Study of austenite-martensite transformation in Ni-Mn-Ga magnetic shape memory alloy

Abstract

\nNi$_{49.7}$Mn$_{29.1}$ Ga$_{21.2}$ magnetic shape memory sample showing magnetic-field-induced strain of 5.7 % in the magnetic field 0.5 T was studied at different conditions during austenite-martensite and martensite-austenite transformations. Transformation temperatures ${\\rm T_A} = 317$ K, ${\\rm T_M} = 308$ K were determined by AC susceptibility measurement. Structure parameters obtained from X-ray measurement are ${\\rm a_A} = 0.584$ nm for austenite and ${\\rm a_M} = {\\rm b_M} = 0.595$ nm, ${\\rm c_M} = 0.561$ nm for martensite which has fivelayered 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.\n