Strain versus field measurements for a ferromagnetic shape memory alloy in the NiMnGa system demonstrate the largest magnetostrictive strains to date of nearJy 1,3%. These strains are achieved in the martensitic state through field-induced variant rearrangement. An experimental apparatus is described that provides biaxial magnetic fields and uniaxial compressive prestress with temperature control while recording microstructural changes with optical microscopy. The magnetostrictive response is found to be sensitive to the initial state induced by stressbiasing the martensitic variant structure, and exhibits rate effects related to twin boundary mobility. Experiments performed with constant stress demonstrate work output capacity. Experimental results are Interpreted by using a theory based on minimization of a micromagnetic energy functional that includes applied field, stress, and demagnetization energies. It is found that the theory provides a good qualitative description of material behavior, but significantly overpredicts the amount of strain produced. Issues concerning the martensitic magnetic anisotropy and variant nucleation are discussed with regard to this discrepancy.
Bibliographical noteFunding Information:
Mnnuscript received November 4, 1998: revised June 1, 1999. This work was supported by ONWARPA (RDJ & TWS: N00014-95-1-1145 and -91-J-4034; M W N00014-95-11071 and -93-10506); the APOSR (RDJ: 49620-96-1-0057), the ARO (MW DAALO3-92-0-0121), and the NSF (MW DMR-93-21185; RDJ: DMS-9505077). R. Tickle, R. D. James, and T. Shield are with the Depnrtment of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN 55455 USA (e-mail: tickle9aemamn.edui. M. Wutiig'is wifh lhe Depimnent of Materials acid Nuclear Engineering, University of Maryland, College Park, MD 20742 USA. V. V. Kokorin is with the Institute for Magnetism, 252680 Kiev, Uluaine. Publisher hem Identiiier S OOi8-9464(99)07904-2.
- Cubic-to-tetragonal transformation
- Ferromagnetic shape memory
- Giant magnetostrictive materials
- Magnetomechanical properties