Toward the final explanation of martensitic transformation in shape memory alloy Co-Ni-Al

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

Great success in Ni₂MnGa derived alloys [1,2] attracted attention to 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). Without regard to structural confusion all samples perform pseudoelastic behaviour at room temperature, which is strongly dependent on crystallographic orientation.

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.

Nevertheless, the role of nanoprecipitates resulting from segregation in oversaturated matrix after annealing or other changes in matrix induced by quenching seems to be indisputable [7]. These precipitates can serve as nucleation centres for the stress induced martensitic transformation. Nanoprecipitates were observed in annealed samples but in both pseudoelastic [8] and non-pseudoelastic samples [9]. The wide variety of nanoprecipitates is described in Ref. [4], nevertheless their necessity for pseudoelastic behaviour was not proven. The many micron-sized non-twinned, single and triple {111}_p twinned precipitates with partial L1₂ ordering were observed in the austenite matrix after annealing at 1373 K for 72 h and quenching [10]. The final description of A1 interdendritic particles dissolution and the role of precipitation after quenching is still under the process.

The more optimistic view we have now on various results of “transformation” given by different methods. The strong magnetoelastic coupling can be documented by the evolution of damping in RUS [11]. Surprisingly the effect of magnetic field in Co-Ni-Al austenite, considering the external field, is very weak [12]. We can expect now that the finalization of the oriented cuboid samples will give us the full elastic constants set in temperature dependence and the story of “martensitic transformation” in Co-Ni-Al alloys would be finally explained.

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7. J. Kopeček, V. Kopecký, M. Landa, O. Heczko, Mat. Sci. Forum, 738-739, 416-420, (2013)

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9. B. Bártová, D. Schryvers, Z. Q. Yang, S. Ignácová, P. Sittner, Scripta Mater., 57 (2007), 37- 40.

10. J.B. Lu, H. Shi, S. Sedláková-Ignácová, R. Espinoza, J. Kopeček, P. Šittner, B. Bártová, D. Schryvers, J. Alloys Comp., 572, 5-10, (2013)

11. H. Seiner, J. Kopeček, P. Sedlák, L. Bodnárová, M. Landa, P. Sedmák, O. Heczko, Acta Mater., 61, 5869-5876, (2013)

12. O. Heczko, H. Seiner, P. Sedlák, J. Kopeček, V. Kopecký, M. Landa, Eur. Phys. J. B, 86:(2), 62-1 – 62-5, (2013

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.