Papers by Keyword: NiMnGa Alloy

Abstract: The martensitic transformation, crystalline structure, microstructure and shape memory effect of the Ni53.25Mn21.75Ga25 (at.%) alloy are investigated by means of Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), Transmission Electron Microscope (TEM) and the standard metal strain gauge technique. The XRD results showed that the Ni53.25Mn21.75Ga25 alloy is composed of cubic parent phase at room temperature. TEM observation proved that the typical twin martensite is tetragonal structure and tweed-like contrast which is typical image for the parent phase. A large reversible transformation strain, about 0.54%, is obtained in this undeformed polycrystalline alloy due to martensitic transformation and its reverse transformation. This transformation strain is also increased to 0.65% by the external magnetic field. It is believed that the effect of the magnetic field on the preferential orientation of martensitic variants increases the transformation strain.

Abstract: The effect of annealing temperature on martensite transformation temperature and magnetic properties has been investigated in a polycrystalline Ni53.69Mn26.06Ga20.25 alloy. The significant variation of the martensite transformation temperature and the magnetic properties is observed as a function of annealing temperature.

Abstract: Textured polycrystalline NiMnGa alloys were prepared by directional solidification. Alloys were chosen to have either the 7M or the 5M modulated martensitic structure after proper heat treatment. Mechanical training allowed to reduce the twin boundary pinning stress to below the magnetically induced stress. Thus, magnetic field induced changes in the mechanical behaviour could be demonstrated. The conditions of preparation and mechanical training will be discussed together with their influence on structure, microstructure, and the magneto-mechanical behaviour.

Abstract: The magnetically weakly anisotropic cubic Ni-Mn-Ga Heusler alloys exhibit martensitic transformation resulting in martensitic phases with elastically soft crystal lattices and strong magnetocrystalline anisotropies. The magnetic state of these martensites is coupled with a highly mobile twin structure through the ordinary magnetoelastic interactions giving rise to a giant magnetic-field-induced-strain effect. This effect is the key ingredient of a new scientific field. In the present article, the basic phenomena and concepts of this field, such as lattice instability, soft-mode behavior, electron concentration, ferromagnetic shape memory effect, magnetic-field-induced superelasticity, twinning strain-induced change of magnetization, and magnetoelastic mechanism of magnetostress are briefly reviewed.