Invar

Invar

Trademark name for an alloy of iron (64% iron, 36% nickel) that expands very little when heated. Invar was formerly used for absolute standards of length measurement and is now used for surveying tapes and in watches and various other temperature-sensitive devices. The name expresses the invariability of its dimensions. It was developed by Charles-Édouard Guillaume (1861–1938), winner of the 1920 Nobel Prize for Physics.

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invar [′in‚vär]

(materials)

An alloy (64% iron-36% nickel) that exhibits almost no thermal expansion over the temperature range of -50 to 150°C (-58 to 302°F).

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Invar 

an iron-based alloy containing 36 percent nickel. It was first made in France in 1896 by C. Guillaume. It has a low coefficient of thermal expansion (1.5 X 10−⁶ 1/°C at temperatures from—80° to 100°C). Its low thermal expansion is explained by the fact that the magnetostrictive reduction in volume during heating compensates for thermal expansion. Invar is used for making geodetic wires and tapes, rulers, parts for measuring and monitoring instruments, and other items. Its melting point is 1430°C and its strength, about 490 MN/m²(49 kgf/mm²). Invar is subjected to cold plastic deformation with subsequent low-temperature heat treatment to increase its strength. After polishing, the alloy becomes corrosion-resistant under atmospheric conditions; protective coatings are applied to articles made of the alloy that are going to be used in hostile environments. Varieties of Invar are superinvar, an alloy with an extremely low coefficient of thermal expansion (less than 1 X 10−⁶ 1/°C) that contains 64 percent iron, 32 percent nickel, and 4 percent cobalt, and stainless invar, an alloy that contains 54 percent cobalt, 37 percent iron, and 9 percent chromium.