Shape memory alloy Co-Ni-Al as complex multiferroic

J. Kopeček¹, M. Jarošová², K. Jurek², J. Drahokoupil¹, I. Kratochvílová¹, L. Fekete¹, L. Bodnárová³, H. Seiner³, P. Sedlák³, M. Landa³, J. Šepitka⁴, J. Lukeš⁴, V. Kopecký¹, O. Heczko¹

¹ Institute of Physics of the AS CR, Na Slovance 2, 182 21 Praha 8, Czech Republic

² Institute of Physics of the AS CR, Cukrovarnická 10/112, 162 00 Praha 6, Czech Republic

³ Institute of Thermomechanics of AS CR, Dolejškova 5, 182 00 Prague 8, Czech Republic

⁴ Laboratory of Biomechanics, CTU in Prague, Technická 4, 166 07, Prague 6, Czech Republic

kopecek@fzu.cz

Keywords: shape memory alloys, martensitic transformation, metallography, SEM, EBSD

Great success in Ni₂MnGa derived alloys [1,2] attracted attention towards similar Heusler alloys including cobalt based CoNiAl and CoNiGa [3,4]. As the NiMnGa alloys suffer due to their strongly intermetallic state (brittleness, poor creep and fatigue properties) the cobalt based alloys seemed to be the interesting candidate for the mechanically stronger and more resistant FSMAs.

The article describes the progress in work on Co₃₈Ni₃₃Al₂₉ alloy [5,6]. The defined crystals with single-crystalline matrix were prepared after long struggling. The influence of annealing on martensitic transformation was investigated. Both post-mortem XRD and in-situ neuron diffraction confirmed the martensitic phase transformation of alloy matrix B2 « L1₀ and stable amount of A1 particles (fcc cobalt solid solution) in alloy, Fig. 1. The image of transformation paths is blurred considering the results of resonant ultrasound spectroscopy (RUS), magnetic susceptibility measurements and various microscopies (LOM, SEM, AFM), which shows transformation temperature significantly higher (about approx. 70 °C). The strong premartensitic phenomena can be documented by the evolution of damping in RUS. Regardless to structural confusion all samples exhibit pseudoelastic behaviour at room temperature, which is strongly dependent on crystallographic orientation as shown in Fig. 2.

Figure 1. The structure of the samples observed by scanning electron microscopy. The precipitates marked 1 are interdendritic A1 fcc cobalt solid solution particles. The precipitates marked 2 are L1₂ ordered precipitates of the phase (Co,Ni)₃Al.

Figure 2 Superelastic behaviour in Co38Ni33Al29 alloy single-crystals is strongly dependent on orientation. The measurements were performed at room temperature with deformation rate 0.1 s^-1.

References

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Karaman, H. Maier, E. Cesari and V. A. Kirillov, Russ. Phys. J., 51, (2008), 1016.

5. J. Kopeček, S. Sedláková-Ignácová, K. Jurek, M. Jarošová, J. Drahokoupil, P. Šittner, V. Novák: Structure development in Co₃₈Ni₃₃Al₂₉ ferromagnetic shape memory alloy, 8^th th European Symposium on Martensitic Transformations, ESOMAT 2009, edited by Petr Šittner, Václav Paidar, Luděk Heller, Hanuš Seiner, 2009, article No. 02013.

6. J. Kopeček, K. Jurek, M. Jarošová, et al., IOP Conf. Sci.: Mater. Sci. Eng., 7, (2010), 012013.

Acknowledgements.

Authors would like to acknowledge the financial support from the Grant Agency of the AS CR project IAA100100920 and Czech Science Foundation projects 101/09/0702, P107/11/0391 and P107/10/0824.