Effect of Magnetic Field on Deformation Properties in Ferromagnetic Ni_2 MnGa Shape Memory Alloy

摘要

Ferromagnetic shape memory alloy Ni-25 at percent Mn-23 at percent Ga prepared by melting in an electric furnace was investigated to study compression properties at various temperatures with and without magnetic field. The magnetic field is applied parallel to the compression axis at a temperature below martensite finish temperature and at a temperature above austenite finish temperature. Triggering stress necessary for rearrangement of martensite variants is reduced by 0.76 approx 1.68 MPa (i.e., 2.5 approx 5.5 percent) when a magnetic field of 0.4 approx 0.6 T is applied to martensite state. The stress reduction is larger at a higher magnetic field. On the other hand, triggering stress for stress-induced martensite transformation is essentially unchanged under the same magnetic field applied to austenite state. This is due in part to very small changes in Gibbs free energy under magnetic field because of large temperature differences between M_s and T_c. and because of essentially no difference in magnetization between martensite and austenite phases.