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Selecting Controlled Expansion Alloys
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Selecting Controlled Expansion Alloys
Alloys with controlled expansion characteristics are used for a broad variety of applications where thermal change of the size of the metal must be considered in a component or parts design. Generally, these controlled expansion alloys expand when heated and contract when cooled and their rate of expansion and contraction is matched with another material. The alloy composition and crystal structure together help to determine the thermal expansion behavior of each controlled expansion alloy. The rate of expansion of the alloy is expressed as an average or mean coefficient of thermal expansion of the alloy. The thermal expansion behaviour of some controlled expansion alloys is not linear. Hence, it is important to specify the upper and lower limits of the temperature when describing the thermal expansion coefficient that is needed in controlled expansion material.Get more news about Controlled Expansion Alloy,you can vist our website!
Low thermal expansion alloys have very low coefficients of thermal expansion - less than 1.8 to approximately 9 x 10-6 per °C - within certain specific temperature ranges. These alloys exhibit unusual expansion behavior. This unique thermal expansion characteristic of this family of alloys is related to ferromagnetism exhibited by the alloy. Each alloy in the family exhibits very low thermal expansivity below its Curie temperature - the temperature below which it is ferromagnetic. This low thermal expansivity anomaly referred to as the "Invar Effect" is related to spontaneous volume magnetostriction where lattice distortion counteracts the normal lattice thermal expansivity. Above the Curie temperature, these alloys have a high rate of thermal expansion as they are no longer ferromagnetic.
Many theories give insight to the Invar Effect in these alloys phenomenon, the mechanism is not sufficiently understood. In addition, with certain nickel-iron compositions, it is possible to obtain a very high magnetic permeability in these alloys. Consequently, the alloys in this family are used in applications requiring high magnetic permeability such as transformers, cores, laminations for efficient motors, relays and solenoids.
All the alloys in this group are iron-nickel or iron-nickel-cobalt alloys with face-centered cubic crystal structure. As the nickel content in these iron-nickel alloys increases from 36%, thermal expansivity and Curie temperature of the alloy also increases. Curie temperature increases from 280°C for 36% nickel to greater than 510°C for 50% nickel. When selecting an alloy from this family, consideration must also be given to the useful temperature range as this might be limited by the Curie temperature of the alloy.
One of the best known low expansion alloys is Alloy 36 (UNS K93601), a 36% nickel-balance iron alloy. It is used in many applications such as radio and electronic devices where dimensional changes due to temperature must be minimal, for structural members in precision optical laser measuring devices and as the low expansion side in bimetal thermostats. Alloys in this family are suitable for unique low expansion requirements. Alloy 36 (UNS K93602), with a slight increase in expansion properties, offers improved machinability for applications where high parts productivity is important. This alloy has been used for aircraft controls and a variety of electronic devices.