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Fe-Ni-Co alloys whose coefficient of linear expansion is close to that of hard glass. The term “fernico” is used for two alloys, both of which are manufactured by US firms. Fernico I is an analog of the alloy Kovar, and fernico II contains 31 percent Ni and 15 percent Co, with Fe constituting the remainder. Fernico II is used for metal-to-glass seals in vacuum-tube devices operating at temperatures lower than –80°C. The content of the principal components (Ni and Co) in fernico II differs from that in fernico I, and this difference ensures the stability of the former’s structure at temperatures down to –180°C. The coefficient of linear expansion of fernico II in the range from 20° to 500°C is 6.5 × 10–6°C–1; the physical and mechanical properties and fabricating characteristics of the alloy are close to those of fernico I.