ISSN:
1089-7550
Source:
AIP Digital Archive
Topics:
Physics
Notes:
Rare-earth/transition-metal alloys can exhibit magnetostrictive strains of order 0.17% in modest fields. Larger strains are of interest for many actuator applications. Certain alloys that undergo martensitic transformations exhibit a shape–memory effect that can yield strains up to 20% upon heating the deformed martensitic phase and they can show superelasticity upon application of a small stress. These methods of activation can be a disadvantage for many applications; magnetic activation of a shape–memory effect is desired. Several magnetic shape–memory alloys exist, among them intermetallics based on Ni2MnGa, which are the subject of this work. These materials experience a 6.6% c-axis contraction on cooling through the martensitic transition temperature, which is near 273 K; this strain is accommodated by formation of an ensemble of strained twin crystals separated by twin boundaries. Strains of several percent could be produced magnetically if the twin boundaries could be moved under application of a magnetic field. We report observations of strains of nearly 0.2% induced along [001] in unstressed crystals of Ni2MnGa with magnetic fields of 8 kOe applied at 265 K (Fig. 1). (This strain is an order of magnitude larger than the magnetostrictive strain we measure in the parent Heusler phase at 283 K.) Our data suggest that these giant strains are associated with the superelastic motion of twin boundaries in the martensitic phase, which is stable below about 274 K; the strength of the measured anisotropy energy density, MSHa/2 is comparable to the elastic energy density, eσ/2, needed for superelastic twin boundary motion.1 The strains we observe are equivalent to those achieved in terfenol-D and represent only a small fraction of the strain available if the twin variants of the martensitic phase can be oriented prior to application of a field. © 1997 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.364556
