The Effect Of Temperature, Magnetic Field And Compressive Stress On The Martensitic Transformation In Ni-Mn-Ga Shape Memory Alloys

The effects of exoteric factor including temperature, compressive stress and magnetic field on the martensitic transformation temperature and strain in Ni-Mn-Ga alloys have been studied by means of resistivity measurement, optical microscopy, strain measurement, atomic force microscopy, AC-susceptibility, DSC and X-ray diffraction. At the same time the microstructure change was observed during the martensitic transformation.The martensitic transformation characteristics and temperature in Ni56.3Mn19.3Ga22.8C1.6 alloy didn't vary in thermal cycle. Uniaxial compressive stress has some effects on the alloy. During the stress increasing within 4.44 MPa, the martensitic transformation temperature decrease and the strain reduced, however, the resistivity increased. The transformation temperature increased when the stress is over 4.44 MPa, which indicates that the compressive stress surpasses a critical stress and initiates another transformation.The difference of martenstic transformation characteristics between single crystal Ni54.08Mn21.17Ga24.75 alloy and the crystal alloy is obvious. The martensitic variants of single crystal have the same orientation and the microstructure is alike at the same temperature in two thermal cycles.Magnetic field has some effects on the martenstic transformation of Ni54.92Mn20.51Ga24.57 alloy. The martenstic transformation temperature decreased and the scale of transformation temperature became wider under the exoteric magnetic field H=5000 Oe, which was parallel to the direction of grain growing. The stress versus strain curve of single crystal Ni54.08Mn21.17Ga24.75 alloy takes on a flat during the compressed process and the value of stress is close to the frictional force. The chemistry energy is resumed by this stress under the station that temperature is the only exotericfactor. The strain induced by magnetic field is similar to that induced by thefrictional force, which indicates the frictional force take an important part in the martensitic transformation.