J. Phys. IV France 115 (2004) 121
Stress-induced variant rearrangement in Ni-Mn-Ga single crystals with nonlayered tetragonal martensitic structureA. Sozinov1, 2, A.A. Likhachev2, N. Lanska2, O. Söderberg1, K. Koho1, K. Ullakko2 and V.K. Lindroos1
1 Helsinki University of Technology, Laboratory of Physical Metallurgy and Materials Science, PO Box 6200, 02015 Hut, Finland
2 Helsinki University of Technology, Laboratory of Biomedical Engineering, PO Box 2200, 02015 Hut, Finland
Three Ni-Mn-Ga alloys with different chemical compositions and nonmodulated tetragonal martensitic phase (NM) were studied. In two of the alloys, the polydomain nonmodulated tetragonal phase was obtained in cubic-tetragonal martensitic phase transformation at temperatures below and above Curie point, while in the third one, NM-phase is a result of intermartensitic transformation (7M-NM). Magnetic susceptibility and differential scanning calorimetry were used to study these phase transformations. X-ray diffraction, optical polarised microscopy and mechanical testing were used to study variant rearrangement in compression. Twinning strain approximately 19% was observed for all alloys. It was found also that twinning stress is high (approximately 6-18 MPa) and the alloys did not show detectable magnetic field-induced strain. The influence of magnetic field on stress-strain curves and temperature dependencies of twinning stress are presented.
© EDP Sciences 2004