Clean Grain Boundary Found in C14/Body-Center-Cubic Multi-Phase Metal Hydride Alloys
The grain boundaries of three Laves phase-related body-center-cubic (bcc) solid-solution, metal hydride (MH) alloys with different phase abundances were closely examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and more importantly, electron backscatter diffraction (EBSD) techniques. By using EBSD, we were able to identify the alignment of the crystallographic orientations of the three major phases in the alloys (C14, bcc, and B2 structures). This finding confirms the presence of crystallographically sharp interfaces between neighboring phases, which is a basic assumption for synergetic effects in a multi-phase MH system.
The structure of TiNi martensite
The crystal structure of the martensite (low-temperature) phase of the intermetallic compound TiNi has been determined based upon single-crystal electron and powder X-ray diffraction data. The structure is monoclinic, P1121/m, with dimensions a = 2.885 (4), b=4.622 (5) ,c=4.120 (5) A and 7= 96.8 (1.0) °. The atomic positions were determined by considering the collapse of the high-temperature structure occurring by a shuffle of its {110} planes, taking into account the constraints imposed by the difference in size of the Ti and Ni atoms. The final structure yielded an R of 0.17 for the first 22 reflections and contains a set of interatomic bond lengths commensurate with those observed in other structures in the Ti-Ni system.
Effects of annealing on Zr8Ni19X2 (X = Ni, Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf): Hydrogen storage and electrochemical properties
Abstract The effects of 8-h annealing at 960 °C on the gaseous phase hydrogen storage and electrochemical properties of partial-Ni substituted Zr 8 Ni 21 alloys were studied. The substituting elements included Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf. Only the main phase of the annealed Sn-substitution remained Zr 8 Ni 21 -structured while those of other substitutions turned into Zr 7 Ni 10 or Zr 2 Ni 7 . The observed trend in maximum gaseous phase hydrogen storage capacity followed the increasing order of B/A ratio of the main phase as orthogonal Zr 7 Ni 10  > tetragonal Zr 7 Ni 10  > Zr 8 Ni 21  > Zr 2 Ni 7 . After annealing, due to the increase in abundance of the main phase, the maximum gaseous phase hydrogen storage capacities of alloys with higher capacities before annealing increased while others decreased. The full discharge capacity also improved in the same increasing order of B/A ratio in the main phase. Hf-substitution showed the highest electrochemical discharge capacity at 200 mAh g −1 . After annealing, all alloys with the same main phase as the as-cast alloys showed degradation in full electrochemical capacity due to the reduction in both number and abundance of the catalytic secondary phases. All supplements assisted in improving surface exchange current from the base binary Zr 8 Ni 21 alloy. Except for La- and Hf-substitutions, annealing reduced the surface exchange current density. The bulk hydrogen diffusion coefficient decreased with most of the supplements except for V- and Sn-substitutions. All supplements, except for Sc, showed improvement in the bulk diffusion after annealing. Furthermore, the maximum gaseous phase hydrogen storage capacity showed a strong correlation to the full electrochemical discharge capacity. Among all alloys in this study, the as-cast Hf-substituted Zr 8 Ni 21 alloy demonstrated the best overall gaseous hydrogen storage and electrochemical properties.