190
H.T. Takeshita et al. / Journal of Alloys and Compounds 311 (2000) 188–193
nitrogen-containing samples, whereas it was monoclinic
for the carbon-containing sample. The amount of these
minor phases in the ternary sample tended to increase from
the oxygen-containing sample to the carbon-containing
one.
Fig. 3 shows a schematic diagram for the crystal
structure of the Ti2Ni-based compound, where the 8a
position is taken as the present origin for the crystal
structure of the E93 type [5]. For the Ti4Ni2O compound,
oxygen atoms occupy the 16c positions which are the
center of the irregular octahedrons composed of six
titanium atoms [5]; however, no information is available
for the positions of the nitrogen and carbon atoms in the
Ti4Ni2N and Ti4Ni2C compounds, respectively. The calcu-
lated diffraction peak patterns of the Ti4Ni2X (X5O, N,
C) compounds in which the non-metal atoms occupy the
16c positions are shown with the calculated pattern of the
Ti2Ni compound without any atom at the 16c position in
Fig. 4. The effect of these light elements on the diffraction
patterns is mainly observed at low diffraction angles.
Compared with the binary compound, the intensity of the
(111), (222) and (331) peaks decreases, whereas the (311)
peak appears and the intensity of the (400) peak increases.
The XRD profiles measured for the four samples are
redrawn in the 2u range between 10 and 358 in Fig. 5. The
tendency observed in the measured profiles is in a good
agreement with that observed in Fig. 4. It is considered
that not only oxygen but also nitrogen and carbon atoms
occupy the 16c positions in the Ti2Ni-based compounds.
3.2. Pressure–composition isotherms of alloy samples
Fig. 6 shows the PCT relations of the oxygen-, nitrogen-
and carbon-containing samples. The three samples re-
versibly absorbed and desorbed hydrogen at temperatures
ranging from 0 to 808C for the change in hydrogen
pressure ranging between 0.003 and 2 MPa. The maximum
hydrogen content at the hydrogen pressure of 2 MPa was
0.45 H/M at 08C for the oxygen-containing sample, 0.67
H/M at 08C for the nitrogen-containing one and 0.59 H/M
at 408C for the carbon-containing one. These samples
exhibited hydrogen pressure plateau-like regions in which
the slopes of the curves were smaller than those in other
hydrogen-content ranges. The plateau-like regions were
observed in the range of hydrogen content from 0.1 in the
atomic ratio of hydrogen to metal (H/M) to 0.4 H/M for
the oxygen-containing sample, from 0.15 H/M to 0.5 H/M
for the nitrogen-containing sample and from 0.3 H/M to
0.5 H/M for the carbon-containing sample. The ‘hyster-
esis’ effect, which was observed as the difference between
hydrogen absorption and desorption pressures, was only
slightly observed in the PCT relations.
Fig. 2. Metallographic structures of alloy samples with nominal com-
positions of Ti57.1Ni28.6X14.3 (X5O, N, C).
˚
oxygen-containing sample, 11.3260.01 A for the nitrogen-
˚
containing one, 11.3660.01 A for the carbon-containing
˚
one and 11.3360.01 A for the binary Ti2Ni compound
prepared for comparison. The averages and the three X
standard deviations for the oxygen, nitrogen and carbon
contents in the Ti2Ni-type phases, which were obtained by
the quantitative SEM-EDS analysis, were 1463, 1462 and
1463 atomic percent, respectively. It was found from this
quantitative analysis that the compositions of the Ti2Ni-
type phases could be expressed as Ti4Ni2X (X5O, N, C).
The minor phases in the ternary samples were found to
be TiNi and TiX (X5O, N, C) based on the XRD and
quantitative SEM-EDS analyses. The crystal structure of
the TiNi phase was cubic (B2-type) for the oxygen- and
Fig. 7 shows the PCT relations of the TiNi and Ti2Ni
compounds prepared for comparison. These compounds
did not show any hydrogen pressure plateau under the
present conditions. The TiNi compound with the mono-