Low Expansion Alloys

Although iron and nickel have very similar coefficients of thermal expansion, the addition of nickel to iron can result in the creation of an alloy in which the coefficient is reduced by an order of magnitude. In 1920 Charles-E Guillaume was awarded the Nobel Prize following his discovery that an iron-nickel alloy, containing around 36% nickel, exhibited virtually no thermal expansion at or close to room temperature. This material was registered under the name Invar®* to signify its invariability / lack of expansion or contraction with temperature changes. This product is now covered by the standards ASTM F1684 / Alloy 36 and K93603.

By modifying the composition of the 36% nickel containing alloy metallurgists have created a number of special materials with unique expansion characteristics to suit particular applications. The alloys include Alloy 42 / K94100, Alloy 48 / K94800 and the Alloy K / K96410 (Kovar®)**.

These alloys have been utilised in modern application areas such electronics, telecommunications, aerospace engineering, medical (laser and x-ray machines) and cryogenic components. Specifically when a metal needs to be joined to glass or ceramic, where the thermal expansion rates of the two materials must coincide to prevent problems within the joint.

Alloy 36 / K93601 / ASTM 1684
Also widely known under several trade names such as Invar®*, is a 36% nickel iron alloy with a near zero thermal expansion from around -100 up to 200°C . This is around a tenth of the expansion rate of carbon steels. It also retains a high strength and toughness at cryogenic temperatures making it suitable for a variety of low temperature applications.

Alloy 42 / K94100 / ASTM F30
This is has a nickel content of 41% nickel and exhibits a low coefficient of thermal expansion (CTE) over a range of temperatures and is utilised in glass-to-metal and ceramic-to-metal sealing applications. The materials thermal expansion is matched to silicon, ceramic materials such as alumina, beryllia and vitreous glass compounds. Most widely used in the manufacture of semiconductor packages, electronic tubes, CRT electron guns and automotive and industrial lamps.

Alloy 48 / K94800 / ASTM F30
This grade has a CTE value which is matched to vitreous soda-lead and soda-lime glass compounds. With a nickel content of 48% it is used extensively in the electronics industry for hermetic sealing in a wide variety of glass-to-metal and ceramic-to-metal sealing combinations and in the manufacture of encapsulated components.

Alloy K / ASTM K94610 / ASTM F15
Also known as Kovar®** this material contains 29% nickel with a 17% cobalt addition with its composition tightly controlled within narrow limits to ensure precise and universal thermal expansion and mechanical characteristics. This helps the material provide high integrity glass-to-metal and ceramic-to-metal seals in applications requiring reliability and resistance to thermal shock. It is used for the manufacture of hermetic seals with both the harder Pyrex or borosilicate glasses and ceramic materials. Applications include microwave tubes, transistors and diodes for lids and closures in a variety of hybrid electronic circuit packages.