SELECTING CONTROLLED EXPANSION ALLOYS

Alloys with controlled expansion characteristics are used for a broad variety of applications where thermal size change of the metal must be considered in component part design. Generally, these alloys expand when heated and contract when cooled.Get more news about Controlled Expansion Alloy,you can vist our website!

Composition and crystal structure, together, help determine the thermal expansion behavior of each alloy. The alloy expansion rate is commonly expressed in terms of an average or mean coefficient of thermal expansion.

The expansion behavior of some alloys is not linear. Therefore, it is important to specify the upper and lower temperature limits of interest when describing the expansion coefficient needed in any material.
Low Expansion Alloys
The low expansion alloys have very low coefficients of thermal expansion - less than 1.8 to approximately 9 x 10-6 per °C (1.0 to approximately 5.0 x 10-6 per °F) - within certain temperature ranges. These alloys exhibit unusual expansion behavior; it is represented by the curve in Fig. 1.

The unique expansion characteristic of this alloy family is related to ferromagnetism. Each alloy exhibits very low thermal expansivity below its Curie temperature: i.e., the temperature below which it is ferromagnetic. This low thermal expansivity anomaly, often 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, the alloys expand at a high rate because they are no longer ferromagnetic. A number of theories have been proposed to explain this "Invar Effect"
Although these theories have given insight to the phenomenon, the mechanism is not yet sufficiently understood. In addition, with certain nickel-iron compositions, it is possible to obtain a very high magnetic permeability. Consequently, the alloys in this family have been useful in applications requiring high magnetic permeability such as transformers, cores, laminations for very efficient motors, relays and solenoids.

All of the alloys in this group are iron-nickel or iron-nickel-cobalt alloys with face-centered cubic crystal structure. As nickel content in the iron-nickel alloys increases from 36 percent, thermal expansivity and Curie temperature also increase. Curie temperature increases from 280°C (536°F) for 36 percent nickel to greater than 510°C (960°F) for 50 percent nickel. Thus, consideration in selecting an alloy also must be given the useful temperature range as might be limited by the Curie temperature.
Two Categories
Uses of low-expansion alloys are divided into two general categories. First are those in which size change due to temperature variation must be minimized. These applications include structural components for measurement and control instruments in which excessive expansion and contraction due to temperature changes would seriously impair accuracy. Typical applications include aircraft and missile control components, laser and optical systems and wave guide tubes.

The second category includes temperature controls utilizing a bimetallic strip. This simple type of control consists of a low expansion alloy metallurgically bonded to a high expansion alloy to produce a bimetallic element. When the strip is heated, the difference in thermal expansion rates between the two alloys causes the element to bend in curvature.

The change in curvature is directly proportional to the difference in coefficient of expansion and the temperature change of the strip components, and inversely proportional to the thickness of the combined components. The amount of bending is also affected by the ratio of the moduli of elasticity of the two components and by their thickness ratio.