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Microstructural Studies of Multiphase (Zr,Ti)(V,Cr,Mn,Co,Ni)2 Alloys for NiMH Negative Electrodes

Published online by Cambridge University Press:  18 January 2011

L. A. Bendersky ,
K. Wang ,
W. J. Boettinger ,
D. E. Newbury ,
K. Young  and
B. Chao
L. A. Bendersky
Affiliation:
Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
K. Wang
Affiliation:
Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
W. J. Boettinger
Affiliation:
Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
D. E. Newbury
Affiliation:
Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
K. Young
Affiliation:
Energy Conversion Devices Inc., Rochester Hills, MI 48309, USA
B. Chao
Affiliation:
Energy Conversion Devices Inc., Rochester Hills, MI 48309, USA
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Abstract

The solidification microstructures of six Laves-based (Zr,Ti)(TM,Ni)2 alloys (TM= V,Cr, Mn,Co) intended for use as novel negative electrodes in Ni-metal hydride batteries were studied here; these alloys often have their best electrochemical properties when in the cast state. Solidification occurs by dendritic growth of a hexagonal C14 Laves phase followed by peritectic solidification of a cubic C15 Laves phase and formation of a cubic B2 phase in interdendritic regions. The observed sequence of Laves phase C14/C15 upon solidification agrees with predictions using effective compositions and thermodynamic assessments of the ternary systems, Ni-Cr-Zr and Cr-Ti-Zr. The paper also examines the complex internal structure of the interdendritic grains formed by solid-state transformation, which plays an important role in the electrochemical charge/discharge characteristics. By studying one alloy it is shown that the interdendritic grains solidify as a B2 (Ti,Zr)44(Ni,TM)56 phase, and then undergo transformation to Zr7Ni10-type, Zr9Ni11-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. Binary Ni-Zr and ternary Ti-Ni-Zr phase diagrams were used to rationalize the formation of the observed domain structure.

Keywords

transmission electron microscopy (TEM)scanning electron microscopy (SEM)phase transformation
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1295: Symposium N – Intermetallic-Based Alloys for Structural and Functional Applications , 2011 , mrsf10-1295-n06-06
Copyright
Copyright © Materials Research Society 2011

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References

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