J. Phys. IV France 112 (2003) 1021
DOI: 10.1051/jp4:20031055

Microstructures and transformation behavior of CoNiGa ferromagnetic shape memory alloys

Y. Kishi¹, M. De Graef², C. Craciunescu¹, T.A. Lograsso³, D.A. Neumann⁴ and M. Wutting<sup>1

1</sup>  Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115, U.S.A.
²  Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, U.S.A.
³  Ames Laboratory, Iowa State University, Ames, IA, U.S.A.
⁴  Center for Neutron Research, Nationallnstitute of Standards & Technology, Gaithersburg, MD 20899, U.S.A.

Abstract
TEM observations and transformation characteristics of Co2Ni, xGal, ferromagnetic shape memory alloys will be presented. The dependence of the martensitic transformation start temperatures on the valance-electron atom ratio is identical to that of NiMnGa alloys. Optical microstructures show typical self-accommodated structures. The domain boundaries are straight indicating only elastic distortions in the product phase. High-density striations are observed in the TEM bright field images of the martensite-phase. Selected area electron diffraction patterns and TEM dark field images revealed that they are thin twin plates. Tweed or striated contrast is observed in the TEM bright field images of the parent phase at temperatures about 70 K higher than the M, temperature.

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