Invar at 100 years

Invar-Effect alloys are a family of nickel-iron alloys that exhibit different expansivities over a broad temperature range. A minimum thermal expansivity is reached at approximately 36% nickel. Increasing the nickel content increases the thermal expansivity and also raises the Curie temperature, above which ferromagnetic properties are lost. Below their Curie temperatures, where they are ferromagnetic, these alloys exhibit anomalously low thermal expansivities. This article discusses the compositions of the most widely used Invar-Effect alloys, the nature of the thermal expansion curve, fabrication processes, and applications.