Examination of Multiphase (Zr,Ti)(V,Cr,Mn,Ni)2 Ni-MH Electrode Alloys: Part II. Solid-State Transformation of the Interdendritic B2 Phase

Abstract

Solidification microstructure of multicomponent (Zr,Ti)-Ni-(V,Cr,Mn,Co) alloys intended for use as negative electrodes in Ni-metal hydride (Ni-MH) batteries was studied in Part I of this series of articles. Part II of the series examines the complex internal structure of the interdendritic grains formed by solid-state transformation and believed to play an important role in the electrochemical charge/discharge characteristics of the overall alloy composition. By studying one alloy, Zr 21 Ti 12.5 V 10 Cr 5.5 Mn 5.1 Co 5.0 Ni 40.2 Al 0.5 Sn 0.3 , it is shown that the interdendritic grains solidify as a B2 (Ti,Zr) 44 (Ni,TM) 56 phase, and then undergo transformation to Zr 7 Ni 10 -type, Zr 9 Ni 11 -type, and martensitic phases. The transformations obey orientation relationships between the high-temperature B2 phase and the low-temperature Zr-Ni-type intermetallics, and consequently lead to a multivariant structure. The major orientation relationship for the orthorhombic Zr 7 Ni 10 type is [011] Zr7Ni10 //[001] B2 ; (100) Zr7Ni10 //(100) B2 . The orientation relationship for the tetragonal Zr 9 Ni 11 type is [001] Zr9Ni11 //[001] B2 ; (130) Zr9Ni11 //(100) B2 . Binary NiZr and ternary Ti-Ni-Zr phase diagrams were used to rationalize the formation of the observed domain structure.

C14 Laves Phase Metal Hydride Alloys for Ni/MH Batteries Applications

“…The TiNi phase further transforms into Zr7Ni10, Zr9Ni11, and ZrNi phases via solid-state reactions [106][107][108][109] (Figure 14). When other modifiers are present, additional secondary phases are also formed.…”

Comparison of C14- and C15-Predomiated AB2 Metal Hydride Alloys for Electrochemical Applications

“…The composition of several representative areas (identified by Roman numerals) in the SEM micrographs were studied by EDS, and the results are summarized in Table 6. SEM micrographs of the C14 alloy shows a very typical multi-phase C14-C15-Zr x Ni y microstructure, which has been extensively studied with transmission electron microscopy (TEM) [76,77] and electron backscattering diffraction (EBSD) [75]. Occasional ZrO 2 inclusions are also seen in the C14 alloy and act as oxygen scavengers [78], which may contribute positively to the bulk diffusion of hydrogen and provide surface protection against oxidation by the electrolyte [79].…”

Studies on Incorporation of Mg in Zr-Based AB2 Metal Hydride Alloys

“…The B/A ratio in the TiNi phase is calculated based on the assumption of V occupying the A-site [27] and the results are still higher than 1, which indicates the possibility of other Zr x Ni y secondary phases with higher B/A ratios. The nature of the Zr x Ni y phase was studied before by transmission electron microscopy [62]. The Mg-rich phase (the fourth phase in each BEI micrograph) has an Mg-content from 45.4 at% to 82.1 at%.…”