Ferromagentic shape memory alloy composites exhibit good qualities as vibration absorbers. Loss ratios in excess of 25% have been measured in polymer samples containing 20 vol% Ni-Mn-Ga. The ability to dissipate large amounts of energy is due to the same mechanism that is also responsible for the large strains observed in single crystals used as actuators, namely twin-boundary motion. The loss ratios of the FSMA-loaded composites are compared to those for pure polymer samples and polymer loaded with inert filler. The effects of the pre-processing of the filler material on its performance are also shown.
The dynamic field-induced strain response at 2Hz is reported for a ferromagnetic shape memory alloy (FSMA), Ni49.8Mn28.5Ga21.7. For the d31 actuation mode, longitudinal strain response was measured as a function of longitudinally applied bias stress and transverse applied field. Under a 1.5MPa compressive bias stress, dynamic strains of 2.6% were achieved at fields of 6 kOe. However, dynamic field-induced strain is largely blocked under a compressive bias stress of 4.2MPa. The 'coercive field' hysteresis in the field versus strain loops was observed to be as low as 100kA/m at 1.5MPa and increase linearly at greater stresses. Peak piezomagnetic d31 coefficients measured from these field versus strain loops approached 1.3 X 10-7 m/A. Dynamic stress versus strain loops were recorded for compressive bias stresses from 0 to 4.2MPa. Stiffnesses of approximately 40MPa in the active twinning stress range were recorded, and the stiffness approached 5 times the twinning stiffness beyond the twinning range. The mechanical loss measured in stress versus strain loops, when normalized to the output strain, resulted in a linear increase of 6.84 kJ/m3 per MPa bias stress. Current investigations are attempting to isolate the factors that contribute to the extraordinary behavior exhibited in these properties of the Ni-Mn-Ga system.
Micro magnetic and analytic models have been sued to describe the equilibrium twin structure and quasistatic actuation behavior of ferromagnetic shape memory alloys. However, these models do not incorporate microscopic aspects of the twin-boundary strain field, interactions with defects or non-equilibrium behavior. A model is described that accounts for the interaction of a 90 degree domain wall with such a twin boundary. Application of a magnetic field can displace the domain wall from a pinned twin boundary with the Zeeman energy being stored elastically in the domain- wall anisotorpy energy. Finally, the departure of the magnetization and twin structure from equilibrium configurations can be incorporated in thermodynamic models to describe AC behavior and hysteresis.
Very large DC field-induced strains ((epsilon) approximately equals 6%) have been reported for Ni-Mn-Ga single-crystal ferromagnetic shape memory alloys (FSMAs) at room temperature. Described here is an AC test system that provides a dynamic bias stress to an FSMA sample. The low- frequency (epsilon) -H curves show a stress dependence consistent with the DC results, i.e. the maximum output strain peaks for a bias stress of order 1.4 Mpa. The AC (epsilon) -H hysteresis at sub-optimal bias stress can be considerably smaller than that for DC actuation. A thermodynamic model of field-induced twin-boundary motion is expanded to include external stress, threshold field and hysteresis in the twin boundary motion. Twin-boundary motion is driven by the Zeeman energy difference across the domain wall, 2MsH, in the high anisotropy limit and is suppressed by domain magnetization rotation in the weak anisotropy limit. The magnitude of the threshold field and hysteresis can be obtained from features on mechanical stress-versus-strain curves. The field dependence and stress dependence of the AC strain are reasonably well accounted for by the model.