Acoustic-Assisted, Magnetic-Field-Induced Actuation of Ni-Mn-Ga Single Crystals

Abstract

Ni–Mn–Ga based ferromagnetic shape memory alloys (FSMAs) have emerged as a promising class of active materials capable of producing a large (up to 10%) magnetic-field-induced strain (MFIS). This large strain is not the familiar anisotropic magnetostriction; it results from field-induced twin-boundary motion and has appropriately been referred to as magnetoplasticity. FSMAs still have several characteristic shortcomings that may limit their potential applications. A threshold field of 150 to 300 kA/m must be overcome to initiate twin-boundary motion and a larger field is required to achieve full strain. The operating window of the stress output from FSMA actuators is limited to the range between 1 and 1.5 MPa. Outside this operating range, the strain output diminishes significantly. This paper addresses these performance limitations and describes an acoustic-assist technique that has been shown to decrease the required threshold field and increase the stress and strain output of FSMA actuation. The application of an acoustic assistance 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 80 kA/m are observed, and the twinning-yield stress can be reduced by up to 0.5 MPa. The effect of acoustic assistance on FSMA actuation can be understood as a form of time varying stress waves that facilitate twin-boundary motion. A stress wave analysis is shown to give a quantitative understanding 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 by acoustic assistance. Without the acoustic assistance, the maximum reversible strain of the sample used here is 3% and appears only in the limited external stress range between 0.7 and 1 MPa. With the acoustic assistance, the maximum reversible strain increases to 4.5% and appears in a broader range of stress output between 0.4 and 1.2 MPa. The reduction in the twinning-yield stress due to the acoustic assistance significantly improves the FSMA cyclic actuation performance; magnetic energy not used to drive twin-boundary motion can be utilized to work against a larger external load.