Martensitic transformation and shape memory effect of Ni_(53.25)Mn_(21.75)Ga_(25) ferromagnetic shape memory alloy

摘要

The martensitic transformation, crystalline structure, microstructure and shape memory effect of the Ni_(53.25)Mn_(21.75)Ga_(25) (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 Ni_(53.25)Mn_(21.75)Ga_(25) 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.