Invar

Invar is an iron -nickel alloy having a very low coefficient of thermal expansion and consists of 64 % iron and 36 % nickel. Invar alloy Nilo 36, Nilvar, NS 36, permalloy D, radio metal 36 Vacodil 36 is also under the names Invar 36, sold. It has the material number 1.3912.

The name is also used as a generic term for a group of alloys and compounds, which possess the remarkable property in certain temperature ranges to have abnormally low or even negative thermal expansion coefficient. Thus, the name is due to the invariance of the elongation with respect to a temperature change.

Invar is a trademark of Imphy Alloys that currently belong to the steel group Arcelor Mittal. Invar alloys have been found in the economy, a wide range of applications and are used where special emphasis is placed on longitudinal stability with temperature variations.

History

Was discovered the effect in 1896 by the Swiss physicist Charles Édouard Guillaume at the Invar alloy Fe65Ni35, in 1920 received the Nobel Prize for Physics for this. He worked in the International Bureau of Weights and Measures and was looking for a cheap material to produce lengths and mass standards. Then these standards, such as the standard meter are made ​​of a platinum-iridium alloy.

Alloys

Fe65Ni35 Invar contains 65 % iron and 35 % nickel. Up to 1% magnesium, silicon and carbon are alloyed in order to modify the mechanical properties. By alloying cobalt 5%, the thermal expansion coefficient can be further reduced. A label for the alloy Inovco, Fe - 33Ni - 4.5Co, α ( 20-100 ° C) of 0.55 ppm / K.

Variants of this alloy have a slightly different thermal expansion coefficients. So Kovar has the coefficient of expansion of about 5 ppm / K.

Nowadays, many other alloys are known in which an Invar effect occurs:

  • Face-centered cubic (fcc ): FeNi, FePt, FePd, FeMn, CoMn, FeNiPt, FeNiMn, CoMnFe uvw.
  • Body-centered cubic (bcc ): CrFe, CrMn
  • Hexagonal close -packed ( hcp ): CoCr

Laves phases and compounds: TiFe2, ZrFe2, Reco2 (RE = rare earths except Eu), FeC, Dy2 ( FeCo ) 17, including

Properties

Using the example of Fe65Ni35 - Invar:

  • Spec. electrical resistivity = 0.75-0.85 Ω · mm2 / m,
  • Modulus = 140-150 GPa,
  • Brinell hardness = 160,
  • Elongation <45%,
  • Tensile strength = 450-590 MPa,
  • Density = 8 g / cm ³,
  • Längenausdehnungskoeffz. at 20-90 ° C = 1.7 to 2.0 · 10-6 K-1,
  • Thermal conductivity at 23 ° C = 13 Wm -1K -1

Physical background

Invar effect is due to the negative volume magnetostriction of the crystal lattice. This means that the grid " bloated " is a rejection of the magnetic moments of the individual atoms of the alloy, so the atomic distances larger. However, this effect decreases with increasing temperature (since the magnetic moments decrease ) and shrinks the crystal lattice thereby. The decrease of the negative volume magnetostriction with increasing temperature thus behaves opposite to the thermal expansion, which increases the interatomic distances. These physical phenomena can compensate so that the interatomic distances do not change effectively and the solid thus no change in length (or volume change ) experiences itself in certain temperature ranges. The Invar effect disappears together with the magnetic moments of the atoms from the respective magnetic ordering temperature of the material, that is the Curie temperature and the Néel temperature.

Area of ​​use

Invar was first used to make cheap mass and length standards. It was also used to make precision clocks and chronometers. One half of bimetallic strips is often made ​​of Invar.

Invar alloys are used for a wide range of products that require high longitudinal stability with temperature variations. Examples are shadow mask for cathode ray tubes, glass -to-metal transitions, tanks of LPG vessels (membrane tanks ), chip - based panels, laser housing, waveguide and astronomical and seismographic instruments. By developing a method for welding of Invar, the applications have been extended. In geodesy wires of invar in Präzisionsnivellierlatten as well as for high-precision distance measurements over short distances (up to about 20 m ) is used, for example in tunnel or dam. In the processing of large CFRP components corresponding Laminating example, partly made of Invar in the aerospace

Iron–nickel alloy Chronometer watch Waveguide (electromagnetism)
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