The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Novel Ni-Mn-Ga Alloys and their Magnetic Shape Memory Behaviour

Outi Söderberg

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Materials Science and Rock Engineering for public examination and debate in Auditorium V1 at Helsinki University of Technology (Espoo, Finland) on the 10th of December, 2004, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-7416-7)   [12582 KB]
Dissertation is also available in print (ISBN 951-22-7415-9)

Abstract

The ternary Ni-Mn-Ga system has already been studied for several decades. Although the present work concentrates on the alloys with the 7-layered near-orthorhombic martensite structure (7M) and the non-modulated tetragonal martensite structure (T), it introduces 37 alloys altogether, including materials that have the 5-layered tetragonal martensite (5M) at ambient temperature. The dissertation shows briefly how the different compositions of ternary Ni-Mn-Ga alloys affect the phase transformations and the crystallographic structures of various phases. This thesis presents new alloys with the 7M structure and, for the first time, the Curie point of such a martensitic structure (TC7M). This TC7M is mainly detected in the cooling of the alloys, showing the co-occurrence of the magnetic transition and the double-step reverse transformation where the hysteresis of the Curie point is connected to the two martensitic phases.

The service temperature region of the MSM alloy depends on the existence of the proper ferromagnetic twinned martensite. Consequently, the transformation sequence of the studied 7M and T alloys is investigated in more detail. According to the magnetic and crystalline transformation, the alloys are divided into six groups: A (7M ambient), B (7M above), C (7M co-transition), D (T high), E (T low) and F (T co-transition). In the groups A, B, D and E, the magnetic and crystalline transitions are separated, while in the groups C and F they co-occur.

The pre-straining processes for obtaining the single-variant state in the 7M and the T martensite are presented. In the compression of the 7M alloys, three deformations to two crystallographic directions are needed, while the T alloys require three deformations to three crystallographic directions. However, it is also shown that with the tensile/compressive cycling of the T phase the full single-variant state has been obtained already, during the second cycle. The twinning stresses (σtw) needed for the martensite variant reorientation could be lowered by pre-straining close to 1 MPa in the 7M phase, while in the T phase only the level 6 MPa was reached, even at elevated temperatures. The magneto-mechanical tests confirmed that the magnetically induced stress (Δσmag) in the 7M structure is 1.5 MPa and in the T structure approximately 1 MPa. By applying the criteria of Δσmag ≥ σtw for the magnetic shape memory effect (MSME), it is obvious that MSME is possible in the 7M structure, but it can not be obtained in the T structure.

This thesis consists of an overview and of the following 8 publications:

  1. Lanska N., Söderberg O., Sozinov A., Ge Y., Ullakko K. and Lindroos V. K., 2004. Composition and temperature dependence of the crystal structure of Ni-Mn-Ga alloys. Journal of Applied Physics 95, number 12, pages 8074-8078. © 2004 American Institute of Physics. By permission.
  2. Söderberg O., Friman M., Sozinov A., Lanska N., Ge Y., Hämäläinen M. and Lindroos V. K., 2004. Transformation behavior of two Ni-Mn-Ga alloys. Zeitschrift für Metallkunde 95, number 8, pages 724-731. © 2004 Carl Hanser Verlag. By permission.
  3. Sozinov A., Likhachev A. A., Lanska N., Söderberg O., Koho K., Ullakko K. and Lindroos V. K., 2004. Stress-induced variant rearrangement in Ni-Mn-Ga single crystals with nonlayered tetragonal martensitic structure. In: Lexcellent C. and Patoor E. (editors), Proceedings of the Euromech-Mecamat 2003, 7th European Mechanics of Materials Conference on Adaptive Systems and Materials: Constitutive Materials and Hybrid Structures (EMMC7). Fréjus, France, 18-23 May 2003. Journal de Physique IV - Proceedings 115, pages 121-128. © 2004 EDP Sciences. By permission.
  4. Sozinov A., Likhachev A. A., Lanska N., Söderberg O., Ullakko K. and Lindroos V. K., 2004. Stress- and magnetic-field-induced variant rearrangement in Ni-Mn-Ga single crystals with seven-layered martensitic structure. In: Friend C. (editor), Proceedings of the European Symposium on Martensitic Transformations and Shape-Memory (ESOMAT 2003). Cirencester, England, 17-22 August 2003. Materials Science and Engineering A 378, numbers 1-2, pages 399-402. © 2004 Elsevier Science. By permission.
  5. Gavriljuk V. G., Söderberg O., Bliznuk V. V., Glavatska N. I. and Lindroos V. K., 2003. Martensitic transformations and mobility of twin boundaries in Ni2MnGa alloys studied by using internal friction. Scripta Materialia 49, number 8, pages 803-809. © 2003 Elsevier Science. By permission.
  6. Söderberg O., Straka L., Novák V., Heczko O., Hannula S.-P. and Lindroos V. K., 2004. Tensile/compressive behaviour of non-layered tetragonal Ni52.8Mn25.7Ga21.5 alloy. Materials Science and Engineering A 386, numbers 1-2, pages 27-33. © 2004 Elsevier Science. By permission.
  7. Sozinov A., Likhachev A. A., Lanska N., Söderberg O., Ullakko K. and Lindroos V. K., 2003. Effect of crystal structure on magnetic-field-induced strain in Ni-Mn-Ga. In: Dimitris C. Lagoudas (editor), Smart Structures and Materials 2003: Active Materials: Behavior and Mechanics. San Diego, California, USA, 2-6 March 2003. Proceedings of SPIE 5053, pages 586-594. © 2003 Society of Photo-Optical Instrumentation Engineers (SPIE). By permission.
  8. Söderberg O., Sozinov A. and Lindroos V. K., 2004. Giant magnetostrictive materials. In: Buschow J. (editor), The Encyclopedia of Materials: Science and Technology. Elsevier Science, in press. © 2004 Elsevier Science. By permission.

Keywords: Ni-Mn-Ga alloys, martensite, intermartensitic transformation, twinning, magnetic shape memory

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© 2004 Helsinki University of Technology

Last update 2011-05-26