It was noted that ferromagnetic shape-memory alloys had exhibited strains of up to 5%, which resulted from the rearrangement of crystallographic variants by twin boundary motion under an applied field. A model was proposed which described twin boundaries that moved suddenly in order to accommodate the mechanical energy of an applied stress or the magnetic interaction energy of an applied field. The model predicted the material response to both fields and loads. The main prediction of the model was that, when the mechanical or magnetic energy became greater than the other the twins would move so as to rearrange the entire sample suddenly to give a variant which was stable under the new conditions. Such abrupt changes in the variant structure were observed experimentally, although the twin boundaries swept through only limited potions of the sample. The experimental field-induced strains reached 2.2%. Samples were also tested in a cyclic field, with constant load, and exhibited cyclic strains of 1.5%.

Model for Discontinuous Actuation of Ferromagnetic Shape Memory Alloy under Stress. S.J.Murray, R.C.O'Handley, S.M.Allen: Journal of Applied Physics, 2001, 89[2], 1295-301