Model for discontinuous actuation of ferromagnetic shape memory alloy under stress

Ferromagnetic shape memory alloys have demonstrated strains up to 5% result ing from the rearrangement of crystallographic variants by twin boundary mo tion under an applied field. A model is proposed that describes twin bounda ries moving abruptly in order to accommodate either the mechanical energy o f an applied stress sigma epsilon (0) or the magnetic interaction energy of the applied field MsH. This model provides predictions of the materials re sponse under both field and load. The principal conclusion of the model is that when one energy, either mechanical or magnetic, becomes larger than th e other, the twins will move to abruptly rearrange the entire sample to a v ariant stable under the new condition. Experiments were performed to verify this model in Ni-Mn-Ga shape memory alloy. The abrupt change of the varian t structure was observed experimentally although the twin boundaries only s wept through limited potions of the sample. Experimental field-induced stra ins reached 2.2%. Samples were also tested under cycled field with constant load and showed cyclic strains of 1.5%. (C) 2001 American Institute of Phy sics.