Field-induced strains up to 10% at room temperature have been observed in magnetic shaep memory alloys based on off-stoichiometric compositions of the intermetallic compound Ni2MnGa. This occurs by the motion of twin boundaries in the ferromagnetic martensitic state under magnetic fields of a few kOe. Some data illustrating the interdependence of strain, stress, and magnetic field are reviewed. Phenomenological models describe many of these observations by minimization of free energy terms including Zeeman energy, magnetocrystalline anisotropy energy, stored elastic energy and fractional twin-boundary distribution. Two important questions have been raised about field-induced strain in FSMAs. They are 1) the role of body forces (due to action of the field on the sample), and 2) the role of magnetostriction (stress/strain in a single variant under magnetization rotation) in the twin boundary motion. These questions are addressed in light of published data and models.
Ferromagnetic shape memory alloys can exhibit magnetic-field-induced strains of several percent at room temperature. These strains have been shown to correlate with the motion of twin boundaries in the crystals. Twin boundaries advance by the motion of stacking faults along the twin boundary. Such mechanical defects have as an upper limit of their velocity, the speed of sound. It is an important matter to understand the mobility of twin boundaries in ferromagnetic shape memory alloys from a scientific perspective. Namely, how does their velocity depend on field strength, crystal structure and perfection, what are the roles of inertia and threshold field, and does the velocity ever approach anything like the speed of sound. From a practical point of view, it is important to know the twin boundary dynamics in order to understand the bandwidth capabilities of these new active materials as well as their response to different field wave forms that may optimize the response for particular applications. In the present paper we describe a pulse field experimental setup and preliminary results that begin to address the issues raised above.
Micro magnetic and analytic models have been sued to describe the equilibrium twin structure and quasistatic actuation behavior of ferromagnetic shape memory alloys. However, these models do not incorporate microscopic aspects of the twin-boundary strain field, interactions with defects or non-equilibrium behavior. A model is described that accounts for the interaction of a 90 degree domain wall with such a twin boundary. Application of a magnetic field can displace the domain wall from a pinned twin boundary with the Zeeman energy being stored elastically in the domain- wall anisotorpy energy. Finally, the departure of the magnetization and twin structure from equilibrium configurations can be incorporated in thermodynamic models to describe AC behavior and hysteresis.
Very large DC field-induced strains ((epsilon) approximately equals 6%) have been reported for Ni-Mn-Ga single-crystal ferromagnetic shape memory alloys (FSMAs) at room temperature. Described here is an AC test system that provides a dynamic bias stress to an FSMA sample. The low- frequency (epsilon) -H curves show a stress dependence consistent with the DC results, i.e. the maximum output strain peaks for a bias stress of order 1.4 Mpa. The AC (epsilon) -H hysteresis at sub-optimal bias stress can be considerably smaller than that for DC actuation. A thermodynamic model of field-induced twin-boundary motion is expanded to include external stress, threshold field and hysteresis in the twin boundary motion. Twin-boundary motion is driven by the Zeeman energy difference across the domain wall, 2MsH, in the high anisotropy limit and is suppressed by domain magnetization rotation in the weak anisotropy limit. The magnitude of the threshold field and hysteresis can be obtained from features on mechanical stress-versus-strain curves. The field dependence and stress dependence of the AC strain are reasonably well accounted for by the model.
Large magnetic field induced strain shave been reported in ferromagnetic shape memory alloys. two such alloys, Ni-Mn-Ga and Fe-Ni-Co-Ti are explored. A single crystal of Ni-Mn-Ga is shown to deformed by bending approximately six degrees under the influence of an applied field. This deformation is caused by the motion of a single twin boundary with stable variants of martensite on either side. This effect was demonstrated using either divergent or homogeneous field. Fe-Ni-Co-Ti is a shape memory steel with high saturation magnetization being developed as a magnetic shape memory material. Material properties in this alloy can be controlled by composition and heat treatment and the effects of both are explored. Variation of the Ni to Co ratio has been found to have a strong effect on the martensite transition temperatures. Aging treatments cause Ni3Ti3 precipitates to form, which affect the martensite transition and subsequently the magnetization. The structure of most of the Fe-Ni-Co-Ti alloys tested showed lenticular martensite at room temperature with a single sample showing retained thin plate martensite in austenite after cooling to 77K.