  The Y33.33Ni66.67-xAlx system has been investigated in the region 0 <= x <=
33.3. The alloys were synthesized by induction melting. Phase occurrence and
structural properties were studied by X-Ray powder Diffraction (XRD). The Al
solubility in each phase has been investigated by XRD and Electron Probe
Micro-Analysis (EPMA). The hydrogenation properties were characterized by
pressure-composition isotherm measurements and kinetic curves at 473 K. For x =
0, the binary Y33.33Ni66.67 alloy crystallizes in the cubic superstructure with
F4-3m space group and ordered Y vacancies. For 1.67 <= x <= 8.33, the alloys
contain mainly Y(Ni, Al)2 and Y(Ni, Al)3 pseudobinary phases; while for 16.67
<= x <= 33.33 they are mainly formed by ternary line compounds Y3Ni6Al2,
Y2Ni2Al and YNiAl. Contrary to the binary Y33.33Ni66.67, Y(Ni, Al)2
pseudo-binary compounds crystalize in C15 phase (space group Fd-3m ) with
disordered Y vacancies. The solubility limit of Al in the C15 YNi2-yAly phase
is y = 0.11 (i.e., x = 3.67). The Y(Ni, Al)3 phase changes from rhombohedral
(PuNi3-type, R-3m) to hexagonal (CeNi3-type, P63/mmc) structure for x
increasing from 5.00 to 6.67. Upon hydrogenation, the disproportion of C15
Y(Ni, Al)2 and losses of crystallinity of YNi and Y2Ni2Al are the main reasons
causing capacity decay of Y33.33Ni66.67-xAlx (0 <= x <= 33.33) alloys upon
cycling.
