313K reported so far. The optimum
composition of the Ti–Cr–V alloys was also discussed and
the alloys with Ti/Cr ratio of 2/3 have the plateau region in
the PCT curve around 0.1MPa (1atm) with highest capacity
at 313K. The alloys will be promising since it contains a low
amount of the expensive V element. Since the BCC alloys
with low content of V exhibit high capacities comparable to
pure vanadium or V-based alloys, it is expected that V free Ti–
Cr alloys with a BCC structure may exhibit a high capacity.
But Ti–60Cr alloys (Ti/Cr = 2/3) have Laves phase as equi-
librium phase in the range of room temperature–1630K and
BCC solid solution phase at high temperature in narrow range
of 1630K–1700K. Therefore it is difficultto obtain only BCC
phase freezing from high temperature in the binary alloys, so
that it is necessary to stabilize the BCC phase in Ti–Cr binary
phase diagram by adding some BCC-forming elements such
Mo, W, Nb, and Ta. Then, the purpose of the present study
∗1
This Paper was Presented at the Autumn Meeting of the Japan Institute
of Metals, held in Fukuoka, on September 23, 2001.
∗2
Graduate Student, Tohoku University, Sendai 980-8579, Japan.
∗3
Graduate Student, Tohoku University, Present address Honda R&D.
is to investigate the formation of BCC phase in the Ti–Cr–
X alloys (X=Mo, W, Nb, Ta) and their protium absorption-
desorption properties.
2. Experimental Procedures
The alloys were prepared from raw materials by arc melt-
ing on a water-cooled copper hearth under pure argon at-
mosphere. The purity of the elements were as follows:
Ti > 99.6at%; Cr > 99.99at%; and V, Mo, W, Nb, Ta >
99.99at%. In our previous study, The Mo or W added Ti–Cr–
V alloys have mainly BCC phase. But Mo or W unmelted ad-
ditives were remained in the alloys with the addition of more
than 10%Mo or 5%W, respectively. The melting point of Mo,
W, Nb and Ta is high temperature such as 2883 K, 3683 K,
2741K and 3269K respectively. Therefore, at first Cr–X al-
loys (X=Mo, W, Nb, Ta) were melted, and then Ti–Cr–X al-
loys were prepared. Sample ingots were remelted three times
to ensure their homogeneity. Some samples were annealed at
1673K for 1min, and quenched into iced water.
Crystal structures and lattice parameters were studied by
X-ray diffractometer (XRD) using Cu-Kα radiation (Philips,
X’pert System). Hydrogenated sample for the XRD experi-
ment was prepared by pressurizing the alloys on 0.1 MPa hy-
drogen gas and soaking protium absorbed alloys in ethanol to
confine the protium in the sample.
PCT curves were measured with a Sieverts-type apparatus
at 313K. Each sample was put into a vessel and was evac-
uated at 313K for 2h, using a rotary vacuum pump. The
alloys absorb protium fully at first hydrogen charge process,
so initial activation treatments were unnecessary. Hydrogen
was introduced gradually into the vessel up to a pressure of
10MPa. The third cycle of the PCT curve is shown as the
protium absorption-desorption property in this study.
3. Results and Discussion
Figure 1 shows XRD patterns of cast state and heat-
treated Ti–60Cr alloys, and Ti–56.5Cr–2.5X heat-treated al-
loys (X=V, Mo, W, Nb, Ta). It was shown that main phase of
Crystal Structure and Protium Absorption Properties of Ti–Cr–X Alloys 471
Fig. 1 XRD patternsof as-castand heat-treated Ti–60Cr and Ti–56.5Cr–2.5X
heat-treated alloys (X=V, Mo, W, Nb, Ta).
the binary alloys were C14-type Laves phase, and BCC phase
is appeared in heat-treated alloys. All Ti–56.5Cr–2.5X alloys
in as-cast state consist of only Laves phase, but the Ti-56.5Cr–
2.5V, Ti–56.5Cr–2.5Mo and Ti–56.5Cr–2.5W heat-treated al-
loys contain only BCC solid solution phase. The Laves phase
was observed with small amount of secondary BCC phase
in the Ti–56.5Cr–2.5Nb and Ti–56.5Cr–2.5Ta alloys. It was
found that the addition of V, Mo or W element to the Ti–Cr
alloys had an effect to form BCC phase in the alloys. Figure
2 shows PCT curves of Ti–60Cr and Ti–56.5Cr–2.5X heat-
treated alloys at desorption process. The protium contents of
the alloys at 10MPa hydrogen increased with V, Mo or W
addition, and V and Mo added alloys exhibited the flattened
plateau regions and the capacities of 3.6mass% protium. It
was found that Mo acts as BCC former similar to V element
in the Ti–Cr alloys. The Ta added alloy had no plateau region
in pressure range of this study, since it consists of mostly C14
type laves phase. It was foundthe capacityof the alloys would
be related with the ability of formation BCC phase, and it is
not necessary to include V, which knows as expensive ele-
ment, by containing Mo element. Mo could possibly replace
V in the Ti–Cr BCC solid solution alloys.
Figure 3 shows XRD patterns of Ti–56.5Cr–2.5Mo heat-
treated alloy in various hydrogenated stages: A) as-prepared
state (0mass%; H/M = 0), B) hydrogenated stage at 0.1MPa
for absorption process (1.1mass; H/M = 0.6) and C) at des-
orption process (3.3mass%; H/M = 1.7). The virgin sam-
ple had BCC phase and a lattice constant of the sample was
0.304nm. The hydrogenated sample (B) with lower pro-
tium content than that of plateau region had also BCC phase
(0.314nm). The XRD pattern of hydrogenated sample (C)
with higher protium content than plateau region showed FCC
phase (0.428nm). Considerations of BCT unit cell in FCC
lattice structure(c), lattice parameters of a-axis and c-axis
were 0.303nm and 0.428 nm respectively. The variation of
these lattice parameters was different from that of vanadium
hydrogenation. Vanadium has two hydrides, VH
∼1
and VH
2
,
0.01
0.1
1
10
4.03.02.01.00.0
V
Mo
W
Nb
Ta
Ti-60Cr
PH2/MPa
Fig. 2 PCT curves of Ti–60Cr and Ti–56.5Cr–2.5X heat-treated alloys at
desorption process (X=V, Mo, W, Nb, Ta).
Fig. 3 XRD patterns of Ti–56.5Cr–2.5Mo heat-treated alloy on varioushy-
drogenated stages: as-prepared state (marked A in PCT curve), hydro-
genated stage on 0.1MPa at absorption process (marked B) and at desorp-
tion process (marked C).
which had different structures of BCT and FCC respectively.
Figure 4 shows PCT curves of Ti-(60 − x)Cr-xMo heat-
treated alloys (x = 0, 0.5, 1.0, 2.0, 2.5) at desorption
process. XRD study showed that the alloys containing
more than 1.0at%Mo consist of mainly the BCC phase,
whereas the alloys without and with 0.5Mo contained a large
amount of Laves phase in addition to the BCC phase. Ti–
Cr–(1.0–2.5)Mo alloys with only the BCC phase yield the
highest capacity of about 3.6 mass% protium and had flat
plateau regions with the capacity of about 2.2mass% pro-
tium. The plateau pressure of the alloys containing more than
1.0at%Mo increased with increasing of Mo content in the al-
loys.
472 A. Kamegawa et al.
0.01
0.1
1
10
4.03.02.01.00.0
Ti-Cr
0.5Mo
1.0Mo
2.0Mo
2.5Mo
PH2/MPa
Fig. 4 PCT curves of Ti-(60 − x)Cr-xMo heat-treated alloys at desorption
process (x = 0, 0.5, 1.0, 2.0, 2.5).
Fig. 5 XRD patterns of Ti-(60 − x)Cr-xMo as-cast alloys (x = 2.5, 5, 10,
20, 30, 50).
Figure 5 shows XRD patterns of Ti-(60 − x)Cr-xMo as-
cast alloys (x = 2.5, 5, 10, 20, 30, 50). The alloys contain-
ing less than 2.5at%Mo consists of only the Laves phase as
mentioned earlier whereas the 5.0Mo alloy contained small
amounts of secondary Laves phase in addition to the BCC
phase as main phase. The as-cast state alloys with more than
10at%Mo had only the BCC phase. Figure 6 shows the cor-
responding PCT curves of these alloys. The all curves of
the alloys showed absorption-desorption hysteresis loops and
sloped plateau regions, even the curve of Ti–10Cr–50Mo al-
loys had small hysteresis loops and the capacity of 1.1mass%
protium. The protium absorbing amount decrease with in-
creasing the Mo content in the alloys.
Figure 7 shows PCT curves of Ti-(60 − x)Cr-xMo heat-
treated alloys (x = 2.5, 5, 10, 20, 30, 50). Only BCC phase
were observed in all heat-treated alloys. The slope of the
plateau region in these alloys decreased after heat-treatment
except for the alloy containing 50at%Mo. The 50Mo alloy
did not change the shape of the curve by heat-treatment at
0.01
0.1
1
10
4.03.02.01.00.0
5Mo
10Mo
20Mo
30Mo
50Mo
P
H
2/MPa
Fig. 6 PCT curves of Ti-(60− x)Cr-xMo as-cast alloys (x = 5, 10, 20, 30,
50).
0.01
0.1
1
10
4.03.02.01.00.0
2.5Mo
5Mo
10Mo
20Mo
30Mo
50Mo
P
H
2
/MPa
Fig. 7 PCT curves of Ti-(60 − x)Cr-xMo heat-treated alloys (x = 2.5, 5,
10, 20, 30, 50).
1673K for 1min, because it was expected that the annealing
temperature was too low to homogenize the alloy or the time
of the annealing was too short. The capacity of the alloys de-
creased with increasing Mo content and the pressure of the
plateau increased. However, XRD study showed that the lat-
tice parameters of the phase in the alloys increased with in-
creasing Mo content. For example, the 2.5, 5, 10, 20, 30 and
50Mo alloys had the lattice parameters 0.302, 0.305, 0.306,
0.308, 0.311 and 0.315nm, respectively. This is because the
atomic radius of Mo is larger than that of Cr. Figure 8 shows
the corresponding PCT curves with the horizontal axis de-
scribed by H/M ratio at desorption process in these alloys.
The H/M ratio of the alloy with less than 10at%Mo was 1.8 at
10MPa hydrogen pressure and was unchanged with Mo con-
tent. The ratio of H/M of the alloy with more the 20at%Mo
decreased with increasing the content of Mo.
4. Conclusions
The protium absorption desorption properties of Ti–Cr–X
alloys (X=Mo, W, Nb, Ta) were investigated and the follow-
ing conclusions were made.
Crystal Structure and Protium Absorption Properties of Ti–Cr–X Alloys 473
0.01
0.1
1
10
2.01.61.20.80.40.0
H / M
2.5Mo
5Mo
10Mo
20Mo
30Mo
50Mo
P
H
2/MPa
Fig. 8 PCT curvesof Ti-(60−x)Cr-xMo heat-treated alloys with horizontal
axis of H/M ratio (x = 2.5, 5, 10, 20, 30, 50).
The addition of V, Mo or W element to Ti–Cr alloys stabi-
lizes BCC phase. The effects of Mo addition to Ti–Cr alloys
give not only flattened plateau regions and also the capacities
of 3.6mass% protium similar to Ti–Cr–V alloys.
The phase change in the Ti–Cr–Mo alloys during the hy-
drogenation process was studied. BCC structure was formed
at the ratio of H/M = 0.6 or below, then, a FCC structure
appeared at higher protium concentration region passing the
plateau region.
The as-cast state alloys with more than 10at%Mo consist
of only the BCC phase. The all curves in the PCT measure-
ments of the alloys showed absorption-desorption hystere-
sis loops and sloped plateau regions, and they are flattened
by heat-treatment. The curve of Ti–10Cr–50Mo alloys show
small hysteresis loops and the capacity of 1.1mass% protium.
The highest value of the capacity in this study yields
3.6mass% protium of Ti–Cr–(1.0–2.5)Mo. However, it was
found that the protium content in the alloys containing more
than 2.5at%Mo decreases with increasing Mo content and the
plateau pressure increased, but there is no change in the H/M
ratio of the alloy with less than 10at%Mo content. The lattice
parameters of the BCC phase in the Ti–Cr–Mo alloys increase
with increasing Mo content.
Acknowledgments
This work has been supported in part by a Grant-in-Aid
for Scientific Research on Priority Area A of “New Protium
Function” from the Ministry of Education, Culture, Sports,
Science and Technology.
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