Galfenol alloys show promise as a new magnetically activated smart material based on their unique combination of
relatively high magnetostrictive performance and good mechanical robustness. Investigations of aluminum additions to
single crystal iron-gallium alloys have been done previously, and the magnetostrictive response seems to follow the rule
of mixtures with decreasing saturation magnetostriction with increasing aluminum content. Aluminum is assumed to
substitute for Ga directly in the alloy. Directionally solidified polycrystalline Galfenol alloys with aluminum additions
were produced to determine the effects on the magnetic properties. Iron-gallium-aluminum alloys were investigated for
two primary reasons: (1) Fe-Al alloys are well established and are typically manufactured using conventional thermo-mechanical
processing techniques such as rolling; it is anticipated that aluminum additions will aid in the development
of Galfenol alloy rolled sheets (2) Gallium prices continue to rise and a cost effective alternative needs to be
investigated.
Several Fe-Ga-Al alloy compositions were prepared using the Free Stand Zone Melting (FSZM) directional
solidification technique. Alloy composition ranges investigated include: Fe80.5GaxAl19.5-x (4.9≤x≤13), Fe81.6GayAl18.4-y
(4.6≤y≤13.8), and Fe85GazAl15-z (3.75≤z≤11.25). Alloys were studied using EDS (chemistry verification), EBSD
(crystallite orientation), and magnetic characterization techniques to determine the effect of aluminum addition on the
polycrystalline binary Fe-Ga system. Magnetic properties such as saturation magnetostriction (λsat), piezomagnetic
constant (d33), and relative magnetic permeability (μr) of directionally solidified Fe-Ga-Al polycrystalline alloys will be compared to binary Fe-Ga alloys including investigations into the crystal orientation effects on these properties. Results suggest that up to 50% aluminum can be substituted in the alloy while maintaining considerable saturation
magnetostriction, > 200 ppm.
A process for inducing uniaxial anisotropy in 18.4 at% Galfenol has been developed, applied, and proven to be reliable.
The stress annealing procedure that produces the most consistent results and induces the highest uniaxial anisotropy in
FSZM Galfenol (6.35 mm &nullset;) is one modified from previous works by the Magnetic Materials Group at Naval Surface
and Warfare Center (NSWC) - Carderock. Further modification in which applied stress is lowered and temperature is
raised has proven most successful for stress annealing Bridgman Galfenol (20.83 mm &nullset;) and, in addition, has produced
an increase in saturation magnetostriction. Model calculations show predicted tensile operation ranges, on average, of 69
MPa for research grade and 40 MPa for production grade FSZM 18.4 at% Galfenol in which full magnetostriction can be
utilized.
Galfenol (Fe100-xGax, x = 6, 12.5, 17, 18.4, 19, 22) and Alfenol (Fe81Al19) alloy rods (~50 mm x 6 mm dia.) were annealed under compressive stresses up to 219 MPa at temperatures from 100°C to 700°C for 10 to 100 minutes. Because of the magnetostriction of the alloys, these heat treatments build in a uniaxial magnetic anisotropy that depends upon annealing stress, annealing temperature, and alloy composition. This built-in uniaxial magnetic anisotropy extends the high power capability of these alloys to operate both in tension and in compression.
Magnetization and magnetostriction measurements of both unannealed and annealed alloys were taken from −100 MPa to +40 MPa. To fit the magnetization and magnetostriction as a function of magnetic field and stress, an energy expression containing a fourth order anisotropy term (cubic term) plus a second order uniaxial term was utilized. The computed magnetizations and magnetostrictions are found by an energy-weighted average using the Armstrong smoothing constant. Excellent fits to the magnetostriction and moment data were obtained. From the model fits m, d33 and k33 were calculated. Since the built-in stresses can be found simply from the model, it is possible to predict the amount of prestress built into the alloys.
We investigate the machining properties of Iron-Gallium alloy for microactuator. Iron-Gallium is ductile
magnetostrictive material with moderate magnetostriction ranging from 100 to 300ppm. The microactuator of Fe-Ga is
expected to have advantages of simple configuration, low voltage driving, high robustness against external force and
high temperature environment, compared with that of PZT. Here the rod of Fe-Ga prepared by FSZM technique was machined
to distributed pillars of 1mm square by milling process. The comparison of magnetostrictions of machined and
non-machined parts by strain gage confirms the strains different in pillars are inherited from the grain distribution
and the milling process does not significantly deteriorate the material properties. The measurement of displacements by
LASER Doppler vibrometer supports the validity of strain measurement. The success of the fabrication of the distributed
pillars of 0.7 and 0.5mm square exhibits the potential of the milling process for Fe-Ga with high aspect ratio suitable
for practical micro applications.
We have been proposing a magnetic force control method using the inverse magnetostrictive effect of magnetostrictive materials. With a parallel magnetic circuit consisting of iron yokes and permanent magnet, the magnetic force exerting on the yoke can be varied by the mechanical stress applied to the magnetostrictive material. The characteristics of the magnetic force, such as stress-sensitivity and range of the variation, are mostly dependent on the material properties of the magnetostrictive material. So far we have mainly investigated the magnetic force using Terfenol-D (Tb-Dy-Fe alloy) and demonstrated its usefulness in practical applications. Recently, Galfenol, Iron-Gallium is widely noticed for alternative for the Terfenol with several advantages. Even lower magnetostriction, it is superior to the Terfenol with high piezomagnetic constant, low hysteresis loss, high saturation and good machinability. In this paper, we investigate the potential of the Galfenol for the magnetic force control method which can enlarge the variation range of the magnetic force and increase the stress-sensitively. The formulation of the magnetic force and experimental results of fundamental material properties and magnetic force of the Galfenol and Terfenol clarifies the merits of the Galfenol inherited from high saturation and high piezomagnetic constant. The correlation between the piezomagnetic constant and bias field is verified, providing magnetic circuit design strategy to make full use of the material properties of the Galfenol for future applications.
The addition of Ga to b.c.c. Fe greatly increases the magnetostriction of Fe in the <100> directions (by a factor of 12 in Fe81Ga19). These Fe-based materials are mechanically tough and thus can be used under both compressive and tensile loading. The object of this study is to examine the effects of temperature aging on Fe81.6Ga18.4 alloys with built-in uniaxial stress anisotropies. To accomplish this, a transverse anisotropy was built into these positive magnetostrictive Fe-Ga (Galfenol) alloys by heat treatment under high compressive stresses. Annealing temperatures between 600 and 635°C and compressive stresses between 100 and 219 MPa produced uniaxial anisotropies between 2 and 9 kJ/m3. It is now possible to obtain magnetostrictions greater than 250 ppm over a broad range of stresses, extending from far into the compressive stress region through zero stress and into the tensile region. In this paper we examine the effect of aging at elevated temperatures on the built-in uniaxial anisotropy and magnetostriction of these alloys. Aging at 150°C for 697 hours left the magnetostriction unchanged. At 200°C most of the uniaxial anisotropy had disappeared after 525 hours. At 250°C, about two-thirds of the uniaxial stress was lost after 168 hours and very little remained after 336 hours.
This investigation focused on dynamic characterization of a laminated polycrystalline sample of Fe81.6Ga18.4 alloy grown by the FSZM process. Previous studies using static characterization methods have shown unique properties of the alloy for use in transducers and active structures. Static characterization values were verified and material properties were contrasted with the "giant" magnetostrictive material Terfenol-D. Common test methods were used for dynamic characterization to calculate Young's modulus, coupling coefficient, magnetostrictive coefficient, and permeability. In addition, mechanical Q and potential efficiency were calculated. Comparison with static testing of single crystal samples showed that modulus, permeability, and magnetostrictive coefficient were very close to static values, with any differences being attributed to test fixture effects and material differences. Coupling coefficient appeared to be quite low, but no correction was applied for the test fixture losses and magnetic circuit effects. Comparisons with Terfenol-D show that with significantly less magnetic field, iron-gallium alloys can be used in high Q systems to achieve large acceleration and force output. The unique structural and magnetic properties of gallium-iron alloys enable applications that would be difficult or impossible with Terfenol-D.
The Zone Melt Crystal Growth Method (FSZM) has been used to produce polycrystalline Galfenol specimens, Fe81.6Ga18.4, with preferred {100} orientation. This crystal growth technique has advantages over conventional Bridgman methods in that zone rates used were at least an order of magnitude greater; 350 mm/hr versus 2-4 mm/hr. This material had measured magnetostrictions ranging from 168 ppm to 220 ppm compared to 290 ppm for a single crystal with a similar composition. It was discovered that upon machining a large increase in magnetostriction occurred, ~15%. Using Orientation Imaging Microscopy (OIM) techniques it was shown that the magnetostriction increase is due to the removal of off-axis grains located on the circumference of the FSZM samples. The room temperature mechanical properties were measured to be 72.4 GPa-86.3 GPa modulus of elasticity, 348 MPa-370 MPa ultimate strength, and elongation values of 0.81% - 1.2% depending upon zoning conditions.
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