Introduction to Permalloys

Permalloys often refer to iron-nickel alloys with a nickel content in the range of 30 to 90%. It is a very widely used soft magnetic alloy. Through the appropriate process, the magnetic properties can be effectively controlled, such as the initial permeability of more than 105, the peak permeability of more than 106, the coercivity as low as 2 ‰ Oster, the rectangular coefficient close to 1 or close to 0. The permalloy with a face-centred cubic crystal structure has a very good plasticity, and it can be processed into an ultrathin strip with a 1 μm diameter and a variety of use forms. Commonly used alloys are 1J50, 1J79, 1J85 and so on.

　　1J50 saturation magnetic induction strength is slightly lower than silicon steel, but the permeability is dozens of times higher than silicon steel, iron loss is also 2 to 3 times lower than silicon steel. Made of higher frequency (400 ~ 8000Hz) transformer, no-load current is small, suitable for the production of 100W below the small higher frequency transformer. 1J79 has a good overall performance, suitable for high-frequency and low-voltage transformers, leakage protection switch core, common-mode inductance core and the current transformer core. 1J85 initial permeability can be up to one hundred thousand (105) or more, suitable for making a weak signal low-frequency or high-frequency input/output transformer. 1J85 initial permeability can be up to one hundred thousand (105) or more, suitable for making weak signal low-frequency or high-frequency input/output transformers. It is suitable for use as low frequency or high frequency input/output transformers for weak signals, common mode inductors and high precision current transformers.

Structural Magnetism

　　Permalloy is a kind of iron and nickel alloy with high permeability under weak magnetic field, nickel content in 30% or more nickel-iron alloys, at room temperature for the single-phase face-centred cubic (γ) structure, but in the vicinity of 30% Ni single-phase structure is very unstable, so the practical iron and nickel soft magnetic alloys with nickel content are more than 36%. Iron-nickel alloys in the vicinity of 75% nickel (atomic fraction), in this single-phase alloy will occur in the Ni3Fe long-range ordered transition, when the alloy's fractional constants and physical properties, such as electrical resistivity and magnetism, etc. will change. Therefore, the effect of the ordered transition on the properties has to be considered. Small amounts of additional elements such as Mo or Cu are usually added to Ni3Fe alloys to inhibit the generation of long-range ordering [1] .

Interrelationships

　　Figure 1 illustrates the saturation magnetisation strength Js, Curie temperature Tc, magnetic crystal anisotropy K1 and magnetostriction constant λ of binary Ni3Fe alloys as a function of nickel content. Fig. 2 illustrates K1 and λ of Ni-(Fe+Cu)-Mo alloys as a function of composition and heat treatment cooling rate. Fig. 3 illustrates the variation of uniaxial anisotropy constants Ku1 and Ku2 with nickel content obtained by rolling and magnetic annealing processes. As can be seen from the figure, K1 is not only dependent on the composition, but also related to the short-range ordering of Ni3Fe (controlled by the heat treatment cooling rate). λ is basically determined by the composition, and the cooling rate has a small effect on λ111 and λ100 only at the composition of Ni3Fe. The Ku2 produced by magnetic field heat treatment at temperatures below Tc is an order of magnitude smaller than the Ku1 produced by slip deformation (during cold rolling.) Both Ku1 and Ku2 are uniaxially anisotropic, so that rectangular hysteresis loops are obtained when magnetised along their preferred direction, whereas flattened loops with a low Br are obtained when magnetised along the perpendicular direction. For alloys with 70%~80% Ni, which have K1≤0, the easy magnetisation direction is <111>, and it is necessary to avoid {100}<001> cubic weaving, random weaving with chaotic orientation. And nickel for 45% ~ 68% of the alloy K1>0, easy magnetisation direction is <100>, so in order to obtain high magnetic properties, should try to obtain cubic weave. Specifically can be used in large under pressure cold rolling and lower temperature (900 ~ 1050 ℃) annealing. The annealing of pozzolanic alloys should be carried out in a pure hydrogen atmosphere without oxygen, with a dew point below -40℃, or in an atmosphere with a vacuum of 10-2～10-3Pa.

Classification properties

　　Permalloys can be divided into four major categories according to composition: 35% to 40% Ni-Fe alloys, 45% to 50% Ni-Fe alloys, 50% to 65% Ni-Fe alloys and 70% to 81% Ni-Fe alloys. Each category can be made with a circular hysteresis loop, rectangular hysteresis loop or flat hysteresis loop material.

35%～40%

　　In the range of 35% to 40% nickel content, the magnetocrystalline anisotropy K1 decreases with the increase of nickel content, and the square ratio Br/Bs also becomes smaller, showing a circular hysteresis loop. This circular return line combined with high resistivity (ρ = 60 μΩ-cm at 40% Ni; and ρ = 45 μΩ-cm at 48%) and fine-grain isotropic microstructure results in lower core losses. For example, a 40% Ni-Fe alloy strip with a thickness of 0.05 mm has a loss of 9 watts per kilogram at 0.1 T and 20 kHz; while the corresponding loss for a 48% Ni-Fe alloy strip is 14 watts per kilogram. These alloys are suitable for square wave transformers, DC converters, etc.

45% to 50%

　　Alloys in this composition range have a high saturation magnetisation strength among the Permalloys with K1>0 and a susceptible magnetisation direction of <100>. Rectangular hysteresis loops can be obtained by forming cubic weaves for use in magnetic amplifiers, chokes, and transformers. Circular hysteresis loops can also be obtained by forming a secondary recrystallised {210} weave, or by forming a fine-grained isotropic microstructure with the aid of primary recrystallisation. This alloy has a high permeability and low coercivity and is used in current transformers, ground-fault circuit breakers, micromotors and relays.

50% to 65%

　　Alloys in this composition range have high Curie temperatures and high saturation magnetisation strengths and are in the ordered state K1≈0. Therefore, the magnetic field heat treatment effect is particularly pronounced and can produce strong induced magnetic anisotropy. Low temperature (Curie point below about 130 ℃) magnetic field heat treatment, the hysteresis loop is rectangular, DC permeability is high, but the dynamic characteristics of the poor; high temperature (Curie point below about 60 ℃) magnetic field heat treatment, the square ratio of the return line has decreased, the DC peak permeability is not high, but the dynamic characteristics of the good. Nickel-iron alloy containing about 55% nickel (plus 2% molybdenum) by high-temperature annealing, the formation of {210}<001> weaving structure or fine grain secondary recrystallisation organisation, and then high-temperature longitudinal magnetic field heat treatment, can significantly improve the μi and μm. 65% nickel iron alloy containing nickel with a fine-grained isotropic microstructure of the longitudinal magnetic field heat treatment, you can get a good dynamic characteristic of the rectangular hysteresis loop material, suitable for the magnetic amplifiers. This alloy by transverse magnetic field heat treatment, can get low Br flat-like return line, permeability in a certain range of magnetic field strength changes very little, known as constant permeability alloy, suitable for inductive components.

70%～81%

　　This composition range of Permalloy has high magnetic permeability. Although the binary nickel-iron alloy K1 and λ can not be reduced to zero at the same time, but in this composition range to add the appropriate amount of alloying elements such as molybdenum, chromium, copper, etc., and then through the control of the cooling rate of the heat treatment, it will be able to make K1 and λ at the same time converge to zero, so as to obtain a very high permeability and very low coercivity. Generally this alloy μi up to 40 ~ 60mH / m. In 1947, the Americans Bozorth (R.M. Bozorth) and others with pure raw materials, vacuum melting and in pure hydrogen in 1200 ~ 1300 ℃ annealed at high temperatures, obtained the μi and μm high Ni79Mo5 alloy, called the super-slope Permalloy. Its μi can reach 150mH / m or more, μm up to 1130mH / m. In the late 1960s, Japan's Masumoto amount of 78% Ni-Fe alloys in the addition of niobium, tantalum, and later added the fourth and fifth elements such as molybdenum, chromium, titanium, aluminium, manganese, etc., to obtain a high hardness and high permeability of the pozzolanic alloys, the hardness of its Hv>200, known as hard pozzolanic alloys. These alloys are suitable for making transformers, chokes, magnetic heads, magnetic shields, etc. In addition, this type of alloy through the formation of cubic weaving structure, the return line can also be rectangular; at the same time, control the degree of order of the alloy, so that K1 ≥ 0, it shows good dynamic characteristics, it is very suitable for doing magnetic modulator. Add 2% of 80% ~ 82% Ni-Fe alloy powder made of pressed powder core, with high resistance and good stability, can be used at a frequency of 300Hz.

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