Role of twin boundary mobility in performance of the Ni-Mn-Ga single crystals

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Kemian tekniikan korkeakoulu | Doctoral thesis (article-based)
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Aalto University publication series DOCTORAL DISSERTATIONS , 109/2011
Ni-Mn-Ga alloys have attracted great interest for more than ten years from the scientific community because of the exceptional properties of their twinned martensitic microstructure, combined with their magnetic properties such as high anisotropy. The high mobility of their martensitic twin variant boundaries enables the magnetic shape memory actuation and the high mechanical vibration damping capacity, to name just a few applications. Their material properties are in several ways unique, such as the large magnetic-field-induced strain of several per cent operating at frequencies of several hundred Hz. The most mobile twin boundaries have been found in high-quality single crystals. However, the twin boundary mobility of different crystals often varies significantly. It may even vary in the same crystal, which is not favorable for their practical use. In this work, twin mobility of several types of Ni-Mn-Ga single crystals is studied under monotonic uniaxial and shear loading as well as in dynamic loading at different temperatures. Furthermore, the performance of the material is studied by mechanical and magneto-mechanical cycling of the twin boundaries to reveal changes in the material properties. The results show that the stress needed for the twin mobility in both uniaxial and shear mode can be very low. In the shear mode the twin boundary motion can start even at 0.07 to 0.23 MPa stress. However, the stress onset for the twin boundary motion in the single variant state can be more than a decade higher than in the state with existing twin boundaries. As demonstrated in this Thesis by 2 × 109 cycles at 2 % strain peak-to-peak, 10M Ni-Mn-Ga has potentially a long fatigue life, however several reasons which may reduce their long-term performance are confirmed or proposed.
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Hannula, Simo-Pekka, Prof.
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Hannula, Simo-Pekka, Prof.
Ni-Mn-Ga, martensite, twinning, fatigue, ferromagnetic shape memory, damping
Other note
  • [Publication 1]: I. Aaltio, O. Heczko, O. Söderberg and S-P. Hannula. Magnetically Controlled Shape Memory Alloys. In CRC Handbook "'Smart Materials"', M. Schwartz (Editor), pp. 20-1–20-8, 2009.
  • [Publication 2]: I. Aaltio, O. Söderberg, Y. Ge, S-P. Hannula. Twin boundary nucleation and motion in Ni-Mn-Ga magnetic shape memory material with a low twinning stress. Scripta Materialia, 62, pp. 9-12, Doi:10.1016/j.scriptamat.2009.09.012, September 2010. © 2009 Acta Materialia. By permission.
  • [Publication 3]: I. Aaltio, M. Lahelin, O. Söderberg, O. Heczko, B. Löfgren, Y. Ge, J. Seppälä and S-P. Hannula. Temperature dependence of the damping properties of Ni-Mn-Ga alloys. Materials Science and Engineering: A, 481-482, pp. 314-317, Doi: 10.1016/j.msea.2006.12.229, December 2008. © 2007 Elsevier. By permission.
  • [Publication 4]: I. Aaltio, K. P. Mohanchandra, O. Heczko, M. Lahelin, Y. Ge, G. P. Carman, O. Söderberg, B. Löfgren, J. Seppälä, S-P. Hannula. Temperature dependence of mechanical damping in Ni-Mn-Ga austenite and non-modulated martensite. Scripta Materialia, 59, pp. 550-553, Doi: 10.1016/j.scriptamat.2008.05.005, May 2008. © 2008 Acta Materialia. By permission.
  • [Publication 5]: I. Aaltio, Y. Ge, X. W. Liu, J. Tellinen, O. Söderberg and S-P. Hannula. Effect of magneto-mechanical cycling on 10M Ni-Mn-Ga magnetic shape memory material. In G. B. Olson, D. S. Liebermann, A. Saxena (Editors), Proceedings of the International Conference on Martensitic Tranformations ICOMAT 2008, pp. 487-491, ISBN 978-0-87339-745-2, 2009. © 2009 The Minerals, Metals & Materials Society (TMS). By permission.
  • [Publication 6]: I. Aaltio, A. Soroka, Y. Ge, O. Söderberg and S-P. Hannula. High-cycle fatigue of 10M Ni-Mn-Ga magnetic shape memory alloy in reversed mechanical loading. Smart Materials and Structures, 19, 075014, 10 pp, Doi:10.1088/0964-1726/19/7/075014, June 2010. © 2010 Institute of Physics Publishing (IOPP). By permission.