Low expansion alloys are iron-nickel alloys that exhibit an extremely low expansion rate at room temperature. These materials have been used in modern applications that require the bonding of metal to glass or ceramic, and in areas where thermal expansion rates of materials must overlap to prevent differential expansion in the joint area. They are used in industries such as electronics, medical (laser and x-ray machines), aerospace engineering, telecommunications, and cryogenic components.

Iron and nickel have very similar thermal expansion coefficients. However, the addition of nickel to iron can result in the formation of an alloy in which the coefficient is reduced by an order of magnitude. Research by Charles Édouard Guillaume, who won the Nobel Prize for Physics in the 1920s, showed that an iron-nickel alloy containing about 36% nickel showed almost no thermal expansion at room temperature. Such material was noted due to its invariance or lack of expansion and contraction with temperature changes. By changing the composition of this 36% nickel alloy, metallurgists have created a number of special materials with unique expansion properties to suit specific applications. The most common low expansion alloys are Invar 36, Kovar, Invar 42, Alloy 52.