LOW-EXPANSION ALLOYS
Alloys, mainly of iron and nickel, having low coefficients of thermal expansion, usually within a specific temperature range. Uses include precision-instrument parts requiring dimensional stability at various temperatures and glass-to-metal sealing applications, in which the thermal expansivity of the metal must closely match that of the glass. The best-known alloy is Invar, also known as Nilvar, an iron–36% nickel composition also containing (as impurities) minute amounts of carbon, manganese, and silicon. Developed in France, it has the lowest coefficient of thermal expansion of all metals in the —459 to 350°F ( — 273 to 177°C) range. In the annealed conditions, the alloy has a coefficient of thermal expansion ranging from about 0.8 X 10~6/°F (1.44 X 10~6/K) at 0 to 77°F ( — 17.8 to 25°C). At 300°F (149°C), the value is still only 1 X 10~6/°F (1.8 X 10~6/K). Expansivity is affected by heat treatment and cold work. Quenching from about 1526°F (830°C), for example, reduces the coefficient of thermal expansion below that of annealed material, as does cold forming. A combination of quenching and cold work can even result in zero or negative coefficients. Invar has a thermal conductivity of 6.4 Btu/(ft • h • °F) [11 W/(m ■ K)] from room temperature to 212°F (100°C) and is quite soft, having a Brinell hardness of about 160. Tensile properties are about 75,000 lb/in2 (517 MPa) ultimate strength, 50,000 lb/in2 (345 MPa) yield strength, and 35 to 40% elongation. The alloy is ferromagnetic at room temperature but becomes paramagnetic with increasing temperature. Because the alloy’s thermal expansivity is rather constant within a specific temperature range, Invar is also known as a
controlled-expansion alloy.
There are many other such alloys, each suited for specific coefficients of thermal expansion within certain temperature ranges. They include iron with 39% nickel, or Fe-39 Ni, Fe-42 Ni (Dumet and Alloy 42), Fe-46 Ni, (Platinite), Fe-48.5 Ni, Fe-50.5 Ni, Fe-42 Ni-6 Cr, Fe-45 Ni-6 Cr, Fe-36 Ni-12 Cr (Elinvar), Fe-22 Ni-3 Cr, and Fe-42 Ni-5.5 Cr-2.5 Ti-0.40 Al (NiSpan C and Elinvar Extra). Besides its low coefficient of thermal expansion, Elinvar is noted for its constant modulus of elasticity over a wide temperature range. Because of their titanium content, NiSpan C and Elinvar Extra can be solution heat-treated and aged to very high strength levels. Tensile yield strengths to 180,000 lb/in2 (1,241 MPa), with a corresponding Rockwell C hardness of 42 have been reported. Some of these alloys are also glass-to-metal sealing alloys. Another is Alloy 27 (Fe-28 Cr–0.6 Mn–0.5 maximum Ni–0.4 silicon–0.05 C), which grips the glass on cooling and is used in electronic and vacuum tubes as well as fluorescent and incandescent lamps. Annealed, it has a tensile yield strength of 55,000 lb/in2 (379 MPa) and 25% elongation.
Cobalt in iron-nickel alloys increases the coefficient of thermal expansion at room temperature but enhances thermal stability over a wider temperature range. Kovar and Fernico, Fe–28 Ni–18 Co alloys, and Fernichrome (Fe–30 Ni–25 Co–8 Cr) are used for applications requiring vacuum sealing to glass. Kovar has a tensile yield strength of 59,500 lb/in2 (410 MPa) and 25% elongation. A Co 54-Fe 37-Cr 9 alloy is noted for its near-zero and sometimes negative coefficient of thermal expansion in the 32 to 212°F (0 to 100°C) range. Elgiloy (40 Co–20 Cr–15.5 Ni–15.3 Fe–7 Mo–2 Mn–0.15 C–0.04 Be), originally a watch-spring alloy, has found many other spring applications, including seals, diaphragms, timers, instruments, surgical implants, and orthodontic fixtures. Produced in strip form by Elgiloy LP, it has an operating temperature range from the cryogenic to 900°F (480°C). Besides dimensional stability, the alloy is noted for its good fatigue strength, corrosion and heat resistance, and nonmagnetic characteristics. Incoloy 903 (42 Fe–38 Ni–15 Co–3 Cb–1.4 Ti–0.7 Al), which is also heat-treatable, is noted for a near-constant coefficient of thermal expansion, about 4 X 10~6/°F (7.2 X 10~6/K) from 212 to 800°F (100 to 427°C) and a near-constant modulus of elasticity from —320 to 1200°F (—196 to 649°C). In the aged condition, the alloy has a yield strength of about 160,000 lb/in2 (1,103 MPa).
Other low-expansion or controlled-expansion alloys that have been developed include Nivar, which contains 54% cobalt; the Swiss alloy Nivarox (Fe–37 Ni–8 Cr with small amounts of manganese, beryllium, silicon, and carbon) and Contracid (60 Ni–15 Cr–15 Fe–7 Mo–2 Mn and small amounts of beryllium and silicon); Nicol (40 Co–20 Cr–16 Fe–15 Ni–7 Mo–2 Mn and small amounts of beryllium and carbon); the French iron-nickel alloys Dilvar and Adr; Super-Invar from Japan, a 5% cobalt iron-nickel alloy; Sylvania 4 (Fe–42 Ni–5.7 Cr with small amounts of manganese, silicon, carbon, and aluminum) and the similar Sealmet HC-4; Niron 52 (52 Ni–48 Fe); Rodar (Fe–29 Ni–17 Co–0.3 Mn); and Nicromet (54 Fe–46 Ni).

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