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Hexagonal Co-rich and L12 ordered precipitates in annealed Co-Ni-Al shape memory alloy
|Barbora Bartova 1, Dominique Schryvers 1, Zhiqing Yang 1, Petr Sittner 2, Silvia Ignacova 2|
1. EMAT, University of Antwerp, Groenenborgerlaan 171, Antwerp B-2020, Belgium
Ferromagnetic shape memory alloys (FSMAs) are presently intensively studied because of their potential applications as smart materials. Martensitic transformations and lattice reorientation processes in FSMAs can be triggered not only by changes in temperature and stress, as in conventional paramagnetic SMAs, but also by applying a magnetic field. Recently, Co-Ni-Al alloys have attracted increasing interest as new FSMAs [1,2].
Conventional transmission electron microscopy and electron diffraction studies together with advanced microscopy techniques (HRTEM, EFTEM) were carried out in order to characterize the microstructure of annealed (1275°C/4h and 1100°C/72h) Co38Ni33Al29. In addition to the major constituents (B2 type ordered β-phase and f.c.c. ordered γ-phase), rod shaped precipitates ranging from 20 to 75 nm of h.c.p. ε-Co were observed in the 1275°C/4h sample (Fig.1). EFTEM measurements confirm that the precipitates are Co-rich (Fig.2). The orientation relationship between the nanosized precipitates and the B2 matrix is found to be the Burgers orientation relation. Martensitic transformation to the tetragonal L1o structure was observed at -40°C which was confirmed by in-situ experiment and DSC measurements. Large precipitates (Co,Ni)3Al with size up to several microns were found in specimen annealed at 1100°C/72h. These precipitates show the L12 ordered structure while a majority contain a single twin boundary (Fig.3).
 Karaca H.E. et al., 2004, Scripta Mater. 51, 261-266
 Kainuma R. et al., 1996, Intermetallics 4, S151-S158
Acknowledgements: B. Bartova and S. Ignacova like to thank MULTIMAT “Multi-scale modeling and characterization for phase transformations in advanced materials”, a Marie Curie Research Training Network (MRTN-CT-2004-505226) for supporting this work.
Presentation: Oral at E-MRS Fall Meeting 2007, Symposium E, by Barbora Bartova
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-10 14:19 Revised: 2009-06-07 00:44