The iron / nickel alloy Fe36Ni today commonly known by its tradename Invar was discovered in 1897. The alloy is roughly 36% nickel and 64% iron (other variations include the addition or substitution of cobalt, platinum, carbon and manganese etc)

Most metals and alloys have a relatively high coefficient of thermal expansion (CTE or α) - which can cause problems if they are used to construct ultra-precise mechanical systems which are exposed to changes in temperature.

Invar has a CTE around twenty times lower that ordinary steel alloys, and is therefore extremely useful in the construction of scientific instruments, advanced electronic components, parts for space vehicles, extreme precision moulds, mechanical clocks, & etc.

Despite more then a century of research and routine use, the physical reasons for the alloy's very low CTE are currently controversial. It's generally thought the alloy has some atomic / magnetic properties which counter-compensate for the 'normal' expansion that would be expected. Many possible mechanisms have been suggested (example) but as yet, none has been generally accepted.

Note that the generalised theory of thermal expansion in 'ordinary' materials is itself poorly understood.

Thermal expansion of materials, defined as the change of volume with respect to temperature under constant pressure, is one of the most common phenomena in nature. It has been studied through the asymmetric feature of interatomic force as a function of atomic distance, i.e., the anharmonic lattice dynamics. Even though ZTE and NTE have been observed for over a century current understanding is still largely qualitative and phenomenological […]

Source : Scientific Reports, 2014; 4: 7043.