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Notes: Ni–Mn–Ga based ferromagnetic shape memory alloys (FSMAs) have emerged as apromising class of active materials capable of producing a large (up to 10%) magnetic-field-inducedstrain (MFIS). This large strain is not the familiar anisotropic magnetostriction; it results fromfield-induced twin-boundary motion and has appropriately been referred to as magnetoplasticity.FSMAs still have several characteristic shortcomings that may limit their potential applications. Athreshold field of 150 to 300 kA/m must be overcome to initiate twin-boundary motion and a largerfield is required to achieve full strain. The operating window of the stress output from FSMAactuators is limited to the range between 1 and 1.5 MPa. Outside this operating range, the strainoutput diminishes significantly. This paper addresses these performance limitations and describesan acoustic-assist technique that has been shown to decrease the required threshold field andincrease the stress and strain output of FSMA actuation. The application of an acoustic assistancefrom a 33-mode piezoelectric stack is shown to improve MFIS of Ni–Mn–Ga single crystals byreducing the required threshold field and twinning-yield stress. Threshold field reductions of up to80 kA/m are observed, and the twinning-yield stress can be reduced by up to 0.5 MPa. The effect ofacoustic assistance on FSMA actuation can be understood as a form of time varying stress wavesthat facilitate twin-boundary motion. A stress wave analysis is shown to give a quantitativeunderstanding of the measured reduction in the twinning-yield stress. For FSMA cyclic actuation,both operating stress and strain outputs of the FSMA actuation are significantly enhanced byacoustic assistance. Without the acoustic assistance, the maximum reversible strain of the sampleused here is 3% and appears only in the limited external stress range between 0.7 and 1 MPa. Withthe acoustic assistance, the maximum reversible strain increases to 4.5% and appears in a broaderrange of stress output between 0.4 and 1.2 MPa. The reduction in the twinning-yield stress due tothe acoustic assistance significantly improves the FSMA cyclic actuation performance; magneticenergy not used to drive twin-boundary motion can be utilized to work against a larger externalload

Notes: Ni-Mn-Ga ferromagnetic shape memory alloys (FSMA) are a potential new class of actuator materials able to respond at higher frequencies (at least 300 Hz) with comparable strains (up to 6 %) in a moderate field (below 1 T)[1]. Magnitude of the strain depends on the values of several critical material parameters, most importantly the martensitic transformation temperature (TMart), Curie temperature (TC) and saturation magnetization (MS)[2]. It is well known that these parameters are strongly dependent on the composition of the alloy. Composition dependence of TMart, TC and MS have been experimentaly explored [3,4]. Therefore, it is possible to compile a more complete, and hence more useful composition map for designing Ni-Mn-Ga FSMAs.Ageing behavior is important in these newly developed FSMAs because ageing can affect the reliability of devices using the alloys. Ni-Mn-Ga FSMAs and Au-Cd[5] alloys have several important common characteristics, including off-stoichiometry alloy composition (designed for operation at ambient temperature) and easy twin boundary motion in the martensite state, thus similar ageing behavior is expected in Ni-Mn-Ga alloys.Ni-Mn-Ga alloys have also demonstrated strong damping due to the motion of twin boundaries[6]. Low-frequency mechanical properties are typically measured using the technique of dynamical mechanical analysis (DMA)[7].In this paper, we present studies of composition design, subtle structure changes associated with ageing, and the temperature dependence of the low-frequency mechanical properties of several Ni-Mn-Ga single crystal alloys

Notes: Composites of Ni–Mn–Ga particles in a polyurethane matrix can be made by mixing theparticles with the polymer, and allowing them to cure under a magnetic field to texture thecomposites. These composites show large hysteresis and mechanical losses, when subjected to acyclic stress, that were far larger than the matrix polymer ones. The additional losses are attributedto the motion of twin boundaries in the filler particles and provide a way for obtaining mechanicalenergy absorption in a wide frequency range. By means of X-ray and neutron diffraction we presentevidence that confirms that twins are present in the particles and that they do move whenmechanically loading the composite