R.D. Greenough

Magnetoelastic effects in rare‐earth iron‐aluminum compounds

Magnetic and magnetoelastic measurements under various conditions of compressive stress and applied field have been carried out on samples with the composition Tb0.3Dy0.7(Fe1−xAlx)1.95, 0≤x≤0.1, prepared by a float‐zone growth technique. Their magnetoelastic performances compare favorably with the highly magnetostrictive ‘‘Terfenol‐D’’ compounds by having large magnetostrictive strains (λ∼1200 ppm at ∼120 kA/m), good magnetomechanical coupling (k33≊62%) and a large strain coefficient (d33≊80 nm/A). The substitution of aluminum for iron produces a material with potential benefits for use in actuators: higher electrical resistivity and improved ductility, which reduce eddy current effects and increase the mechanical strength.

Actuation and transduction by giant magnetostrictive alloys

Abstract Since the discovery and understanding of the magnetic properties of the rare earth elements, the transduction properties of magnetostrictive alloys has improved significantly over earlier nickel based alloys. Alongside the advances in permanent magnet technology, novel compounds containing terbium, dysprosium and iron are now available in commerical quantities. Given giant magnetostrictive strains (∼1200 ppm) coupled with a high transduction efficiency (>50%) and a large load bearing capability, all of which can be realised in modest magnetic fields, these new materials are now being employed in a variety of applications. High frequency generation is still dominated by piezoelectrics. However, the new materials have an excellent low frequency response, operate at low voltages and can withstand stress cycling. Non-linear and hysteretic behaviour has been controlled by state of the art digital control techniques and algorithms to such an extent that active vibration cancellation has been successfully demonstrated.

The properties and applications of magnetostrictive rare-earth compounds

Abstract The exceptional magnetoelastic properties of rare-earth iron compounds (Terfenol) offer great potential for a variety of applications. Different techniques for the characterisation of their magnetoelastic performance have been developed which provide consistent data, for instance, for the magnetochemical coupling efficiency. Measurements have been made of the field and pressure dependencies of the magnetomechanical coupling (k33), magnetostrictive strain (λ) and magnetostrictive strain coefficient (d33), so delineating the materials optimum operating conditions for use in Terfenol actuators. A self-adaptive variable structure controller has been designed to cater for variations in actuator response which arise from dynamic loading.

Magnetomechanical behaviour of rare earth iron-aluminium compounds

The DC field and pressure dependencies of the magnetic induction (B) and magnetostrictive strain ( lambda ) have been measured at room temperature in Terfenol-D (Tb/sub 0.3/Dy/sub 0.7/Fe/sub 1.95/) and in the compound Tb/sub 0.3/Dy/sub 0.7/(Fe/sub 0.9/ Al/sub 0.1/)/sub 1.95/. From these data the static differential permeability ( mu ) and strain coefficient (d/sub 33/=d lambda /dH) have been obtained. A comparison of their magnetostrictive responses indicates that the magnetocrystalline anisotropy is reduced by the addition of Al to Terfenol-D. A slight lowering of d/sub 33/ with Al content is attributed to a reduction in magnetoelastic coupling, also seen in a lower magnetomechanical coupling coefficient. The work done by either compound against the applied pressure for different isostatic fields has been calculated. From these curves the residual stress levels, T/sub 0/, are substantially lower than those reported previously in Tb/sub 0.27/Dy/sub 0.73/Fe/sub 2/, indicating that in the present compounds T/sub 0/ is due mainly to inhomogeneities and defects, rather than anisotropy. >

Actuation with Terfenol-D

The magnetomechanical properties of Terfenol have been determined by two independent experimental techniques. Measurements on several samples of the same nominal compositions made by free-float zoning reveal variations in quality. With a novel technique for scanning along the length of a Terfenol rod, structural defects can be detected which may explain these variations. A difference between magnetostrictive surface and bulk strains has been observed, and the frequency response of the former is substantially different from theoretical predictions, casting doubt on techniques for the characterization of Terfenol which involve the use of dynamic (AC) fields to measure the magnetostrictive strain coefficient. A description of the control strategies used to implement the actuation of Terfenol-based transducers is given. >

Pressure dependencies of magnetostrictive strain and d coefficient in Terfenol‐D after thermal or magnetic annealing

The pressure dependence of the magnetostrictive strain coefficient, d33, and maximum strains in applied fields of 120 kA m−1 have been measured in samples of Terfenol‐D before and after thermal or magnetic annealing. Application of an annealing field, Ha, parallel to the 〈111〉 axes which are normal to grains oriented along the [112] axis, leads to an increase in d33 of as much as 81%, with applied uniaxial prestresses as low as 3 MPa. The variation of optimum d33 values as a function of stress applied along the [112] axis shows anomalous oscillations after magnetic annealing. The thermal and magnetic effects of the annealing procedures are discussed and a mechanism to explain magnetic annealing is proposed.

Control of Terfenol‐D under load

A vibration control case study is described which demonstrates the application of magnetostrictive (Terfenol‐D) actuators. The magnetomechanical properties of the material suggests that when incorporated in devices for applications a nonlinear response will result. This case study compares the performance of two kinds of control strategy (both discrete), a conventional proportional and integral (PI) algorithm and a variable structure algorithm (DVSC) in both servo and cancellation/regulation roles. Both strategies were implemented on an INMOS transputer based microcontroller with a sampling period of 300 μs. The results presented show that the control behavior of the DVSC strategy offers significant advantages over the PI strategy when controlling actuators with nonlinear characteristics, i.e., the rejection of a 5 Hz disturbance with a gain of −36 dB compared to a gain of −15 dB when using the PI strategy.

The effects of magnetic annealing and compressive stress on the magnetic properties of the rare earth-iron compound Terfenol-D

Abstract The rare earth-iron compound Terfenol-D (Tb x Dy 1− x Fe y with x ≈ 0.3 and y ≈ 1.92) is known to be highly magnetostrictive (1500–2000 ppm) and to possess a magnetomechanical coupling coefficient of around 0.7, properties which are affected by the application of compressive stress. Measurements have been made of the variation in the magnetostriction (λ), induction ( B ), magnetostrain coefficient ( d 33 ), permeability (μ σ ) and magnetomechanical coupling ( k 33 ) as a function of applied field and uniaxial compressive stress in the range 0 to 20 MPa and remeasured after being subjected to a magnetic heat treatment. After magnetic annealing, increases in the magnetostriction in the absence of an applied pre-stress are evident. However, under the conditions used in these annealing experiments, this improvement in the magnetostriction is at the expense of a reduced μ σ and d 33 .

Magnetisation and magnetostriction in rare earth-iron alloys

Abstract As a function of composition, the magnetostriction λ and d 33 (=dλ/d H ) in polycrystalline Dy 0.73 Tb 0.27 Fe x for 1.7≤ x ≤2.3 show a double peak with maxima at x ⋍1.85 and 1.975. The maximum in d 33 is much more pronounced than that in λ, and corresponds to a magnetisation curve with a higher maximum permeability and sharper knee.

Magnetomechanical coupling in Dy0.73Tb0.27Fe2 alloys

Abstract Dynamic and static measurements of the magnetomechanical coupling coefficient, k 33 , have been performed on as cast random polycrystalline terfenol. The values obtained from the two techniques are consistent ( k 33 ≈ 0.33) but are small in comparison with previously published values from similar material. Although a significant improvement could be achieved by employing refined preparation techniques to produce grain oriented material, the magnetic properties of the present material have been investigated to explore the reasons for low coupling. Data for the relative differential permeability at constant stress, μ σ , and magnetostrictive strain coefficient, d 33 , show that the coupling is spoiled by a low maximum permeability and an exceedingly small value for d 33 . The difficulties in making measurements on samples which give information on material quality alone, regardless of the experimental conditions, are discussed.