Magnetic alloys

5.8.4 Magnetic alloys

The work done in moving a domain boundary depends on the energy of the boundary, which in turn depends on the magnetic anisotropy. The ease of magnetization also depends on the state of internal strain in the material and the presence of impurities. Both these latter factors affect the magnetic ‘hardness’ through the phenomenon of magnetostriction, i.e. the lattice constants are slightly altered by the magnetization so that a directive influence is put upon the orientation of magnetization of the domains. Materials with internal stresses are hard to magnetize or demagnetize, while materials free from stresses are magnetically soft. Hence, since internal stresses are also responsible for mechanical hardness, the principle which governs the design of magnetic alloys is to make permanent magnetic materials as mechanically hard and soft magnets as mechanically soft as possible.

Magnetically soft materials are used for transformer laminations and armature stampings, where a high permeability and a low hysteresis are desirable: iron–silicon or iron–nickel alloys are commonly used for this purpose. In the development of magnetically soft materials it is found that those elements which form interstitial solid solutions with iron are those which broaden the hysteresis loop most markedly. For this reason, it is common to remove such impurities from transformer iron by vacuum melting or hydrogen annealing. However, such processes are expensive and, consequently, alloys are frequently used as ‘soft’ magnets, particularly iron–silicon and iron–nickel alloys (because silicon and nickel both reduce the amount of carbon in solution). The role of Si is to form a γ-loop and hence remove transformation strains and also improve orientation control. In the production of iron– silicon alloys the factors which are controlled include the grain size, the orientation difference from one grain to the next, and the presence of non-magnetic inclusions, since all are major sources of coercive force. The coercive force increases with decreasing grain size because the domain pattern in the neighborhood of a grain boundary is complicated owing to the orientation difference between two adjacent grains. Complex domain patterns can also arise at the free surface of the metal unless

278 Physical Metallurgy and Advanced Materials these are parallel to a direction of easy magnetization. Accordingly, to minimize the coercive force,

rolling and annealing schedules are adopted to produce a preferred oriented material with a strong ‘cube texture’, i.e. one with two procedure is extremely important, since transformer material is used in the form of thin sheets to minimize eddy-current losses. Fe–Si–B in the amorphous state is finding increasing application in transformers.

The iron–nickel series, Permalloys, present many interesting alloys and are used chiefly in com- munication engineering, where a high permeability is a necessary condition. The alloys in the range 40–55% nickel are characterized by a high permeability and at low field strengths this may be as high as 15 000 compared with 500 for annealed iron. The 50% alloy, Hypernik, may have a permeability which reaches a value of 70 000, but the highest initial and maximum permeability occurs in the

composition range of the FeNi 3 superlattice, provided the ordering phenomenon is suppressed. An interesting development in this field is in the heat treatment of the alloys while in a strong magnetic field. By such a treatment, the permeability of Permalloy 65 has been increased to about 260 000.

This effect is thought to be due to the fact that, during alignment of the domains, plastic deformation is possible and magnetostrictive strains may be relieved.

Magnetically hard materials are used for applications where a ‘permanent’ magnetic field is required, but where electromagnets cannot be used, such as in electric clocks, meters, etc. Mate- rials commonly used for this purpose include Alnico (Al–Ni–Co) alloys, Cunico (Cu–Ni–Co) alloys,

ferrites (barium and strontium), samarium–cobalt alloys (SmCo 5 and Sm 2 (Co, Fe, Cu, Zr) 17 ) and Neomax (Nd 2 Fe 14 B). The Alnico alloys have high remanence but poor coercivities; the ferrites have rather low remanence but good coercivities, together with very cheap raw material costs. The rare- earth magnets have a high performance but are rather costly, although the Nd-based alloys are cheaper than the Sm-based ones.

In the development of magnetically hard materials, the principle is to obtain, by alloying and heat treatment, a matrix containing finely divided particles of a second phase. These fine precipitates, usually differing in lattice parameter from the matrix, set up coherency strains in the lattice which affect the domain boundary movement. Alloys of copper–nickel–iron, copper–nickel–cobalt and aluminum–nickel–cobalt are of this type. An important advance in this field is to make the particle size of the alloy so small, i.e. less than 100 nm diameter, that each grain contains only a single domain.

Then magnetization can occur only by the rotation of the direction of magnetization en bloc. Alnico alloys containing 6–12% Al, 14–25% Ni, 0–35% Co, 0–8% Ti, 0–6% Cu in 40–70% Fe depend on this feature and are the most commercially important permanent magnet materials. They are precipitation- hardened alloys and are heat treated to produce rod-like precipitates (30 nm × 100 nm) lying along

the direction of the field, when the remanence and coercivity are markedly increased; Sm 2 (Co, Fe, Cu, Zr) 17 alloys also rely on the pinning of magnetic domains by fine precipitates. Clear correlation exists between mechanical hardness and intrinsic coercivity. SmCo 5 magnets depend on the very high magnetocrystalline anisotropy of this compound and the individual grains are single-domain particles. The big advantage of these magnets over the Alnico alloys is their much higher coercivities.

The Heusler alloys, copper–manganese–aluminum, are of particular interest because they are made up from non-ferromagnetic metals and yet exhibit ferromagnetic properties. The magnetism in this group of alloys is associated with the compound Cu 2 MnAl, evidently because of the presence of manganese atoms. The compound has the Fe 3 Al-type superlattice when quenched from 800 ◦

C, and in this state is ferromagnetic, but when the alloy is slowly cooled it has a γ-brass structure and is non-magnetic, presumably because the correct exchange forces arise from the lattice rearrangement on ordering. A similar behavior is found in both the copper–manganese–gallium and the copper– manganese–indium systems.

The order–disorder phenomenon is also of magnetic importance in many other systems. As dis- cussed previously, when ordering is accompanied by a structural change, i.e. cubic to tetragonal,

Physical properties 279

400 50 Nd 2 Fe 14 B (Neomax)

240 2 (Co–Fe–Cu–Zr) 17 30

Sm

(KJ m Sm 2 (Co–Fe–Cu) 17 (MGOe)

Sm–Pr–Co 5

) max ) max (BH 160

Sintered SmCo 5 20 (BH

Columnar Alnico

KS-Steel MK-Steel

1980 1990 Figure 5.33 The variation of (BH) max with time over this century (courtesy of I. R. Harris).

coherency strains are set up which often lead to magnetic hardness. In FePt, for example, extremely high coercive forces are produced by rapid cooling. However, because the change in mechanical properties accompanying the transformation is found to be small, it has been suggested that the hard magnetic properties in this alloy are due to the small particle-size effect, which arises from the finely laminated state of the structure.

While the much cheaper but lower performance magnets such as ferrites have a significant market share of applications, the rare-earth (RE) magnets have revolutionized the properties and applications of permanent magnets. A parameter which illustrates the potential of these materials is the maximum energy product (BH) max . The larger the value of (BH) max the smaller the volume of magnet required to produce a given magnetic flux. This is illustrated in Figure 5.33, where the neodymium–iron– boron materials have (BH) max in excess of 400 kJ m −3 , an order of magnitude stronger than the

ferrites. In the drive for miniaturization the Nd 2 Fe 14 B materials are unrivaled. They are also finding applications where a very strong permanent field is required, such as MRI scanners. The main process route for these magnets, shown in Figure 5.34, consists of powdering the coarse-grained cast ingot, aligning the fine powder along the easy magnetization axis, compacting and then sintering to produce

a fully dense magnet. The alloy is very active with respect to hydrogen gas so that, on exposure at room temperature and around 1 bar pressure, the bulk alloy absorbs the hydrogen, particularly in the grain boundary region. The differential and overall volume expansion results in the bulk alloy ‘decrepitating’ (separating into parts with a crying sound) into very friable particulate matter which

consists of fine, grain boundary debris and grains of Nd 2 Fe 14 B which are of the order of ∼100 µm in size. The phenomenon of ‘hydrogen decrepitation’ (HD) has been incorporated into the process route. Apart from economically producing powder, the HD powder is extremely friable, which substantially

aids the subsequent jet-milling process. With this modified processing route substantial savings of between 15% and 25% can be achieved in the cost of magnet production. The majority of NdFeB magnets are now made by this process.

280 Physical Metallurgy and Advanced Materials

Ingot material

Coarse crushing

Nd 2 Fe 14 B Nd-rich

Fine crushing

⫹ Hydrogen

Jet milling

Nd 2 Fe 14 B hydride

Aligning⫹pressing

Nd hydride

Sintering

Heat treatment

Hydrogen decrepitation

Grinding & slicing

A schematic representation of the HD process (courtesy of I. R. Harris).

Physical properties 281