Acoustic-Assisted, Magnetic-Field-Induced Actuation of Ni-Mn-Ga Single Crystals |
| Journal |
Materials Science Forum (Volume 583) |
| Volume |
Advances in Shape Memory Materials |
| Edited by |
V.A. Chernenko |
| Pages |
147-168 |
| DOI |
10.4028/www.scientific.net/MSF.583.147 |
| Online since |
May, 2008 |
| Authors |
Ratchat Techapiesancharoenkij,
S.M. Allen,
Robert C. O'Handley
|
| Keywords |
Acoustic Assistance, Ferromagnetic Shape Memory Alloy (FSMA), Magnetic Field-Induced Strain, NiMnGa , Piezoelectric, Threshold Field, Twinning-Yield Stress |
| 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. |
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