Permalloy


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permalloy

[′pər·mə‚lȯi]
(metallurgy)
A trade name for any of several highly magnetically permeable iron-base alloys containing about 45-80% nickel.

Permalloy

 

the trademark of a group of nickel-iron alloys that are characterized by high magnetic permeability μ, low coercive force Hc, and low loss of energy upon hysteresis. Permalloys are classified as soft-magnetic materials. The first reports on Permalloys appeared in the USA after World War I, and Permalloys were first used in industry in the 1920’s. A distinction is made between low-nickel Permalloys, which contain 40–50 percent Ni (Permenorm is a typical example), and high-nickel Permalloys, which contain 70–83 percent Ni.

The conditions under which heat treatment is carried out play an important role in determining the internal structure that accounts for the good magnetic properties of Permalloys, which are usually heat treated in a vacuum or hydrogen atmosphere, and sometimes under an applied magnetic field. To achieve a high μ and low Hc, high-nickel Permalloys are rapidly cooled from 600°C at a rate of 30° to 80°C per sec, which prevents the occurrence of any structural changes that may adversely affect magnetic properties. To reduce the rate of cooling and increase electrical resistance, high-nickel Permalloys are usually alloyed with other elements, for example, Mo, Cr, Cu, and Si. Molybdenum Permalloy is a typical high-nickel Permalloy, containing approximately 79 percent Ni, 17 percent Fe, and 4 percent Mo; its physical properties include an initial magnetic permeability μa ≥ 22,000, a maximum magnetic permeability μmax of 150,000, an Hc ≤ 0.012 ampere per cm, a saturation magnetization of 0.85 tesla, and a Curie point of 400°C. Of all soft-magnetic materials, the highest μ values are found in Supermalloy, which contains approximately 79 percent Ni, 16 percent Fe, and 5 percent Mo; the values μa ≥ 100,000 and μmax ≥ 1,000,000 are obtained with careful heat treatment and by using furnace charges of the highest purity.

Permalloys and similar alloys are manufactured mainly in the form of strips 0.003–0.5 mm thick that are used in sciences that employ weak currents, for example, wire and wireless engineering. The magnetization of Permalloys that contain 65–68 percent Ni and, as a rule, 2–3 percent Mo is characterized by a rectangular hysteresis loop. These Permalloys are used in automated equipment and in computer technology.

REFERENCES

Bozorth, R. Ferromagnetizm. Moscow, 1956. (Translated from English.)
Pretsizionnye splavy: spravochnik Moscow, 1974.

A. I. ZUSMAN

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