Studies on the fatigue behaviour of magnetic shape memory materials

Magnetic shape memory (MSM) materials based on Ni-Mn-Ga alloys are promising candidates to drive electromagnetic actuators requiring fast frequency response, large stroke, and long lifetime. Typical MSM materials are single crystalline rectangular sticks, with well-defined crystallographic axes. The MSM effect is attributed to the re-orientation of martensite twin variants in materials having low enough twinning stress. In this case the magnetic field of the order of 0.6 T (i.e. typical for electromagnets) is sufficient to induce motion of suitable oriented relative to the field twins. A great advantage of MSM compared to the traditional thermal shape memory alloys is their considerably higher fatigue life. While in thermal shape memory alloys a maximum amount of cycles measured up to few million cycles (depending on strain and alloy), in MSM materials cycles well above 100 million have been reported. In the present work we investigate the fatigue properties of Ni-Mn-Ga alloys running up to 100 million cycles at frequencies up to 50 Hz. Various MSM samples and test benches have been used. The results are analysed in term of magneto-mechanical (M-M) properties, while structural modifications are studied using microscopy methods. The M-M properties (e.g. magnetic field-induced strain, twinning stress, work output, etc.) as a function of the cycle number are studied in detail, and the reason of the changes are discussed. A universal model predicting the life time of MSM materials is concluded, too.