Thermal expansion alloys, such as Invar and Kovar, are made up of an iron-nickel base and are known for their low coefficient of thermal expansion (CTE). Iron and nickel have very similar CTE, however with these alloys, thermal expansion is affected by the addition of nickel to iron.
Invar, which was founded in 1896 by Swiss physicist Charles Edouard Guillaume and trademarked by Imphy Alloys, is made up of 36% nickel and 64% iron. With a similar look and feel to steel, Invar has had a significant impact on the development of scientific instruments and incandescent light bulbs. Today, it is used in a vast number of household appliances and controls.
Kovar, an iron-nickel-cobalt alloy, is another alloy that exhibits the “Invar effect.” Compared to other alloys, Kovar expands at a variable rate with increasing temperature with very little change in dimension. Kovar is often found in applications requiring a hermetic seal between the metal and glass or ceramic component. Additionally, primary applications include for x-ray and microwave tubs, and light bulb ends to name a few.