Dynamic Response in the Low-kHz Range and Delta-E Effect in Ferromagnetic Shape Memory Ni-Mn-Ga

Recent work on ferromagnetic shape memory nickelmanganese-gallium (Ni-Mn-Ga) has demonstrated several characteristics which make this material attractive as an active element for the next generation of intelligent transducers. Alloys of martensitic Ni-Mn-Ga can strain up to 6% as a result of the rotation of twin variants and associated twin boundary motion which occur in these materials in response to magnetic fields. The magnetic actuation holds promise in transducer design because it can lead to enhanced frequency response compared with shape memory alloys with comparable strains. In this paper, we report on experimental measurements collected from a Ni50Mn28.7Ga21.3 sample which has been tested in a solenoid transducer by means of a novel drive configuration consisting of a collinear uniaxial fielduniaxial stress pair. We have observed that the elastic modulus of a Ni-Mn-Ga sample driven in these conditions changes substantially in response to varying bias field. In this paper, we further investigate the dependence of the elastic modulus on ac field intensity and mechanical load as well as bias field. Quasistatic, white noise, and swept-sine excitations were employed to examine the behavior of Ni50Mn28.7Ga21.3 driven under various combinations of magnetic fields and mechanical loads. Mechanically free quasi-static tests demonstrate reversible strains of 6300 με which are consistent with prior measurements on samples with similar composition near the Heusler stoichiometry. Dynamic measurements reveal a significant stiffness increase, of ∗Address all correspondence to this author ([email protected]). up to 209%, with dc bias field. This frequency shift or ∆E effect is shown to originate in the Ni-Mn-Ga sample and is believed to stem from the reorientation of twin variants in response to varying dc field. These results might facilitate a new class of solenoid-based Ni-Mn-Ga transducers for tunable vibration absorber applications, and lay the ground work for developing methods and criteria for the implementation of broadband NiMn-Ga transducer technologies. INTRODUCTION Ferromagnetic shape memory alloys (FSMAs) in the NiMn-Ga system have been shown to produce large strains up to 6% when exposed to magnetic fields [1]. These strains result from the reorientation of twinned martensitic variants and associated twin boundary motion which occur as the easy crystallographic axis aligns with external magnetic fields. While these strains are on the same order of those seen in shape memory alloys (SMAs), the rotation of twin martensitic variants in response to magnetic activation is faster and thus can lead to faster response than those achieved through martensite-austenite phase transformations [2]. Upon removal of the external magnetic field, however, there is no restoring force to drive the twin boundary in the opposite direction and the field-induced strain is not recoverable. To achieve large reversible field-induced strains, a compressive stress is typically applied perpendicular to the field direction that favors variants with the c-axis aligned along the compres1 Copyright c © 2003 by ASME