Controlled Expansion Alloys

Controlled thermal expansion alloys are iron-nickel alloys that display a very low expansion rate at room temperature. These materials are utilized in advanced applications that require metal to be joined to ceramic or glass, and areas where the thermal expansion rates of the materials have to coincide to prevent differential expansion within the joint area. They are used in industries such as telecommunications, medical (X-ray and laser machines), aerospace engineering, electronics, and cryogenic components.

Iron and nickel have extremely similar coefficients of thermal expansion. However, the incorporation of nickel to iron can lead to the creation of an alloy in which the coefficient is decreased by an order of magnitude. Research in the 1920s discovered that an iron-nickel alloy containing about 36% nickel displayed almost no thermal expansion at, or close to, room temperature. This type of material became popular for its invariability or lack of expansion and contraction with temperature varies. By altering the composition of this 36% nickel alloy, metallurgists have developed several special materials with unique expansion features to suit specific applications.

Alloy 36 (Invar®) / K93603 / ASTM F1684

Widely known under several trades names such as Nilo 36** and Invar®*, this grade is a 36% nickel-iron alloy. It displays a near zero rate of thermal expansion from about –100 °C up to 200 °C. This is about a tenth of the expansion rate of carbon steels. Alloy 36 also shows a high retention of toughness and strength at cryogenic temperatures, making it ideal for a range of low temperature or low expansion applications. The alloy is perfectly suited to areas where dimensional changes because of temperature must be minimal, such as radio and electronic devices, tooling for aerospace composites, and structural members in precision equipment such as lasers, thermostats, measuring devices, and cryogenic instrumentation. Alloy 36 can also be utilized along with a high expansion material to produce mechanical movement with temperature in thermomechanical controls and switchgear.

Alloy 42 / K94100 / ASTM F30

A binary nickel-iron containing about 41% nickel. This grade displays a low and normally consistent coefficient of thermal expansion across the range 20-300 °C. This enables its application in glass-to-metal or ceramic-to-metal sealing applications and tooling for aerospace composites. Alloy 42 also has a coefficient of thermal expansion matched to silicon, and ceramic materials such as beryllia, alumina, and vitreous glass compounds. It can be found as a sealing material in semiconductor packages, CRT electron guns, thermostat rods, microelectronic components, electronic tubes, vacuum devices and electric industrial lamps.

Alloy 52 / N14052

Alloy 52 contains 52% nickel and 48% iron and is extensively used in the telecommunications sector. It also finds application in a wide range of electronic applications, especially for glass seals.

Alloy K / K94610 / ASTM F15

Also known as Kovar®*** and Nilo K**, this alloy has 29% nickel with a 17% cobalt addition. Its composition is tightly controlled within narrow limits to ensure precise and universal thermal expansion and mechanical features. This helps the material provide high integrity glass-to-metal and ceramic-to-metal seals in applications requiring resistance to thermal shock and reliability. Alloy K is frequently used for the producing of hermetic seals with both the harder Pyrex or borosilicate glasses and alumina-type ceramic materials. It finds wide-ranging application in the production of transistors and diodes for lids and closures in a range of hybrid electronic circuit packages and also in the production of microwave tubes.

Technical Data

Coefficient of Thermal Expansion

Alloy 36 Alloy 42 Alloy 52 Alloy K
30-150 °C (μm/m°C) 1.2 - 2.7
30-300 °C (μm/m°C) 4.0 - 4.7
30-400 °C (μm/m°C) 4.6 - 5.2
30-450 °C (μm/m°C) 6.7 - 7.4 9.7 - 10.2 5.1 - 5.5
30-550 °C (μm/m°C) 10.0 - 10.5

Typical Physical Properties

Alloy 36 Alloy 42 Alloy 52 Alloy K
Density (g/cm3) 8.13 8.15 8.30 8.25
Melting Point (°C) 1450 1425 1425 1450
Curie Point (°C) 230 330 510 425
Thermal conductivity (W/m°C) 10.5 12.5 13.4 17.0
Specific Heat (J/g°C) 0.51 0.50 0.50 0.50
Young’s Modulus (MPA) 140,000 145,000 165,000 139,000

Round Bar Weight and Stock Sizes

Diameter Weight Diameter Weight Diameter Weight
mm kg/ft kg/m mm kg/ft kg/m mm kg/ft kg/m
3 0.02 0.06 16 0.50 1.64 50 4.88 16.00
6 0.07 0.23 20 0.78 2.56 60 7.02 23.04
8 0.13 0.41 25 1.22 4.00 70 9.56 31.37
9.5 0.18 0.58 30 1.76 5.76 75 10.98 36.01
10 0.20 0.64 35 2.39 7.84 80 12.49 40.97
12 0.28 0.92 40 3.12 10.24 90 15.80 51.85
12.7 0.32 1.03 45 3.95 12.96 100 19.51 64.01

NB Weight data for guidance only

*Invar® is a registered trademark of Arcelor Mittal
** The Nilo® alloy family is a registered trademark of Special Metals Ltd
***Kovar® is a registered trademark of Carpenter Technology Corporation

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