Defense Intelligence Reference Document Metallic Glasses For Aerospace Applications

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Metallic Glasses
STRUCTURE
The atomic-scale structure of most metals and alloys is crystalline; that is, the atoms
are arranged in a highly ordered manner on a lattice that is periodic in three
dimensions, as depicted in Figure l(a). In contrast to this crystalline structure, metallic
glasses lack the long-range order of a lattice and are therefore said to be amorphous,
as depicted in Figure l{b). Although the word "amorphous" implies a complete lack of
structural order, in fact the atomic structure of metallic glasses is not truly random.
Constraints on atomic packing provide strong short-range order; for instance, on
average the atoms have a particular number of nearest atomic neighbors at a well-
defined distance. But this short-range order persists only over distances of a few
atoms; there is no long-range order as there is in a crystalline alloy. In many ways, the
atomic-scale structure of metallic glasses more closely resembles the highly disordered
structure of a liquid than the structure of a crystalline alloy.
(b)
Crystalline Amorphous (glass)
Figure 1, Amorphous Versus Crystalline Structure. Schematic atomic-scale structure of crystalline
(a) and amorphous (b) metals. In a crystalline structure, order- persists over long distances (many
atomic dimensions}. In a glass, there is short range order but no long-range order.
A corollary of this difference in structure is that the nature of structural defects is quite
different between crystalline and amorphous alloys. Crystalline alloys, for example,
have extended linear defects in the crystal structure, called dislocations, that are (in
large part) responsible for determining mechanical behavior. The lack of crystalline
order precludes the existence of dislocations in metallic glasses, but other sorts of
defects can be present and may influence properties and behavior.
From an applications point of view, the amorphous structure of metallic glasses has two
principal implications. First, the mechanical properties of amorphous alloys are
significantly different from those of their crystalline counterparts; some of these
differences are advantageous, but others are not. Second, because metallic glasses are
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