Terfenol-D

This shiny grey alloy is widely known and used for its magnetostrictive properties; when exposed to a magnetic field, magnetostrictive materials can change their shape or dimensions through the process of magnetisation. This happens because ferromagnetic materials have a structure which is divided into magnetic domains; each domain acts like its own tiny bar magnet. When a magnetic field comes near, the domain boundaries shift, and the domains rotate to align with the field. Rotating domains corresponds to a reorganisation of the atoms inside the material, which results in the object changing shape.

Terfenol-D is named after the three different metals from which it is composed, Terbium (Tb), Iron (Fe), Dysprosium (Dy), and from the laboratories where it was first generated (Naval Ordnance Laboratory - NOL). Magnetostrictive materials like Terfenol-D are often used in actuators or transducers, where the change in the magnetic field can generate some kind of motion or an electrical or mechanical response. For example, Terfenol-D was first used to make underwater acoustic devices in naval sonar systems, and has since been introduced as a new technology for fuel injectors in diesel engines where the expansion or contraction of this alloy regulates the flow of fuel through the injector. Its low ductility and fracture resistance makes it vulnerable to breaking, as has happened in this sample.

Magnetostrictive effects are sometimes undesirable. For example, magnetostriction can lead to energy losses by magnetic cores in motors or generators when the material deforms and produces heat due to friction. It is also responsible for the humming sound of transformers; when oscillating electric currents produce a changing magnetic field to make the magnetic core rapidly change shape and vibrate like a loudspeaker.