Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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Revised Pages

18.9 Soft Magnetic Materials • 743

MATERIAL OF IMPORTANCE

An Iron-Silicon Alloy That is Used in Transformer Cores

A

s mentioned earlier in this section, trans-

former cores require the use of soft magnetic

materials, which are easily magnetized and demag-

netized (and also have relatively high electrical re-

sistivities). One alloy commonly used for this appli-

cation is the iron–silicon alloy listed in Table 18.5

(97 wt% Fe-3 wt% Si). Single crystals of this alloy

are magnetically anisotropic, as are also single crys-

tals of iron (as explained above). Consequently, en-

ergy losses of transformers could be minimized if

their cores were fabricated from single crystals such

that a [100]-type direction [the direction of easy

magnetization (Figure 18.17)] is oriented parallel

to the direction of an applied magnetic field; this

configuration for a transformer core is represented

schematically in Figure 18.20. Unfortunately, single

crystals are expensive to prepare, and, thus, this

is an economically impractical situation. A better

alternative—one that is used commercially, being

more economically attractive—is to fabricate cores

from polycrystalline sheets of this alloy that are

anisotropic.

It is often the case that the grains in polycrys-

talline materials are randomly oriented, with the

result that their properties are isotropic (Section

3.19). However, one way of developing anisotropy

Iron alloy core

Secondary

winding

Primary

winding

B field

Figure 18.20 Schematic diagram of a transformer core,

including the direction of theBfield that is generated.

in polycrystalline metals is via plastic deformation,

for example by rolling (Section 14.2, Figure 14.2b);

rolling is the technique by which sheet transformer

cores are fabricated. A flat sheet that has been

rolled is said to have arolling(orsheet)texture,

or there is a preferred crystallographic orientation

of the grains. For this type of texture, during the

rolling operation, for most of the grains in the

sheet, a specific crystallographic plane (hkl) be-

comes aligned parallel (or nearly parallel) to the

surface of the sheet, and, in addition a direction

[uvw] in that plane lies parallel (or nearly paral-

lel) to the rolling direction. Thus, a rolling texture

is indicated by the plane–direction combination,

(hkl)[uvw]. For body-centered cubic alloys (to in-

clude the iron–silicon alloy mentioned above), the

rolling texture is (110)[001], which is represented

schematically in Figure 18.21. Thus, transformer

cores of this iron–silicon alloy are fabricated so that

the direction in which the sheet was rolled (corre-

sponding to a [001]-type direction for most of the

grains) is aligned parallel to the direction of the

magnetic field application.^3

The magnetic characteristics of this alloy may

be further improved through a series of deforma-

tion and heat-treating procedures that produce a

(100)[001] texture.

Rolling plane

Rolling direction

[110] Plane [001] Direction

Figure 18.21 Schematic representation of the

(110)[001] rolling texture for body-centered cubic iron.

(^3) For body-centered cubic metals and alloys, [100] and [001] directions are equivalent (Section 3.l4)—that is, both
are directions of easy magnetization.