Like other nickel/iron compositions, Invar is a solid solution; that is, it is a single-phase alloy, consisting of around 36% nickel and 64% iron. Common grades of Invar have a coefficient of thermal expansion of about 1.2 × 10−6 K−1, while ordinary steels have values of around 11–15 ppm. Extra-pure grades can readily produce values as low as 0.62–0.65 ppm/°C. Some formulations display negative thermal expansion characteristics. Though it displays high dimensional stability over a range of temperatures, it does have a propensity to creep.
Applications
Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, television shadow-mask frames, valves in engines and large aerostructure molds. One of its first applications was in watch balance wheels and pendulum rods for precision regulator clocks. At the time it was invented the pendulum clock was the world's most precise timekeeper, and the limit to timekeeping accuracy was due to thermal variations in length of clock pendulums. The Riefler regulator clock developed in 1898 by Clemens Riefler, the first clock to use an invar pendulum, had an accuracy of 10 milliseconds per day, and served as the primary time standard in naval observatories and for national time services until the 1930s. In land surveying, when first-order elevation leveling is to be performed, the level staff used is made of Invar, instead of wood, fiberglass, or other metals. Invar struts were used in some pistons to limit their thermal expansion inside their cylinders. In the manufacture of large composite material structures for aerospacecarbon fibre layup molds, invar is used to facilitate the manufacture of parts to extremely tight tolerances.
Variations
There are variations of the original Invar material that have slightly different coefficient of thermal expansion such as:
Inovco, which is Fe–33Ni–4.5Co and has an α of 0.55 ppm/°C.
FeNiCo alloys—named Kovar or Dilver P—that have the same expansion behaviour as borosilicate glass, and because of that are used for optical parts in a wide range of temperatures and applications, such as satellites.
Explanation of anomalous properties
A detailed explanation of Invar's anomalously low CTE has proven elusive for physicists. All the iron-rich face-centered cubic Fe–Ni alloys show Invar anomalies in their measured thermal and magnetic properties that evolve continuously in intensity with varying alloy composition. Scientists had once proposed that Invar's behavior was a direct consequence of a high-magnetic-moment to low-magnetic-moment transition occurring in the face centered cubic Fe–Ni series ; however, this theory was proven incorrect. Instead, it appears that the low-moment/high-moment transition is preceded by a high-magnetic-moment frustrated ferromagnetic state in which the Fe–Fe magnetic exchange bonds have a large magneto-volume effect of the rightsign and magnitude to create the observed thermal expansion anomaly.
