An Fe-36%Ni alloy. Its magnetovolume effect cancels ordinary thermal expansion, giving a very small linear expansion coefficient of about 1×10⁻⁶ K⁻¹ near room temperature.
Japanese name: インバー
Invar appears in 1 practice question across 1 topic, including Thermal properties of materials.
Answer: B. Below the Curie temperature, the magnetic volume expansion shrinks as magnetization weakens on heating, canceling normal expansion
Invar is a ferromagnet with a large magnetovolume effect (spontaneous volume magnetostriction), in which spontaneous magnetization lengthens the interatomic distance and enlarges the volume. As temperature rises, the magnetization weakens, so this magnetic expansion decreases and nearly cancels the ordinary thermal expansion from lattice vibration. As a result, the linear expansion coefficient is as small as about 1×10⁻⁶ K⁻¹ below the Curie temperature, and Invar is used in precision measuring instruments, containers for liquefied natural gas and the like. Above the Curie temperature the cancellation disappears and it expands like an ordinary metal. Invar is single-phase FCC at room temperature; atomic vibration grows with temperature as usual, and no phase transformation that offsets the expansion takes place.
Invar is a ferromagnet with a large magnetovolume effect (spontaneous volume magnetostriction), in which spontaneous magnetization lengthens the interatomic distance and enlarges the volume.
Which statement correctly explains why the linear thermal e…Coefficient of linear thermal expansion Curie temperature Debye temperature Dulong–Petit law Phonon Thermal diffusivity Umklapp process Wiedemann–Franz law