Ni_(49.7)Mn_(29.1)Ga_(21.2) magnetic shape memory sample showing magnetic-field-induced strain of 5.7% in the magnetic field 0.6 T was studied at different conditions during austenite-martensite and martensite-austenite transformations. Transformation temperatures T_A = 317 K, T_M = 308 K were determined by AC susceptibility measurement. Structure parameters obtained from X-ray measurement are a_A = 0.584 nm for austenite and a_M = b_M = 0.595 nm, c_M = 0.561 nm for martensite which has five-layered modulated structure. Mechanical stress up to 4 MPa and magnetic field up to 1 T were applied in order to study their influence on the transformations. Totally 39 transformation cycles were measured. The thermoelastic strains during transformations vary from -3.6% to +1.8%. Simultaneous change of magnetization and strain has been observed. Thermomechanical and magnetic history of the sample seems to have no or only very weak influence on the character of austenite-martensite transformation and no clear trend emerges in our experiments.
展开▼
