Examination of multiphase (ZR,TI)(V,CR,MN,NI)2 NI-MH electrode alloys: Part II. Solid-state transformation of the interdendritic B2 phase

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 ₂₁Ti _12.5V ₁₀Cr _5.5Mn _5.1Co5.0Ni40.2Al0.5Sn0.3, it is shown that the interdendritic grains solidify as a B2 (Ti,Zr)44(Ni,TM)56 phase, and then undergo transformation to Zr ₇Ni ₁₀-type, Zr ₉Ni 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 ₇Ni ₁₀ type is [011] _Zr7Ni10//[001] _B2; (100) _Zr7Ni10///(100)B2. The orientation relationship for the tetragonal Zr9Ni11 type is [001] _Zr9Ni11//[001] _B2; (130) _Zr9Ni11//(100)B2. Binary Ni-Zr and ternary Ti-Ni-Zr phase diagrams were used to rationalize the formation of the observed domain structure.