The actuation performance and twin-boundary velocity in bulk NiMnGa magnetic shape memory single crystals at various frequencies were investigated. A dynamic actuation experimental setup with the ability to apply mechanical stress and a high-frequency alternating magnetic field was developed in tandem with time-resolved optical microscopy. Reversible twin-boundary motion activated in the frequency range up to 600Hz was directly observed. The maximum field-induced strain increased with actuation frequency. Strain hysteresis curves measured at different frequencies showed similar onset fields of strain. The data implied that twin-boundary velocity was not the limiting factor on actuation performance. The results provided significant information on twin-boundary performance during high-frequency actuation.

Direct Observation of AC Field-Induced Twin-Boundary Dynamics in Bulk NiMnGa. Y.W.Lai, R.Schäfer, L.Schultz, J.McCord: Acta Materialia, 2008, 56[18], 5130-7