Understanding and Development of Combined Acoustic and Magnetic Actuation of Ni2MnGa Single Crystals

Ni–Mn–Ga based ferromagnetic shape memory alloys (FSMAs) have emerged as a promising new class of active materials capable of producing a large (several %) magnetic-field-induced strain (MFIS). FSMAs still have several characteristic shortcomings that may limit their potential applications. A threshold field of 2 to 4 kOe must be overcome to initiate twin boundary motion and a larger field is required to achieve full actuation. The operating window of the stress output from FSMA actuators is narrow and limited to the range between 0.5 and 2 MPa. Outside the operating range, the strain output diminishes significantly. This thesis addresses these limitations and reports potential techniques to decrease the required threshold field and increase the stress and strain output of FSMA actuation. The demagnetizing field due to magnetic poles on the surface of the sample is found to significantly influence the maximum field needed for full MFIS. The demagnetizing field decreases the effective internal field inside the FSMA sample; as a result, for a given external field, the magnetic driving force is reduced by the demagnetizing field. For a small demagnetization factor, full MFIS can be achieved at a field as low as 0.5 kOe. However, for a high demagnetization factor, full MFIS may require a field as high as 3.5 kOe. A phenomenological free energy model with an approximate magnetostatic term included properly describes this. The application of an acoustic assist from a 33-mode piezoelectric stack is shown to improve MFIS of Ni-Mn-Ga single crystals by reducing the required threshold field and twinning-yield stress. Threshold field reductions of up to 1 kOe are observed, and the twinning-yield stress is reduced by up to 0.5 MPa. The piezo assist on FSMA actuation can be understood as a form of time varying stress waves that facilitate twin boundary motion. The stress-wave theory and FEM analyses, based on assumption of an elastic and isotropic material, are used to estimate the amplitude of stress waves. The stress values determined from the wave theory and FEM are comparable to the observed reduction in twinning-yield stress (0.6 to 2 MPa). The empirical stress wave amplitude is generally lower than the calculated one, because the actual stress waves generated in FSMA are limited by inelastic and anisotropic nature of the FSMA samples.