SECTION 14.2. SURVEY OF MATERIALS 155Nanocrystalline alloys. Nanocrystalline alloys have a microstructure consisting of
ultrafine grains in the nanometer range. The first step in the manufacturing ofnanocrystalline
alloys is the same as used for amorphous alloys. Subsequently, these alloys are given
a heat treatment above the corresponding crystallization temperature. The composition
of nanocrystalline alloys has been slightly modified with respect to that of soft-magnetic
metallic glasses and contains small additions of Cu and Nb. A well-known composition is for
instance The effect of the additions is to control the nucleation and
growth rates during crystallization. The result is a homogeneous, ultrafine grain structure.
In the example mentioned, the grains consist of (or rather having a grain
diameter of typically 10 nm. This structure leads to relatively high electrical resistivities
and makes these alloys suitable for high-frequency applications. In fact, nanocrystalline
alloys fill the gap between amorphous alloys and conventional polycrystalline alloys and
offer the possibility of tailoring superior soft-magnetic properties for specific applications.
In Fig. 14.2.8, the soft-magnetic properties of various groups of materials are compared.
It was mentioned already at the beginning of this chapter that a major requirement
for the attainment of superior soft-magnetic properties is generally a low or vanishing
magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant of the ultra-
fine grains is related to the crystal symmetry; the local easy axis of magnetization being
grainsdetermined by the crystal axis. The anisotropy constant is about for
the of that form the main constituent phase in nanocrystalline
This is much too large to explain by itself the low coercivity
and the high permeability
The key to the understanding of the superior soft-magnetic properties of the nanocrys
talline alloys mentioned is that the anisotropy contribution of the small, randomly oriented,