A new nickel-aluminum bronze alloy with low magnetic permeability
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I.
J 5 Xm z H
INTRODUCTION
FOR certain applications, low magnetic properties of a material are required. This means the magnetic permeability of the material may not exceed a value of 1.05 and, for certain applications, even a value of 1.01. Although the commonly used nonferrous alloys are paramagnetic, they have a permeability of more than 1.05, which makes them unsuitable for this specific application. It is necessary that a new material with low magnetic properties retain the characteristics of a nickel-aluminum bronze alloy, i.e., good mechanical properties, high corrosion resistance, and an excellent corrosion-fatigue behavior. Therefore, investigations have been made to determine the effect of variation of composition on the magnetic permeability and other properties of nickel-aluminum bronze. The magnetic permeability of the alloy is greatly affected by iron, which is precipitated in the structure as an ironrich k phase. This phase acts as a grain refiner during solidification and slow cooling with a favorable effect on the mechanical properties. To achieve a low magnetic permeability, the iron content in this alloy has to be limited, with a consequent coarser grain structure and significant loss of the mechanical properties. Furthermore, to retain good ductility and corrosion resistance of the alloy, nickel must be lowered in proportion to the iron content. Thus, it is essential to understand the microstructure of nickel-aluminum bronze in relation to its composition, in order to develop an alloy with low magnetic properties, a fine grain structure, and adequate other properties. II. MAGNETIZATION AND MAGNETIC PROPERTIES OF METALS AND ALLOYS The magnetic properties of metals and alloys are defined physically by the magnetic susceptibility and practically by the magnetic permeability of the material. When an object is placed in a magnetic field, it becomes magnetized and a force is exerted on it. The intensity of magnetization J is indicated by the strength of this force and is determined by the magnetic field strength H and the magnetic susceptibility Xm (chi) of the material: P. WENSCHOT, formerly Research Associate, Boliden LDM, P.O. Box 42, 5150 AA Drunen, The Netherlands, is Private Materials Consultant. Manuscript submitted December 11, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
[1]
All metallic elements exhibit magnetic phenomena in the form of ferromagnetism (e.g., iron and nickel), paramagnetism (e.g., aluminum), or diamagnetism (e.g., copper and lead). Most of the copper alloys are paramagnetic and have a small positive Xm and are only weakly attracted by a magnetic field. The magnetization of the material gives a small contribution to the magnetic field strength indicated by the magnetic induction B, according to the relation B 5 m0 z H 1 J 5 m z H
[2]
where B 5 magnetic induction or magnetic flux density, Wb/m2 (5104 G); H 5 magnetic field strength, A/m (54.1023 Oe); m0 5 absolute magnetic permeability in vacuum; and m 5 absolute magnetic permeability of a material. The relation betw
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