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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Magnetic and Magneto-Mechanical Properties of Ni-Mn-Ga Magnetic Shape Memory Alloys

Ladislav Straka

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics for public examination and debate in Auditorium F1 at Helsinki University of Technology (Espoo, Finland) on the 15th of June, 2007, at 12 noon.

Overview in PDF format (ISBN 978-951-22-8820-5)   [1119 KB]
Dissertation is also available in print (ISBN 978-951-22-8819-9)

Abstract

Ni-Mn-Ga alloys close to stoichiometric Ni50Mn25Ga25 (at. %) composition have recently gained considerable interest due to the possibility of rearrangement of their martensite microstructure in magnetic field. The rearrangement is accompanied by large strains of up to 10%. This effect is different from ordinary magnetostriction and it is referred to as magnetic shape memory effect (MSME).

The Thesis presents the first attempt to study the temperature limits of irreversible and reversible MSME by exploiting a theoretical model and experimentally determined temperature dependences of magnetic and other material properties governing the existence of MSME. The obtained predictions are compared with direct observations of MSME. Extraordinary magneto-mechanical effects in Ni-Mn-Ga alloys, not discussed previously, are investigated and compared with theoretical models in this Thesis. These effects include reversible MSME with strain close to 6%, magnetic field controlled superelasticity with strain close to 6%, and up to 30% changes of magnetization during loading in static magnetic field. Unique simultaneous measurements of strain and magnetization on Ni-Mn-Ga alloys are presented for various experiments such as, e.g., during MSME and reversible MSME.

The broad spectrum of experiments presented in the Thesis corroborates the important role of magnetic anisotropy, twinning stress and temperature for existence and reversibility of MSME. The simultaneous measurement of strain and magnetization in various experiments confirms experimentally the close relation between martensite microstructure and its magnetic properties and demonstrates the interplay between martensite microstructure and magnetic field. Good agreement of all presented experimental results with the used theoretical model supports validity of the model and shows that the model is suitabile for predicting temperature and stress limits of MSME or reversible MSME, and for modelling of magnetic-field induced superelasticity. Some of the presented experiments can additionally be considered as application examples. The original findings presented in this Thesis broaden the general scientific understanding of MSME and can serve as informative source when considering possible engineering usage of Ni-Mn-Ga alloys as actuators, sensors, or intelligent material.

This thesis consists of an overview and of the following 7 publications:

  1. L. Straka, O. Heczko, V. Novák, and N. Lanska, Study of Austenite-Martensite Transformation in Ni-Mn-Ga Magnetic Shape Memory Alloy, Journal de Physique IV 112 (2003) 911-915. © 2003 EDP Sciences. By permission.
  2. O. Heczko, L. Straka, N. Lanska, K. Ullakko, and J. Enkovaara, Temperature Dependence of Magnetic Anisotropy in Ni–Mn–Ga Alloys Exhibiting Giant Field-Induced Strain, Journal of Applied Physics 91 (2002) 8228-8230. © 2002 American Institute of Physics. By permission.
  3. L. Straka, O. Heczko, and K. Ullakko, Investigation of Magnetic Anisotropy of Ni–Mn–Ga Seven-Layered Orthorhombic Martensite, Journal of Magnetism and Magnetic Materials 272-276 (2004) 2049-2050. © 2004 Elsevier Science. By permission.
  4. O. Heczko and L. Straka, Temperature Dependence and Temperature Limits of Magnetic Shape Memory Effect, Journal of Applied Physics 94 (2003) 7139-7143. © 2003 American Institute of Physics. By permission.
  5. L. Straka and O. Heczko, Superelastic Response of Ni–Mn–Ga Martensite in Magnetic Fields and a Simple Model, IEEE Transactions on Magnetics 39 (2003) 3402-3404. © 2003 IEEE. By permission.
  6. L. Straka and O. Heczko, Magnetization Changes in Ni–Mn–Ga Magnetic Shape Memory Single Crystal During Compressive Stress Reorientation, Scripta Materialia 54 (2006) 1549-1552. © 2006 Elsevier Science. By permission.
  7. L. Straka, O. Heczko, and S.-P. Hannula, Temperature Dependence of Reversible Field-Induced Strain in Ni–Mn–Ga Single Crystal, Scripta Materialia 54 (2006) 1497-1500. © 2006 Elsevier Science. By permission.

Keywords: Ni-Mn-Ga, martensite, magnetic shape memory, temperature dependence

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© 2007 Helsinki University of Technology

Last update 2011-05-26