Defense Intelligence Reference Document Metallic Glasses For Aerospace Applications

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Other Properties: Magnetic, Electrical, Optical, Thermal, and Acoustic
Although most of the current interest in metallic glasses centers on their mechanical
properties, it is appropriate to consider other properties of potential utility. Of these,
the magnetic properties of ferromagnetic metallic glasses stand out. 24 A variety of
ferromagnetic glass-forming alloys exist, mostly based on transition metals (iron,
nickel, and cobalt). The presence of alloying elements (necessary to make the material
glass-forming) means the saturation magnetization of metallic glasses is not as large as
that of the pure elements. However, some amorphous alloys have very low coercivity (a
measure of how strong a magnetic field must be to change the direction of
magnetization of the material) owing to the lack of crystalline defects (such as grain
boundaries) and magnetocrystalline anisotropy. In addition, the relatively high electrical
resistivity of amorphous alloys (see below) minimizes eddy current losses caused by
high-frequency magnetization/demagnetization. Some amorphous alloys also have
strong magnetoelastic effects (coupling between magnetic properties such as
susceptibility or magnetization and elastic strain). Current and potential future
applications of these magnetic properties are discussed below.
Like crystalline alloys, metallic glasses have conduction electrons that make them both
electrically and thermally conductive,25 although their structural disorder and high alloy
content make them poor conductors. In addition, in a behavior that is useful in some
applications, the conductivity of metallic glasses is not very sensitive to temperature;
an exception is near absolute zero, where some amorphous alloys become
superconducting.
Another consequence of the amorphous structure of metallic glasses is that they tend to
have very low acoustic damping. This may be useful in applications such as vibrating-
structure gyroscopes for vehicle orientation. 26
A common misperception among those hearing about metallic glasses for the first time
is to think they are transparent. This is not the case; amorphous alloys are highly
reflective, with a shiny luster similar to that of other metals (Figure 7). This is a result
of the presence of the conduction electrons, which scatter and absorb incident light.
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