Magnetostrictive materials can convert magnetic energy into
kinetic energy, or the reverse, and are used to build
actuators and
sensors. The property can be quantified by the magnetostrictive coefficient, λ, which may be positive or negative and is defined as the fractional change in length as the magnetization of the material increases from zero to the
saturation value. The effect is responsible for the familiar "
electric hum" () which can be heard near
transformers and high power electrical devices. Cobalt exhibits the largest room-temperature magnetostriction of a pure element at 60
microstrains. Among alloys, the highest known magnetostriction is exhibited by
Terfenol-D, (Ter for
terbium, Fe for
iron, NOL for
Naval Ordnance Laboratory, and D for
dysprosium). Terfenol-D, {{chem2|Tb_{
x}Dy_{1−
x}Fe2}}, exhibits about 2,000 microstrains in a field of 160 kA/m (2 kOe) at room temperature and is the most commonly used engineering magnetostrictive material.
Galfenol, {{chem2|Fe_{
x}Ga_{1−
x}|auto=1}}, and
Alfer, {{chem2|Fe_{
x}Al_{1−
x} }}, are newer alloys that exhibit 200-400 microstrains at lower applied fields (~200 Oe) and have enhanced mechanical properties from the brittle Terfenol-D. Both of these alloys have easy axes for magnetostriction and demonstrate sufficient ductility for sensor and actuator applications. Another very common magnetostrictive composite is the amorphous alloy {{chem2|Fe81Si_{3.5}B_{13.5}C2|auto=1}} with its trade name
Metglas 2605SC. Favourable properties of this material are its high saturation-magnetostriction constant, λ, of about 20
microstrains and more, coupled with a low
magnetic-anisotropy field strength, HA, of less than 1 kA/m (to reach
magnetic saturation).
Metglas 2605SC also exhibits a very strong ΔE-effect with reductions in the effective
Young's modulus up to about 80% in bulk. This helps build energy-efficient magnetic
MEMS. Cobalt
ferrite, (CoO·Fe2O3), is also mainly used for its magnetostrictive applications like sensors and actuators, thanks to its high saturation magnetostriction (~200 parts per million). In the absence of
rare-earth elements, it is a good substitute for
Terfenol-D. Moreover, its magnetostrictive properties can be tuned by inducing a magnetic uniaxial anisotropy. This can be done by magnetic annealing, magnetic field assisted compaction, or reaction under uniaxial pressure. This last solution has the advantage of being ultrafast (20 min), thanks to the use of
spark plasma sintering. In early
sonar transducers during World War II,
nickel was used as a magnetostrictive material. To alleviate the shortage of nickel, the Japanese navy used an
iron-
aluminium alloy from the
Alperm family.
Mechanical behaviors of magnetostrictive alloys Effect of microstructure on elastic strain alloys Single-crystal alloys exhibit superior microstrain, but are vulnerable to yielding due to the anisotropic mechanical properties of most metals. It has been observed that for
polycrystalline alloys with a high area coverage of preferential grains for microstrain, the mechanical properties (
ductility) of magnetostrictive alloys can be significantly improved. Targeted metallurgical processing steps promote
abnormal grain growth of {011} grains in
galfenol and
alfenol thin sheets, which contain two easy axes for magnetic domain alignment during magnetostriction. This can be accomplished by adding particles such as
boride species and
niobium carbide () during initial chill casting of the
ingot. For a polycrystalline alloy, an established formula for the magnetostriction, λ, from known directional microstrain measurements is: λs = 1/5(2λ100+3λ111) During subsequent
hot rolling and
recrystallization steps, particle strengthening occurs in which the particles introduce a "pinning" force at
grain boundaries that hinders normal (
stochastic) grain growth in an annealing step assisted by a atmosphere. Thus, single-crystal-like texture (~90% {011} grain coverage) is attainable, reducing the interference with
magnetic domain alignment and increasing microstrain attainable for polycrystalline alloys as measured by semiconducting
strain gauges. These surface textures can be visualized using
electron backscatter diffraction (EBSD) or related diffraction techniques.
Compressive stress to induce domain alignment For actuator applications, maximum rotation of magnetic moments leads to the highest possible magnetostriction output. This can be achieved by processing techniques such as stress annealing and field annealing. However, mechanical pre-stresses can also be applied to thin sheets to induce alignment perpendicular to actuation as long as the stress is below the buckling limit. For example, it has been demonstrated that applied compressive pre-stress of up to ~50 MPa can result in an increase of magnetostriction by ~90%. This is hypothesized to be due to a "jump" in initial alignment of domains perpendicular to applied stress and improved final alignment parallel to applied stress.
Constitutive behavior of magnetostrictive materials These materials generally show non-linear behavior with a change in applied magnetic field or stress. For small magnetic fields, linear piezomagnetic constitutive behavior is enough. Non-linear magnetic behavior is captured using a classical macroscopic model such as the
Preisach model and Jiles-Atherton model. For capturing magneto-mechanical behavior, Armstrong proposed an "energy average" approach. More recently, Wahi
et al. have proposed a computationally efficient
constitutive model wherein constitutive behavior is captured using a "locally linearizing" scheme. ==Applications==