Magnetic shape memory fatigue

Abstract

ABSTRACT Single crystal specimens of having compositions close to Ni 2 MnGa and exhibiting magnetic shape memory effect(MSME) were tested in a rotating magnetic field at a frequency of 5.7 Hz. The applied magnetic field, about 0.7 T wasstrong enough to induce the MSME. Test of one specimen was discontinued because of the structural failure of thespecimens after 0.5 million cycles. Second specimen was tested up to 37x10 6 cycles. The evolution of the martensiticmorphology and crack propagation were observed by optical microscopy. To characterize the magnetic shape memorybehavior the simultaneous measurements of the field-induced strain and magnetization as a function of the magneticfield and external load was used. The full MSM effect, abou t 6% obtained prior the test, decreased to about 3% duringthe first million cycles. This value stayed then approximately constant up to 37x10 6 cycles of rotating magnetic field.The magnetic field needed to initiate the MSME increased. The observed behavior is discussed within the framework ofobserved martensitic band structure in the specimens and the existence of initial cracks and other obstacles formartensitic twin boundary motion.Keywords: magnetic shape memory, ferromagnetic shape memory alloys, Ni-Mn-Ga alloys, magnetic shape memoryeffect fatigue, giant magnetic-field-induced strain