In 1917, GWElmen discovered that (30～90) Ni-Fe alloy has good soft magnetic properties under low and medium magnetic fields, and the initial permeability of 78Ni-Fe ( μ0) is the highest and is called Permalloy. In 1921, he discovered the Ni-Fe alloy Perminvar containing a certain amount of cobalt. Its remanence (Br) and coercive force were low, and its magnetic permeability was almost unchanged under a low magnetic field, and the hysteresis loop was honeycomb-shaped. In 1924, W.S. Smith made Mumetal, a high Ni-Fe alloy containing copper. In 1931, Elmen added 4% Mo to 78Ni-Fe alloy to increase the resistivity (ρ), improve the magnetic properties, simplify the heat treatment operation, and make 4-79 molybdenum permalloy. In 1933, Germany O.Dahl and others made Isoperm (50Ni-Fe) permanent magnetic alloy. In 1934, Ellis (W.G.Ellis) and others made 81 Ni-2 Mo-Fe powder magnetic core. At the beginning of the Second World War, Germany made a 50 Ni-Fe alloy with a rectangular loop grain orientation, called Permenorm 5000 Z (called Deltamax in the United States). In 1947, O.L. Boothly and others made 5-79 molybdenum permalloy with the highest initial permeability and maximum permeability (μm), called Super-malloy. In 1956, G.H. Howe made 65Ni-2 Mo-Fe alloy with rectangular loops, which had high magnetic permeability and low coercivity, which was called Dynamax. Ni-Fe alloys have different magnetic properties due to different nickel content. Figures 1 and 2 show the effect of nickel content on the saturation magnetic induction (Bs), Curie temperature (Tc) and initial permeability of Ni-Fe alloys. Ni-Fe series soft magnetic alloys have many varieties and wide applications; all countries and manufacturers have their own brands. Since 1961, China has successively produced Ni-Fe soft magnetic alloys with various properties.
Ni-Fe alloys can be roughly divided into the following categories (see table for typical magnetic properties):
High resistivity 36Ni-Fe alloy resistivity is about 75μΩ·cm, low eddy current loss , The initial permeability is low (2000～3000) and almost unchanged under low magnetic field. 34.5 Ni-Fe alloy is added with 2% Mo, and after cold rolling and annealing with a reduction rate of more than 90%, a secondary recrystallized texture (see preferred orientation) is obtained, which can reduce the saturation magnetostriction and the initial permeability is obvious By increasing (55000), the resistivity is also increased to 90μΩ·cm, which is suitable for application at intermediate frequency.
High saturation magnetic induction (45～60) Ni-Fe alloy, saturation magnetic induction is about 1.6T, and the initial permeability is higher (3000～6000). Through cold rolling and annealing at a reduction rate of about 95%, a secondary recrystallization texture is obtained, and μ0 can be increased to 10,000. The most commonly used (47-50) Ni-Fe alloy, magnetic field heat treatment can further increase the value of μ0 and μm by 3 to 4 times.
High magnetic permeability (74～82) Ni-Fe alloy, the initial permeability is 30,000～100,000, the maximum permeability can reach more than 100,000～300,000, and the coercive force is very low. The cooling rate after annealing has a great influence on the magnetocrystalline anisotropy and permeability, which is related to the orderly transformation to form an ordered structure of FeNi3. In order to reduce the orderly transformation speed and increase the resistivity, a small amount of molybdenum, chromium and copper are often added. Molybdenum and chromium atoms replace part of iron atoms, and copper atoms replace part of nickel atoms. These can prevent the formation of FeNi3, thereby simplifying the heat treatment operation and increasing the magnetic permeability. The most commonly used high-permeability Ni-Fe alloys are 4-79 molybdenum permalloy, Mumetal (5% Cu, 2.75% Cr and 77% Ni or 4% Mo, 5% Cu and 77% Ni) and containing 5% Mo Super Permalloy. Add 2 to 3% Ti or 3 to 7% Nb to the high magnetic permeability Ni-Fe alloy to form very fine precipitates, hinder the movement of dislocations, improve the hardness and wear resistance, and maintain high magnetic permeability. It is hard permalloy and is often used as a magnetic head material.
High squareness ratio Ni-Fe alloy grain orientation (enhanced magnetocrystalline anisotropy), magnetic field heat treatment (induced uniaxial anisotropy) and large deformation cold working (induced slip induced anisotropy) The opposite sex) can change the shape of the hysteresis loop and affect the magnetism. Grain orientation 50 Ni-Fe alloy (Deltamax and Per-menorm 5000Z) is the most important rectangular loop material. After 95% reduction rate cold rolling and 1000～1200℃ primary recrystallization annealing, {100} cubic texture is obtained, and its squareness ratio (Br/Bm) value is very high. After 65Ni-Fe and 65Ni-2.5Mo-Fe (Dynamax) alloys are cold-rolled and annealed, and then subjected to a magnetic field heat treatment, their loops are rectangular, with high Br/Bm and μ values, and low Hc values. The presence of trace oxygen in the alloy has a great influence on the formation of anisotropy induced by magnetic field treatment. Because oxygen can prevent the formation of a long-range ordered structure of FeNi3, it is beneficial for FeNi3 atom pairs to be preferentially arranged in a certain direction. 4-79 Moly Permalloy or Mumetal alloy can also have a rectangular loop after being heat-treated in a magnetic field. 4-79 molybdenum permalloy is cold-rolled into thin strips with a reduction ratio of greater than 93% or cold-drawn into filaments with a compression ratio of 99%. The resulting slip-induced anisotropy can also change the hysteresis loop Into a rectangle.
The characteristic of the constant magnetic Ni-Fe alloy is that the magnetic permeability under low magnetic field does not change with the magnetic field strength, the hysteresis loop is oblique, and the remanence is very low. 50Ni-Fe Isoperm alloy (sometimes with a small amount of copper or aluminum) is cold rolled with a reduction rate of over 90% and annealed at 1000°C, and then cold rolled with a reduction rate of 50%, magnetized along the rolling direction, the loop is oblique , The Br value is very low, and the magnetic permeability hardly changes with the size of the magnetic field, that is, it has constant magnetic permeability. 65Ni-Fe alloy (or 1% Mn), Perminvar type alloy [(34～45)Ni-(25～29)Co-Fe or 2～3% Mo and Nb] and 79Ni-4Mo-Fe alloy After the magnetic field heat treatment, the loop is also oblique, and the magnetic permeability is higher and constant. The 65Ni-Fe alloy containing 1% Mn (Chinese brand 1J66) has the best comprehensive performance such as permeability stability, temperature stability and effective permeability.
81Ni-2Mo-Fe alloy powder magnetic core has high resistivity, can be used at 300kHz frequency, good temperature stability, but low permeability.
Ni-Fe soft magnetic alloy is usually used in strip form. Strips with a thickness of 0.15～0.35mm are mainly used to make laminated iron cores with a frequency of 50～400Hz; those with a thickness of 0.10～0.25mm are mainly used to make winding cutting iron cores and winding iron cores with a frequency of less than 25kHz; Very thin strips of 0.003-0.025mm can be used to manufacture shaft-wound cores with frequencies up to 500kHz (Figure 3).
Ni-Fe alloy has good plasticity and can be rolled into ultra-thin strips with a thickness of 0.0025mm (see foil rolling) or drawn into filaments with a diameter of 0.01mm.
Bibliography
RMBozorth, Ferromagnetism,Nostrand,Princeton,NJ,1951.
GYChin & JHWernick, Soft Magnetic Metallic Materials in Ferromagnetic Materials ,Vol.2,pp.57～188,North-Holland,Amsterdam,1980.